Austenitic stainless steel

Abstract

An austenitic stainless steel having good corrosion resistance, particularly in chloride environments; this is achieved by the use of a rare earth element, preferably lanthanum, singly or in combination with nitrogen, along with nickel and molybdenum at relatively low levels for an austenitic stainless steel. The composition includes 15 to 25% chromium, greater than 16 to 25% nickel, 3 to 7% molybdenum, with a rare earth element consisting of lanthanum within the range of 0.005 to 0.05% in combination with 0.1 to 0.5% nitrogen.

Description

There is a need for highly corrosion resistant stainless steels for use broadly in marine and severe chemical environments. More specifically, construction of power and chemical plants that utilize seawater for coolants, recent developments in the pulp and paper industry that increase the chloride concentrations in these applications and the installation of pollution control equipment have created applications for stainless steels that are more corrosion resistant than the conventional AISI Type 304 and Type 316 stainless steels.

For these applications, and particularly to provide corrosion resistance in chloride-type environments, it is known to use stainless steels having relatively high combinations of nickel and molybdenum. Nickel and molybdenum in recent years have become increasingly more expensive so that a need exists for a stainless steel having the required corrosion resistance in chloride environments without resorting to higher nickel and/or molybdenum contents to achieve this.

It is accordingly the primary object of the present invention to provide an austenitic stainless steel having good corrosion resistance in chloride environments, wherein nickel and molybdenum are maintained at relatively low levels and the rare earth element lanthanum is used in combination with nitrogen to enhance corrosion resistance and hot-workability.

This and other objects of the invention, along with a more complete understanding thereof, may be obtained from the following description, specific examples and drawing.

The single FIGURE of the drawing is a bar graph showing the criticality of lanthanum compared to cerium in the alloy of the invention from the standpoint of hot-workability, and specifically hot-workability.

Broadly, the composition of the austenitic stainless steel in accordance with the present invention consists essentially of, in weight percent, 0.01 to 0.1 carbon, manganese 12 max., preferably 3 max., silicon 1 max., chromium 15 to 25, preferably 17 to 23, nickel greater than 16 to 25, preferably greater than 16 to 20, molybdenum 3 to 7, preferably 3 to 5.5, lanthanum within the range of 0.005 to 0.05, nitrogen 0.1 to 0.5, preferably 0.1 to 0.3, and the balance iron.

Copper may be added for acid corrosion resistance in amounts up to about 3%.

It is understood that for stabilization purposes and depending upon the carbon and nitrogen content of the alloy conventional stabilizing elements such as titanium, columbium, vanadium, zirconium and tantalum may be present alone or in combination. Also, the conventional deoxidizers such as aluminum, calcium, boron and magnesium may be used. With respect to the composition recited in the claims, these are considered to be incidental elements and their use for stabilization and deoxidation, respectively, is considered to be within the scope of the claims. It is also understood that for purposes of providing good hot workability and resistance to weld hot cracking, elements known to be detrimental to these properties, such as sulfur, phosphorus, lead, and tin may be controlled to very low levels.

For purposes of defining the limits of the invention, and by way of specific example thereof, six 50-pound vacuum induction heats were melted. These heats contained approximately 18% chromium with variations in the nickel, copper, nitrogen and lanthanum. The desired molybdenum content was achieved by using split heats. These heats were employed along with additional samples, including conventional commercial alloys. The heats were processed by casting 17-pound ingots which were held at 2100.degree. F. for two hours. They were then forged into sheet bar of 31/2.times.7/8".times.length. After conditioning they were heated at 2200.degree. F. for one hour and then hot rolled to 0.250" hot band. The hot band was heated at 2150.degree. F. for one half hour and then water quenched. After shot blasting and pickling the material was cold rolled to 0.125" strip, which was heated for 15 minutes at 2150.degree. F. and water quenched. The material was then cold rolled to achieve a further reduction to 0.060" strip, which strip was heated at 2150.degree. F. for 10 minutes, water quenched, shot blasted and pickled.

TABLE I sets forth the chemical composition of these heats as well as the other alloys used for evaluation. Also listed in TABLE I are the results of the microstructural evaluation of the alloys.

TABLE I__________________________________________________________________________CHEMICAL COMPOSITION AND MICROSTRUCTUREOF EXPERIMENTAL ALLOYS Weight Percent Micro- C Mn S P Si Cu Cr Ni Mo N Other structure.sup.2__________________________________________________________________________Heat No.Type - Laboratory Heats 18 Cr 16 Ni3D37 .027 1.50 .005 .015 .41 0.07 18.30 16.10 4.10 .023 A3D38 .sup.1 18.57 16.10 5.51 A + LS3D39 .sup.1 18.47 15.95 7.52 A + HS 18 Cr 16 Ni N3D24 .020 1.63 .005 .016 .46 0.08 18.47 15.25 3.54 .13 A3D21 .018 1.44 .005 .017 .46 0.08 18.21 15.82 4.02 .12 A3D25 .sup.1 18.99 15.32 4.12 .13 A3D22 .sup.1 18.43 16.13 4.57 .11 A3D26 .sup.1 18.98 15.39 5.29 .13 A + LS3D23 .sup.1 18.45 15.97 5.69 .12 A + MS 18 Cr 20 Ni3D40 .028 1.57 .006 .010 .29 0.08 17.64 20.18 4.03 .022 A3D41 .sup.1 17.93 20.30 5.43 A + MS3D42 .sup.1 17.88 20.05 7.19 A 18 Cr 20 Ni La3D46 .025 1.54 .003 .013 .38 0.08 17.48 19.86 4.03 .023 La 0.07 A3D47 17.68 19.82 5.78 .024 La 0.04 A3D48 17.53 19.58 7.37 .023 La 0.009 A + HS 18 Cr 18 Ni Cu3D43 .016 1.49 .005 .013 .36 2.03 17.66 18.14 4.27 .031 A3D44 .sup.1 2.07 18.10 18.37 5.89 A + LS3D45 .sup.1 1.36 2.07 17.99 18.30 7.07 A + HS Competitive Alloys3983 .020 1.80 .011 .013 .56 0.14 25.25 25.20 3.78 .022 B 0.001 A3C14 .018 1.72 .011 .013 .51 25.32 22.80 2.21 .12 B 0.0006 A + LSSandvik 2RE693C 15 .014 1.66 .014 .014 .56 21.43 25.37 5.58 Ti 0.22HaynesMOD 20GradeCommercial Steels90840 (AL 6X) 20.00 24.00 6.00 A + LS6X 20.80 25.58 6.21 Ce 0.0074 A + LS La 0.0042JS700 .03 1.70 21.00 25.00 4.50 Cb 0.30 A904L .02 1.75 1.40 20.00 25.00 4.50 A20Cb3 .04 1.70 3.50 20.00 33.70 2.50 Cb 0.35 A316L .025 1.70 17.00 12.50 2.25 A317L .025 1.70 18.40 13.20 3.20 A__________________________________________________________________________ .sup.1 Not Analyzed, Split Heat .sup.2 A = Austenite LS = Light Second Phase MS = Medium Second Phase HS = Heavy Second Phase

For purposes of crevice corrosion testing, test specimens were prepared by making autogenous gas tungsten arc cross-welds on the samples and then cutting them into 1".times.3" test specimens. A hole was drilled at the cross in the welds. The surfaces of the specimens were ground with a 120 grit belt, cleaned, measured and weighed. Serrated teflon blocks were fastened to the specimen with titanium bolts and uniformly tightened with a torque wrench. The tests evaluate the base metal, heat-affected zone and the weld. The tests were performed in a solution of synthetic seawater containing 1% potassium ferricyanide. The test temperatures were 86.degree. F. and 104.degree. F. for 120 or 124 hours, respectively. Weight loss per square inch of specimen, as well as visual examination of the specimen, were the evaluation criteria.

TABLE II lists the results of the corrosion tests conducted at 86.degree. F. Each alloy tested was ranked according to weight loss. Alloys displaying no weight loss were ranked according to the degree of etching or discoloration as determined by visual and macroscopic examination.

TABLE II______________________________________CREVICE CORROSION TEST RESULTSIN SYNTHETIC SEAWATER* Wt. LossNominal Composition, Weight % Mg/sq.Heat Cr Ni Mo Others in. Rank Order______________________________________3D48 17.53 19.58 7.37 La .009 0 1 Best3D26 18.98 15.39 5.29 N .13 0 23D23 18.45 15.97 5.69 N .12 0 33D47 17.68 19.82 5.78 La .04 0 420Cb-3 20.00 33.70 2.50 Cu 3.5 1.3 Cb 0.35 53982 20.34 25.05 6.24 1.7 6(6X)3D25 18.99 15.32 4.12 N .13 2.4 73D41 17.93 20.30 5.43 4.3 83D46 17.48 19.86 4.03 La .07 4.8 93D44 18.10 18.37 5.89 Cu 2.07 6.0 10JS700 21.00 25.00 4.50 Cb .30 6.4 113D45 17.99 18.30 7.07 Cu 2.07 6.6 123D42 17.88 20.05 7.19 6.7 133D39 18.47 15.95 7.52 7.1 14UD904L 20.00 25.00 4.50 Cu 1.5 8.2 153D21 18.21 15.82 4.02 N .12 8.3 163D43 17.66 18.14 4.27 Cu 2.03 8.3 173D38 18.57 16.10 5.51 9.3 183D24 18.47 15.25 3.54 N .13 9.4 193D37 18.30 16.10 4.10 9.6 203D22 18.43 16.13 4.57 N .11 11.8 213D40 17.64 20.18 4.03 12.0 22317L 18.40 13.20 3.20 13.5 23316L 17.00 12.50 2.25 22.4 24 Poorest______________________________________ *Synthetic Seawater containing 1% potassium ferricyanide 30.degree. C. (86.degree. F.) 120 hours.

As may be seen from the results presented on TABLE II with alloys containing nominally 4% molybdenum, nitrogen addition was beneficial from the corrosion resistance standpoint. Alloys containing 4.5 to 5.5% molybdenum in combination with nitrogen were superior to the commercial austenitic stainless steels tested. With respect to the alloys containing 18 to 20% nickel, at all the molybdenum levels tested, copper provided no benefit from the chloride corrosion standpoint. A lanthanum addition to these alloys was beneficial at all molybdenum levels tested. Specifically, a small lanthanum addition to the 5.7% molybdenum-containing steel (3D47) resulted in better crevice corrosion resistance than alloys 6X, JS700 and UD904L. Little benefit is obtained by increasing molybdenum above about 7%. However, the nitrogen or lanthanum modified alloys containing more than 5.25% molybdenum are more resistant to crevice corrosion than the higher nickel Cb-3 or 6X alloys. Similar with regard to the 40.degree. C. test data of TABLE III this shows that again increasing the molybdenum is beneficial but there is little benefit in using more than about 5.5% molybdenum.

TABLE III__________________________________________________________________________CREVICE CORROSION TEST RESULTS IN SYNTHETIC SEAWATERCONTAINING 1% K.sub.3 Fe(CN).sub.6 - 40.degree. C. (104.degree. F.) Nominal Composition Weight Loss (Weight Percent) (mg/in..sup.2)Alloy Cr Ni Mo Others 124 hrs. 120 hrs. Average Rank__________________________________________________________________________90840 (AL6X) 20.0 25.0 6.0 -- 1.1 -- 1.1 Best 13D48 17.5 19.5 7.37 La .009 1.7 3.0 2.3 23982 (6X) 20.0 25.0 6.24 -- 1.1 4.1 2.6 33D26 19.0 15.0 5.29 N .13 1.9 3.6 2.8 43D47 18.0 20.0 5.78 -- 1.5 5.1 3.3 53C15 21.0 25.0 5.58 Ti .2 3.4 -- 3.4 6(Haynes)Comm 6X 21.0 25.0 6.21 -- 4.1 -- 4.1 73983 25.0 25.0 3.78 -- 4.4 4.7 4.5 83D23 18.0 16.0 5.69 N .12 4.8 -- 4.8 93C14 25.0 23.0 2.21 -- 5.6 -- 5.6 10(Sandvik)20Cb3 20.0 34.0 2.5 Cu 3.5 6.6 6.2 6.4 Poorest 11(Carpenter)__________________________________________________________________________

In TABLE IV The compositions of three heats are reported; all are of essentially the same composition except for the lanthanum and cerium contents. Heat 3G31A contains essentially no lanthanum or cerium; Heat 3G28A contains lanthanum but no cerium; and Heat 3G29A contains cerium but essentially no lanthanum. From ingots of each of the heats reported in TABLE IV hot bands were produced by conventional practice including hot rolling from a temperature of 2275.degree. F. After hot rolling the hot band from each heat was examined for edge cracking. From this examination, a bar graph constituting the single FIGURE of the drawing was prepared. This FIGURE shows that the lanthanum-containing Heat (3G28A) exhibits significantly less edge cracking than the lanthanum- and cerium-free heat (3G31A) and the cerium-containing heat (3G29A).

TABLE V summarizes weight loss corrosion test data for annealed hot bands from the heats of TABLE IV alloys in both boiling 10% sulfuric acid (H.sub.2 SO.sub.4) and crevice corrosion tests using acidified 10% ferric chloride (FeCl.sub.3). In the former test the lanthanum-containing alloy exhibits about one half the weight loss than either of the other two alloys. The results were similar in the crevice corrosion tests reported on TABLE V.

TABLE IV__________________________________________________________________________CHEMICAL COMPOSITION OF LANTHANUM-AND CERIUM-CONTAINING HEATSHeat Weight PercentNumber C Mn S Si Cr Ni Mo N La Ce Fe__________________________________________________________________________3G31A .038 1.74 .010 .58 20.60 18.04 5.87 .24 .001 N.D. Bal.3G28A .035 1.74 .005 .65 20.25 17.89 5.90 .26 .013 N.D. Bal.3G29A .030 1.73 .006 .67 20.53 17.96 5.87 .25 .001 .026 Bal.__________________________________________________________________________ N.D. = Not Detected TABLE V______________________________________EFFECT OF LANTHANUM AND CERIUMON THE WEIGHT LOSS CORROSION OFAUSTENITIC STAINLESS STEEL Weight Loss (mg/in..sup.2) Crevice Corrosion 10% H.sub.2 SO.sub.4 Acidified 10% FeCl.sub.3 120 Hours 24 Hours 120 HoursHeat Number Boiling 37.5.degree. C. 46.degree. C. 55.degree. C.______________________________________3G31A 500 0.5 9.9 17.63G28A 292 0.2 6.3 13.9(0.013% La)3G29A 525 0.7 9.2 19.5(0.026% Ce)______________________________________

Claims

1. An austenitic stainless steel having good corrosion resistance in chloride environments at relatively low nickel and molybdenum levels, said steel consisting essentially of, in weight percent, carbon 0.01 to 0.1, manganese 12 max., silicon 1 max., chromium 15 to 25, nickel greater than 16 to 25, molybdenum 3 to 7, a rare earth element consisting of lanthanum 0.005 to 0.05, nitrogen 0.1 to 0.50 and balance iron.

2. The steel of claim 1 having copper up to about 3%.

3. The steel of claim 1 having 0.1 to 0.3% nitrogen.

4. The steel of claim 1 having manganese 3% max.

5. The steel of claim 1 having molybdenum within the range of 3 to 5.5%.

6. The steel of claim 1 having nickel within the range greater than 16 to 20%.

7. An austenitic stainless steel having good corrosion resistance in chloride environments at relatively low nickel and molybdenum levels, said steel consisting essentially of, in weight percent, carbon 0.01 to 0.1, manganese 12 max., silicon 1 max., chromium 15 to 25, nickel greater than 16 to 20, molybdenum 3 to 5.5, a rare earth element consisting of lanthanum 0.005 to 0.05, nitrogen 0.1 to 0.50 and balance iron.

8. The steel of claim 7 having manganese 3% max.

9. The steel of claim 7 having copper up to about 3%.