HEAT RESISTANT ALLOYS

James Kelly Director of Technology

TABLE OF CONTENTS INTRODUCTION...............................................................................................1 DEFINITIONS What Are Heat Resistant Alloys? .........................................................2 Rolled Alloys Products Nominal Compositions ..................................3 Heat Resistant Alloy Specifications .....................................................4 EFECT OF ALLOYING ELEMENTS ............................................................5 RESISTANCE TO THE ENVIRONMENT .....................................................12 Oxidation ..................................................................................................13 Laboratory Oxidation Testing ..............................................................17 Carburization............................................................................................22 Metal Dusting/Catastrophic Carburization/Carbon Rot.....................26 Nitriding.....................................................................................................29 Sulphidation .............................................................................................30 Halogen Gas Hot Corrosion ...............................................................34 Molten Salts .............................................................................................38 Molten Metals ..........................................................................................40 Magnetism................................................................................................45 STRENGTH AT TEMPERATURE..................................................................48 Tensile Strength ......................................................................................48 Elastic Modulus.....................................................................................48 Yield Strength..........................................................................................49 Ductility.....................................................................................................49 Creep and Rupture .................................................................................50 Creep-Rupture Testing ..........................................................................52 10,000 hour Rupture Strength Data ....................................................54 0.0001% per hour Minimum Creep Rate Data ...................................55 THERMAL FATIGUE........................................................................................56 WEAR .................................................................................................................58 Erosion .....................................................................................................58 Galling ......................................................................................................58 PHYSICAL METALLURGY .............................................................................60 Sigma .......................................................................................................60 Grain Growth ..........................................................................................63 HEAT RESISTANT ALLOY GRADES...........................................................65 Iron-Chromium Alloys ............................................................................65 Fe-Cr-Ni alloys, Ni under 20%.............................................................67 Fe-Ni-Cr alloys, Ni 30 to 40%..............................................................69 Ni-Cr-Fe alloys, Ni 45 to 60%..............................................................71 Ni-Cr-Fe alloys, Ni over 60%, 15 to 25%Cr.......................................72 Cast heat resistant grades.....................................................................74 DESIGN ..............................................................................................................78 Thermal Strain.........................................................................................79 Weldments ...............................................................................................81 Thermal Expansion ................................................................................82 Thermal Expansion Coefficients ..........................................................84 Section Size .............................................................................................85

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SELECTING THE ALLOY ...................................................................................86 Temperature..............................................................................................86 Atmosphere ...............................................................................................87 CUTTING AND FORMING..................................................................................88 Shearing ....................................................................................................90 Bending and Forming................................................................................90 Spinning and Deep Drawing .....................................................................93 Machining ..................................................................................................94 Forging ......................................................................................................95 WELDING ............................................................................................................96 Carbon Steel vs Stainless.........................................................................97 Shielding Gases ........................................................................................98 Cold Cracking versus Hot Cracking..........................................................99 Distortion ...................................................................................................100 Penetration ................................................................................................101 Fabrication Time........................................................................................101 Welding Austenitic Alloys ..........................................................................102 Alloys under 20% Nickel ...........................................................................103 Alloys over 20% Nickel..............................................................................104 Gas Metal Arc Welding .............................................................................105 Flux Cored Welding...................................................................................106 Shielded Metal Arc Welding......................................................................107 Gas Tungsten Arc Welding .......................................................................108 Plasma Arc Welding..................................................................................109 Submerged Arc Welding ...........................................................................111 Resistance Welding ..................................................................................111 Weld Filler Selection .................................................................................113 Dissimilar Metal Joints ..............................................................................114 Heat Resistant Alloy Weld Filler Metals ....................................................115 BRAZING AND SOLDERING..............................................................................116 APPLICATIONS Bolts...........................................................................................................119 Cast Link Belts ..........................................................................................120 Muffles .......................................................................................................123 Radiant tubes ............................................................................................125 Rotary Retorts & Calciners .......................................................................126 Salt Pots ....................................................................................................126 Springs ......................................................................................................133 FIELD FAILURES................................................................................................134 The Melted Radiant Tube .........................................................................134 The Hole in the Box...................................................................................135 The Culprit Copper ....................................................................................136 THUMBNAIL BIOGRAPHIES OF RA ALLOYS ..................................................139 CHEMICAL SYMBOLS........................................................................................140 BIBLIOGRAPHY..................................................................................................141 HISTORY.............................................................................................................143 TRADEMARKS....................................................................................................144 GERMAN STANDARDS COMPARED WITH AMERICAN TEMPERATURE CONVERSION CHART

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INTRODUCTION We cannot but marvel at the fact that fire is necessary for almost every operation. By fire minerals are disintegrated, and copper produced, in fire is iron born and by fire it is subdued, by fire gold is purified. Pliny the Elder, Natural History, Book XXXVI, 200 Rolled Alloys has specialized in supplying wrought heat and corrosion resistant alloys for a half century now. We have an experienced sales force, laboratory personnel and a more detailed inventory of heat and specialty corrosion resistant alloys than any other supplier. Our technical expertise, to which this paper is an introduction, includes documented field experience and laboratory studies back to 1952. Our current laboratory data, both oxidation testing and metallography, has been generated under the direction of Timothy J. Carney During these years, Rolled Alloys worked with the industrial furnace builders, and with those fabricators who also specialize in heat resistant alloy fabrication, to create the present market for RA330, RA333, RA 253 MA, RA 353 MA, and RA 602 CATM alloys. We have modified the chemistry and mill processing of RA330 on three separate occasions to maximize its effectiveness in heat treat applications. Beginning in the 1970s, Rolled Alloys initiated and drafted eleven separate ASTM specifications for our proprietary and semi-proprietary alloys. We, together with our suppliers, generated the data to obtain ASME Code coverage of RA330 to 1650F (900C). Although RA330 and RA333 are both sold to published ASTM, UNS or AMS chemistries, our internal purchasing specifications are designed for more rigorous quality control levels than required by these industry-wide specifications. We currently stock over a dozen different grades of heat resisting alloys, ten more aerospace grades used in gas turbine engines, specialty welding fillers and weld overlay wires, along with several alloys designed for corrosion applications. Several of these are proprietary to Rolled Alloys, and we have significant market share in others. Bulletin 401 Rolled Alloys 2005 minor corrections May 13, 2005 Note: This document continues to be developed and expanded. If your copy is more than two years old, a newer edition may be available. Contact Rolled Alloys Technology & Marketing Services, +1-734-847-0561, FAX +1-734-847-3915. James Kelly

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What are heat resistant alloys? Heat Resistant alloys, from our perspective, are those solid solution strengthened alloys (intended) for use at temperatures over 1400F (760C) and limited in the extreme to 2400F (1316C), which is near the melting point of these materials. These materials cannot be strengthened by heat treatment, as they are used over very broad temperature ranges, and above the temperatures where hardening mechanisms other than solid solution, carbides or nitrides are effective. There are two fundamental types of heat resistant alloys, the ferritic and the austenitic. Nearly all heat resistant alloys of interest to us are austenitic. The ferritic alloys, such as RA446, are simply iron with anywhere from 11% to about 26% chromium added. They have a body-centered cubic crystal structure, the same as does iron. These ferritic grades also have a little manganese, silicon, carbon and nitrogen, mostly included for their benefits in hot working the alloy at the steel mill. Ferritic grades are very weak and often brittle, and may be difficult to weld. They are magnetic. In spite of poor strength, ferritic heat resistant alloys may be used for their good resistance, at red heat, to sulphur bearing atmospheres, and to attack by low melting point metals, including molten copper. Notethe ferritic grades have some resistance to hot SO2 or H2S gas, but they are not resistant to aqueous corrosion by sulphuric acid. When enough nickel is added to the ironchromium mix, the alloy becomes austenitic. One example is RA310, with 25% chromium, just like RA446. With the addition of 20% nickel to a 25%Cr-iron base, the alloy becomes austenitic, with roughly ten times the strength of RA446, and much greater ductility. Austenitic alloys of interest to us cover the range from about 8% to 80% nickel. The austenitic alloys all have much greater creep-rupture strength than the ferritics. At room temperature the austenitics are more ductile and generally easier to fabricate. They are non-magnetic as supplied, although after certain high temperature service conditions some may become magnetic. It is these austenitic alloyswhich are of primary interest to us. The straight chromium grades also divide into two more classes of stainless steel, the martensitic and the precipitation hardening. However, neither of these are of any use above 1200F (~650C). An addition of carbon permits the straight chromium grades to be hardened by heat treatment, and constitute the martensitic stainlesses. These include types 410, and the 440s A, B and C. Martensitic stainlesses can be heat treated to maintain high strength through about 900F (482C), with a maximum use temperature of 1200F (649C). Like the ferritic stainlesses, all straight chromium grades embrittle severely after being held in the 700 to 1000F (370 to 540C) temperature range, the so-called 885F (475C) embrittlement. There is a class of very low carbon martensitic stainlesses which obtain their high strength by an age-hardening, or precipitation hardening, process. An addition of copper (such as in 17-4PH), molybednum or titanium is responsible for the precipitation hardening mechanism.

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Rolled Alloys Products, Nominal Composition Cr Ni Si Mo Co W Al Ti Other Heat Resistant RA333 25 45 1.0 3 3 3 - - - - 0.05C 18Fe RA330 19 35 1.25 - - - - - - - - - - 0.05C 43Fe RA330HC 19 35 1.25 - - - - - - - - - - 0.40C 43Fe RA 253 MA 21 11 1.7 - - - - - - - - - - 0.17N 0.08C 0.04Ce 65Fe RA 353 MA 25 35 1.2 - - - - - - - - - - 0.16N 0.05C 0.05Ce 36Fe RA 602 CA 25 63 - - - - - - - - 2.2 - - 0.1Y 0.08Zr 9.5Fe RA800H/AT 21 31 0.4 - - - - - - 0.4 0.6 0.07C 45Fe RA309 23 13 0.8 - - - - - - - - - - 0.05C 62Fe RA310 25 20 0.5 - - - - - - - - - - 0.05C 52Fe RA600 15.5 76 0.2 - - - - - - 0.2 0.2 0.08C 8Fe RA601 22.5 61.5 0.2 - - - - - - 1.4 - - 0.05C 14Fe RA446 25 - - 0.5 - - - - - - - - 0.08 0.15N 73Fe Stainless RA410 12 - - 0.3 - - - - - - - - - - 0.14C 87Fe RA410S 12 - - 0.3 - - - - - - - - - - 0.06C 87Fe RA17-4 15.5 4.7 0.3 - - - - - - - - - - 3.3Cu 0.3Cb 0.05C 75Fe RA321 17.3 9.3 0.7 - - - - - - - - 0.2 0.01C 70Fe RA347 17 9.5 0.7 - - - - - - - - - - 0.5Cb 0.04C 70Fe Corrosion Resistant AL-6XN 20.5 24 0.4 6.3 - - - - - - - - 0.22N 0.02C 48Fe RA20 20 33 0.4 2.2 - - - - - - - - 3.3Cu 0.5Cb 0.02C 40Fe RA2205 22.1 5.6 0.45 3.1 - - - - - - - - 0.16N 67Fe Aerospace RA X 22 47 0.3 9 1.7 0.6 - - - - 0.08 19Fe RA625 21.5 61 0.1 9 - - - - 0.4 0.4 3.6 Cb 0.05C 5Fe RA718 19 52 0.1 3 - - - - 0.5 0.9 5Cb 19Fe C-263 20 51 - - 5.8 20 - - 0.4 2.2 0.02Zr 0.001B Ren 41 19.3 52.7 0.07 9.4 10.6 - - 1.5 3.2 0.08C 3Fe WASPALOYTM 19 57 - - 4 13 - - 1.4 3 0.06C 2Fe RA188 22 22.5 - - - - 37.7 14 - - - - 0.10C 2Fe L-605 20 10.5 - - - - 50 15 - - - - 0.10C 2Fe

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Heat Resistant Alloy Specifications Alloy UNS Product Form ASME ASTM AMS W.Nr./EN (W.Nr.) RA333 N06333 Plate, sheet, strip - - B 718 5593 2.4608 (2.4608) Bar - - B 719 5717 Smless pipe, tube - - B 722 Welded pipe - - B 723 Welded tube - - B 726 RA330 N08330 Plate, sheet, strip SB-536 B 536 5592 1.4886 (1.4886) Bars & shapes SB-511 B 511 5716 Billets & bars - - B 512 Smless pipe, tube SB-535 B 535 Welded pipe SB-710 B 710 Welded tube - - B 739 Fusion weld pipe - - B 546 RA 253 MA S30815 Plate, sheet, strip SA-240 A 240 - - 1.4893 (1.4893) Bars and shapes SA-479 A 479 1.4835 Pipe SA-312 A 312 Welded tube SA-249 A 249 ASME Code Case 2033-1 RA 353 MA S35315 Plate, sheet, strip - - A 240 - - 1.4854 (1.4854) Bars and shapes - - - - Pipe - - A 312 RA 602 CA N06025 Plate, sheet, strip - - B 168 2.4633 (2.4633) Rod, bar, wire - - B 166 RA800H/AT N08811 Plate, sheet, strip SB-409 B 409 - - - - (N08810) Rod and bar SB-408 B 408 - - (1.4876) Smlss pipe &tube SB-407 B 407 RA309 S30908 Plate, sheet, strip SA-240 A 240 - - 1.4833 (1.4833, Bars and shapes SA-479 A 479 1.4833 1.4933) Pipe SA-312 A 312 RA310 S31008 Plate, sheet, strip SA-240 A 240 5521 1.4845 (1.4845) Bars and shapes SA-479 A 479 5651 1.4845 Pipe SA-312 A 312 RA446 S44600 Plate, sheet, strip - - A 176 - - 1.4763 (1.4763) RA600 N06600 Plate, sheet, strip SB-168 B 168 - - 2.4816 (2.4816) Rod, bar, wire SB-166 B 166 5665 Smlss pipe & tube SB-167 B 167 RA601 N06601 Plate,sheet, strip SB-168 B 168 5870 2.4851 (2.4851) Rod, bar, wire SB-166 B 166 Bar, forgings,rings - - - - 5715 Smlss pipe & tube SB-167 B 167

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EFFECT OF ALLOYING ELEMENTS Starting with a base of iron, the most important alloying elements in heat resisting alloys are: chromium (Cr) for oxidation resistance and nickel (Ni) for strength and ductility. Other elements are added to improve these properties, but heat resistant alloys are primarily alloys of iron, chromium and nickel, and a few are mainly nickel and chromium. Silicon is one of the most effective elements in contributing carburization resistance. CHROMIUM (symbol Cr) Chromium is the one element present in all heat resisting alloys. Oxidation resistance comes mostly from the chromium content (the same is true of aqueous corrosion resistance). Chromium adds to high temperature strength, and to carburization resistance. Metallurgically speaking, chromium tends to make the atomic structure ferritic, that is, with a body centered cubic (BCC) crystal structure. High chromium also contributes to sigma formation. RA446, which is essentially 25% chromium, 75% iron, is a ferritic alloy. Both the tendency to form ferrite, and to form sigma, are counteracted by nickel. NICKEL (symbol Ni) Nickel is present, anywhere from 8% up to 80%, in all of the austenitic heat resistant alloys. When added to a mix of iron and chromium, nickel increases ductility, high temperature strength, and resistance to both carburization and nitriding. Nickel decreases the solubility of both carbon and nitrogen in austenite. High nickel is bad for sulphidation resistance. Again speaking metallurgically, nickel tends to make the atomic structure austenitic, that is, with a face centered cubic (FCC) crystal structure. Nickel counteracts, but doesnt necessarily stop, the tendency for an alloy to form sigma. While RA446 with 25%Cr 75%Fe (iron) is a ferritic alloy, rather weak, if one substitutes 20% nickel for some of that iron one gets a 25% chromium, 20% nickel, 55% iron alloy called RA310. RA310 is much stronger and more ductile than RA446. IRON (symbol Fe) Heat resistant alloys may contain anywhere from 8 to 75% iron. In some proportions iron is a strengthening element, but it is easily oxidized and carburized unless protected by other elements. Metallurgically speaking, iron is a ferritizing element. Iron itself has a ferritic, or body centered cubic (BCC) crystal structure. Iron base alloys require a certain amount of nickel to be added before they become austenitic.

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EFFECT OF ALLOYING ELEMENTS, continued THE NEXT GROUP of alloying elements present in all heat resisting alloys is silicon, carbon, nitrogen, sulphur and phosphorus. All may be considered impurities arising from the steel making process. They may either be tolerated at some level as undesirable impurities, or controlled for their effects on metal properties.

Silicon, for example, affects the fluidity of the molten metal, which is an important variable in the steelmaking process. Carbon is controlled within certain limits in heat resisting alloys as a strengthening element, normally above 0.04%. In corrosion resistant grades carbon is considered an undesirable element, and is kept as low as practical, under 0.03%. Nitrogen may be controlled like carbon as a strengthening element in both the heat and the corrosion resistant grades. When not used deliberately, there may be about 0.05% or so N in austenitic stainless and nickel alloys. Sulphur is generally undesirable, but some sulphur is used to improve machinability. Phosphorus is quite harmful to weldability in nickel alloys. SILICON (symbol Si) Silicon improves both carburization and oxidation resistance, as well as resistance to absorbing nitrogen at high temperature. At high enough levels, silicon improves resistance to alkali metal hot corrosion.Silicon can decrease weldability in some, not all, alloys. In the U.S., Rolled Alloys has long been the only company to produce wrought heat resistant alloys containing silicon. RA330 has 1.2%Si, RA333 about 1% silicon. All the cast heat resistant alloys have silicon, in part because it increases fluidity of the molten metal. In Europe silicon is used to improve a number of heat resistant alloys, such as the AvestaPolarit inventions, RA 253 MA and RA 353 MA, and the German alloys 314 (1.4841) and 1.4828. The metallurgical effects of silicon are that it tends to make the alloy ferritic, or to form sigma. In RA330, the 35% nickel content is more than enough to prevent any embrittling sigma to form from the Si. Silicon decreases the solubility of carbon in the metal (technically it increases the chemical activity of carbon in the alloy). A silica (silicon oxide) layer, just under the chromium oxide scale on the alloy, is what helps the alloy resist carburization. CARBON (symbol C) Carbon, even a few hundredths of a percent, is a strengthening element. As the carbon level increases, the alloy becomes stronger, but it also becomes less ductile. Most wrought heat resisting alloys contain around 0.05 to 0.10% carbon, with RA 602 CA near 0.2%, and RA330HC at 0.40% C. The cast heat resisting alloys usually have from 0.35% up to 0.75% carbon. While strong, the cast alloys are not very ductile. Corrosion resistant grades, by contrast, have less than 0.03% carbon, and sometimes much less.

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CARBON (symbol C), continued Carbon is an austenitizing element, and tends to retard or prevent formation of ferrite and sigma. Carbon may actually be dissolved in the alloy, or, more commonly, it is present as small, hard particles called carbides. These are chemical compounds of carbon with chromium, molybdenum, tungsten, titanium, ziconium or columbium (niobium). NITROGEN (symbol N) A small amount of nitrogen serves to strengthen austenitic heat resisting alloys. Too much nitrogen can embrittle them. Nitrogen is used to strengthen Outokumpus heat resistant grades 153 MA, 253 MA and 353 MA, likewise Haynes HR-120. Nitrogen is also an austenitizing element. It tends to retard or prevent ferrite and sigma formation. A small amount of nitrogen is specified in RA446. This causes a little austenite to form (in with the ferrite) while it is being hot worked. This in turn helps keep the grain size from getting too large. Nitrogen at 0.23% is used in the corrosion resistant alloy AL-6XN to prevent sigma formation. It also raises the tensile and yield strengths of AL-6XN, and increases its resistance to chloride pitting corrosion. SULPHUR (symbol S) Sulphur is normally regarded as an impurity, and is commonly below 0.010% in most nickel alloys. It has the benefit of improving machinability, so for 304 and 316 bar it is kept up around 0.02%. Free machining stainless steels, such as 303, may have much higher sulphur, 0.3%. To improve hot workability, and therefor maximize yields, the steel mill normally refines the metal to a very low sulphur content. This is fairly easy to do with current melting processes such as the AOD (argon-oxygen decarburization) or ESR (electro-slag remelt) furnaces. AL-6XN is refined to extremely low sulphur, 0.001% being not uncommon. Sulphur is also detrimental to weldability. Along with simply removing the sulphur in the refining process, the harmful effects of S on hot working and welding may be reduced to a degree by the addition of some manganese. PHOSPHORUS (symbol P) Phosphorus is harmful to weldability. Phosphorus cannot be removed during the refining process. To produce alloys with low phosphorus, one must start with low phosphorus raw materials. Because phosphorus is so harmful to nickel alloy weldability, the nickel weld fillers themselves are normally specified to have no more than 0.015% phosphorus, and even lower P would be preferred.

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EFFECT OF ALLOYING ELEMENTS, continued OTHER METALLIC ALLOYING ELEMENTS include cobalt, manganese, tungsten, molybdenum, titanium, aluminum, columbium (also called niobium), zirconium, the rare earth elements such as cerium, lanthanum and yttrium, and boron. Copper and vanadium are used in some corrosion resistant alloys but not in the heat resistant grades. Some of these elements are added for strength, others like aluminum and the rare earth elements are largely for oxidation resistance. COBALT (symbol Co) Cobalt at the 3% level in RA333 improves strength slightly and enhances oxidation resistance at extreme temperatures. Larger amounts are required for a significant strengthening, such as the 15%Co in cast Supertherm or 12.5% in the LBGT combustor alloy 617. Cobalt base alloys L605 and 188 are very strong, but oxidation resistant only to 1800 or 2000F (1000 or 1100C). Cobalt is an austenitizing element, like nickel. High cost and variations in availability tend to limit the use of cobalt alloys to gas turbine engine applications. MANGANESE (symbol Mn) Manganese is used in steelmaking to improve hot workability. It is mildly detrimental to oxidation resistance, so is limited to 2% maximum in most heat resistant alloys, and restricted further, to 0.80% max, in RA 253 MA. Manganese improves weldability, and is added to many austenitic weld fillers. RA330-04 achieves its hot cracking resistance from about 5% manganese added to the 35%Ni 19%Cr base. Manganese is usually considered an austenitizing element. It increases solubility for nitrogen and has for decades been used in the Nitronic series of stainless steels from AK Steel (formerly Armco), both as a partial substitute for nickel and to permit a substantial nitrogen addition. TUNGSTEN (symbol W) Tungsten is a large, heavy atom used as a strengthening addition, 3% in RA333, 5% in the cast alloy Supertherm and 14% in Haynes alloy 230. Tungsten is a carbide forming element, that is, it reacts with the carbon in the alloy to form a hard particle, which may incorporate other carbide forming elements such as chromium. Tungsten also promotes formation of sigma, and of ferrite. Tungsten metal, with thoria or rare earth oxide additions, is used for the electrode in gas tungsten arc welding. It is tungstens very high melting point, 6170F (3410C), which is required for this application. Tungsten oxidizes in air readily above 950F (510C), but is prevented from doing so by the argon or helium weld shielding gas.

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MOLYBDENUM (symbol Mo) Molybdenum is another large, heavy atom used to increase high temperature creep-rupture strength. 3% Mo is used in RA333. This is about as much Mo as can be tolerated in a heat resistant alloy without serious oxidation problems in heat treat furnace applications. Alloys X, 625 and 617 all contain 9% molybdenum, which is very good for strength but not so good for oxidation at extreme temperatures (2100F/1150C). Molybdenum promotes sigma formation, unless counterbalanced by austenitizing elements such as nickel, cobalt, etc., and is a ferritizer. Molybdenum is also a carbide forming element. Molybdenum helps weldability in austenitic alloys, both stainless and nickel base. Commercially pure molybdenum metal is used for vacuum furnace fixturing, because of its very high melting point, 4730F (2610C), and high temperature strength. However, molybdenum metal has no oxidation resistance above 800F (427C) and would literally disappear in a cloud of white smoke if exposed to air at red heat. TITANIUM (symbol Ti) Titanium is added in small amounts, about 0.3- -0.7%, for strength in austenitic alloys. Around 0.10.2%Ti is used, as part of steel mill melting practice, in deoxidation of nickel alloys. Ti is a strong carbide former, and it is the titanium carbides that strengthen RA800AT. Titanium also promotes sigma and ferrite, but it is normally used in such small amounts as to be inconsequential in this respect. In aqueous corrosion alloys titanium is referred to as a stabilizing element. Age hardening alloys used in aerospace, such as A-286, X750, C-263, the various Nimonic alloys, Ren 41, WASPALOYTM and 718, depend upon some larger amount of titanium, to 3% or so, for their age hardening properties. Titanium metal itself, although it has a very high melting point (3040F/1671C), is not really a heat resistant metal. Titanium alloys are used up to about 600F (316C) in aerospace applications. ALUMINUM (symbol Al) Aluminum is added at the 1 to 5% level for oxidation resistance. RA 602 CATM has 2.2% Al, alloy 601 contains 1.4% Al, and Haynes 214 4.5% aluminum. Aluminum is a ferritizing element, and promotes sigma formation. It is used in the age hardening (precipitation hardening) alloys. At around 0.1 to 0.4%, aluminum is added to most nickel alloys as a deoxidizing agent, in the last stages of AOD refining.

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COLUMBIUM (symbol Cb) . . . also called NIOBIUM (symbol Nb) Columbium is added at the 0.4 to 0.8% level for strength in several heat resisting alloys, and to prevent corrosion after welding in 347 stainless and nickel corrosion resistant alloys 20Cb-3, G-3 and G-30. This low amount of Cb is harmful to weldability, while higher amounts are beneficial. About 2 to 2.7%Cb is used in various high nickel weld fillers (82, 182), while the 3.6% Cb in 625 is good for both strength and weldability. Columbium is very harmful to oxidation resistance, practically speaking around 1800F/980C and higher. For this reason we limit the amount of residual Cb that may be present in RA330, and in RA333. Columbium is a strong carbide former, a ferritizing element and promotes sigma formation. At the 5% level, it is the age hardening element in alloy 718. ZIRCONIUM (symbol Zr) Zirconium is a strong carbide former. It is added in very small amounts, less than 0.1%, to increase strength in RA 602 CATM, and in alloy 214. THE RARE EARTH ELEMENTS cerium, lanthanum and yttrium are used singly or in combination to increase oxidation resistance in austenitic alloys both wrought and cast, and in the newer ferritic heat resistant alloys. The technology has been known, but little used, since about 1940 in Germany. CERIUM (symbol Ce) Cerium is the major rare earth element responsible for the excellent oxidation resistance of RA 253 MA. The cerium is added as an alloy of several rare earths, called mischmetal. For chemistry control purposes, the mill analyzes only for Ce. Mischmetal is encountered in everyday life as the flint in a cigarette lighter. It oxidizes (burns) very readily. In RA 253 MA and RA 353 MA the Ce helps chromium form a thinner, tighter and more protective oxide scale. Residual cerium oxides in the metal may contribute to creep-rupture strength. LANTHANUM (symbol La) Used at the 0.02 to 0.05% range to for oxidation resistance in Haynes alloys 230, S, 556 and 188. YTTRIUM (symbol Y) Used at the 0.005 to 0.1% level for oxidation resistance in 214 and RA 602 CA. A larger amount, 0.5%, of yttria (Y2O3) is used as an oxide dispersion strengthening element in the ODS ferritic alloys such as Inconel MA956. As yttria, it also increases oxidation resistance.

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BORON (symbol B) Boron increases creep-rupture strength, and is used at rather low concentrations, 0.002% is typical. Boron is somewhat harmful to weldability of nickel alloys, so nickel alloy weld filler is often made without boron, even though the matching base metal alloy has a boron addition. This is specifically true of alloys such as RA333, X and 230. Boron is an interstitial element and tends to concentrate at the grain boundaries. It is used in high temperature braze alloys, specifically the Nickel-Silicon-Boron braze alloys developed by Dr. Robert Peaslee.

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RESISTANCE TO THE ENVIRONMENT Corrosion resistance at high temperatures is a general term for resistance to a variety of hot gaseous or liquid environments that can eat holes through the metal, turn it completely into a pile of scale or seriously embrittle a formerly ductile alloy. It includes (but is not limited to) the effects of oxygen, nitrogen, chlorine, carbon, sulfur, phosphorus, various molten salts and low melting metals. In many high temperature environments one significant effect of corrosion is to continuously change the actual chemistry of the alloy throughout its life. With the exception of selective leaching (parting corrosion), the examples being dezincification of copper-zinc alloys1, or denickelification2 of the 67Ni-31Cu alloy Monel 400, this is rather uncommon in aqueous corrosion of nickel base alloys or stainless steels. Nevertheless in high temperature corrosion selective removal of one or more alloying elements through the life of the equipment is the norm. Or, the metal may increase in carbon and nitrogen content, with consequent major change in mechanical properties, and no loss in cross section. Heavily carburized metal actually increases in volume. The types of high temperature corrosion most commonly encountered are oxidation, carburization, sulfidation, hot salt corrosion, chlorination and attack by low melting metals. Nitriding may be important, but is less often considered, if not encountered. It is difficult to run high temperature corrosion tests in the laboratory and obtain results that can be used to predict metal behavior in service. Even two laboratories running the same type of test may not come up with numerical results that agree with one another, although the alloy rankings should be similar. Based on extensive experience and lab work, we have confidence that our oxidation data may be used to compare relative performance of one alloy with another, at that test temperature. Still, good performance of a new alloy in this test only indicates that the alloy MAY perform well in service. As oxidation rates vary with thermal cycling, among other variables, the data are not directly useful for predicting metal wastage/corrosion rates of high temperature equipment in service. It is even more difficult to obtain data useful as an engineering tool to predict life of equipment in sulphidation, carburization, liquid metal environments and other types of high temperature corrosion. We must emphasize that laboratory data are a necessary first step, but the laboratory test itself must be validated by documented service experience for it to be a useful engineering tool. In the following pages we will present the results of both laboratory testing, controlled service experience and experience reported by others. Read these data, as well as those of other suppliers, with a critical eye. References 1. The Corrosion Handbook, page 69, edited by Herbert H. Uhlig, Ph.D., John Wiley & Sons, New York, 1948 2. D.R. Lenard and R.R. Welland, Corrosion Problems with Copper-Nickel Components in Sea Water Systems, NACE Corrosion 98, Paper Number 599, NACE Houston, Texas

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OXIDATION For our purposes, this means the high temperature chemical reaction of a metal with the oxygen in the air. Simply put, most metals can burn when they get hot enough. Some, like magnesium (once used in flash bulbs) and titanium do burn in the conventional sense and cause serious industrial fires. Even iron burns. Of course, lighting a match to a nail does absolutely nothing. But if one takes very fine iron wirespecifically, 0000 steel woolit may indeed be ignited by a match. There is no actual flame, but a red hot coal develops and enough sparks fly to endanger clothing. There are two basic ways in which a metal may be resistant to oxidation. First, it may be inert and simply not react chemically with oxygen in the air. Two examples come to mind, the precious metals gold (Au) and platinum (Pt). Because of its high melting point, 3217F (1769C), coupled with oxidation resistance, platinum is actually used for some laboratory ware and other items which must withstand extreme temperature. The second way a metal may resist oxidation, and the one of interest to us, is that the metal or alloy may form an adherent oxide film, which protects it from further oxidation. The element most often used to form such a protective oxide layer, or scale, is chromium. It forms the oxide Cr2O3, otherwise known as chromia. Although chromium itself oxidizes even more readily than iron, the oxide it forms is very thin, and adheres tightly to the metal. This oxide layer forms very quickly at high temperature, but once formed it protects the metal against further oxidation. The chromia scale also protects the alloy against carburization and sulfidation, to a degree. The protection is by no means perfect. The scale contains defects through which oxygen and other elements may pass, to continue to react with the alloy. It also cracks from both thermal and mechanical strains, and small pieces spall off each time the metal is cooled down. For a high temperature alloy to have useful oxidation resistance, the scale must be able to heal these defects, by more chromium diffusing to the surface to form a new protective film. Other elements are added to the alloy to improve the protective nature of this oxide film or scale. One of the most effective is silicon. Silicon oxidizes to SiO2, or silica. If enough silicon is present, the silica forms a sub-scale underneath the chromium oxide scale. This silica subscale is how silicon provides resistance to carburization, in alloys such as RA330. At the 1.2% Si level in RA330, silicon also contributes to oxidation resistance. In RA85H, which is no longer produced, the silicon was much higher, 3.5%. At this high level silicon appeared to offer resistance to molten alkali salt corrosion. The effectiveness of the chromium oxide scale may be improved by very small additions of rare earth elements, such as cerium. Cerium promotes a thinner scale, which is more protective against oxidation because it cracks and spalls off less than would a thicker scale. It is the 0.04% cerium in RA 253 MA that is responsible for the excellent oxidation resistance of this rather lean 21Cr, 11Ni alloy.

- 13 -

OXIDATION, continued Aluminum is also used to improve oxidation resistance. In order to actually develop an Al2O3, or alumina, scale a rather high amount of aluminum is required. At 2.2% aluminum, RA 602 CA alloy will form an alumina subscale. This contributes to the oxidation resistance of RA 602 CA. RA 602 CA does not oxidize internally. The 1.7% Al typical in alloy 601 is not enough to form an alumina scale, but it is enough to enhance oxidation resistance of 601. Because of aluminum at this somewhat lower level, 601 oxidizes internally. This is not a problem in plate gauges, though perhaps it may be a consideration in thin sheet. The 4.5% aluminum in Haynes alloy 214 is enough to form an alumina scale, and 214 is extremely oxidation resistant (above 1800F/982C).

The Protective Film While chromium is given credit for promoting oxidation resistance and is without question the most effective element in this respect, it is actually the most easily oxidized. This may sound like double talk, but it really isnt. When pure chromium or a chromium-bearing alloy is exposed to oxygen, even at room temperature, it is oxidized and a layer of chromium oxide (and oxides of other elements as well) is formed. Even the chromium plate on automobiles, or the cutlery on our dinner tables, has a microscopically thin and transparent film of chromium oxide present. When formed at high temperatures, the oxide coating becomes green, black, blue or yellow, depending upon its thickness and which of the numerous chromium oxide compounds is formed. This in turn depends upon the temperature and availability of oxygen to combine with chromium. The oxide layer is dense, is inclined to be tightly adhering, and effectively seals out the air or oxygen from the metal underneath. So long as the oxide layer is intact, the metal is protected and further oxidation proceeds very slowly. Several things may tend to destroy our protective layer: Expansion and contraction, as the result of heating and cooling, will pop the oxide layer, because the base metal and the oxide expand and contract at different rates. The more rapid the rate of expanding and contracting, or the more quickly the metal is heated and cooled, the more hazard there is of the protective coating flaking off. Certain combinations of chromium, iron, nickel, silicon and other oxides are more tightly adhering than others at different temperatures. With some alloys it is possible to reach a temperature where the scale or oxide is no longer tightly adhering and will be loose, thereby offering little or no protection. Thus, an excessive temperature for the specific alloy can destroy the protection normally offered by the oxide layer. Some examples are 321, which is acceptable at 1600F (870C) but scales unacceptably at 1800F (980C), and 309, which isnt very useful above 1900F (1040C).

- 14 -

The Protective Film, continued

Composite radiant tube, RA333 for 4 feet (1.2metre) on the firing end, middle portion RA330 and exhaust end fabricated of RA309. Used at a nominal furnace operating temperature 1750F ( 955C) for annealing malleable iron castings. It is to be expected that the tube metal temperature would be perhaps 100150F (5585C) higher. A jam-up in the furnace broke the tube. Note the crater-like appearance of local oxidation, or warts, on the RA309. When an alloy is used at a temperature exceeding its capabilities the scale may breakdown locally, a condition sometimes called warts. We have observed this on 309 (above) and 310, occasionally on RA 253 MA, RA330 and 600 alloy. On one occasion we saw warts on an RA333 brazing muffle. Upon investigation we found that the alloy had been heated in service to the incipient melting temperature. The grains were sliding apart so as to leave voids at the triple points, resulting in apparent porosity of the 11gage (3mm) muffle wall. Mechanical deformation and creep, such as the stretch of a bar under load, may also destroy the protection. While the metal is ductile and yields in creep, the oxide coating is fragile and brittle and will spall off. In service, a given item may appear to have insufficient oxidation resistance, whereas that particular property would have been more than adequate had the strength been sufficient to avoid excessive creep. Laboratory data which do not duplicate cyclic conditions or stresses imposed in actual service can be misleading as a measurement of an alloys oxidation resistance.

- 15 -

The Protective Film, continued Of great concern are environments that promote the destruction of the protective layer by some chemical reaction. For example, we know of one case where minute amounts of potassium nitrate/nitrite austempering salts were present on fixturing used in a carburizing atmosphere. The salts attacked the protective oxide coating, so that a normally carburization resistant alloy carburized very quickly and uniformly. In years past, we knew of a few cases where parts being heat-treated were first coated with sal ammoniac (ammonium chloride). The presence of this chloride salt resulted in a chemical attack upon the protective oxide coating, so that the alloys normally selected for the strength, oxidation resistance and thermal shock resistance requirements were not suitable. Another form of chemical destruction that may be encountered is corrosion from welding fluxes. Fluoride-bearing fluxes from coated welding electrodes must be carefully and thoroughly removed. Otherwise they continue to function as a flux, damaging not only oxidation resistance, but also carburization resistance. Green rot might be considered one form of destruction of the protective oxide coating. To the best of our knowledge, green rot tends to be more prevalent in alloys containing about 65% or more nickel. Green rot is the result of the alloy being alternately exposed to oxidizing and reducing conditions. When the alloy is exposed to the oxidizing environment, a protective oxide coating is formed, as we previously discussed. When the alloy is exposed to highly reducing conditions, the nickel and other less stable oxides may be reduced to pure metal, which disappears as a powder; but the chromium oxide, being more stable, is not reduced. Upon exposure of the alloy to an oxidizing environment once more, the oxygen is free to penetrate to the metal and form another layer of oxide, since there are now voids in the coating where some of the oxides previously existed. With continuous exposure to the two conditions, a mass is eventually formed consisting only of porous chromium oxide, with or without other oxides that may have been sufficiently stable to resist reducing. This actually has little strength and no ductility. It has the characteristic greenish-black color of chromium oxide and, upon fracture, has the appearance of rotten wood. Hence the name, green rot. Catastrophic oxidation is, as its name implies, oxidation which proceeds so rapidly that complete failure of the material occurs in an extremely short time. Certain elements, such as molybdenum, columbium (niobium) and vanadium, form oxides that are volatile at relatively low temperatures. If these oxides are formed and retained in the scale, they act as fluxes and destroy the protective film1, 2. The effect of molybdenum is important enough that we would like to quote directly from the late Howard S. Averys classic work on heat resistant alloys, Cast Heat-Resistant Alloys for High-Temperature Weldments: Where in fact the addition of molybdenum has conferred

- 16 -

The Protective Film, continued better hot strength, the chief problem may be surface stability, especially in the 18002300F (9801260C) range. This is most serious under those conditions that cause catastrophic oxidation which stems from the volatile nature of molybdenum oxide (MoO3). This oxide is likely to form in stagnant atmospheres, with a threshold for trouble around 14001500F (760816C). Catastrophic oxidation may be a serious problem under certain operating conditions. That is, a stagnant atmosphere, or solid deposits under which the atmosphere is of course stagnant, and extreme temperatures. Alloy X (N06002, W.Nr. 2.4665), containing 47% nickel, 22% chromium and 9% molybdenum, may completely disappear from catastrophic oxidation when heated for some months at 2200F (1200F). At lower temperatures in free-flowing atmospheres alloy X is highly oxidation resistant. It has, after all, served for decades as the primary alloy used in gas turbine flight engine combustors. However, alloy X may not well tolerate stagnant conditions or temperature extremes. Laboratory Oxidation Testing In order to evaluate new and competitive alloys we perform considerable laboratory oxidation testing at Rolled Alloys, at temperatures up to 2200F (1204C)3, 4 . We measure weight gain, that is, the total amount of oxygen (and nitrogen) that has reacted with the test specimens. Specimens are usually of plate gages, and the tests are cyclic. Samples are heated in porcelain crucibles, 4 to 6 in a tray, for about 160 hours (one week) at temperature. The tray is then removed from the furnace, lids are quickly placed on the crucibles to contain spalling oxide, and the assembly allowed to air cool to room temperature. The crucible, containing specimen and scale, is then weighed every cycle. Results are reported as weight gain, in milligram/centimeter2 . The numerical results are valid only for the specific conditions of the test. Which means they are not useful for predicting metal wastage of components in actual service. However they are of value when one compares the data from new alloys, with those of existing grades. For example, we have a great deal of experience with the good performance of RA333 and RA330. Likewise, 309 is about the only one of our heat resistant alloys that occasionally gives disappointing performance, generally around 1900F (1040C) or above. If an alloy performs well on test, that means it MAY perform well in service. As a simple coupon test for 3000 hours or so does not simulate all the things that can happen in service, it is possible for an alloy to look very good in the laboratory and not at all so good in production equipment. One thing the coupon test does not simulate is the effect of creep strain spalling off the scale. Another is the effect of stagnant atmospheres, which may occur in certain areas of electrically heated equipment, or underneath solid deposits. Alloys with high molybdenum contents are subject to catastrophic oxidation under these conditions.

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OXIDATION, continued And, finally, the laboratory test does not properly simulate time. 3000 hours seems a reasonable length of time to run a test in our laboratory, but that is still only about 4 months. If one expects the equipment to last 1, 2 or 10 years, it would be hard to make a case that a 4 month test adequately represents service conditions. The specimen continually changes chemistry throughout the test (it loses chromium, silicon and aluminum by scaling). Thin samples, simply from having less total chromium, may show greater oxidation rates than thick specimens. In our considered view, significant extrapolations of oxidation, or other high temperature corrosion data, are not valid. Nevertheless the declining availability of experienced engineers in the U.S.A. has generated pressure to extrapolate such data, valid or not. Data shown on the following bar graphs is all for 3000 hour (~18 weeks) exposure, in order to compare all alloys for about the same exposure time. All but the RA309 tests were run for 3000 hours, that one being extrapolated from a 1600 hour run. As this is weight gain data, high numbers mean heavy oxidation, small numbers a relatively light degree of oxidation. One would expect to use these numbers, along with service experience, as a guide to an alloys usefulness. However, unlike what is assumed about aqueous corrosion rates, oxidation data ought in our opinion be viewed qualitatively. Numbers under 20 may give assurance that the alloy, in plate form, should not lose structural integrity due to metal loss. One might want a little actual service background when considering alloys with weight gains in the 100-300 mg/cm2 range. One example is 304 stainess, which gains 64 mg/cm2 at 1600F (871C), and in the neighborhood of 300 mg/cm2 at 1800F (982C). By experience, we know that 304 1/4 plate will simply disappear in 2-3 months when used in air around 1700-1800F (930980C). We would look at the higher alloys, for more elevated temperatures, somewhat differently. Note 600 alloy which shows a 153 mg/cm2 weight gain at 2100F (1149C). Nevertheless, alloy 600 plate is a useful material for retorts and muffles operating in the 2100-2200F (11501200C) temperature range. Likewise RA 353 MA is used quite successfully at such temperatures. RA 602 CA is clearly the best by far in our test series. The good resistance to scaling of RA 602 CA in test has also been borne out by service experience in rotary calciners and CVD retorts. Bear in mind that these data still represent simple laboratory oxidation testing, which does not take into account many of the ways by which the protective oxide scale may be damaged. The alloys were cycled to room temperature once a week. More rapid thermal cycling would not only increase oxidation rates but might also change the relative performance of some alloys. In static 1000 hour oxidation testing, for example, 310 is somewhat superior to RA330. When thermal cycling is added, RA330 better retains its protective oxide. Another point to remember is that alloys high in molybdenum and columbium may be sensitive to catastrophic oxidation, particularly under stagnant atmospheres.

- 18 -

References 1. Leslie and Fontana, Transactions ASTM, Vol 41, pages 1213-1247, 1949, ASTM

Philadelphia, Pennsylvania 2. H.S. Avery, Cast Heat-Resistant Alloys for High-Temperature Weldments, WRC Bulletin

143, August 1969, Welding Research Council, New York, New York 3. Gene Rundell and James McConnell, Oxidation Resistance of Eight Heat-Resistant Alloys

at 870, 980 1095 and 1150C, Oxidation of Metals, Vol. 36, Nos. 3/4, 1991 4. J.C. Kelly and J.D. Wilson, Oxidation Rates of Some Heat Resistant Alloys, Heat-

Resistant Materials II, Conference Proceedings of the 2nd International Conference on Heat-Resistant Materials 11-14 September, 1995 Gatlinburg, Tennessee

- 19 -

- 20 -

Exposed for 3000 HoursCycled Every 160 Hours

64

1 1

19

5 4

2032

200

113

54

11*

154*

53

0

50

100

150

200

250

RA304 RA309 RA310 RA 253 MA RA330

Alloys

Wei

ght G

ain

(mg/

cm2 )

1600F1800F2000F2100F

- 21 -

Exposed For 3000 HoursCycled Every 160 Hours

32

4 4

143

83

1121 18

12 11

295

145153

58

36 3421 18

232

156

50 49

333324

138

61

0

50

100

150

200

250

300

350

RA800H RA625 RA600 RA X RA 353 MA RA333 RA601 RA 602 CA

Alloys

Wei

ght G

ain

(mg/

cm2 )

1800F2000F2100F2200F2250F

CARBURIZATION

The femper of Iron for Files

It must be made of the best Steel, and excellently tempered, that it may polish, and fit other iron as it should be: Take Ox hoofs, and put them into an Oven to dry, that they may be powdered fine: mingle well one part of this with as much common Salt, beaten Glas, and Chimney-soot, and beat them together, and lay them up for your use in a wooden Vessel hanging in the Smoak; for the Salt will melt with any moisture of the place or Air. The powder being prepared, make your iron like to a file: then cut it checquerwise, and crossways, with a sharp edged tool: having made the Iron tender and soft, as I said, then make an Iron chest to lay up your files in, and put them into it, strewing on the powders by course, that they may be covered all over: then put on the cover, and lute well the chinks with clay and straw, that the smoak of the powder may not breath out; and then lay a heap of burning coals all over it, that it may be red-hot about an hour: when you think the powder to be burnt and consumed, take the chest out from the coals with Iron pinchers, and plunge the files into very cold water, and so they will become extream hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters. But I shall teach you to temper them excellently G. B. Della Porta, 1589, Sources for the History of the Science of Steel 15321786, Ed. Cyril Stanley Smith Carburizing is one of the most commonly performed steel heat treatments. For perhaps three thousand years it was performed by packing the low carbon iron parts in charcoal, then raising the temperature of the pack to red heat for several hours. The entire pack, charcoal and all, was then dumped into water to quench it. The surface became very hard, while the interior or core of the part retained the toughness of low carbon steel. Pack hardening is uncommon today. Now, low carbon steel parts are heated in a prepared furnace atmosphere that provides the carbon which diffuses into the surface layers of the steel. Temperatures are usually around 1750F (950C). This atmosphere has traditionally been endothermic, made by partially burning natural gas. Typical composition1 of an endothermic gas (Class 302) is 39.8% nitrogen, 20.7% carbon monoxide, 38.7% hydrogen and 0.8% methane, with a dew point 5F (20C). This carrier gas is subsequently enriched by a small, controlled addition of a hydrocarbon gas, such as propane, or an easily vaporized liquid, which is the source of carbon. 100% nitrogen, from bulk tanks, may also be used as a carrier gas, with propylene or other hydrocarbon injected to provide the necessary carbon. The end result is that low carbon steel parts acquire a high carbon steel surface. When the steel is quenched it combines the hardness and wear resistance of this high carbon steel case with the toughness of the low carbon steel interior (core). The alloy bar frame baskets2, radiant tubes and other fixturing in the furnace also pick up carbon through many, many heat treat cycles. The fixtures are made of carburization resistant alloys. Even though the atmosphere is reducing to iron, it is still oxidizing to the chromium and silicon which provide most of the alloys resistance to carburization. Even a carburization resistant alloy eventually carburizes. Austenitic alloys do not harden when quenched, like the steel work pieces. Nevertheless, once the heat resistant alloy has picked up sufficient carbon, its room temperature ductility will be greatly reduced.

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Carburization embrittles high temperature alloys, so that they can not be straightened or weld repaired. The degree of embrittlement depends upon the amount of carbon absorbed3, and upon the microstructure. Generally speaking, once an alloy has absorbed about 1% carbon it will no longer have measurable room temperature ductility. We once examined a sample of 310 sheet which contained 4% carbon, and could readily be broken by hand. Enough ductility may remain while at red heat for the metal to perform its task. This, so long as it is not excessively strained at high temperature, or impacted at room temperature.

Alloy 601 used in a powdered iron sintering muffle, grain growth is from the operating temperature. Brittle fracture at room temperature comes from the large amount of carbon, 2.34%, absorbed during service. The nitrogen-hydrogen atmosphere is not supposed to be carburizing. Carbon enters the atmosphere from the organic compounds used when the green powder compact is pressed. Carburization resistance in an alloy is conferred almost entirely by the protective oxide scale4, along with the nickel content. The oxide scale is primarily chromia, with silicon being a very potent assist5. Nickel lowers the solubility of carbon in the alloy, so that a very high nickel grade simply will not carburize to the same level as will a lower nickel material. RA330 usually does the best job for the money. RA333, RA600, RA 353 MA, RA601 and RA 602 CA are all more carburization resistant but also more expensive. 800H does not well tolerate the effects of carburization, in part because it lacks silicon but also, and more importantly, because it is invariably coarse grained. RA 253 MA has worked as furnace fixturing because it is strong, but RA 253 MA is not resistant to carburization. Even RA309 has somewhat better carburization resistance than RA 253 MA, which is of practical consequence in steel coil annealing covers. These lower alloys such as RA309 and RA310, and the common stainlesses do not possess adequate resistance to carburization for use as fixturing in commercial carburizing heat treat furnaces. Ferritic grades such as 446 are quite poor in carburization resistance.

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Carburization, continued When nickel heat resisting alloys become carburized, it happens that many also become magnetic. A pocket magnet, then, becomes a handy tool to judge whether or not alloy fixturing has enough ductility remaining to be weld repaired or straightened. Not due to carburization, but a purely mechanical problem that may occur in a carburizing atmosphere is of some concern. Soot may deposit from the atmosphere and coke in any crevices, such as cracks in weld joints or surface defects on castings. The growth of this soot deposit acts like tree roots growing in rock. It literally pries open lack of fusion in the weld or turns small pin holes into large cavities. In the case of wrought alloys, which are free of surface defects, we emphasize the need to have designs and weldments that do not provide crevices in which carbon deposition may occur. This is one reason why full penetration welds of the return bend to straight leg are essential for maximum life in radiant tubes. On low fire soot may deposited in the root crevice (as well as in surface defects of cast return bends). On high fire this soot burns out, locally overheating and weakening the metal. Carburization testing Laboratory carburization testing must be carried out in some approximation of the industrial atmosphere of interest. The test temperature should be similar to that anticipated in service. In addition it would be a good idea to include thermal cycles about like the expected service conditions6. Finally, test time is important. We have noted that carburization resistance depends upon the chromia scale, and, in some grades, the silica subscale. For this reason the test atmosphere must contain an oxygen partial pressure comparable to the expected service atmosphere, in order to form a similar protective scale7. One may also wish to consider the nitrogen level, as nitrogen from the atmosphere can react with alloying elements such as chromium, and may affect carburization. There have been laboratory carburization tests run in an atmosphere of hydrogen2% methane, with no control of oxygen partial pressure. As the alloy will not develop much of a protective scale, such an atmosphere is an excellent way to achieve the objective of actually carburizing the alloy. As it is the oxide scale which is primarily responsible for carburization resistance, it is unlikely that such a test should rank alloys as they perform in actual service. Whether ethylene tubes or heat treat fixtures, some partial pressure of oxygen is almost always present.

- 24 -

Carburization testing, continued In the absence of dew point (oxygen partial pressure) control the results may not even be repeatable. Very small amounts of oxygen can form enough alumina or titania scale, for example, to inhibit brazing. Even alloy 800H contains enough titanium to turn light gray in a common vacuum heat treat furnace. In order to braze even stainless steel (with no Al or Ti) in hydrogen it is normally considered that the dew point should be 60F (51C) or lower8. This is necessary to dissociate the oxides of most alloying elements. Alumina and titania will not be dissociated by this atmosphere. One might expect that grades such as N06601, N06025 and N0811 would form aluminum and titanium oxide films in a nominal hydrogenmethane atmosphere. Such films may affect carburization. When the atmosphere simulates that of industrial interest, carburization testing may require long exposure. There is some period of time during which significant carbon absorption does not take place. Experience related to us from one furnace company indicated that the test had to be run for at least 1000 hours, before the results correlated with service experience. The following results, from that same company, are from tests conducted in an electrically heated industrial carburizing furnace. The higher temperature results, 1900F (1038C) are from a composite electric heating element made of the five alloys shown, and the 1750F (954C) results are from plate samples exposed to the actual furnace operating temperature. In both cases the total exposure hours were distributed as follows: 20% of the time in endothermic gas enriched with natural gas to carbon potential 1.0-1.2%C relative to iron, 70% of the hours in nitrogen, and 10% of the time reflected air burnout cycles at 100F (56C) reduced temperature. Various depth of cuts were machined in the samples and the carbon contents analyzed. Results here are reported at 0.045 (1.14mm) depth on the element and 0.20 (0.508mm) depth on the plate sample. 1900F (1038C) 1750F (954C) 2260 hour exposure 4300 hour exposure alloy %carbon %carbon RA333 1.53 0.344 RA330 3.03 0.443 617 2.86 1.6 601 2.98 1.096 600 1.56 - - 310 - - 3.92

- 25

METAL DUSTING A somewhat aggravating problem in carburizing atmospheres is metal dusting, a.k.a. catastrophic carburization, or carbon rot. This occurs at lower temperatures, typically 8001200F (430650C) in heat treating furnaces. Such temperatures exist in a carburizing furnace (nominal 1750F/950C) where alloy tube hangers, atmosphere sampling tubes or electrical leads pass through furnace walls, and in some areas of Ipsen furnace chains. The exact mechanism may be disputed, but the effect is that the metal disappears. A bar may look just like a beaver had chewed away on it. In other cases, the metal literally appears worm-eaten on the surface. In the petrochemical industry, a small amount of sulphur (4050 ppm H2S) is sometimes added to the process gas stream to poison the high temperature chemical reaction that is metal dusting. Alloys vary greatly in susceptibility to metal dusting. RA333, by experience and several years testing in the heat treat industry, is the best known choice. RA85H was also good, though not quite so resistant as is RA333. RA330 is so-so, 800H perhaps worse, and 600 alloy is the least resistant. Neither 310 nor 601 will solve metal dusting problems. One direct alloy comparison, below, shows two RA333 GMAW beads with only minor surface smoothing, while the 3/16 (4.8mm) RA310 plate between them suffered nearly complete loss of section9.

This example is from a rotary retort used to carburize small parts at an operating temperature of about 1750F (940C). Spiral flights of RA310 welded to the inside served to transport work pieces through the retort. As the retort was externally fired, the 3/8 (7.9mm) 600 alloy shell was above the temperature range for metal dusting. Metal dusting was a serious problem with flights at the entry end of the retort. Here the cold work pieces chilled the RA310 flights down into the metal dusting temperature range.

- 26 -

METAL DUSTING, continued

Furnace chain severely attacked by metal dusting. This is an application where RA333 has given the best service life in original equipment.

- 27

METAL DUSTING, continued RA330 carburizing furnace anchor bolt, 3/4 (19 mm) diameter. Failure by metal dusting. RA330 gave better life than 600 alloy in this application. Three possible solutions here, depending upon the cost of furnace down-time, are to: 1.) Simply keep replacing the part in RA330 2.) Alonize the replacement RA330, or 3.) Replace the part in RA333. The following test results are from a direct comparison of alloys for 25,594 hours (3 years) at temperature, in the metal dusting zone of a Surface Combustion carburizing furnace. 1 (25.4mm) Sch 40 oxygen probes of various alloys with different surface treatments were inserted through the furnace roof. The atmosphere is endothermic enriched with 0.70.8% methane to a 1.20% carbon potential, operating temperature 1700F (927C). Metal dusting occurs in the region where temperatures are roughly 1100F (600C), as the pipe passes through the refractory. Alloy Condition Results RA333 As received Dark, no pits at 27,594 hours Preoxidized Some pits at 16,183 hours RA85H As received Black, no pits at 8122 hours Preoxidized Black, no pits at 7549 hours RA330 As received Pitted, test stopped at 19,472 hours 214TM As received Many pits, test stopped at 19,472 hours Preoxidized Many pits, test stopped at 19,472 hours HR-120TM As received Pittedremoved from test at 11,264 hours HR-160TM As received Pitting started at 24,422 hours Preoxidizing treatments provided no benefits or were counter productive. Although aluminum diffusion coatings are usually considered to provide resistance to metal dusting, the 4.5% nominal aluminum content of alloy 214 was ineffective.

- 28 -

NITRIDING Nitrogen reduces alloy ductility in a manner similar to carbon. A great deal of attention is given to carbon-pickup in alloys at high temperature, but the nitrogen content is rarely analyzed. An increase in nitrogen content may occur during high temperature service in air, though not usually sufficient to cause of failure.

RA446 plate exposed 3000 hours in air at 2100F (1150C). Initial nitrogen content 0.089%, after exposure nitrogen reached 1.15%. Chromium nitrides in this photomicrograph are the needles at 60 angles. Surface, at top, shows some internal oxidation. Absence of nitrides probably due to chromium depletion from oxidation. Mill certification, Jessop Steel Co. Heat 26445 0.18C 0.70Mn 0.47Si 0.26Ni 24.84Cr 0.03Mo 0.03Cu 0.089N

Nitrogen has been associated with blistering and severe reduction of creep-rupture strength in carburized HL (30Cr 20Ni) steam-methane reformer tubes10. Carburization decreases nitrogen solubility in Ni-Cr-Fe alloys, by removing chromium from the matrix. Because of the reduced solubility, nitrogen then diffused ahead of the advancing carburized front. This locally concentrated the nitrogen, and contents as high as 0.462% were measured. It was postulated that this could result in high nitrogen gas pressure, which the authors associated with microvoids and cracks. A lamellar phase near the grain boundaries was apparently a nitride phase. Commercial nitriding, e.g. the Floe process, usually is done with RA600 alloy fixturing. Carbo-nitriding is a process carried out in an atmosphere containing both carbon and nitrogen. Temperatures are usually higher than for nitriding but lower than carburizing, roughly 13001650F (705900C), and times are shorter. The life of alloy fixturing in a carbo-nitriding application cannot be expected to equal that in a straight carburizing environment, probably for two reasons. First, because the embrittling effect of carbon and nitrogen combined is more drastic. Second, because the cycles are much shorter. The hours or years of exposure are not the important things affecting an alloys (quenching fixture) life, but rather the number of cycles it receives. Thermal fatigue cracking gradually develops and grows with each cycle. A part in a carbo-nitriding environment will receive many more cycles in a given length of time than if it were in a carburizing application, and its life will be shortened accordingly.

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NITRIDING, continued References 1. Furnace Atmospheres, Metals Handbook Ninth Edition, Volume 4 Heat Treating, ASM, Metals Park, Ohio 1981 2. G. R. Rundell, Evaluation of Heat Resistant Alloys in Composite Fixtures, Corrosion 86 Paper Number 377, National Association of Corrosion Engineers, Houston, Texas 1986 3. D. E. Wenschof and J. A. Harris, The Influence of Carburization on the Mechanical Properties of Wrought Nickel Alloys, Corrosion/77 Paper No. 9, National Association of Corrosion Engineers, Houston, Texas 1977 4. R. H. Kane, Carburization of Cast Heat-Resisting Alloys in Synthetic Petrochemical Environments, Corrosion/83 Paper Number 266, National Association of Corrosion Engineers, Houston, Texas 1983 5. D. B. Roach, Carburization of Cast Heat-Resistant Alloys, Corrosion/76 Paper No. 7, National Association of Corrosion Engineers, Houston, Texas 1976 6. D. J. Hall, M. K. Hossain, and J. J. Jones, Factors affecting carburization behavior of cast austenitic steels, Materials Performance, January 1985, Houston Texas 7. R. H. Kane, Effects of Silicon Content and Oxidation Potential on the Carburization of Centrifugally Cast HK-40, Corrosion/80 Paper Number 168, National Association of Corrosion Engineers, Houston, Texas 1980 8. Brazing of Heat-Resistant Alloys, Low-Alloy Steels, and Tool Steels, ASM Handbook Volume 6, Welding, Brazing and Soldering, ASM International, Metals Park, Ohio 1993 9. James Kelly, Metal Dusting in the Heat Treating Industry, Stainless Steel World 99 Comference, The Hague, Netherlands 1999 10. J. R. Schley and F. W. Bennett, Destructive Accumulation of Nitrogen in 30 Cr 20Ni Cast Furnace Tubes in Hydrocarbon Cracking Service at 1100C, Corrosion, September, 1967 National Association of Corrosion Engineers, Houston, Texas SULPHIDATION Environments containing sulphur may rapidly attack high nickel alloys. The problem is more severe under reducing, or low oxygen, environments. The higher the nickel the more sensitive the alloy is to sulphidation attack. If sulphur is a problem, we do not suggest using any alloy with more than 20% nickel. RA310, with 25% chromium and 20% nickel, is useful in many sulphur bearing environments. RA309, at 13% nickel, may be preferred for some applications. Under the most severe conditions an alloy completely free of nickel, such as RA446 may be required, in spite of other disadvantages it has. When the environment is oxidizing the alloy is more likely to form a protective chromium oxide scale, rather than a chromium sulphide. Under reducing environments the alloy forms chromium sulphide, which is non-protective.

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SULPHIDATION, continued An oxidizing environment is one in which sulphur is present as sulphur dioxide (SO2), and there is some excess oxygen (O2), or even carbon dioxide (CO2) and/or water vapor (H2O). In reducing environments sulphur is in the form of hydrogen sulphide (H2S), there may be hydrogen (H2), carbon monoxide (CO), methane (CH4) or other sources of carbon, and rather little CO2 or H2O. Sometimes the distinction isnt obvious. For example, there may be solid deposits on metal in an oxidizing environment. Underneath those deposits, in contact with the metal the actual amount of oxygen available to form a scale may be very, very small. We have heard it stated that oxygen partial pressures are about 10-8 underneath calcium sulphate deposits on some fluidized bed components. If the deposit contains sulphur, then the metal may be heavily attacked under the deposit, regardless of how much oxygen is in the atmosphere above it. An example of under deposit attack is shown below. This is 1/4 (6.35mm) thick RA 253 MA from a kiln processing ferrous sulphate monohydrate to red iron oxide pigment. Atmosphere air plus the SO2 and SO3 driven off in the process, operating temperature 1840F (1004C). After about a year the RA 253 MA kiln shell had developed holes roughly 3/4 (20mm) across, some rather long. Previously used RA310 had failed by more uniform thinning, and lasted 2 to 2 1/2 years.

From Rolled Alloys Report Number 94-72

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SULPHIDATION, continued

Types of Scale Developed on Type 310 Stainless Steel as a Function of Oxygen and Sulfur Partial Pressures in the Gas Environment at Temperatures of 750, 875, and 1000C. Conversion factor: 1 atm = 0.101356 MPa. ANL Neg. No. 306-79-6251 Most sulphidation failures occur under highly reducing conditions. That is, where a source of carbon, such as methane (CH4) is present along with the hydrogen sulphide Even 1/2% of H2S can be quite destructive. One example is in carbon black manufacture. Low-grade oil is heated, with very little oxygen, to break it down into sootwhich is carbon- or lamp black. The oil used as feed stock normally contains up to 3 percent sulphur. High nickel alloys are quite unsuited for high temperature service in the sulphidizing environments of carbon black plants. Specifically, RA330, RA333, alloys X, 800H, 600, 601, 617 and even some of the high cobalt alloys may fail by sulphidation.

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SULPHIDATION, continued Nickel reacts chemically with sulphur very readily. Unlike metal oxides, which at least are solid, metal sulphides, or metal-metal sulphide eutectics, are often molten at operating temperature. If sufficient molten metal sulphide forms underneath the chromium oxide scale, it may literally wash that scale away. Valid laboratory corrosion testing for sulphidation resistance requires very long time exposure. In general, the corrosion rate in sulphidation may be more or less parabolic for some period of time. Eventually, for a variety of reasons, corrosion enters a break-away mode2 , where corrosion rates accelerate dramatically. It is the time to break-away corrosion, rather than the linear or parabolic corrosion rate, that is significant. Melting points of some metal-metal sulphide eutectics are3 : 1175F (635C) for Ni-Ni3S2, 1611F (877C) for Co-Co4S3 and 1810F (988C) for Fe-FeS. The Fe0-FeS eutectic melts at 940C4 . CrS-Cr2S3 doesnt melt until 2462F (1350C)5. References 1. K. Natesan, CORROSION AND MECHANICAL BEHAVIOR OF MATERIALS FOR COAL GASIFICATION APPLICATIONS, ANL-80-5, Argonne National Laboratory, Argonne, Illinois U.S.A. 1980 2. Maurice A. H. Howes, High-Temperature Corrosion in Coal Gasification Systems, Final Report (1 October 1972-31 December 1985) as subcontractor to The Materials Properties Council, Inc., New York, New York. 3. Binary Alloy Phase Diagrams, Thaddeus B. Massalski, Editor, 1986, American Society for Metals, Metals Park, Ohio 4. Stanislaw Mrowec, Teodor Werber, Gas Corrosion of Metals, translation published by the Foreign Scientific Publications Department of the National Center forScientific, Technical and Economic Information, Warsaw, Poland 1978 5. Handbook of Chemistry and Physics, 65th Edition, CRC Press Inc., Boca Raton, Florida 19841985

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HALOGEN GAS HOT CORROSION Unlike oxides, metal halides are volatile. When halogens are present in high temperature environments any oxide scale present becomes porous and non-protective. In order to form a protective scale it is generally considered that the metal chloride vapor pressure must be below 10-4 atmosphere. For CrCl3 and NiCl2 that would be about 600C, just slightly lower for CoCl2. For FeCl3 the limit is much lower, about 160C, and lower yet for MoCl5, 50C, and AlCl3, about 75C. As a practical matter, the high nickel alloys 600 (UNS N06600) and 400 (N04400) are most commonly chosen for hot halogen gas resistance. Good discussions of this subject are given in the old INCO Corrosion Engineering Bulletins CEB-3 for HCl and Cl2, and CEB-5 for HF and F2. Data for 100% Cl2, and for HCl follow.

Corrosion in dry Chlorine Gas Metal Approximate temp, F, at which given Suggested corrosion rate, mils/year, is exceeded in short upper temp time tests in dry Cl2 limit for continuous Corrosion in Dry Chlorine, 100% service, F 30 60 120 600 1200 Nickel 950 1000 1100 1200 1250 1000 alloy 600 950 1000 1050 1200 1250 1000 alloy 400 750 850 900 1000 1000 800 316 600 650 750 850 900 650 304 550 600 650 750 850 600 Platinum 900 950 1000 1050 1050 500 CopperA 350 450 500 500 550 400 SteelB 250 350 400 450 450 400 Gold 250 300 350 400 400 - - Silver 100 150 250 450 500 - - Acopper metal ignites in hot Cl2 at about 600F Bcarbon steel ignites in hot Cl2 at about 450-500F

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Corrosion in Dry Hydrogen Chloride, 100% Metal Approximate temperature, F, at which given Suggested corrosion rate, mils/year is exceeded in short upper temp time tests in dry HCl limit for continuous Corrosion rate, mil/year, in dry HCl service, F 30 60 120 600 1200 Nickel 850 950 1050 1250 1300 950 alloy 600 800 900 1000 1250 1350 900 alloy 400 450 500 650 900 1050 450 316 700 700 900 1100 1200 800 304 650 750 850 1100 1200 750 Platinum 2300 - - - - - - - - 2200 Copper 200 300 400 600 700 200 Steel 500 600 750 1050 1150 500 Gold 1800 - - - - - - - - 1600 Silver 450 550 650 850 - - 450 Both of these tables were abstracted from INCO Bulletin CEB-3. The data were obtained from short-time laboratory tests and offer only a rough guide to maximum practical temperature limit of materials. The original data from which INCO developed their table was published in 1947, M.H. Brown, W.B. DeLong and J.R. Auld, Corrosion by Chlorine and by Hydrogen Chloride at High Temperatures, Ind. & Eng. Chemistry, Vol 39, No. 7 pp 839-844 At lower halogen concentrations alloys forming a chromia layer can tolerate higher temperatures. Data in Bender and Schtze, Paper 00239 Corrosion 2000, show that alloy 600 can form a protective oxide at 800C in 0.1%Cl2, 100 hour test. At 2%Cl2, same temperature, the alloy does not develop a protective scale. Grain size has an effect, 600 with finer grains, 75m (ASTM 4.5), being superior to 600 with coarser grain size, 125m (ASTM 3). Fine grain size increases diffusion rate of chromium to the surface.

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Longer time tests show lower corrosion rates. The following industrial data were obtained from 30 day test exposures.

100% Chlorine Gas, type 304/321 stainless, 30 day test

Temp mils/yr mm/yr F C 572 300 6 0.15 617 325 7 0.18 662 350 9 0.23 707 375 15.5 0.39 752 400 33 0.84 797 425 115 2.9

100% Chlorine Gas, alloy 600, 30 day test

Temp mils/yr mm/yr F C 977 525 8 0.2 1022 550 12 0.3 1067 575 15 0.38 1112 600 24 0.61 1157 625 47 1.2 Corrosion of nickel alloys by hot 100% F2 gas is given in Table 14, CEB-5. Most of that data is reproduced below.

Corrosion by dry fluorine gas F 80 400 700 1000 Temperature C 27 204 370 538 Material Exposure Corrosion Rate, mils per year time, hours 400 5 2.4 0.5 1.9 29.8 24 0.5 0.5 1.7 11.3 24* -- 0.7 2.4 21.3 120 0.2 0.1 1.2 7.2 200 nickel 5 1.0 3.3 1.7 24.5 24 0.9 0.5 1.2 16.1 24* -- 0.3 0.5 44.5 120 0 0.1 0.4 13.8 304 5 1.7 6.1 1565 -- 304L 24 0.6 7.5 6018 -- 120 0 25.4 -- -- 347 5 2.7 4.0 4248 -- 600 5 1.1 0.6 78.0 3451

All tests were made in flowing fluorine gas, except * which were conducted in bombs at initial pressure of 250 psi.

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The original source of this fluorine data was: R.B. Jackson, General Chemical Division, Allied Chemical Company, Corrosion of Metals and Alloys by Fluorine, Contract AF 04 (611)-3389

Corrosion Tests in Hydrogen Fluoride Gas

Temperature: 930 to 1110F (500 to 600C). Test duration 36 hours. From Table 17, INCO CEB-5 Material Corrosion Rate Comments mils/year mm/yr Hastelloy alloy C 0.3 0.008 iridescent tarnish film Inconel alloy 600 0.7 0.02 Hastelloy alloy B 2 0.05 black film Nickel 200 9 0.2 Nickel 201 14 0.36 Monel alloy 400 13 0.33 adherent dark film Monel alloy K-500 16 0.41 70-30 Copper-Nickel 16 0.41 Hastelloy is a registered trademark of Haynes International Inconel and Monel are registered trademarks of Special Metals, Inc. In atmospheres containing a significant partial pressure of oxygen these laboratory data in pure halogens or halide gases have limited utility as the basis for alloy selection. The heat resistant alloy X (UNS N06002) has outperformed alloy 600 in oxidizing gases containing HCl. Alloy 59 (N06059) has performed satisfactorily in an oxidizing atmosphere with HF, where alloy 617 weld filler was inferior to alloy 600. Examination of the 600 alloy part, removed from service after many years life, showed some corrosion from sulphur and phosphorous as well. At this writing, August 2002, it is not clear to us whether it is the better oxidation resistance of the higher chromium alloys, or some molybdenum effect, that is responsible. If the customer intends to perform tests in his environment, we would suggest including a heat resistant alloy such as the 3%Mo alloy RA333 (N06333). RA333 has shown good resistance to hot corrosion by the fluoride flux used in aluminum salt bath brazing environments.

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MOLTEN SALT CORROSION Hot chloride salts, and particularly salt fumes mixed with air, are very corrosive to heat resistant alloys. In general the higher nickel alloys, such as 600, are preferred, although we have seen tolerable results from the 1.7% silicon grade, RA 253 MA. Corrosion in Molten Chloride Heat Treat Salts, 1100-2200F (600-1200C) Depth of Intergranular Attack Grade Nickel, weight Silicon, weight mm inch % % RA85H 15 3.5 0.11 0.0044 RA 253 MA 11 1.7 0.18 0.0069 RA600 76 0.2 0.19 0.0075 RA309 13 0.8 0.32 0.0125 RA330 35 1.2 0.35 0.0138 Plate samples were exposed in a commercial heat treat salt line. They saw 210 to 252 cycles in preheat salts 700C (1290F) and 815C (1500F), high heat salt 1200C (2200F), quench in 600C (1100F) nitrate/nitrite salt, air cool. Preheat and high heat salts were mixtures of potassium, sodium and barium chlorides. The alkali metals in the salt turn the protective chromium oxide scale into an alkali chromate, which is non-protective and water soluble. As fast as the scale is removed, more chromium diffusing to the surface reforms the scale. Eventually most of the chromium may be rem