HIGH TEMPERATURE NICKEL BASE ALLOY WITH IMPROVED STABILITY

Abstract: A nickel-chromium-molybdenum base alloy characterised by exceptional structural stability when exposed at temperatures upwards of 1800°F (980°C) for prolonged periods of time, such as 10 000 hours. and consisting of about 19 to 30% chromium, less than 0.25% silicon, 0.05 to 0.15% carbon, 7.5 to 9% molybdenum, about 7.5 to 20% cobalt, up to 0.6% titanium, about 0.8 to 1.5% aluminum, up to 0.006% boron, up to 0.1% zirconium, up to 5% iron, up to 5% tungsten and the balance being essentially nickel, said alloy being further characterized by an average grain size coarser than about ASTM 5.

Description: This invention is directed to a nickel-chromium-molybdenum (Ni-Cr-Mo) alloy, and particularly to a Ni-Cr-Mo alloy which manifests a combination of exceptional impact strength and ductility upon exposure to elevated temperature, e.g. 1000°C (1832°F) for prolonged periods of time, 3000 hours or more, while concomitantly affording high tensile and stress-rupture strengths plus good resistance to cyclic oxidation at high temperatures. US-A-3 859 060 and the corresponding GB-A-1 336 409 disclose nickel-base alloys containing, by weight, 20 to 24% chromium, 0.8 to 1.5% aluminum, 9.5 to 20% cobalt, 7 to 12% molybdenum, not more than 0.15% carbon, up to 0.6% titanium, up to 0.006% boron, up to 0.1% zirconium, up to 0.05% magnesium and up to 0.15% in all of cerium and/or lanthanum, the balance, apart from impurities and incidental elements, being nickel. Examples of alloys containing up to 0.41% silicon as impurity are disclosed. The alloys of these prior patents are stated, inter alia, not to develop embrittling phases after prolonged exposure to temperatures in the range of 650 to 870°C. A commercial embodiment of the alloys of these prior patents is the alloy known as alloy 617, which has been produced and marketed for a number of years. Nominally, this alloy contains about 22% chromium, 9% molybdenum, 1.2% aluminum, 0.3% titanium, 2% iron, 12.5% cobalt and 0.07% carbon, as well as other constituents that include 0.5% silicon and one or more of boron, manganese and magnesium, the balance being nickel. The virtues of alloy 617 include (i) good resistance to scaling and to cyclic oxidation in oxidising environments at elevated temperature, (ii) excellent stress rupture strength, and (iii) good tensile strength and ductility at both ambient and elevated temperatures. Alloy 617 also possesses structural stability under, retrospectively speaking, what might be characterized as, comparatively speaking, moderate service conditions. But as it has turned out it is this characteristic which has given rise to a problem encountered commercially for certain intended and desired applications, e.g., high temperature gas feeder reactors (HTGR). This is to say, when the alloy was exposed to more stringent operating parameters of temperature (982°C) (1800°F) and time (1000-3000+ hours) an undesirable degradation in structural stability occurred, though stress rupture, tensile and oxidation characteristics remained satisfactory. Apparently, what happened was that before the 982°C (1800°F)/1000+ hour operating conditions came into consideration, the test temperature for the study of stability was usually not higher than 870°C (1600°F), or if higher temperatures were considered, short exposure periods of about 100 hours were used. Longer periods (about 10,000 hours or more) were only used at the lower temperatures, i.e., not more than 700-760°C (1300°F-1400°F). Apart from temperature/time operating conditions, the problem would not surface because in many applications structural stability was not critically important, e.g., boats used for catalyst-grid supports, heat treating baskets, reduction boats used in refining certain metals, etc. Accordingly, the problem became one of ascertaining the cause(s) for the stability deterioration at upwards of 980-1090°C (1800°F-2000°F) for periods well exceeding 1000 hours, and evolving, if possible, a new alloy which would result in enhanced stability under such operating conditions but without incurring a detrimental sacrifice in stress-rupture/oxidation/ tensile properties. We have found that silicon and molybdenum when present to the excess can adversely affect the stability of Alloy 617. We have also found that carbon, if beyond the range specified below herein, can, depending upon chemistry, exercise a negative influence. Moreover, it has been determined that grain size plays a significant, if not the major, role, grain size being influenced by composition and processing, particularly annealing treatment. Grain size, chemistry, particularly silicon, molybdenum and carbon, and annealing temperature are interrelated or interdependent as will become more clear infra. The invention herein involves the critical controlling of these related aspects. According to the invention, a nickel-chromium-molybdenum base alloy having at temperatures of 982°C (1800°F) and higher (i) a high level of structural stability as determined by its ability to absorb energy over prolonged periods of time of at least 3000 hours at such temperatures, (ii) good ductility together with (iii) satisfactory tensile strength and (iv) satisfactory stress-rupture strength as well as (v) resistance to oxidation, including cyclic oxidation, consists, by weight, of 19 to 30% chromium, less than 0.25% silicon, 0.05 to 0.15% carbon, 7.5 to 8.75% molybdenum, 7.5 to 20% cobalt, up to 0.6% titanium, 0.8 to 1.5% aluminum, up to 0.006% boron, up to 0.1% zirconium, up to 0.075% magnesium, up to 5% iron, up to 5% tungsten, up to 1% copper and up to 1% manganese, the balance, apart from impurities, being nickel, and has an average grain size coarser than ASTM 5.5. In carrying the invention into practice, and in endeavoring to achieve consistent results, care must be exercised in respect of compositional control; in particular, molybdenum, silicon and carbon should be interrelated and controlled as indicated hereinafter. Silicon has been found to act subversively, particularly at high molybdenum and carbon contents. In retrospect, virgin materials were used in the research stage of Alloy 617. Thus, silicon was at a low level. But in commercial production scrap materials are used wherever possible to reduce costs. As a consequence, higher percentages of silicon would have been employed since the overall adverse effect of silicon in conjunction with molybdenum/carbon, grain size/annealing temperature at 982-1093°C (1800-2000°F) was neither known nor understood prior to the present invention. As indicated above, a typical commercial nominal silicon content is 0.5% and there are current commercial "specifications" where the silicon can be as high as 1% with molybdenum being as high as 11%. Morphologically speaking, the subject alloy is of the solid-solution type and further strengthened/hardened by the presence of carbides, gamma prime hardening being minor to insignificant. The carbides are of both the M₂₃C₆ and M₆C types. The latter is more detrimental to room temperature ductility when occurring as continuous boundary particles. The higher levels of silicon tend to favor M₆C formation. This, among other reasons, dictates that silicon be as low as practical though some amount will usually be present, say, 0.01%, with the best of commercial processing techniques. For optimum stability, as measured by the standard parameters of Charpy-V-Notch impact strength and tensile ductility, Molybdenum should not exceed 8.75%, particularly at the higher silicon levels. As will be shown infra, molybdenum contents even at the 10% level detract from CVN impact strength, particularly at silicon levels circa 0.2-0.25%. Molybdenum contributes to elevated temperature strength and thus at least about 8% should preferably be present. Tests indicate that stress-rupture life is not impaired at the 1093°C (2000°F) level though a reduction (acceptable) may be experienced at 871°C (1600°F) in comparison with Alloy 617. Given the foregoing, it is advantageous that the silicon content should be less than 0.15% and that the silicon and molybdenum be correlated so that the molybdenum content is less than 8.5% when the silicon content is greater than 0.15%. With regard to carbon, a range of 0.05 to 0.1%, particularly 0.05 to 0.07%, is advantageous. Carbon contributes to stress-rupture strength but detracts from structural stability at high percentages. Lower levels, say 0.03-0.04%, particularly at low molybdenum contents, result in an unnecessary loss of stress-rupture properties. Carbon also influences grain size by limiting the migration of grain boundaries. As carbon content increases, higher solution temperatures are required to achieve a given recrystallized grain diameter. Where optimum corrosion resistance is required, chromium can be used up to 30%. But at such levels chromium together with molybdenum in particular may lead to forming an undesired volume of the embrittling sigma phase. It need not exceed 28% and in striving for structural stability a range of 19 to 23% is beneficial. The content of iron, if present, should not exceed 5%, and preferably it does not exceed 2% to avoid impairing stress-rupture strength at temperatures such as 1093°C (2000°F). The presence of tungsten can be tolerated up to 5%, and copper and manganese, if present, should not exceed 1%, respectively. Residual deoxidising cleansing elements may be present as impurities, and any sulphur and phosphorus present as impurities should be maintained at low levels, say not more than 0.015% and 0.03% respectively. In addition to the foregoing, it has been determined that grain size has a marked influence on toughness. Chemistry and processing control, mainly annealing temperature, are interdependent in respect of grain size. While it has been customary to final anneal Alloy 617 at 1190-1204°C (2175-2200°F) commercially, in accordance with the present invention annealing should be conducted below about 1177°C (2150°F) and above 1093°C (2000°F). The effect of annealing temperature on a commercial size, 10 t (22,000 lb), melt is given in Tables IV and V. An annealing temperature of, say 1204°C (2200°F), promotes the formation of the coarser grains but stress-rupture properties are higher. On the other hand, very low annealing temperatures, say 1038-1080°C (1900-1975°F), offer a finer grain size but stress-rupture is unnecessarily adversely impacted. Accordingly, it is preferred that the annealing temperature be from 1107°C (2025°F) to less than 1093°C (2150°F) with a range of 1107°C (2025°F) to about 1162°C (2125°F) being preferred. While the grain size may be as coarse as ASTM 0 or 00 where the highest stress-rupture properties are necessary, it is preferred that the average size of the grains be finer than about ASTM 1 and coarser than about ASTM 5.5, e.g., ASTM 1.5 to ASTM 4. To give those skilled in the art a better appreciation of the invention, the following information and data are given: 14 kg vacuum induction laboratory heats were made, then forged at about 2200°F to 13/16 inch squares for hot rolling (2200°F) to 9/16 inch rounds. Respresentative compositions are given in TABLE I. Alloys AA through DD are outside the invention. Alloy No. C Mn Fe Si Ni Cr Al Ti Co Mo B Zr 1 0.07 0.011 1.33 0.06 56.23 21.98 1.08 0.61 10.99 7.60 0.004 0.014 2 0.11 0.005 0.74 0.04 54.90 22.54 1.17 0.48 11.89 8.19 0.003 0.014 3 0.08 0.008 0.69 0.21 54.34 22.63 1.17 0.41 12.00 8.47 0.002 0.014 4 0.13 0.008 0.67 0.22 54.43 22.73 1.22 0.41 12.01 8.28 0.001 0.014 AA 0.07 0.007 0.68 0.23 52.81 22.59 1.21 0.42 12.00 10.11 0.003 0.014 BB 0.11 0.008 0.67 0.23 52.51 22.71 1.21 0.41 12.00 10.33 0.002 0.014 CC 0.06 0.008 0.71 0.04 53.04 22.46 1.17 0.44 11.99 10.17 0.003 0.014 DD 0.12 0.009 0.69 0.04 52.58 22.76 1.19 0.43 11.97 10.29 0.002 0.014 Annealing temperatures were 2125°F and 2250F, respectfully, the specimens being held thereat for 1 hour, then air cooled. The alloys were exposed at 1832°F (100°C) for 100, 1000, 3000 and 10,000 hours and air cooled as set forth in TABLE II which sets forth the data obtained i.e., grain size, Rockwell hardness (Rb), yield (YS) and tensile strengths (TS), elongation (El.), Reduction of (RA) and Charpy V-Notch Impact Strength (CVN), the latter serving to assess structural stability. Concerning the data above given, Alloys AA and BB resulted in markedly lower impact levels than Alloys 1-4, especially low silicon, low molybdenum Alloys 1 and 2, particularly when annealed at 1232°C (2250°F). Alloys AA and BB had, comparatively speaking, high percentages of both silicon and molybdenum together with a coarse grain varying from ASTM 0 to 1. Alloys CC and DD while better than AA and BB due, it is deemed to much lower silicon percentages, were still much inferior to Alloys 1-4 given a 1163°C (2125°F) anneal. While the Charpy-V-Notch impact data for Alloys AA-DD appear to be good for the 1163°C (2125°F) anneal, our investigations have indicated that with commercial size heats impact strengths for alloys of high molybdenum significantly drop off. Also, there is danger/risk of not controlling annealing temperature and the 1232°C (2250°F) anneal reflects what can be expected in terms of anticipated structural stability. In Table III are reported stress rupture data for the Alloys In Table I. In this case the annealing temperature was 1177°C (2150°F). While the stress (34.5 mPa) used at 1000°C (1832°F) is fairly high for that temperature level, stress rupture properties for the alloys within the invention are satisfactory. Alloy No. C Si Mo ASTM GS # Temp (°C) Stress (mPa) Life (h) EL (%) RA (%) 1 .07 .06 7.60 7.5 650 414 1317.5 24.5 26.5 760 207 651.5 53 71 870 97 40.7 68.5 89.5 1000 34.5 29.4 51 62 2 .11 .04 8.19 5 650 414 453.7 10.5 14 760 207 473.4 47 45 870 97 22.1 61.5 77 1000 34.5 24 45.5 52 3 .08 .21 8.47 5 650 414 203.6 16 14.5 760 207 374.6 17 44 870 97 17.8 63.5 83 1000 34.5 114.1 38 39 4 .13 .22 8.28 6.5 650 414 430.7 13.5 15 760 207 424.1 35.5 65.5 870 97 26.0 91.5 69 1000 34.5 56.2 35.5 40 AA .07 .23 10.11 6 650 414 1468.3 22.5 24 760 207 808.3 44 76.5 870 97 30.9 92 90 1000 34.5 62.2 57 66 BB .11 .23 10.33 8 650 414 1729 33.5 35.5 760 207 520.7 49 72 870 97 30.7 120.5 87.5 1000 34.5 39.9 46.6 66.5 CC .06 .04 10.17 7 650 414 655.8 18.5 20.5 760 207 643.3 40 64 870 97 42.2 79 87.5 1000 34.5 169.6 39 33.5 DD .12 .04 10.29 6.5 650 414 2592.5 23 28 760 207 567.8 44.5 59 870 97 124.3 65.5 82 1000 34.5 65.3 31.5 42 Tables IV and V pertain to a 10 t (22,000 lb) commercial size heat which was produced using vacuum induction melting followed by electroslag refining. The material was processed into 1.9 cm (3/4") dia. hot rolled rounds for testing and evaluation. The as-hot-finished rod stock was used for an annealing evaluation/grain size study evaluation. The composition of the heat, Alloy 5, is given below in Table IV with annealing temperature and grain size reported in Table V. Element, Wt.% Element, Wt.% chromium 21.88 iron 0.21 cobalt 12.48 manganese 0.01 molybdenum 8.62 boron 0.002 carbon 0.05 magnesium 0.001 silicon 0.07 sulphur 0.001 aluminum 1.26 phosphorus 0.002 titanium 0.23 copper 0.01 nickel 55.18 Anneal 1 hour at Temperature (°C) Followed By Water Quench Grain Size, ASTM Grain No. 1093 7.5 1121 4.0 1149 1.5 1163 1.5 1177 1.0 1190 1.0 1204 0 1219 0 1232 0 As reflected by Table V, given the chemistry in IV, an annealing temperature above 1190°C (2175°F, e.g. 1204°C (2200°F), and above resulted in an excessively coarse grain structure whereas annealing at 1093°C (2000°F) gave too fine a grain. As indicated above herein, a final annealing should be conducted above 2000°F to about 2150°F. The effect of annealing temperatures (1093°C (2000°F), 1121°C (2050°F), 1163°C (2125°F), 1232°C (2250°F)) and grain size on structural stability as indicated by the Charpy-V-Notch test size is shown in Table VI, and is graphically depicted in Figure 1. Table VI includes tensile properties, stress rupture results being given in Table VII. Stress Rupture Properties Ann. Temp °C/l h, WQ ASTM G.S. No. Test Temp. (°C) Test Stress (MPa) Life (h) El (%) RA (%) 1093 7.5 871 89.7 23.9 96.8 89.1 1121 4.0 39.9 83 91.5 1163 1.5 50.3 87 77.5 1232 0 47.2 85.5 69 1093 7.5 1093 20.7 14.2 137.5 80 1121 4.0 18.1 115.5 76 1163 1.5 76.6 98 56.5 1232 0 96.0 46 56.5 The impact energy data at 1000°C (1832°F) in Table VI confirms the superior results of a commercial size heat of an alloy composition/annealing temperature within the invention. For an exposure period of 10,000 hours and an annealing temperature of 1232°C (2250°F), Alloy 5 manifested a borderline impact strength of 54 J/cm² (32 ft. lbs), versus, for example, 98 J/cm² (58 ft. lbs.), when annealed at 1163°C (2125°F). It is deemed that the impact energy level at 1000°C (1832°F) and 10,000 hours exposure should be at least 68 J/cm² (40 ft. lbs) and preferably 85 J/cm² (50 ft. lbs) although, as suggested above 50 J/cm² (30 ft. lbs) is marginally acceptable. The 1093°C (2000°F) anneal afforded high impact strength at 10,000 hours but as shown in Table VII stress-rupture life suffered, being 23.9 hours vs. 50 hours when annealed at 1163°C (2125°F). The difference is even more striking at the 1093°C (2000°F) test condition. Apart from the foregoing and based on welding data at hand, the instant alloy is deemed readily weldable using conventional welding practices as will he demonstrated below. As a matter of general observation from the tests conducted, no base metal microfissuring was observed in the heat affected zone (HAZ) of a Gas Metal Arc (GMA) weldment. This test resulted in a slight loss of strength in the as-welded and annealed condition as would be expected but, more importantly, the deposit exhibited greatly improved ductility and impact strength after exposure to aging temperature, given corresponding properties for commercial Alloy 617. Gas shielded metal arc (GSMA) deposits made using filler metals of the invention alloy as a core wire in a coated welded electrode manifested improved ductility and impact strength in comparison with weld deposits using filler metal of commercial Alloy 617. In this connection, a significant loss of ductility was experienced after exposure and this was attributed to the elements, notably carbon and silicon, introduced in the deposit by the flux coating. It is deemed that such constituents are sufficient to induce high temperature reaction which are believed responsible for the ductility loss in the deposit. With regard to the welding tests, plate 8.8 mm (0.345 inch) thick taken from hot band of Alloy 5 was annealed at both 982°C (1800°F) and 1204°C (2200°F) to provide material of different grain sizes. (The 982°C (1800°F) anneal would not cause a change in grain size, the original grain size being ASTM 2.5). The 1204°C (2200°F) anneal (which is not a recommended annealing treatment) gave a grain size beyond about ASTM 00. This was done with the purpose that an alloy of limited weldability, given the variation in grain size, would be expected to manifest some variation in base metal microfissuring. A weldment was deposited between two specimens of the plate (one of each anneal) by GMAW - spray transfer with 1.1 mm (0.045 inch) diameter filler metal from Alloy 5, the following parameters being used. Diameter 1.1 mm Joint Design V-Butt - 60° Opening Current 220 A Voltage 32 V Wirefeed 10.7 m/min Position Flat - 1G Flow Rate 1.4 m³/h. Travel Speed 30-38 cm/min. (Manual) Transverse face, root and side bend specimens, centered in both the weld and heat affected zones (HAZ) were tested, (i.e., usually 3 specimens were taken from the weld plate per test conditions. Liquid penetration inspection revealed no fissuring in the welds or the HAZ. Using specimens bent over a thickness twice that of the specimens (2T), only one face bend test showed any fissuring; however, the fissures did not intersect the fusion line and were thus deemed not weld related but were probably due to plate surface. No other fissuring was detected in either liquid penetration or metallographic examination. Filler metals of Alloy 5 were made in wire diameters of 1.1 and 2.4 mm and then used in Gas Metal Arc Welding (GMAW) spray transfer and Gas Tungsten Arc Welding (GTAW), respectively. A third wire, 3.2 mm in diameter was used as a core wire for producing a covered electrode for Shielded Metal Arc Welding (SMAW). Room temperature impact data from weldments of each of the GMAW, GTAW and SMAW are reported in Table VIII with mechanical properties being given in Table IX. The parameters for the GTAW and SMAW were as follows: GTAW Diameter - 2.4 mm. Electrode Type/Diameter - 2% Thoriated Tungsten / 3/32" Current - 180 A DCEN Voltage - 12-14 V Shielding Gas - Argon Flow Rate - 0.7 m³/h. Joint Design - V-Butt 60° Opening Position - Flat - 1G Travel Speed - 10-15 cm/min (Manual) SMAW Diameter - 3.2 mm. Current - 90 A Voltage - 23 V Joint Design - V-Butt - 60° Opening Position - Flat - 1G Travel Speed - 25-30 cm/min. (Manual) Room Temperature Tensile Data Condition Process UTS (MPa) 0.2% YS (MPa) Elong. (%) Red. of Area (%) Hardness (Rb) A GMAW 705 452 50 63.1 94/95 A GMAW 718 437 50 57.0 90/91 A GMAW 727 448 47 55.6 92 B GMAW 717 320 65 70.9 82/83 C GMAW 827 353 41 42.5 89/92 D GMAW 752 300 49 40.2 83/86 A GTAW 753 492 44 60.0 94/96 B GTAW 737 314 61 71.1 84 C GTAW 831 349 46 51.9 89/91 D GTAW 771 295 51 45.1 85/87 A SMAW 782 476 41 37.9 97 B SMAW 761 360 49 45.5 91 C SMAW 812 361 21 20.6 94/95 D SMAW 664 325 13 12.2 91/93 The subject alloy can be melted in conventional melting equipment such as air or vacuum induction furnaces or electroslag remelt furnaces. Vacuum processing is preferred. The alloy is useful for application in which its predecessor has been used, including gas turbine components such as combustion liners.

Claims: A nickel-chromium-molybdenum base alloy having at temperatures of 982°C (1800°F) and higher (i) a high level of structural stability as determined by its ability to absorb energy over prolonged periods of time of at least 3000 hours at such temperatures, (ii) good ductility together with (iii) satisfactory tensile strength and (iv) satisfactory stress-rupture strength as well as (v) resistance to oxidation, including cyclic oxidation, that consists, by weight, of 19 to 30% chromium, less than 0.25% silicon, 0.05 to 0.15% carbon, 7.5 to 8.75% molybdenum, 7.5 to 20% cobalt, up to 0.6% titanium, 0.8 to 1.5% aluminum, up to 0.006% boron, up to 0.1% zirconium, up to 0.075% magnesium, up to 5% iron, up to 5% tungsten, up to 1% copper and up to 1% manganese, the balance, apart from impurities, being nickel, and has an average grain size coarser than ASTM 5.5. An alloy according to claim 1 in which, when the silicon content exceeds 0.15%, the molybdenum content does not exceed 8.5%. An alloy according to claim 1, consisting of 20 to 30% chromium, silicon up to 0.15%, 0.05 to 0.1% carbon, 7.5 to 8.75% molybdenum, 7.5 to 20% cobalt, up to 0.6% titanium, 0.8 to 1.5% aluminum, up to 0.006% boron and up to 0.1% zirconium, the balance, apart from impurities, being nickel. An alloy according to any preceding claim in which the silicon content is less than 0.1%, the carbon content is from 0.05 to 0.07% and the molybdenum content is at least 8%. An alloy according to any preceding claim in which the chromium content is from 19 to 23% and the content of iron, if present, is not greater than 2%. An alloy according to any preceding claim having an average grain size of ASTM 1.5 to 4.5. A method of producing an alloy according to any preceding claim which includes a final annealing treatment at a temperature above 1093°C (2000°F) but less than 1177°C (2150°F). A method according to claim 7 in which the final annealing treatment is performed at from 1106 to 1162°C (2025 to 2125°F). A method according to claim 7 or claim 8 in which the alloy composition, grain size and annealing treatment are so correlated that the alloy has a Charpy V-Notch impact strength of at least 50 J/cm² (30 ft.lb) when exposed at 1000°C (1832°F) for a period of 10 000 hours. A method according to claim 9 in which the correlation is such that the alloy has a Charpy V-Notch impact strength of at least 85 J/cm² (50 ft.lb.)when exposed at 1000°C (1832°F) for a period of 10 000 hours. The use of an alloy according to, or made by the method of, any preceding claim for articles or parts for exposure in use to temperatures of at least 982°C for a period of at least 3000 hours.

Inventors: BASSFORD, THOMAS HARVEY, CLATWORTHY, EDWARD FREDERICK, SMITH, DARRELL FRANKLIN, JR.
Applicants: INCO ALLOYS INTERNATIONAL, INC.
Application year: 1987
Main classification: C22C19
IPC codes: B23K35, C22C19