Type 654 SMO®

UNS S32654

Outokumpu Stainless

Design Features

Outstanding resistance to pitting, crevice, and general corrosion

Extremely high resistance to chloride stress corrosion cracking

Twice the strength of common austenitic stainless steels

Excellent ductility and toughnessSuperior workability, formability, and

weldability

Product Forms Available

PlateSheetStripBarPipe and TubingWelding Consumables

Applications

Critical Seawater and Severe Brackish WaterHandling Systems

Chlorine and Chlorine Dioxide Pulp BleachingSystems

Chemical Processing EquipmentDesalination SystemsMunicipal Waste Incineration SystemsPlate Heat Exchangers

ASTMElement S32654 Typical

Carbon 0.020 max 0.010Chromium 24.0-25.0 24.0Nickel 21.0-23.0 22.0Molybdenum 7.0-8.0 7.3Nitrogen 0.45-0.55 0.50Copper 0.30-0.60 0.50Sulfur 0.005 max 0.001Phosphorus 0.030 max 0.020Silicon 0.50 max 0.40Manganese 2.00-4.00 3.0Iron Balance Balance

Composition, wt. pct. Table 1

Specifications

Solidification Range 2500-2445Scaling Temperature in Air 1830Sigma Phase Formation 1300-1800Carbide Precipitation 840-1470Hot Forming 2200-2000Solution Annealing 2100 min. water quenchStress Relief Annealing 2100 min. water quench

Temperature °F

Characteristic Temperatures Table 2

General Characteristics

With unusually high levels of chromium, molybdenum, and nitrogen, Outokumpu Stainless 654 SMO® achieves levels of chloride pitting andcrevice corrosion resistance not previously possiblewith an austenitic stainless steel. OutokumpuStainless was the first stainless steel producer tointroduce a nitrogen-alloyed 6% molybdenumaustenitic stainless steel with its innovative 254 SMO® steel in the 1970s. Now OutokumpuStainless leads the way to a new level of stainlesssteel technology with the introduction of 654 SMO.

Structure

As shown in Table 2, 654 SMO is solutionannealed at 2100°F minimum to achieve a fullyaustenitic stainless steel structure, although fainttraces of intermetallic phases (sigma or chi phase)may be present and tolerated in the center of theheavier sections. These phases may also form on the grain boundaries in the metal during exposuresin the range of 1100 to 1825°F, with detrimental

Plate, Sheet, Strip A 240, A 480 SA-240, SA-480Pipe A 312, A 358 SA-312, SA-358Tubing A 249, A 269 SA-249, SA-269Bar A 276, A 479 SA-479ASME Code Case 2195-1NACE MR0175

UNS S32654 ASTM ASME

effects on corrosion resistance and toughness. Theprocedures for forming, welding, and heat treatmentare designed to prevent this undesirable precipitationof intermetallic phase.

Critical Crevice Temperatures Table 4

Corrosion ResistancePitting and Crevice Corrosion

Pitting and crevice corrosion are the most commonforms of corrosion for stainless steels. Both types ofattack result in a highly localized form of corrosionwhich can lead to perforation in a short time withrelatively little total weight loss. Both pitting andcrevice corrosion are accelerated by more acidicconditions and by higher temperatures. With itshigh levels of chromium, molybdenum, and nitrogen, 654 SMO is the most resistant stainlesssteel ever produced.

One method of estimating the pitting resistanceis to use the pitting resistance equivalent (PRE), an index of pitting resistance based on statisticalanalysis of the effect of composition of stainlesssteels on a particular parameter assessing pitting resistance, most usually the critical pitting

Type 654 SMO®2

temperature (CPT) for a selected environment andtest procedure. As seen in Table 3, the PRE for 654 SMO is far higher than it is for any of theother stainless steels.

The critical pitting temperature is the highesttemperature at which a stainless steel will resistattack in a particular environment. Two test procedures are common, one being ASTM G 48,Practice A, which measures the CPT by exposingcoupons to a solution of 10% FeCl3• 6H2O at aseries of increasing temperatures until pitting corrosion is observed. Another newer test is an electrochemical test ASTM G 150 using a test cellspecially designed by Outokumpu Stainless Research.Both of these methods have been effective for allother stainless steels, including the 6Mo austeniticstainless steels, such as Outokumpu Stainless 254 SMO. However, these tests are of limited usefor 654 SMO because it is completely resistant toattack in these tests at 204°F and 217°F, the respective boiling points of the chloride media for these tests. So the best evaluation for 654 SMOis obtained by the more aggressive crevice corrosion tests.

Table 4 shows critical crevice temperatures for654 SMO in comparison with those for Alloy 625and Alloy C-276, and for 254 SMO® austeniticstainless steel. The test environments includeASTM G 48, Practice B, and the so-called “GreenDeath” solution designed to discriminate amongthe nickel-base alloys and their high levels ofcorrosion resistance. These tests were made withmultiple crevice washers made of PTFE andimposed on the surface by bolts torqued to 1.16 ft-lb, providing very severe crevice conditions.These tests demonstrate that the 654 SMO is atleast as resistant as Alloy C-276 to crevice corrosionin these acidic, oxidizing, high-chloride solutions.

To simulate a flue gas desulfurization scrubberenvironment, an acidic sodium chloride–containingsolution at 176°F was bubbled with sulfur dioxidefor two hours. The pH of the solution was 1.0–1.5at the start of the test but decreased with time to0.5–1.0. The lowest chloride level necessary tocause corrosion was determined for each material.As shown in Table 5, 654 SMO is superior to allstainless steels, comparable to Alloy 625, but inferiorto Alloy C-276.

654 SMO® 56.1254 SMO® 432507 42.2904L 362205 Code Plus Two® 34.5316L 25.2

PRE* for Stainless Steels(Pitting Resistance Equivalent) Table 3

654 SMO® ≥140 ≥154 ≥140Alloy C-276 140 149 131Alloy 625 ≥68 ≥82 ≥68254 SMO® 100 109 100

Grade CCT, °FASTM G 48 11% H2SO4Practice B 1.2% HCl 4% NaCl10% FeCl3• 1% FeCl3 0.1%Fe2(SO)6

Solution 6H20 1.0% CuCl2 0.01M HCl

*PRE = %Cr + 3.3 x %Mo + 16 x %N

Grade PRE

Type 654 SMO® 3

It is concluded from these and other observationsthat 654 SMO is comparable to Alloy C-276, beingslightly superior in some environments and slightlyinferior in others, depending on details to the particular exposure.

Chloride Stress Corrosion Cracking (SCC)

654 SMO austenitic stainless steel also possessesexcellent resistance to chloride-induced transgranularstress corrosion cracking. This resistance is a resultof the high molybdenum and nickel contents, incombination with the extremely high resistance to pitting.

One method to rank stainless steels by theirSCC resistance is to test them in accordance withthe drop-evaporation test (DET). Uniaxially loaded,electrically heated specimens are exposed to a dripping dilute (0.1 M) sodium chloride solution.The drip rate is adjusted so that one drop is evaporated just before the specimen is hit by thenext drop. The applied stress is varied in steps of10% up to 100% of the yield stress at 392°F untilcracking occurs or up to a maximum of 500 hours.As shown in Table 6, 654 SMO is resistant tocracking at 100% of its yield strength at the maximum exposure of 500 hours.

654 SMO is not immune to SCC in boiling42% magnesium chloride. However, it is immuneto SCC in boiling 25% sodium chloride and thewick test, both shown to be well correlated withpractical experience in resisting SCC in severe heattransfer conditions at ambient pressures.

General Corrosion

A discussion of the resistance of a stainless steel togeneral corrosion must address both pure chemical

environments, e.g., the strong mineral acids, andthose environments with small to moderate levels ofcontamination with halides. The presence of halides,such as chlorides, bromides, iodides, or fluorides,can significantly accelerate general corrosion, especially in nonoxidizing acids.

As shown in Figures 1 and 2, 654 SMO is superior to both 904L and 254 SMO in pure sulfuric acid over most of the concentration range.Neither the 6Mo nor the 7Mo stainless steel is especially good for 96% sulfuric acid. But with contamination of the sulfuric acid with 2000 ppmchloride, the 654 SMO maintains a much highercorrosion resistance than the other stainless steels.

An overview of the performance of 654 SMO in a large number of chemical environments is provided by the Materials Technology Institute(MTI) procedure for comparing new grades. Newmaterials are compared with standard alloys testedat the same time under identical conditions bydetermining the lowest test temperature at whichthe corrosion rate exceeds 5 mpy. Outokumpu Stainless 654 SMO stainless steel has been evaluatedby the MTI procedure and the results are shown inTable 7. 654 SMO shows outstanding corrosion performance, exceeding that of the other specialduplex and austenitic stainless steels in each of these environments.

Stainless steels such as 316L cannot be used inhydrochloric acid even at very low concentrationsbecause of the risk of localized and general corrosion.However, as shown in Figure 3, 654 SMO may beused in dilute hydrochloric acid even at fairly high temperatures, and at room temperature up to about8% concentration. As shown in Figures 4 and 5,the resistance of 654 SMO to hydrofluoric acid andfluosilic acids is also very good.

654 SMO® 12,500316 50904L 500254 SMO® 5,0002507 1,500Alloy 625 4,000 to 15,000Alloy C-276 27,500

Critical Chloride Concentrations Table 5

654 SMO® 48.5 100 >500, >500316 3.0 10 155, 158904L 21.9 70 >500, >500254 SMO® 32.1 90 >500, >500

Applied Stress Time to Failure% Yield

Grade ksi Strength Hours

Stress Corrosion Cracking Resistance (SCC) in Drop Evaporation Tests at 392°F Table 6

Grade Critical Chloride Concentration, ppm

Type 654 SMO®4

strongly affected by the presence of minor chemicalconstituents that may have been omitted from thelaboratory test environment. Therefore, these datashould be considered as the starting point for gradeselection, to be supported by in-process coupontests in a pilot plant or operating facility.

Intergranular Corrosion

The very low carbon content of 654 SMO in combination with its generally very good corrosionresistance make the steel immune to intergranularcorrosion caused by chromium carbide precipitation.For this reason, the test methods normally used to detect the susceptibility of a stainless steel tointergranular corrosion, such as the methods basedon copper sulfate and sulfuric acid mixtures (Strausstest), are not meaningful for 654 SMO.

The improper heat treatment of 654 SMO can cause other types of precipitates that may bedetrimental to corrosion resistance. These precipitates are better detected by examination ofthe microstructure or by tests for critical pitting or crevice temperatures in strong chloride environments, and not by the common tests forintergranular corrosion.

Isocorrosion Curves (4 mpy) in sulfuric acid containing 2000 ppm chloride Figure 2

Isocorrosion Curves (4 mpy) in pure sulfuric acid Figure 1

When selecting any stainless steel for a particularchemical environment, it should be rememberedthat the corrosion performance of any grade may be

Isocorrosion Curves (4 mpy) in pure hydrochloric acid Figure 3

Type 654 SMO® 5

Isocorrosion Curves (4 mpy) in pure fluosilic acid Figure 5

654 254 Type 316L Type 2205 CodeCorrosion Environment SMO® SMO® 904L (2.7 Mo) 304 2507 Plus Two® 2304

0.2% Hydrochloric Acid >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling1% Hydrochloric Acid 203 158 122 86 86p >Boiling 185 13110% Sulfuric Acid 158 140 140 122 — 167 140 14960% Sulfuric Acid 104 104 185 <54 — <57 <59 <<5596% Sulfuric Acid 86 68 95 113 — 86 77 5985% Phosphoric Acid 194 230 248 203 176 203 194 20310% Nitric Acid >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling >Boiling65% Nitric Acid 221 212 212 212 212 230 221 20380% Acetic Acid >Boiling >Boiling >Boiling >Boiling 212p >Boiling >Boiling >Boiling50% Formic Acid 158 212 212p 104 ≤50 194 194 5950% Sodium Hydroxide 275 239 Boiling 194 185 230 194 20383% Phosphoric Acid + 2% Hydrofluoric Acid 185 194 248 149 113 140 122 9560% Nitric Acid + 2% Hydrochloric Acid >140 140 >140 >140 >140 >140 >140 >14050% Acetic Acid + 50% Acetic Anhydride >Boiling >Boiling >Boiling 248 >Boiling 230 212 1941% Hydrochloric Acid + 0.3% Ferric Chloride >Boiling, p 203ps 140ps 77p 68p 203ps 113ps 68p10% Sulfuric Acid + 2000ppm Cl- + N2 149 104 131 77 — 122 95 <5510% Sulfuric Acid + 2000ppm Cl- + SO2 167 140 122 <<59p — 104 <59 <<50WPA1, High Cl- Content 203 176 122 ≤50 <<50 203 113 86WPA2, High F- Content 176 140 95 ≤50 <<50 167 140 95

ps = pitting can occurps = pitting/crevice corrosion can occur

Lowest Temperature (°F) at Which the Corrosion Rate Exceeds 5 mpy Table 7

WPA P2O5 Cl- F- H2SO4 Fe2O3 Al2O3 SiO2 CaO MgO

1 54 0.20 0.50 4.0 0.30 0.20 0.10 0.20 0.702 54 0.02 2.0 4.0 0.30 0.20 0.10 0.20 0.70

Isocorrosion Curves (4 mpy) in pure hydrofluoric acid Figure 4

Maximum Allowable Stress Values, ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, 1999 Addenda, 3.5 Safety Factor Table 10

Tensile Properties at ElevatedTemperatures Table 9

Type 654 SMO®6

Erosion Corrosion

654 SMO possesses excellent resistance to erosioncorrosion in seawater. This behavior is attributedboth to its superior corrosion resistance and to itshigh surface hardening, which resists the effects ofabrasive particles in the fluid.

Fabrication Design

654 SMO is extremely strong, as shown in Table 8,but it still retains the very high ductility and toughness expected of an austenitic stainless steel.Minimum tensile properties for 654 SMO up to750°F are given in Table 9.

It may be possible to use the high strength of654 SMO for reducing the section thicknesses inpractical equipment relative to those required for316L, 904L, and even 254 SMO. Table 10 givesthe allowable design stresses for 654 SMO inSection VIII, Division 1 construction in accordancewith ASME Code Case 2195.

654 SMO should not be used above about1100°F because of the danger of precipitation ofintermetallic phases and the consequent loss of corrosion resistance and ambient temperaturetoughness. However, 654 SMO can be used indefinitely at the moderate temperatures typicallyencountered in chemical processing and heatexchanger service.

Table 11 lists some physical properties of 654 SMO as a function of temperature.

It is anticipated that the high strength and corrosion resistance of 654 SMO will give excellentcorrosion fatigue resistance. As shown in Table 12,

initial tests performed in air at 20 Hz indicate that654 SMO is at least comparable with the duplexstainless steels.

Cold Forming

654 SMO possesses very good cold formability.Most common stainless steel forming methods canbe used for cold forming 654 SMO. Because of thehigh strength and high work hardening rate for 654 SMO, fabricators will find that higher formingforces and increased allowance for spring back arenecessary. However, the high ductility of 654 SMOhas proven useful, as in the case of stamping ofsharply formed heat exchanger plates from thinsheets of 654 SMO. The work hardening rate isquite rapid, as shown by the increase in strength as a function of cold working in Figure 6.

0.2% Yield Strength, ksi 62 56 51 46 44 431.0% Yield Strength, ksi 68 62 57 51 49 48Tensile Strength, ksi 109 105 99 90 85 81

Temperature °F 68 122 212 392 572 752

654 SMO® 31.1 28.5 27.3 26.6 26.4 26.3 26.1 25.9254 SMO® 26.9 24.3 23.5 23.0 22.8 22.7 22.6 —904L 20.3 13.8 12.7 11.9 11.6 11.4 — —316L 16.7 15.7 14.8 14.0 13.7 13.5 13.2 —2205 Code

Plus Two®

(S31803) 25.7 23.9 23.3 23.1 — — — —

Grade -20 to 100°F 400°F 500°F 600°F 650°F 700°F 750°F 800°F

Mechanical Properties Table 8

68°F

0.2% Yield Strength, ksi 62 minTensile Strength, ksi 109 minElongation, in 2 inches, % 40 minBrinell Hardness 250 maxCharpy V-notch Impact Strength, ft-lb 106 min

Fatigue Strength, ksi Table 12

Type 654 SMO® 7

Hot Forming and Annealing

Hot working of 654 SMO should be carried out inthe 2200°F to 2000°F range. Higher temperatureswill reduce workability and may produce heavyscaling. To deal with intermetallic phases that mayprecipitate during the hot forming, it is necessary toanneal at a minimum of 2100°F, followed by waterquenching. Too slow a cooling rate may cause precipitation of intermetallic phases, reducing corrosion resistance.

Machining

The high strength, toughness, and work hardeningof 654 SMO make machining of this grade substantially more difficult than that for the common austenitic grades. Powerful, rigid equipment, reduced speeds and feeds, and superiorlubrication are necessary for machining 654 SMO.Practices for several types of machining as appliedto 654 SMO are discussed in the separate datasheet, “Machining of Type 654 SMO® AusteniticStainless Steel.”

Welding

654 SMO possesses good weldability and can bewelded using the conventional welding methodsapplied to the common austenitic stainless steels.

654 SMO mill products are delivered with ahomogeneous composition. Remelting of the parentmetal, as may occur during welding without fillermetal, may cause microscopic segregation of elements such as chromium, nickel, and especially molybdenum. This phenomenon occurs in all highlyalloyed austenitic stainless steels, but becomesincreasingly pronounced with the more highlyalloyed grades. These variations may reduce the corrosion resistance of the weld. As a general

Modulus of Elasticity psi x 106 27 27 26 25 24Coefficient of Thermal

Expansion (68°F to T) x10-6/°F — 8.3 8.6 8.8 9.0Thermal Conductivity Btu/h ft °F 5.0 5.7 6.5 7.3 8.4Heat Capacity Btu/lb°F 0.120 0.124 0.129 0.133 0.136Electrical Resistivity Ω-in x 10-6 30.7 31.9 33.9 35.8 37.8Density lb/in3 0.289 — — — —Magnetic Permeability 1.003 — — — —

Temperature °F 68 212 392 572 752

Physical Properties Table 11

654 SMO® 143.1 41.6 ±34.13RE60 104.1 41.2 ±33.92205 Code

Plus Two® 103.6 37.7 ±30.5

Tensile MeanGrade Strength Strength Amplitude

Mechanical Properties after Cold Working Figure 6

Type 654 SMO®8

principle, 654 SMO should not be welded withoutfiller metal unless the weld will be subsequentlyfully annealed.

When the weld is not to be subsequentlyannealed, an overalloyed filler metal should be used. Because of the high corrosion resistance of654 SMO, the degree of overalloying required isunusually high. The preferred filler metal, designatedOutokumpu Stainless P16, is shown in Table 13.

The following welding guidelines will ensureoptimal corrosion resistance and mechanical properties in the as-welded condition.1. Outokumpu Stainless P16 should be used in

all welding methods. Autogenous welding shouldbe avoided unless a subsequent full anneal ispossible, or in certain limited circumstancesafter qualification of the resulting structure.

2. The geometry of the weld zone should be set up to establish full penetration of the fillermetal with minimal dilution from the base metal.

3. For gas tungsten arc welding (GTAW or TIG),pure argon or argon mixed with 3–5% nitrogenshould be used as the torch gas. If GTAW isused for autogenous welding, argon with 3–5%nitrogen should be used as the torch gas toobtain the best corrosion resistance.

4. For plasma arc welding (PAW), pure argon or argon mixed with 10% nitrogen and 5%hydrogen should be used as the plasma gas,whether the weld is made with filler or autogenously. The shielding gas should consistof argon mixed with 10–20% nitrogen.

5. For backing gas, nitrogen mixed with 10%hydrogen should be used. The steel is sensitiveto oxidation, so root purging should be performed with care.

6. When welding with filler metal by GTAW orPAW, the filler wire should be fed evenly and continuously to avoid variations in compositionof the weld.

7. The arc should be struck in the prepared weldzone in order to avoid the point of autogenouswelding, with its associated reduced corrosionresistance, outside of the weld.

8. The weld procedure should use low heat input,including small diameter wire, no weaving in

the horizontal position, low amperage, andthinner electrodes.

9. In multi-pass welding, the workpiece should beallowed to cool to approximately 212°F beforethe next weld pass.

10. Pipe or crater cracking may occur on abrupt termination of welds, as with other austenitic stainless steels. Such defects can be removed bywelding, or they can be substantially avoided bycarefully terminating, by backstepping the electrode, and by lifting gently through the slag pool.

11. Heat treatment is not normally required afterwelding, provided that P16 filler has been used.

12. To obtain optimal corrosion resistance, post-weldcleaning should be thorough, preferably withabrasive cleaning followed by careful pickling.

13. Abrasive contact with copper/brass fixturesshould be avoided because copper metal penetration into grain boundaries may causecracking during welding.Additional details for the various welding

methods are given in the brochure, “How to WeldType 654 SMO®.”

Welding Consumables

Our welding unit provides coated electrodes; wiresfor GTAW, GMAW, PAW, FCW, and SAW; andwelding fluxes, all of which have been formulatedto produce excellent results when welding 654 SMO.For these products, call our welding unit at 1-800-441-7343.

Welding Wire 0.02 0.2 0.5 23 Bal. 16Covered Electrode 0.02 0.5 0.7 25 Bal. 14

OutokumpuStainless P16 C max. Si Mn Cr Ni Mo

Welding Consumables Table 13

Cleaning and Passivation

654 SMO mill products are delivered with a surfacethat is cleaned, most frequently by pickling, toremove oxide, embedded iron, and other foreignmaterial. It is essential for maximum corrosion

Type 654 SMO® 9

resistance that this cleanliness be maintained orrestored after handling and fabrication. A majorsource of surface contamination is iron transferredfrom handling equipment, shears, dies, work tables,or other metal equipment. In service this iron cancorrode and initiate a pit. Other sources of contamination include slag entrapment in welds,weld spatter, heat tint, forming lubricants, dirt, or paint.

To maximize the corrosion resistance of stainlesssteel fabrications, including those of 654 SMO,acid passivation should be used to remove surfacecontaminants. For 654 SMO the suggested practiceis to immerse the piece in a solution of 20–40%nitric acid in water for about 30 minutes at120–140°F. Further guidelines for these proceduresare given in ASTM A 380.

If the surface of the steel is oxidized, it may benecessary to use mechanical cleaning or pickling torestore maximum corrosion resistance. Some further guidance is provided in the brochure, “How to Weld Type 654 SMO®.”

Technical Support

Outokumpu Stainless, Inc. assists users and fabricators in the selection, installation, operation,and maintenance of 654 SMO austenitic stainlesssteel. Technical personnel, supported by theresearch laboratory of Outokumpu Stainless, candraw on years of field experience with 654 SMO to help you make the technically and economically correct materials decision.

Outokumpu Stainless is prepared to discuss individual applications and to provide data andexperience as a basis for selection and application of 654 SMO.

Outokumpu Stainless works closely with its distributors to ensure timely availability of 654 SMO in the forms, sizes, and quantitiesrequired by the user. For assistance with technicalquestions, and to obtain top quality 654 SMO,please call Outokumpu Stainless, Inc. at 1-800-833-8703.

Type 654 SMO®10

Type 654 SMO® 11

Type 654 SMO®121095E

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Outokumpu Stainless, Inc., 425 North Martingale Road, Suite 1600, Schaumburg, IL 60173-2218 USA Tel. 1-800-833-8703 Fax 1-800-545-8617 stainless.info@outokumpu.com

Outokumpu Stainless

Outokumpu Stainless is a core businesswithin Outokumpu, a dynamic metalsand technology group operating worldwideand marketing its metals, metal products,technology and services to customers ina wide range of industries.

www.outokumpu.com/stainless/NA

254 SMO and 654 SMO are trademarks of Outokumpu Stainless.

2205 Code Plus Two is a trademark of Outokumpu Stainless, Inc.

Information given in this brochure may be subject to alteration without notice. Care has been taken to ensure

that the contents of this publication are accurate, but Outokumpu Stainless and its subsidiary companies do not

accept responsibility for errors or for information which is found to be misleading. Suggestions for or descriptions

of the end use or application of products or methods of working are for information only and the company and

its subsidiaries accept no liability in respect thereof. Before using products supplied or manufactured by the com-

pany, customers should satisfy themselves of their suitability. If further assistance is required, the company, which

has extensive research facilities, will often be able to help.