Corrosion-resistant nickel-chromium-molybdenum alloys

Abstract: A homogenization heat treatment minimizes the formation of Mu phase in nickel-base alloys of high combined percentages of chromium, e.g., 19 to 25% and molybdenum,. e.g., 12 to 18% particu­ larly together with tungsten. Also described is an advantageous alloy composition containing less than 2.5% iron, low carbon and a titanium to carbon ratio greater than 1 which is particularly adapted to be effectively treated by the homogenization heat treatment. The single or two staged homogenization takes place from 1141°C to 1316°C for a period of at least 5 hours.

Description: The present invention is directed to corrosion-resistant nickel alloys and more particularly to nickel-base alloys of high chromium/molybdenum content which are capable of affording outstanding corrosion resistance in a host of diverse corrosive media. INVENTION BACKGROUND As is generally recognized in the art, nickel-base alloys are used for the purpose of resisting the ravages occasioned by various corrodents. Notable in this regard are the nickel-chromium-molybdenum alloys as is set forth in the Treatise "Corrosion of Nickel and Nickel-Base Alloys", pages 292-367, authored by W.Z. Friend and published by John Wiley & Sons (1980). Among such alloys might be mentioned INCONEL® alloy 625, INCOLOY® alloy 825, Alloy C-276, Multiphase® alloy MP35N, HASTELLOY® alloys C, C-4 and the recently introduced alloy C-22®. Alloys of the type mentioned above are exposed to service conditions where, inter alia, severe crevice and pitting corrosion are encountered as well as general corrosion. Representative of such situations would be (a) pollution control applications, e.g., flue gas desulfurization scrubbers for coal fired power plants, (b) chemical processing equipment such as pressure vessels and piping, (c) the pulp and paper industry, (d) marine environments, particularly sea water, (e) oil and gas well tubing, casings and auxiliary hardware, etc. This is not to say that other forms of corrosive attack do not come into play under such operating conditions. In endeavoring to develop a highly useful and practical alloy for the above applications/service conditions, there seems to have been an emphasis in the direction of using chromium and molybdenum levels as high as possible, and often together with tungsten. (See, for example, Table I below which gives the nominal percentages of various well known commercial alloys.) Alloy Cr plus Mo plus W Alloy 625 21.5 Cr + 9 Mo C-276 15.5 Cr + 16 Mo + 3.75 W MP35N 20 Cr + 10 Mo C 15.5 Cr + 16 Mo + 3.75 W C-4 18 Cr + 15.5 Mo C-22 22 Cr + 13 Mo + 3 W X 22 Cr + 9 Mo + 0.6 W While high chromium, molybdenum and tungsten would be desirable, it can also give rise to a morphological problem, to wit, the formation of the Mu phase, a phase which forms during solidification and on hot rolling and is retained upon conventional annealing. There is perhaps not complete agreement as to what exactly constitutes Mu phase, but for purposes herein it is deemed to be appreciably a hexagonal structure with rhombohedral symmetry phase type comprised of (Ni, Cr, Fe, Co, if present ) 3 (Mo,W) 2 . P phase, a variant of Mu with an orthorhombic structure, may also be present. In any case, this phase can impair the formability and detract from corrosion resistance since it depletes the alloy matrix of the very constituents used to confer corrosion resistance as a matter of first instance. It is this aspect to which the present invention is particularly directed. It will be observed from Table I that when the chromium content is, say, roughly 20% or more the molybdenum content does not exceed about 13%. It is thought that the Mu phase may possibly be responsible for not enabling higher molybdenum levels to be used where resistance to crevice corrosion is of paramount concern. The foregoing aside, in striving to evolve the more highly corrosion resistant alloy, other considerations must be kept in focus. That is to say, corrosion resistance notwithstanding, such alloys not only must be hot workable but also cold workable to generate required yield strengths, e.g., upwards of 689 to 862 or 1035 MPa, together with adequate ductility. In addition, alloys of the type under consideration are often subjected to a welding operation. This brings into play corrosive attack at the weld and/or heat-affected zones (HAZ), a problem more pronounced where elevated operating temperatures are encountered, e.g., in the chemical process industry. Without a desired combination of mechanical properties and weldability an otherwise satisfactory alloy could be found wanting. GB-2 080 332 describes a broad range of nickel-based corrosion-resistant alloys containing Cr, Mo and W. GB-1 186 908 describes nickel-based corrosion-resistant alloys containing Cr, Mo and up to 2% W and also describes soaking such alloys at 1205°C. DRAWINGS The beneficial effect of the present invention is illustrated by a comparison of the figures of the drawing in which - Figure 1 is a reproduction of a photomicrograph at 500 power of an alloyconventionally processed, and Figure 2 is a similar reproduction at the same magnefication of a photomicrograph of the same alloy processed using the homogenization treatment of the present invention. INVENTION SUMMARY It has now been discovered that a special heat treatment, a homogenization treatment as described more fully herein, minimizes the tendency of the Mu phase to form such that higher combined percentages of chromium, molybdenum, e.g., 19-22% Cr, 14-17% Mo, together with 2.5 to 4% tungsten can be utilized. As a consequence, crevice/pitting corrosion resistance in various media is improved and manufacturing operations, including both hot and cold working, can be carried forth to produce product forms such as plate, strip and sheet which, in turn, can be fabricated into desired end products. INVENTION EMBODIMENT The present invention provides a process as set out in the accompanying claims 1 to 9 and a Ni-base alloy as set out in claims 10 to 13. The use of the Ni-base alloy is defined in claim 14. Generally speaking and in accordance herewith, the present invention contemplates the production of nickel-base alloys high in total percentage of chromium, molybdenum and tungsten having a morphological structure characterized by the absence of detrimental quantities of the subversive Mu phase, the alloys being subjected to a homogenization (soaking) treatment above 1149°C, e.g. at 1204°C, (optionally prior to and/or after hot working) for a period sufficient to inhibit the formation of deleterious Mu phase, i.e., at least about 5 hours. Advantageously, this heat treatment is carried out in two stages as described infra. Alloy Compositions In terms of chemical composition it is preferred that the nickel-base alloy contain in percent by weight, at least 19% chromium and at least 14 or 14.25%, molybdenum, together with at least 2.5% tungsten, the more preferred ranges being 20 to 23% chromium, 14.25 or 14.5 to 17% molybdenum and 2.5 to 4% tungsten. It is still further preferred that molybdenum levels of, say, 15 or 15.25 to 17%, be used with the chromium percentage of 19.5 to 21.5%. Conversely, the higher chromium percentage of, say, 21.5 to 23% should be used with molybdenum contents of 14 to 15%. While chromium levels of up to 24 or 25% might be employed and while the molybdenum may be extended up to 17 or 18%, it is deemed that excessive Mu phase may be retained during processing though such compositions might be satisfactory in certain environments. With regard to other constituents, carbon should preferably not exceed 0.05% and is preferably maintained below 0.03 or 0.02%. In a most preferred embodiment it should be held to less than 0.01%, e.g. 0.005% or less. Titanium is present in the alloy in the range of 0.01 to 0.25% and, as set forth hereinafter, is present in a minimum amount correlated to the carbon content. Iron can be present up to 10% and it is to advantage that it be from 0 to 6 or 7%. Incidental elements, if present, are generally in the range of up to 0.5% of manganese and up to 0.25% silicon, advantageously less than 0.35 and 0.1%, respectively; up to 5% cobalt, e.g., up to 2.5%; up to 0.5 or 1% copper; up to 0.5 or 0.75% niobium; up to 0.01% boron, e.g., 0.001 to 0.007%; up to 0.1 or 0.2% zirconium; up to 0.5% aluminum, e.g., 0.05 to 0.3%; with such elements as sulfur, phosphorus being maintained at low levels consistent with good melt practice. Sulfur should be maintained below 0.01%, e.g., less than 0.0075%. Homogenization Treatment The homogenization treatment is a temperature-time interdependent relationship. The temperature should exceed 1149°C and is advantageously at least about 1190°C, e.g., 1204°C, since the former (1149°C) is too low in terms of practical holding periods. On the other hand a temperature much above 1316°C would be getting too close to the melting point of the alloys contemplated and is counterproductive Holding for about 5 or 10 to 100 hours at 1204°C and above gives satisfactory results. However, it is deemed beneficial that a temperature of 1218 to 1245 or 1260°C be employed for 5 to 50 hours. As will be understood by the artisan, lower temperatures require longer holding times with the converse being true, it being recognized that not only is there a time-temperature interdependency, but section size (thickness) and segregation profile of the material treated also enters into the relationship. As a general rule, holding for about 1 hour for each 2.54cm in thickness at 1204-1260°C plus 5 to 10 hours additional gives satisfactory results. In addition to the above, it is preferable to homogenize in at least two stages, e.g., 5 to 50 hours at, say, 1093 to 1204°C and then 5 to 72 hours at above 1204°C, e.g., 1218°C and above. This is to minimize segregation defects. The first stage treatment tends to eliminate low melting point eutectics, and the higher temperature second stage treatment encourages more rapid diffusion resulting in a smaller degree of segregation. Hot Working/Annealing Hot working can be carried out over the temperature range upwards of 1038°C, particularly 1121 or 1149°C, to 1218°C. During the course of hot working, e.g., hot rolling, temperature does decrease and it may be prudent to reheat to temperature. With regard to the annealing operation, in accordance herewith it is desirable to use high temperatures to ensure resolutionizing as much Mu phase as possible. In this regard, the anneal, while it can be conducted at, say, 1149°C, it is more advantageous to use a temperature of 1177°C, e.g., 1191°C, to 1216°C or 1232°C. The following information and data are given to afford those skilled in the art a better perspective in respect of the invention. A series of 45 Kg. melts were prepared using vacuum induction melting, the compositions of which are given in Table II. Alloys 1-11 were each cast into separate 23 Kg ingots. The ingot "A" series (non homogenized) was soaked at 1149°C for 4 hours prior to hot rolling which was also conducted at 1149°C. The series "B" ingots were soaked at 1204°C for 6 hours whereupon the temperature was raised to 1246°C, the holding time being 10 hours. (This is representative of the two-stage homogenization treatment.) The furnace was then cooled to 1149°C and the alloys were hot rolled to plate at that temperature. Ingots were reheated at 1149°C while hot rolling to plate. Plate was annealed at 1204°C for 15 minutes and water quenched prior to cold rolling to strip (Tables V, XIII and XIV). Sheet was produced from strip by cold rolling 33% and then 42% to a final thickness of about 0.25 cm. This was followed by annealing at 1204°C for 15 minutes and then water quenching. Air cooling can be used. Microstructure analysis (and hardness in Rockwell units) are reported in Tables III, IV and V for the as-hot-rolled plate, hot rolled plus annealed plate and cold rolled plus annealed strip conditions, respectively. Alloys 1-7 and 10 were hot rolled to 5.72 cm square and overhauled prior to rolling to 0.66-1.09 cm plate. Alloys 8 and 9 were hot rolled directly to 1.65 cm plate with no overhaul. (Highly alloyed Alloy 7 did not satisfactorily roll to plate for reasons unknown. This is being investigated since based on experience it is considered that acceptable plate should be produced.) While cracking occurred in some heats, it was not detrimental. More important are the resulting microstructures. As can be seen from Table III, microstructure was significantly affected in the positive sense by the homogenization treatment, the size and quantity of Mu phase being considerably less as a result of the homogenization treatment. This is graphically illustrated by a comparison of the photomicrograph Figures 1 (not homogenized) and 2 (homogenized) concerning Alloy 2. Magnification is at 500X, the etchant being chromic acid, electrolytic. Figure 2 depicts only a slight amount of fine Mu particles. Of note is the fact that the homogenized compositions manifested lower hardness levels than the non-homogenized materials. Alloy Chemical Composition Cr Mo W Fe C Si Mn B Al Ti S Ni 1 20.19 15.19 3.43 4.65 .004 .004 .24 .0010 .15 .020 .001 Bal. 2 21.01 15.25 3.45 4.65 .004 .010 .24 .0010 .15 .024 .012 Bal. 3 22.15 15.42 2.66 4.69 .005 .005 .24 .0010 .15 .025 .0008 Bal. 4 21.12 15.82 3.39 4.61 .004 .006 .24 .0011 .15 .024 .0006 Bal. 5 20.94 16.35 3.47 4.67 .005 .000 .24 .0014 .15 .032 .0010 Bal. 6 20.93 15.40 3.92 4.65 .005 .008 .24 .0012 .16 .032 .0009 Bal. 7 21.12 16.20 3.94 4.65 .005 .000 .25 .0013 .15 .026 .0007 Bal. 8 20.59 14.71 3.15 4.66 .003 .060 .25 .0013 .16 .026 .001 Bal. 9 20.41 14.76 3.18 4.70 .004 .058 .24 .0021 .16 .044 .001 Bal. 10 20.76 14.54 3.67 4.50 .002 .046 .25 .0012 .14 .02 .001 Bal. 11 20.76 14.70 3.66 4.53 .042 .25 .0012 .14 .02 -- Bal. As-Hot-Rolled Plate Properties Alloy % by Wt. 1149°C Initial Hot Roll (A/B) (cm) As Hot Rolled @ 1149°C (2nd Rolling) Cr Mo W A (No Homogenizetion) B (Homogenized 2275°F) Gauge (cm) Rc Micro Gauge (cm) Rc Micro 1 20.2 15.2 3.4 5.7/5.7 0.767 41 1, large, mod. 0.838 38 1, fine, light 2 21.0 15.2 3.4 5.7/5.7 0.657 44 1, large, mod. 0.876 22 1, fine mod. 3 22.2 15.4 2.7 Stop/Stop 0.858 36 2, large, heavy 0.721 30 2, fine, mod. 4 21.1 15.8 3.4 5.7/5.7 0.739 34 1, large, mod. 0.742 42 2, fine, heavy 5 20.9 16.4 3.5 Stop/Stop 1.097 31 1-2, large, heavy 0.864 35 2, fine, heavy 6 20.9 15.4 3.9 5.7/Stop 0.777 43 1, large, mod. 0.800 25 2, fine, mod. 7 21.1 16.2 3.9 5.7/Stop 0.876 36 1, large heavy 2.985 26 Different Phase 8 20.6 14.7 3.2 1.65/1.65 0.737 35 1, fine heavy -- -- -- 9 20.4 14.7 3.1 1.65/1.65 -- -- 0.737 26 1, fine, light Similar results were obtained for plate annealed at temperatures of 1149°C and 1204°C, Table IV. Again, the significant beneficial effect of the homogenized alloys is evident. While the absolute optimum microstructures were not attained for the most highly alloyed compositions, the small amount of fine precipitate is more than satisfactory. Also, compare Figures 3 and 4 which depict Alloy 6 in the non-homogenized and homogenized conditions, respectively. Hot Rolled + Annealed Plate Properties Alloy % by Wt A (No Homogenization) B (Homogenized) Cr Mo W HR + 1149°C 1/4hr. WQ HR + 1204°C 1/4hr. WQ HR + 1149°C 1/4hr. WQ HR + 1204°C 1/4hr. WQ Rb Micro Rb Micro Rb Micro Rb Micro 1 20.2 15.2 3.4 92 large, mod. 89 fine, light 89 fine, light 87 OK 2 21.0 15.2 3.4 93 large, mod. 91 fine, mod. 95 fine,mod. 83 OK 3 22.2 15.4 2.7 92 large, mod. 89 large, mod. 97 fine,heavy 85 fine, light 4 21.1 15.8 3.4 94 large, heavy 90 large, mod. 99 fine,heavy 88 fine,very light 5 20.9 16.4 3.5 95 large, heavy 92 large, heavy 101 fine,heavy 91 fine,mod. 6 20.9 15.4 3.9 96 large, mod. 92 large, mod. 97 fine,heavy 84 fine,very light 7 21.1 16.2 3.9 98 large, heavy 93 large, heavy 98 different phase 92 different structure 8 20.6 14.7 3.2 91 large, mod. 87 fine, light -- -- -- -- 9 20.4 14.7 3.1 91 -- -- -- 84 OK -- OK 10 20.8 14.5 3.7 -- fine, mod. -- -- -- OK -- -- As was the case with plate, the homogenization treatment was beneficial to strip as reflected in Table V. Non-homogenized Alloys 3 and 5 did not roll satisfactorily as was the case with Alloy 7. However, no attempt has been made to optimize processing parameters since the focus was on microstructure and crevice/pitting corrosion resistance. Cold Rolled + Annealed Strip Properties Annealed at 1204°C/1/4 Hr, WQ Alloy % by Weight A (No Homogenization) B (Homogenized) Cr Mo W Hardness Micro Hardness Micro As CR CRA As CR CRA Rc Rb Rc Rb 1 20.2 15.2 3.4 38 87 fine,light 38 84 fine,light 2 21.0 15.2 3.4 40 88 large,mod. 38 86 fine,light 3 22.2 15.4 2.7 -- -- -- 38 85 fine,light 4 21.1 15.8 3.4 41 88 large,mod. 39 85 fine,light 5 20.9 16.4 3.5 -- -- -- 39 88 large,light 6 20.9 15.4 3.9 40 90 large,mod. 39 83 fine,light 7 21.1 16.2 3.9 41 92 large,heavy -- -- -- Corrosion Results Tables VI, VII and VIII reflect the beneficial effects in terms of corrosion resistance in 2% boiling hydrochloric acid (VI) and in the "Green Death" test (VII and VIII), the conditions being set forth in the Tables. Alloy 12 was a 9091 kilogram commercial size heat the alloy containing 20.31% Cr, 14.05% Mo, 3.19% W, 0.004% C, 4.41% Fe, 0.23% Mn, 0.05% Si, 0.24% Al, 0.02% Ti, the balance nickel. Both the commercial and laboratory size heats performed well. It should be pointed out that temperatures of 125 and 130°C was used for the so-called "Green Death" test since the conventionally used test temperature of 100°C did not reveal any crevice corrosion over the test period of 24 hours. No pitting or general corrosion was observed. General Corrosion Resistance Boiling 2% HCL - 7 Day Test With Duplicate Specimens 0.152-0.254cm Sheet Alloy Condition Corrosion Rate, micrometers/Yn No. 1 No. 2 Average 12 B 1270 1270 1270 1 A 660 635 660 B 635 635 635 6 A 610 711 660 B 203 254 229 Condition A - No homogenization prior to hot rolling Condition B - Homogenized at 1246°C/10 hr prior to hot rolling Crevice Corrosion Date for Conventionally Processed Commercial Sheet and Plate, Evaluated in the Green Death* for 24 Hours at 125°C Alloy Mill Form Percent of Crevices Attacked Maximum Crevice Pit Depth Micrometers 12 1/16" sheet (a) 21 1651 (b) 29 1219 Average 25 1448 12 1/4" plate (a) 4 51 (b) 0 51 (c) 4 0 (d) 25 1016 Average 9 279 Green Death: 11.9%H 2 SO 4 + 1.3%HCl + 1%FeCl 3 + 1%CuCl 2 balance water (% by wt.) Crevice Corrosion Test Results Laboratory Produced Strip and Plate - Annealed Creviced Specimen Exposed to Green Death Environment for 24 Hr at Temperature Indicated Alloy Condition Temp.,°C Percent of Crevices Attacked Max. Crevice Depth Micrometers 10 A 125 0,4 0, 75 A 125 0,4 0, <02 10 B 125 0,8 0, 152 B 125 0,0 0, 0 11 A 125 0,50 0, 635 B 125 0,0 0, 0 6 A 125 0,0 0, 0 B 125 0,0 0, 0 6 A 130 0,4,17 0, <50, <50 B 130 0,0,4 0, 0, <50 Condition A - No homogenization prior to hot rolling. Condition B - Homogenized at 1246°C prior to hot rolling. Various alloys were also subjected to the ASTM G-28, Practice "B" test, a discriminating test used to assess corrosion of the intergranular type. Test specimens were exposed over what is considered to be the sensitization temperature or temperature range, roughly 760 to 982°C, this temperature being deemed a yardstick as to predicting corrosion attack, and then immersed in boiling 23% H 2 SO 4 + 1.2 % HCl + 1% CuCl 2 + 1% FeCl 3 balance water for the standard 24 hour period. Practice "B" is considered more severe and reliable than the G-28, Practice "A" test procedure in predicting attack. (Practice A procedure employs a corroding solution made up by dissolving 25 grams of Fe 2 (SO 4 ) 3 9H 2 O in 600 ml of an aqueous solution containing 50% H 2 SO 4 by weight). Data are presented in Tables X and XI. Included is Alloy X which corresponds to Alloy C-276 and the chemistry is given in Table IX. Alloy Cr Mo W Fe C Si Mn B Al Ti Ni X 15.05 15.55 3.76 5.79 .001 .051 .45 -- .47 .02 Bal. Intergranular Attack Resistance in ASTM G-28, Practice B Laboratory Produced 0.254cm Strip Annealed at 1204°C Alloy Condition Corrosion Rate micrometers per year As Ann. 760/1 871/1 982/1 8 and 9 A 228 254 11,760 1,041 B 203 254 2,565 356 1 A 279 508 4,648 1,067 B 254 432 1,422 711 6 A 254 6,248 85,725 84,734 B 254 254 1,295 660 10 A -- 34,696 56,388 44,171 B -- 3,783 66,853 3,505 X A 1981 -- 23,596 27,940 X A 1524 -- 30,632 31,775 NOTE : Alloy 10 annealed at 1149°C Condition A - No homogenization prior to hot rolling at 1149°C Condition B - Homogenized at 1246°C/10 hr prior to hot rolling at 1149°C As depicted in Table X, the homogenization treatment is generally beneficial even in respect of intergranular attack. Alloy 10 was annealed at 1149°C. It did not behave as well as the alloys annealed at 1204°C. The effect of reheating on commercial plate and sheet is given in Table XI below. Effect of Reheat Temperature on Intergranular Attack in ASTM G-28, Practice B Commercially Produced Plate and Sheet Condition Corrosion Rate Plate Alloy 12 Sheet Alloy 12 MA + 649°C/1hr 178 2,038 MA + 760°c/1hr 228 51,358 MA + 871°C/1hr 686 50,342 MA + 982°C/1hr 228 1,905 MA + 1093°C/1hr 203 203 MA - Mill Anneal While the principal thrust of the subject invention is directed to corrosion of the crevice/pitting type as well as general corrosion, it is considered that the invention would be of advantage in respect of other forms of corrosive attack, including intergranular, stress-corrosion cracking induced by, for example, chlorides, sulfide stress cracking, etc. In addition, while the subject invention is concerned by far and large with the high chromium/molybdenum/tungsten alloys described herein, it is deemed that alloys of lower levels of such constituents, e.g., down to 15% chromium and down to 12% molybdenum and up to 4% tungsten can be treated in accordance herewith. In addition to the foregoing, it has also been discovered that by controlling the amount of iron and the weight ratio of titanium to carbon in nickel-base alloys amenable to the special heat treatment of the present invention, highly advantageous results in terms of corrosion resistance can be achieved when such alloys are heat treated as described hereinbefore. The additional discoveries involved holding the iron content of the alloys to less than 2.5% (by weight) and preferably to less than 1% by weight. When iron is thus controlled the molybdenum content of the alloys can be as high as 17%, e.g., 12 to 17% while still attaining excellent corrosion resistance. The discoveries also involve maintaining in the alloys a weight ratio of titanium to carbon of at least 1 and up to 10 or higher. When the Ti/C is maintained above 1 and, especially when carbon is maintained below a maximum of 0.015% by weight, advantageous results are obtained, in terms of resistance to intergranular corrosive attack as measured by standard tests with alloys heat treated in accordance with the process of the present invention. By virtue of these discoveries, the present invention contemplates novel alloy compositions as set out in the accompanying claims. Advantageously, the novel alloy compositions contain less than 0.02% carbon and the weight ratio of titanium to carbon is from 3 to 1, to 15 to 1, e.g., 10 to 1. For reasons not fully understood, low iron content, e.g., below 2.5% especially together with a high Ti/ C weight ratio results in alloys which are particularly resistant to the formation of Mu phase after homogenization as disclosed hereinbefore and reheating in the range of 760°C to 982°C. This resistance, as evidenced by resistance to intergranular corrosion attack under the conditions of ASTM G28 practice B test, is set forth hereinafter. Alloy compositions as set forth in Table XII were produced as described hereinbefore in connection with Table II and treated by homogenization as were the series B ingots discussed hereinbefore, i.e., soaked 1204°C for 6 hours followed by holding for 10 hours at 1246°C. Alloy C Mn Fe Si Ni Cr Al Ti Nb Mo W 10 .002 .25 4.50 .05 55.67 20.76 .14 .021 .001 14.54 3.67 13 .002 .24 5.98 .08 56.59 19.49 .21 .027 .004 13.89 3.24 14 .008 .27 3.72 .13 57.39 20.44 .19 .035 .009 14.24 3.34 15 .002 .24 2.46 .06 58.55 20.44 .21 .0005 .005 14.32 3.33 16 .004 .25 1.13 .07 59.67 20.38 .21 .022 .007 14.50 3.36 17 .003 .24 .65 .06 60.16 20.46 .22 .0003 .001 14.40 3.35 18 .005 .26 .24 .06 60.62 20.46 .22 .036 .006 14.30 3.34 19 .003 .24 1.01 .06 57.22 20.56 .20 .0014 .001 16.30 3.89 20 .003 .24 .01 .05 58.72 20.42 .20 .0093 .002 16.53 3.37 Alloy Nos. 15, 16, 18 and 20 in Table XII are examples of the highly improved novel alloys which have been discovered. Alloy 17 and 19 with low iron have low weight ratios of titanium to carbon. Table XIII sets forth results of ASTM-G28 Practice B test on alloys of Table XII which, after initial homogenization followed by hot rolling, have been cold rolled, annealed at 1204°C for ¼ hour water quenched and reheated for one hour as specified. Corrosion Rate in Micrometers per year - ASTM G-28, B Cold Roll + Anneal at 1204°C + Reheat °C/hr Alloy No. Iron % Ti/C 760/1 871/1 982/1 Average 13 6.0 13.5 254 1,194 103,022 17,907 229 2,413 305 10 4.4 10.5 1,143 84,379 7,036 35,433 457 88,849 1,905 64,287 14 3.7 4.4 69,875 63,017 483 45,923 58,903 47,980 356 15 2.5 0.25 11,151 254 889 1,905 356 254 229 16 1.1 5.5 203 229 279 203 178 203 203 17 0.7 0.10 1,575 71,297 279 17,628 8,712 40,970 330 18 0.2 7.2 203 254 305 229 178 203 203 203 19 1.0 0.5 305 508 813 533 533 20 0.0 3.1 279 279 508 356 305 Results similar to those presented in Table XIII but obtained on identically treated alloy samples tested in the less discriminating ASTM G28 practice A test as set forth in Table XIV. Corrosion Rate in micrometers per year -ASTM G-28, A Cold Roll + Anneal at 1204°C + Reheat °C/hr Alloy No. Iron % Ti/C 760/1 871/1 980/1 Average 13 6.0 13.5 1,829 1,854 1,930 1,879 10 4.4 10.5 1,413 3,150 3,404 2,870 3,479 14 2.7 4.4 2,311 4,902 2,134 3,632 5,156 15 2.5 0.25 1,702 2,464 1,321 2,438 4,293 16 1.1 5.5 1,575 1,295 1,118 1,321 1,321 17 0.7 0.10 1,651 1,270 1,930 1,524 1,270 18 0.2 7.2 1,219 1,270 1,168 1,219 1,219 19 1.0 0.47 3,251 5,563 10,566 6,553 6,883 20 0.0 3.1 2,540 3,200 5,944 3,937 4,064 Together, Tables XIII and XIV show that Alloys Nos. 15, 16 and 18 to 20 exhibit advantageous corrosion resistance attributable to iron contents less than 2.5% together with titanium to carbon ratios in excess of 0.2. When iron is low, carbon is less than 0.01%, e.g., less than 0.008% and the titanium to carbon ratio is in excess of 1, e.g., greater than about 3 as in alloys Nos. 16, 18 and 20 the best results are obtained. An additional advantage of the alloys of the present invention is demonstrated by the data in Table XV. Oxidation - Air + 5% H2O at 1100°C Mass Loss (Mg/cm 2 ) in hours indicated Alloy No. Iron % 168 hr. 336 hr. 504 hr. 528 hr 696 hr. 840 hr. 1032 hr. 1200 hr. 13 5.98 1.8 3.9 -- 9.6 15.3 20.9 37.3 75.0 18 0.24 1.0 3.0 -- 4.6 6.5 9.9 16.4 23.2 625 2.5 -- -- 238.0 -- -- -- -- -- C-276 5.5 -- -- 328.0 -- -- -- -- -- The data in Table XV shows that alloy 18 is roughly 3 times more resistant to oxidation in moist air at 1100°C than alloy 13 and between 1 and 2 orders of magnitude more resistant to the same conditions than are well-known corrosion-resistant commercial alloys. It is to be noted that the homogenization treatment of the present invention is particularly effective when carried out prior to hot working, e.g., rolling and even more so when carried out both before and after hot working. Nevertheless, some useful improvement in corrosion resistance may be attained by homogenization after hot working.

Claims: A process for enhancing crevice and pitting corrosion resistance of nickel-base alloys of high combined percentages of chromium and molybdenum in various corrosive media by minimizing the formation of deleterious quantities of Mu phase which comprises subjecting an alloy containing in weight percent from 19 to 25% chromium, 14 to 18% molybdenum, 2.5 to 4% tungsten, up to 0.1% carbon, up to 0.5% manganese, up to 0.25% silicon, 0 to 10% iron, titanium up to 0.25% in such an amount that the weight ratio of titanium to carbon is at least 1, up to 5% cobalt, up to 1% copper, up to 0.75% niobium, up to 0.01% boron, up to 0.2% zirconium and up to 0.5% aluminium with the balance being nickel plus impurities to a homogenization treatment over the temperature range of above 1149°C to 1316°C for a holding period of at least 5 hours. A process according to claim 1 in which the holding period is from about 10 to 100 hours. A process according to claim 1, wherein the alloy contains from 19 to 23% chromium, 14 to 17% molybdenum, 3.4 to 4% tungsten, and 0 to 2.5% iron. A process according to claim 1 or claim 3 in which the homogenization temperature is from 1190°C to 1260°C and the holding period is from 5 to 50 hours. A process according to any one of claims 1, 3 and 4 in which the homogenization treatment is carried out in two stages comprising heating the alloy at from 1093°C to 1204°C for 5 to 50 hours and thereafter heating the alloy for 5 to 72 hours at 1204°C to 1316°C. A process according to any preceding claim, wherein the alloy contains 20 to 23% chromium, 14.25 to 17% molybdenum, 3.4 to 4% tungsten, up to 0.05% carbon and 0 to 2% iron. A process according to any one of claims 1 to 5 in which the alloy contains chromium from 21.5 to 23%. A process according to any one of claims 1 to 5 in which the alloy contains 19.5 to 21.5% chromium and 15 to 17% molybdenum. A process according to any preceding claim in which after homogenization the alloy is hot worked and conventionally processed. A nickel-base alloy possessing enhanced oxidation resistance, enhanced crevice and pitting corrosion resistance and no deleterious quantities of Mu phase after homogenization within the temperature range of 1149°C to 1316°C for a period of 5 to 100 hours even when reheated in the range of 760 to 982°C consisting of, in weight percent, 19 to 23% chromium, 14 to 17% molybdenum, 2.5 to 4% tungsten, 0 to 0.1% carbon, titanium up to 0.25% in such an amount that the weight ratio of titanium to carbon is at least 1, 0 to 2.5% iron, up to 0.5% manganese, up to 0.25% silicon, up to 5% cobalt, up to 1% copper, up to 0.75% niobium, up to 0.01% boron, up to 0.2% zirconium and up to 0.5% aluminium, the balance being nickel together with impurities. A nickel base alloy homogenized within the temperature range of 1149°C to 1316°C for a period of 5 to 100 hours, said alloy having enhanced oxidation resistance, enhanced crevice and pitting corrosion resistance and having no deleterious quantities of Mu-phase after that homogenization treatment even when reheated in the range of 760 to 982°C, comprising 19 to 23% chromium, 14 to 17% molybdenum, 2.5 to 4% tungsten, 0 to 0.1% carbon, titanium up to 0.25% in such an amount that the weight ratio of titanium to carbon is less than 1, 0 to 2.5% iron, up to 0.5% manganese, up to 0.25% silicon, up to 5% cobalt, up to 1% copper, up to 0.75% niobium, up to 0.01% boron, up to 0.2% zirconium and up to 0.5% aluminium, the balance being nickel plus impurities. A nickel-base alloy according to claim 10 or claim 11, containing less than 0.02% carbon and less than 2% iron. A nickel-base alloy according to claim 12 containing less than about 1% iron, less than 0.01% carbon and having a titanium to carbon weight ratio greater than 3. The use of an alloy as set forth in and homogenized according to any preceding claim for articles and parts exposed in use to corrosive conditions.

Inventors: CRUM JAMES ROY, HIBNER EDWARD LEE, POOLE JON MICHAEL, CRUM, JAMES ROY, HIBNER, EDWARD LEE, POOLE, JON MICHAEL
Applicants: INCO ALLOYS INT, INCO ALLOYS INTERNATIONAL, INC.
Application year: 1990
Main classification: C22F1
IPC codes: C22F1, B21B3, C22C19