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Heat Treatment Method for Reducing Polythionic Acid Stress Corrosion Cracking

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Heat Treatment Method for Reducing Polythionic Acid Stress Corrosion Cracking Europaisches Patentamt 0 398 761 European Patent Office © Publication number: A1 Office europeen des brevets EUROPEAN PATENT APPLICATION © Application number: 90305445.0 © int. a* C22F 1/10 © Date of filing: 18.05.90 © Priority: 19.05.89 US 354310 © Applicant: INCO ALLOYS INTERNATIONAL, INC. @ Date of publication of application: 3200 Riverside Drive 22.11.90 Bulletin 90/47 Huntington West Virginia 25702(US) © Designated Contracting States: © Inventor: Crum, James Roy AT BE DE FR GB IT SE 157 Cheyenne Trail Ona, West Virginia 25545(US) Inventor: Lipscomb, William Grant 124 Brady Drive Barboursville, West Virginia 25504(US) Inventor: Ganesan, Pasupathy 6 Partridge Court Huntington, West Virginia 25705(US) © Representative: Hedley, Nicholas James Matthew et a I TZ Gold & Company 9 Staple Inn London WC1V 7QH(GB) © Heat treatment method for reducing polythionic acid stress corrosion cracking. © A heat treatment of alloy 617 generally including 732° C-927° C (1350° F-1700" F) for about one hour. The resultant discontinuous carbide network in the grain boundaries inhibits stress corrosion crack growth in polythionic acid environments. Alloy 617 generally comprises 20-24% Cr, 9.5-20% Co, 7-12% Mo, 0.8-1.5 Al, and less than 0.15% carbon, the balance being iron. to 00 CO Q_ LU Xerox Copy Centre EP 0 398 761 A1 HEAT TREATMENT METHOD FOR REDUCING POLYTHIONIC ACID STRESS CORROSION CRACKING TECHNICAL FIELD The instant invention relates to heat treatment techniques in general and more particularly to method for 5 reducing polythionic acid (H2SxOs) stress corrosion cracking in a nickel-base alloy. BACKGROUND ART TO INCONEL® alloy 617 (trademark of assignee) is a solid solution nickel-chromium- cobalt-molybdenum alloy exhibiting excellent high temperature strength and resistance to oxidizing and reducing environments. See U.S. patent 3,859,060. The alloy displays excellent resistance to a wide range of corrosive environ- ments and is readily formed by and welded by conventional techniques. It is used, amongst other places, in rs demanding petrochemical applications where it generally provides excellent service. The nominal chemical composition of INCONEL alloy 617 is shown below (in weight per cent). BROAD PUBLISHED 20 Nickel Bal 52 Chromium 20-24 22 Cobalt 9.5-20 12.5 Molybdenum 7-12 9.0 Iron 1 .5 25 Aluminum 0.8-1.5 1.2 Carbon S.15 0.1 Manganese 0.5 Silicon 0.5 Titanium 0.3 30 Copper 0.2 The alloy is, however, susceptible to intergranular polythionic acid stress corrosion cracking. Cracking caused by polythionic acid can occur in the annealed condition or after long term exposure at simulated 35 operating temperatures up to about 649° C (1200° F). Polythionic acid may be present in petrochemical environments. Failure of components made from INCONEL alloy 617 due to this or any other condition clearly cannot be tolerated. 40 SUMMARY OF INVENTION Accordingly, there is generally provided a heat treatment for about 732° C-927 C (1350 F-1700 F) per one hour. The heat treatment apparently produces a discontinuous carbide network in the grain boundaries 45 which inhibits crack growth and appears to be effective even after long time simulated service tempera- tures. BRIEF DESCRIPTION OF THE DRAWING 50 Figure 1 is a 300x power scanning electron microscope photomicrograph of alloy 61 7 exposed to a polythionic acid environment in the as-annealed condition. Figure 2 is a 2000x power scanning electron microscope photomicrograph of alloy 617 annealed plus EP 0 398 761 A1 heat treated at 649° C (1200° F) for 100 hours. Figure 3 is a 4000x power scanning electron microscope photomicrograph of alloy 617 heat treated at 900° C (1650° F) for one hour plus 649° C (1200° F) for 100 hours. PREFERRED MODE FOR CARRYING OUT THE INVENTION Alloy 617 is approved for ASME Boiler and Pressure Vessel Code use in Section I and Section VIII, Division 1 welded construction service by Code Case 1956-1. Maximum operating temperature is 899 C 70 (1650° F). Alloy 617 seamless pipe and tube (12.7 cm [5 inch)] OD and under) is approved for Section VIII, Division 1 service to 982° C (1800° F) by Code Case 1982. The effects of long term exposure at 538° C to 760° C (1000° -1400° F) on both room temperature and operating temperature mechanical properties is a common question when designing a system. Precipitation of secondary phases in alloy 617 during long term exposure at 538-760° C (1000-1400° F) produces an 75 increase in the room temperature yield and tensile strengths as well as reducing elongation. These phases, carbides plus potential gamma prime, are responsible for the increase in room temperature yield and tensile strengths. As with any strengthening process there is a reduction in elongation. It is important to point out that the reductions in elongation are at room temperature and elongations are not affected at operating temperature. 20 Expansion joints for elevated temperature service represent a demanding application for an alloy. Added to the problems caused by elevated temperature environmental corrosion attack and elevated temperature strength demands is elevated temperature fatigue. Low cycle fatigue strength is very important in elevated temperature service. Alloy 617 possesses excellent low cycle fatigue properties. 25 U-bend stress corrosion cracking ("SCC") and coupon intergranular attack tests were conducted on INCONEL alloy 617 cold rolled sheet and hot rolled plate 0.81 - 4.75 mm (0.032 - 0.187 inch)in the mill annealed (1177° C [2150° F] for about fifteen minutes) or mill annealed plus the heat treated conditions specified. All specimens were prepared for testing by sanding to a 120 grit surface finish prior to testing. Polythionic acid solution was prepared by bubbling sulfur dioxide gas through distilled water for 3 hours 30 followed by hydrogen sulfide gas for one hour. The resulting solution was checked for potency by exposing an AISI 304 stainless steel U-bend sensitized by heat treatment at 677° C (1250° F) for two hours. Cracking within one hour indicated a solution suitable for use (Ref. ASTM G 35). All SCC tests were conducted at ambient temperature for a period of 720 hours. In evaluating the U-bend specimens, time to crack was defined as the time required for formation of 35 cracks large enough to be visible at 20X magnification. Time to fail was defined as the time required for cracking to advance to the point where tension was lost in the legs of the U-bend specimen. Intergranular attack (sensitization) tests were conducted in boiling 65% nitric acid (ASTM A 262, Practice C) and sulfuric acid-ferric sulfate (ASTM G 28, Practice A) tests. In order to correlate the stress corrosion cracking response of alloy 617 in a polythionic acid ("PTA") 40 environment with microstructure, selected samples in the as-annealed and annealed plus heat treated conditions were examined in optical and scanning electron microscopes (SEM). Also the phases present in the microstructure of the selected samples were identified by X-ray diffraction (XRD) analysis of the residues obtained by dissolving the matrix electrolytically in a 10% HCI-methanol solution. Samples for metallography and SEM were obtained from either leg or U-bend portion of the specimens and prepared 45 following standard procedures for mounting and polishing. All the samples were then swab etched with Kalling's reagent. Samples for XRD analysis were obtained from leg portions of the selected samples. CORROSION AND MICROSTRUCTURE RESULTS 50 A series of INCONEL alloy 617 heats were fabricated for testing purposes. Their chemical compositions are presented in Table 1. Two heats of INCOLOY® (trademark of assignee) alloy 800H from an earlier study were used for comparison purposes. 55 TABLE 1 EP 0 398 761 A1 The results of PTA SCC testing of alloy 617 with various heat treatments representing a wide range of possible service temperatures are listed in Tables 2-4. The results for alloy 800H are included for comparison. TABLE 2, 3 & 4 Heat treatments from 427-982° C (800-1800° F) for up to 100 hours and the mill annealed condition are w: examined in Table 2. It is possible for alloy 617 to crack in the mill annealed condition and after exposure to temperatures of 649° C (1200° F) or below. No cracking occurred in the higher range of 704-899° C (1300- 1650° F) but cracking did occur after exposure at 982° C (1800° F). This data represents 73 tests conducted on six heats of alloy 617. Only 18 of the 73 cracked and only 2 of the 73 cracked severely enough to cause complete failure of the specimen. For comparison, most of the alloy 800H specimens from the 649-899 °C TS (1200-1650° F) temperature range cracked and many failed completely. The absence of cracking after exposure to the 704-899 °C (1300-1650° F) range suggests that a microstructural change may be occurring which inhibits PTA SCC. To explore this possibility, 760 °C (1400° F) and 899 °C (1650° F) heat treatments were applied prior to longer time service temperature simulations, Tables 3 and 4. The 20 TABLE 1 Chemical Composition of Corrosion Test Materials 25 Heat C Mn Fe S Si Cu Ni Cr Al Ti Co Mo 1 .07 .03 .41 .002 .15 .09 Bal 21.89 1.21 .30 12.54 9.03 2 .08 .04 1.18 .001 .17 .12 Bal 21.89 1.03 .25 12.42 9.29 3 .06 .01 .87 .002 .11 .01 Bal 22.23 1.17 .30 12.47 9.20 4 .07 .02 .15 .007 .08 .01 Bal 22.31 1.06 .35 12.46 9.09 30 5 .06 .18 1.03 .001 .12 .02 Bal 21.93 .96 .49 11.71 8.84 6 .06 .07 2.13 .001 .19 .17 Bal 21.90 1.10 .23 12.43 8.89 7 .06 .89 Bai .002 .22 .61 31.24 20.22 .52 .50 .05 .30 8 .08 1.01 Bal .002 .36 .64 31.26 20.43 .37 .39 .08 .38 35 Note: Heats 1-6 are INCONEL alloy 617 Heats 7 and 8 are INCOLOY alloy 800H 40 45 50 55 EP 0 398 761 A1 TABLE 3 PTA U-bend Test Results - Effect of a 760° C (1400° F) Heat Treatment 5 Time to Crack (TTC)/Time to Fail (TTF), in Hours (Hr) 760 °C (1400° F) 1 Hr + Following Heat No.
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