654 SMO

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Type 654 SMO UNS S32654Design Features

SpecificationsUNS S32654Plate, Sheet, Strip Pipe Tubing Bar ASME Code Case 2195-1 NACE MR0175

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 toughness Superior workability, formability, and weldabilityProduct Forms Available

ASTMA 240, A 480 A 312, A 358 A 249, A 269 A 276, A 479

ASMESA-240, SA-480 SA-312, SA-358 SA-249, SA-269 SA-479

General Characteristics

Plate Sheet Strip Bar Pipe and Tubing Welding ConsumablesApplications

Critical Seawater and Severe Brackish Water Handling Systems Chlorine and Chlorine Dioxide Pulp Bleaching Systems Chemical Processing Equipment Desalination Systems Municipal Waste Incineration Systems Plate Heat ExchangersComposition, wt. pct.ElementCarbon Chromium Nickel Molybdenum Nitrogen Copper Sulfur Phosphorus Silicon Manganese Iron

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

Table 1 Typical0.010 24.0 22.0 7.3 0.50 0.50 0.001 0.020 0.40 3.0 Balance

ASTM S326540.020 max 24.0-25.0 21.0-23.0 7.0-8.0 0.45-0.55 0.30-0.60 0.005 max 0.030 max 0.50 max 2.00-4.00 Balance

As shown in Table 2, 654 SMO is solution annealed at 2100F minimum to achieve a fully austenitic stainless steel structure, although faint traces of intermetallic phases (sigma or chi phase) may be present and tolerated in the center of the heavier sections. These phases may also form on the grain boundaries in the metal during exposures in the range of 1100 to 1825F, with detrimentalCharacteristic TemperaturesTemperature FSolidification Range Scaling Temperature in Air Sigma Phase Formation Carbide Precipitation Hot Forming Solution Annealing Stress Relief Annealing 2500-2445 1830 1300-1800 840-1470 2200-2000 2100 min. water quench 2100 min. water quench

Table 2

Outokumpu Stainless


Type 654 SMO

effects on corrosion resistance and toughness. The procedures for forming, welding, and heat treatment are designed to prevent this undesirable precipitation of intermetallic phase.Corrosion Resistance Pitting and Crevice Corrosion

Pitting and crevice corrosion are the most common forms of corrosion for stainless steels. Both types of attack result in a highly localized form of corrosion which can lead to perforation in a short time with relatively little total weight loss. Both pitting and crevice corrosion are accelerated by more acidic conditions and by higher temperatures. With its high levels of chromium, molybdenum, and nitrogen, 654 SMO is the most resistant stainless steel ever produced. One method of estimating the pitting resistance is to use the pitting resistance equivalent (PRE), an index of pitting resistance based on statistical analysis of the effect of composition of stainless steels on a particular parameter assessing pitting resistance, most usually the critical pittingPRE* for Stainless Steels (Pitting Resistance Equivalent)Grade 654 SMO

Table 3 PRE 56.143 42.2 36 34.5 25.2

254 SMO 2507 904L 2205 Code Plus Two 316L

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

Critical Crevice TemperaturesGrade ASTM G 48 Practice B 10% FeCl3 6H20 CCT, F 11% H2SO4 1.2% HCl 1% FeCl3 1.0% CuCl2 154149 82 109

Table 4


4% NaCl 0.1%Fe2(SO) 6 0.01M HCl 140131 68 100

654 SMO 140Alloy C-276 140 Alloy 625 68 254 SMO 100

temperature (CPT) for a selected environment and test procedure. As seen in Table 3, the PRE for 654 SMO is far higher than it is for any of the other stainless steels. The critical pitting temperature is the highest temperature at which a stainless steel will resist attack in a particular environment. Two test procedures are common, one being ASTM G 48, Practice A, which measures the CPT by exposing coupons to a solution of 10% FeCl3 6H2O at a series of increasing temperatures until pitting corrosion is observed. Another newer test is an electrochemical test ASTM G 150 using a test cell specially designed by Outokumpu Stainless Research. Both of these methods have been effective for all other stainless steels, including the 6Mo austenitic stainless steels, such as Outokumpu Stainless 254 SMO. However, these tests are of limited use for 654 SMO because it is completely resistant to attack in these tests at 204F and 217F, the respective boiling points of the chloride media for these tests. So the best evaluation for 654 SMO is obtained by the more aggressive crevice corrosion tests. Table 4 shows critical crevice temperatures for 654 SMO in comparison with those for Alloy 625 and Alloy C-276, and for 254 SMO austenitic stainless steel. The test environments include ASTM G 48, Practice B, and the so-called Green Death solution designed to discriminate among the nickel-base alloys and their high levels of corrosion resistance. These tests were made with multiple crevice washers made of PTFE and imposed on the surface by bolts torqued to 1.16 ft-lb, providing very severe crevice conditions. These tests demonstrate that the 654 SMO is at least as resistant as Alloy C-276 to crevice corrosion in these acidic, oxidizing, high-chloride solutions. To simulate a flue gas desulfurization scrubber environment, an acidic sodium chloridecontaining solution at 176F was bubbled with sulfur dioxide for two hours. The pH of the solution was 1.01.5 at the start of the test but decreased with time to 0.51.0. The lowest chloride level necessary to cause corrosion was determined for each material. As shown in Table 5, 654 SMO is superior to all stainless steels, comparable to Alloy 625, but inferior to Alloy C-276.

Type 654 SMO


It is concluded from these and other observations that 654 SMO is comparable to Alloy C-276, being slightly superior in some environments and slightly inferior in others, depending on details to the particular exposure.Chloride Stress Corrosion Cracking (SCC)

Stress Corrosion Cracking Resistance (SCC) in Drop Evaporation Tests at 392FApplied Stress % Yield ksi Strength

Table 6 Time to Failure Hours >500, >500155, 158 >500, >500 >500, >500

Grade 654 SMO

48.53.0 21.9 32.1

10010 70 90

654 SMO austenitic stainless steel also possesses excellent resistance to chloride-induced transgranular stress corrosion cracking. This resistance is a result of the high molybdenum and nickel contents, in combination with the extremely high resistance to pitting. One method to rank stainless steels by their SCC resistance is to test them in accordance with the 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 the next drop. The applied stress is varied in steps of 10% up to 100% of the yield stress at 392F until cracking occurs or up to a maximum of 500 hours. As shown in Table 6, 654 SMO is resistant to cracking at 100% of its yield strength at the maximum exposure of 500 hours. 654 SMO is not immune to SCC in boiling 42% magnesium chloride. However, it is immune to SCC in boiling 25% sodium chloride and the wick test, both shown to be well correlated with practical experience in resisting SCC in severe heat transfer conditions at ambient pressures.General Corrosion

316 904L 254 SMO

A discussion of the resistance of a stainless steel to general corrosion must address both pure chemicalCritical Chloride ConcentrationsGrade 654 SMO

Table 5

Critical Chloride Concentration, ppm 12,50050 500 5,000 1,500 4,000 to 15,000 27,500

316 904L 254 SMO 2507 Alloy 625 Alloy C-276

environments, e.g., the strong mineral acids, and those environments with small to moderate levels of contamination 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 ppm chloride, the 654 SMO maintains a much higher corrosion 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. New materials are compared with standard alloys tested at the same time under identical conditions by determining the lowest test temperature at which the corrosion rate exceeds 5 mpy. Outokumpu Stainless 654 SMO stainless steel has been evaluated by the MTI procedure and the results are shown in Table 7. 654 SMO shows outstanding corrosion performance, exceeding that of the other special duplex and austenitic stainless steels in each of these environments. Stainless steels such as 316L cannot be used in hydrochloric acid even at very low concentrations because of the risk of localized and general corrosion. However, as shown in Figure 3, 654 SMO may be used in dilute hydrochloric acid even at fairly high temperatures, and at room temperature up to abou