Austenitic stainless steel having improved corrosion resistance in hot water

Abstract: An austenitic stainless steel having excellent corrosion resistance in hot aqueous medium is disclosed. The steel essentially consists by weight of C : not more than 0. 08% Si: 2. 5 - 4. 0% Mn: not more than 0. 8% P : not more than 0. 045% S : not more than 0. 005% Cr: 16 - 25% Ni: 6 - 20% Cu: 1. 5 - 4. 0% N : not more than 0. 05% and the balance Fe and unavoidable incidental impurities.

Description: This invention relates to an austenitic stainless steel which has excellent crevice corrosion resistance and stress corrosion cracking resistance and is suitable for use in low concentration chloride environments. Austenitic stainless steels represented by SUS304 and SUS316 have corrosion resistance to city service water and industrial water which may contain a slight amount of chlorides, and have excellent workability and weldability. Therefore, they are widely used for various kinds of water heaters, heat exchange tubes, chemical plant equipment, etc. However, they have a defect in that pitting and/or crevice corrosion occurs at welded parts, etc. at relatively high temperatures if even a slight amount of chloride ions are present, and said pitting and/or crevice corrosion may lead to stress corrosion cracking. Prevention of stress corrosion cracking of stainless steels has been studied by many researchers and many measures for prevention thereof have been reported. But the reported effects of alloying elements vary considerably depending upon composition of test solutions, test conditions, etc. It is well known that P, Mo and N are deleterious but Cu is effective for prevention of stress corrosion cracking in low concentration chlorides environments. However, reduction of the P content to the safety level for stress corrosion cracking requires a special refining technique, which invites increase in manufacturing cost. On the other hand, Mo and N are considered to be effective for improvement of resistance to localized corrosion such as pitting and crevice corrosion. We previously proposed in JP-B59-45751(1974) a 18Cr-9Ni steel containing a suitable amount of Cu and W, which is provided with both stress corrosion cracking resistance and crevice corrosion resistance without reduction of the P content, but this steel was not satisfactory for use at temperatures near 100°C. In this specific field of the stainless steel technology, JP-A61-9557(1986) (Kawasaki) discloses an austenitic stainless steel containing C≦0.06%, Si≦1.0%, Mn≦0.8%, 16%≦Cr≦25%, 6%≦Ni≦20%, 1.5%<Cu<2.5%, 0.05%<N<0.15% and 0.2%<Mo<0.6%, wherein the S content is restricted to ≦0.005%. The critical temperature for stress corrosion cracking resistance of this steel is around 80°C and, therefore, nor suitable for use in hot water like our above-mentioned previously known steel. Also JP-A59-185763(1984) (Nippon Stainless Steel) discloses an austenitic stainless steel which contains C≦08%, 2.0%<Si<4.0%, Mn≦2.00%, 16.00%≦Cr≦20.00%, 8.00%≦Ni≦13.00%, 0.30%≦Cu≦2.00%, 0.05%≦N≦0.30%, 0.30%≦Mo≦1.50%, and optionally Nb≦0.10%, wherein the B content is restricted to not more than 0.00020%. This steel contains significantly high concentrations of N and Mo and its critical temperature for stress corrosion cracking resistance is higher than 80°C But it is inferior in the crevice corrosion resistance. In FR-A-2 048 370 there is described a stainless steel having an increased corrosion and weld cracking resistance essentially being chracterized in that it consists of 0,01 - 0,1% C, 2 - 6% Si, 0,01 - 3% Mn, 7 - 20% Ni, 13 - 25% Cr, 0,5 - 5% Cu the rest being Fe and inevitable impurities, whereby the contents of said constituents of the stainless steel correspond to the following proportion and in GB-A-2 177 113 there is described a high strength stainless steel consisting essentially of not more than 0,10% C, more than 1,0% and not more than 3.0% Si, less than 0,5% Mn, not less than 4,0% and not more than 8,0% Ni, not less than 12,0% and not more than 18,0% Cr, not less than 0,5% and not more than 3,5% Cu, not more than 0,15% N and not more than 0,004% S, wherein the total content of C and N is not less than 0,10%, the balance being Fe and incidental impurities. In addition to the above-mentioned elements, the stainless steel disclosed in GB-A-2 177 113 may contain not more than 0.020% of Al, not more than 0.020% of REM's, not more than 0.040% of P, not more then 0.020% of Ti and not more than 0.01% Ca. We have further proceeded with our study and found that when stainless steel sheets are used for heat exchange apparatuses, water-heating apparatuses for instance, heat is transmitted through the used stainless steel sheets, supplied to portions susceptible to corrosion and promotes corrosion. In particular, it was learned that crevice corrosion is caused at the parts where sheets are joined by spot welding, and leads to stress corrosion cracking, and the temperature of the side of a stainless steel sheet contacting liquid, that is, the side susceptible to corrosion, reaches 100°C or higher. Therefore, higher temperatures must be taken into consideration for prevention of stress corrosion cracking. Under the circumstances, this invention is intended to provide an inexpensive stainless steel which is provided with resistance to crevice corrosion and does not suffer stress corrosion cracking at temperatures over 100°C. Our detailed study on the mechanism of stress corrosion cracking and its relation to alloying elements revealed that Mo and N are strongly passivating elements which repassivate the corroded parts. But locally non-repassivated portions remain and these small portions suffer excessive corrosion because the surrounding parts are strongly repassivated. This rather makes steel more susceptible to stress corrosion cracking. Also it was revealed that CO, Si and Al relatively uniformly deposit on the holes of pittings or spots of crevice corrosion and inhibit dissolution of metal although very moderately. That is, these elements tend to work favorably toward prevention of stress corrosion cracking. Further it was found that when Si is contained in an amount of about 3%, the steel becomes less susceptible to stress corrosion cracking, and if Al is added, stress corrosion resistance is improved and that corrosion depth in crevice corrosion is reduced. This invention provides an austenitic stainless steel having excellent corrosion resistance in hot water comprising by weight C: not more than 0.08%, Si: 2.5 - 4.0%, Mn: not more than 0.8%, P: not more than 0.045%, S: not more than 0.005%, Cr: 16 - 25%, Ni: 6 - 20%, Cu: 1.5 - 4.0%, N: not more than 0.05%, Al: 0.05 - 3.0% and optionally Mo: more than 0.3% and less than 1.5% and optionally at least one of rare earth metals (REM): 0.005-0.1%, and the balance Fe and unavoidable impurities. Preferrably the Al-content is 0.1 - 2.0% and optionally the Mo-content is 0.3 - 1.2%. It is further preferred that the Al-content is 0.1 - 1.0% and optionally the Mo-content is 0.3 - 1.0%. Preferred is that the Al-content is 0.1 - 2.0% and optionally the Mo-content is 0.3 - 1.2% and REM is 0.01 - 0.08%. It is finally preferred that the Al-content is 0.1 - 1.0% and optionally the Mo-content is 0.3 - 1.0% and REM is 0.03 - 0.05%. Generally speaking, this invention has a character of an improvement of the steel of JP-A59-185763. The steel of the present invention is distinguished from said known steel in that the N content is lower and Mo, Al and REM can be contained. This characteristic will be demonstrated later. The reason why the composition of the steel is defined as described above is as follows: C: Carbon is a strong austenite-stabilizer and does not so adversely affect stress corrosion resistance and crevice corrosion resistance. However, it enhances intergranular corrosion sensitivity at welded parts. Therefore, the upper limit is defined as about 0.08%. The C content is preferably not more than 0.06%, and more preferably not more than 0.05%. Si: Silicon is a necessary and important element in the steel of the present invention and improves stress corrosion cracking resistance in the presence of Cu. This is a very important element which supports the crevice-corrosion-resistance enhancing effect of Mo without hindering the stress-corrosion-cracking-resistance enhancing effect of Mo. It also has some effect to improve pitting resistance. At least about 2.5%, preferably 2.8% of Si is required therefor to exhibit the above-described effect. However, Si is a strong ferrite-former and therefore the upper limit content thereof is defined as 4% in order to minimize the required Ni content. The Si content is more preferably not less than 3.0%. Mn: Manganese forms sulfide inclusions, which are apt to be starting points of corrosion and thus deteriorate the crevice-corrosion resistance and stress-corrosion-cracking resistance of the steel. Therefore, the lower the Mn content, the better the steel property. However, Mn is an unavoidable impurity element in steelmaking and it invites high cost in selection of raw materials and operation to reduce the Mn content extremely low. The upper limit of the content thereof is defined as 0.8%, which is the content level of the inevitably involved Mn in the ordinary steelmaking. Preferably, the Mn content should be not more than 0.5% when high crevice corrosion resistance is desired because the content of Mo which is effective for the crevice corrosion resistance is limited as described below. More preferably, the Mn content is not more than 0.4%. P: The content of phosphorus need not be lowered in particular in the steel of the present invention. However, phosphorus has adverse effect on stress corrosion cracking resistance, and, therefore, the upper limit of the content is defined as 0.045%. S: Sulfur forms sulfide with Mn, which is deleterious for crevice corrosion resistance and stress corrosion cracking resistance. The lower the content thereof, the better the property. The upper limit is defined as 0.005%. Cr: Chromium is an indispensable element in the stainless steel. For the stainless steel of the present invention which is used in chloride-containing hot environments, addition of at least 16% Cr is requisite. The higher the Cr content, the better the corrosion resistance. However, in order to maintain austenite phase, a greater amount of Ni is required. Also Cr impairs forgeability, rollabiliy and workability of the steel. From consideration of these factors, the upper limit is defined as 25%. The Cr content is preferably 17 - 22%, and more preferably 18 - 20%. Ni: Nickel is an element necessary to maintain austenite phase and at least 6% Ni is required for that purpose. On the other hand, more than 20% Ni only unnecessarily increases the cost of the steel. For this reason, the Ni content is defined as 6 - 20%. In this range, Ni does not particularly affect stress corrosion cracking but it is effective for improvement of crevice corrosion resistance. Therefore, Ni preferably should be contained at least in an amount of 10% for an application in which good crevice corrosion resistance is desired. The Ni content is preferably 10 - 18% and more preferably 12 - 16%. Cu: Copper is an important element in the steel of the present invention. Cu is effective for improving stress corrosion cracking resistance of the steel in NaCl-containing hot water environment. The higher the Cu content, the greater the effect. In the steel of the present invention, Cu must be contained at least in an amount of 1.5%. The effect of Cu saturates at 4.0% and more than 4% Cu impairs the steel's hot workability. The Cu content is preferably 2 - 4%, and more preferably 3 - 4%. N: As mentioned above, N is known to be deleterious to stress corrosion cracking resistance but effective for prevention of pitting and crevice corrosion. In the case of the present invention, Si and Cu are contained at higher levels. This enables reduction of the content of N which is deleterious in terms of stress corrosion cracking resistance and hardens the steel. In this sense, the N content is restricted to not more than 0.05%, preferably not more than 0.04% and more preferably not more than 0.03%. Mo: Molybdenum is not an essential element in the steel of the present invention Although Mo is very effective for improvement of crevice corrosion resistance and pitting, it impairs stress corrosion cracking resistance. In the case of the present invention, up to 1.5% of Mo can be present because of the effect of combined addition of Cu and Si (and Al). On the other hand, at least 0.3% Mo is necessary in order for it to exhibit its crevice corrosion resistance improvement effect. The Mo content is preferably 0.3 - 1.2% and more preferably 0.3 - 1.0% when contained. Al: Aluminum improves stress corrosion cracking resistance and raises the limit temperature for stress corrosion cracking prevention in the presence of Cu and Si. Also Al reduces depth of corrosion pits in crevice corrosion. However, addition of a large amount of aluminum deteriorates hot and cold workability. Addition of 0.05 - 3.0% is proper. Preferred amount is 0.1 - 2.0% and more preferred amount is 0.1 - 1.0%. REM: Rare earth metals are effective for improvement of hot workability. In the steel of the present invention, optionally not less than 0.005% of at least one rare earth metal is added in support of the effect of Al. Throughout the description and claims the term REM means "at least one of rare earth metals". Addition of not less than 0.1% REM may increase formation of inclusions. Preferred content is 0.01 - 0.08% and more preferred content is 0.03 - 0.05%. The stainless steel of this invention is provided with both excellent stress corrosion cracking resistance and excellent crevice corrosion resistance and these properties are realized without unduly increased in manufacturing cost. This steel is suitable as a material for apparatuses for treating hot neutral salt solutions. Now the invention will be explained specifically by way of working examples. Steels of the compositions indicated in Table 1 were prepared by vacuum melting and forged and hot-rolled, and thereafter cold-rolled into 1mm thick sheets by the conventional procedure. In Table 1, Samples A1 - A6 are comparative samples, wherein A1 is SUS304 and A2 is SUS316. Samples B3, B4 and B7 are steels of the present invention. Table 2 shows the stress corrosion cracking resistance and crevice corrosion resistance of all the samples. The stress corrosion cracking resistance was determined by the autoclave test and the heat transfer test. The autoclave test was carried out as follows. A larger piece and a smaller piece were fixed together by spot welding and the thus prepared samples were placed in an autoclave containing a 50ppm Cl⁻ solution and kept for 10 days at various temperatures. The critical temperature for stress corrosion cracking resistance was determined by checking occurrence of cracking. Also, spot-welded portions were cut out and depth of corrosion pits caused by crevice corrosion was measured. In this test, the stress corrosion cracking resistance limit temperature over 100°C is regarded as effective. The heat transfer surface test was carried out as follows. Against one surface of the above-mentioned spot-welded test pieces, a copper rod around which a Nichrome™ wire was wound was held, the other surface was contacted with a 50ppm Cl⁻ solution of 80°C and kept for 10 days with the temperature of the surface to which heat is applied varied. Thus the critical temperature for stress corrosion cracking resistance was determined by checking occurrence of cracking. In this test, the stress corrosion cracking resistance limit temperature over around 200°C is regarded as effective. The crevice corrosion resistance test was carried out in accordance with the test method described in D. B. Anderson: "Statistical Aspect of Crevice Corrosion in Seawater", ASTM-STP 576, p. 231, 1976, using a 1.75% NaCl solution containing 2% H₂O₂ as an oxidizing reagent. Test pieces were placed in the corrosion test solution at 40°C for 48 hours, and corrosion weight loss and number of corroded test pieces were determined. Samples B3 and B4 containing Al exhibits excellent stress corrosion cracking resistance (140°C and 250°C or therearound) as well as excellent crevice corrosion resistance although Sample B3 which substantially does not contain Mo is inferior to Sample B4 in the corrosion weight loss. Sample B7, which is similar to Sample B3 but contains a lower level of Al, is inferior to Sample B3 in the stress corrosion cracking resistance. In contrast, Sample A1 and A2, which contain high levels of Mn and low levels of Si and Cu, suffered stress corrosion cracking at 80°C. (These samples did not undergo the heat transfer test.) Samples A3 has good stress corrosion cracking resistance but inferior in the crevice corrosion resistance because the Si content is still low. Sample A4, which is similar to Sample A3 except that it contains Mo, exhibits considerably good crevice corrosion resistance but still is inferior in the stress corrosion cracking resistance, and does not reach the level of the inventive steels in the general corrosion resistance. Sample A5 contains Mo at a level exceeding the limitation of the present invention and, therefore, is inferior stress corrosion cracking resistance. Sample A6 contains N at a level exceeding the limitation of the present invention and, therefore, it suffers localized corrosion, that is, the maximum corrosion depth is great, although it exhibits good stress corrosion cracking resistance.

Claims: An austenitic stainless steel having excellent corrosion resistance in hot aqueous medium comprising by weight C: not more than 0.08% Si: 2.5 - 4.0% Mn: not more than 0.8% P: not more than 0.045% S: not more than 0.005% Cr: 16 - 25% Ni: 6 - 20% Cu: 1.5 - 4.0% N: not more than 0.05% Al: 0.05 - 3.0% and optionally Mo: more than 0.3% and less than 1.5% and Optionally at least one of rare earth metals (REM): 0.005 - 0.1% and the balance of Fe and unavoidable impurities. Stainless steel according to Claim 1, characterized in that the Al-content is 0.1 - 2.0% and optionally the Mo-content is 0.3 - 1.2%. Stainless steel according to Claim 2, characterized in that the Al-content is 0.1 - 1.0% and optionally the Mo-content is 0.3 - 1.0%. Stainless steel according to Claim 2, characterized in that the Al-content is 0.1 - 2.0% and optionally the Mo-content is 0.3 - 1.2% and REM is 0.01 - 0.08%. Stainless steel according to Claim 3, characterized in that the Al-content is 0.1 - 1.0% and optionally the Mo-content is 0.3 - 1.0% and REM is 0.03 - 0.05%.

Inventors: ADACHI TOSHIRO, FUJII ATSUSHI, YOSHII TSUGUYASU, YOSHIMURA ISAMU, ADACHI, TOSHIRO, FUJII, ATSUSHI, YOSHII, TSUGUYASU, YOSHIMURA, ISAMU
Applicants: NISSHIN STEEL CO LTD, NISSHIN STEEL CO., LTD.
Application year: 1988
Main classification: C22C38
IPC codes: C22C38