Case hardening corrosion resistant steel alloy and article made therefrom

Abstract: A case hardening corrosion resistant steel alloy contains about 0.05-0.1 w/o C, 0.05 w/o max. N, 1.5 w/o max. Mn, 1 w/o max. Si, 11-15 w/o Cr, 1-3 w/o Mo, 1.5-­ 3.5 w/o Ni, 3-8 w/o Co, 0.1-1 w/o V, 0.010 w/o max. P, 0.005 w/o max. S and the balance Fe. In a preferred embodi­ ment, the alloy is balanced so that: 3(w/o Cr) + 6(w/o Si) + 2.5 (w/o Mo) + 6(w/o V) - 25(w/o C+N) - 6(w/o Ni) - 2(w/0 Mn) - 2(w/o Co) - 21 ≦ 0. When case hardened and heat treated the alloy and articles made therefrom have high case hardness, corrosion resistance, high temperature capability and metal-to-metal wear resist­ ance and high core ductility, impact toughness and fracture toughness.

Description: This invention relates to a case hardening, corrosion resistant, martensitic steel alloy and articles made therefrom and, more particularly, to such an alloy which, when case hardened and heat-treated, has an outstanding combination of case properties including high hardness with high temperature capability, corrosion resistance and metal-to-metal wear resistance; combined with outstanding core properties including ductility, impact toughness and fracture toughness. Articles such as bearings and gears which possess a combination of excellent toughness, high temperature capability, and corrosion resistance as well as excellent case hardness and core ductility have been in demand to meet the more exacting operating conditions of aircraft engines and transmissions now under development. Presently known stainless steels, such as those described in U.S. Patent No. 2,590,835, issued to H.W. Kirkby and C. Sykes on April 1, 1952, and in U.S. Patent No. 3,154,412 issued to A. Kasak et al. on October 27, 1964, do not possess this requisite combination of properties and therefore are unsuitable for use in the fabrication of parts such as aircraft engine bearings. Recent developments in the design of aircraft jet and turboprop engines and transmissions have placed more stringent demands on parts including bearings and gears; for instance, improvements in engine performance will require such parts to operation under increasingly severe loads and temperatures. Additionally, some new engine designs are more open to the environment than their predecessors, leading to increased risk of corrosion of parts such as bearings due to greater exposure to moisture and, for aircraft used in marine environments, chlorides during shutdown. Thus, a significant problem encountered by the aerospace industry is that engine and transmission bearings and gears fabricated from presently known alloys do not possess the combination of toughness, high temperature capability, and corrosion resistance as well as case hardness and core ductility required of such components by the aircraft designs currently under development. GB-A-1179046 relates to a cast steel for ship propellers. The document broadly describes an alloy having the following composition in weight percent (w/o): The document teaches that when cooled from elevated temperatures to room temperature, the matrix material of the disclosed steel undergoes transformation to either ferrite or martensite. The document does not relate to a case hardenable steel and does not recognize the undesirability of ferrite in such a steel. US-A-3340048 relates to a cold workable, stainless steel having the following broad composition in weight percent: wherein the equivalent nickel index (ENI) is 19.5 to 22. ENI is defined such that : ENI=%Ni+0.68(%Cr)+0.55(%Mn)+0.45(%Si)+27(%C+%N)+0.2(%Co)+Mo. The composition is balanced to provide a combination of strength and sharp-notch ductility and to be readily transformable by mechanical working in the temperature range of about 5F to 100F. C.J. Smithells et al., Metal Reference Book , 5th ed. (1976) reports the compositions of two steels under the heading "Martensitic stainless and heat resisting steels". The steels, identified as 12CrNiMoV steel and 12CrCoMoVNb steel have the following compositions in weight percent: The steels are indicated to have utility in gas turbine components. It is, therefore, a principal object of this invention to provide a stable martensitic steel alloy which can be prepared, case hardened and heat treated utilizing conventional techniques to provide an outstanding combination of properties including high hardness, high temperature capability, corrosion resistance, and metal-to-metal wear resistance of the case and ductility, impact toughness and fracture toughness of the core. It is a further object of this invention to provide articles made of such a martensitic steel alloy which, when case hardened and heat treated, have outstanding case properties including high hardness, high temperature capability, corrosion resistance and metal-to-metal wear resistance, and excellent core properties including ductility, impact toughness and fracture toughness. A more specific object of this invention is to provide such a martensitic steel alloy and articles made therefrom which, when case hardened and heat treated, leave an essentially ferrite-free core having a hardness of at least about HRC 35, which at room temperature have a longitudinal Charpy V-notch impact toughness of at least about 60 ft-1b (about 81J), and a fracture toughness of at least about 80 ksi in (about 88MPa√m), combined with a room temperature case hardness of at least about HRC 60, said case also having a hot hardness of at least about HRC 60 at about 400F (about 205C), good metal-to-metal wear resistance and essentially no retained austenite. The foregoing objects and advantages of the present invention are largely attained by providing a case hardenable, corrosion resistant alloy as indicated in the broad range in Table I. Further or additional advantages are obtained using the intermediate and preferred ranges in Table I. Best freedom from ferrite is attained by balancing the alloy to satisfy Equation 1 (Eq. 1): Here and throughout this application the term "essentially ferrite-free" and synonymous expressions mean that free ferrite constitutes no more than 5 volume percent (v/o) of the alloy. Case hardened and heat treated articles, made from the present alloy, have a unique combination of case and core properties. For all stated ranges, the balance of the alloy is iron, preferably at least 70 w/o iron, apart from incidental impurities which do not detract from the desired properties. For example, up to 0.010 w/o phosphorus, up to 0.005 w/o sulfur, and less than 0.01 w/o of each of the elements titanium and aluminum are tolerable in the alloy as incidental impurities. The forgoing tabulation is provided as a convenient summary and is not intended thereby to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use solely in combination with each other or to restrict the broad, intermediate, or preferred ranges of the elements for use solely in combination with each other thus, one or more of the broad, intermediate, and preferred ranges can be used with one or more of the other ranges for the remaining elements. In addition, a broad, intermediate, or preferred minimum or maximum for an element can be used with the maximum or minimum for that element from one of the remaining ranges. Throughout this application, unless otherwise indicated, all compositions in percent will be in percent by weight (w/o). Further objects and advantages of the present invention will be apparent from the following detailed description thereof. Detailed Description of the Invention Carbon is a powerful austenite former and largely contributes to the attainable hardness of the alloy in the hardened condition. To that end at least 0.05 w/o, preferably 0.06 w/o, carbon is present in this alloy so that the alloy can be balanced so as to contain essentially no free or delta ferrite. When the amount of carbon present is too low, other elements would have to be adjusted to such an extent to preclude free ferrite that the resulting composition would not be suitable for case hardening. Increasing carbon has the undesired effect of reducing the impact toughness and increasing the ductile/brittle transition temperature of the alloy. Therefore, carbon is limited to no more than 0.1 w/o, better yet to no more than 0.09 w/o and preferably to no more than 0.08 w/o. As will be more fully pointed out hereinbelow, carbon and the remaining elements are carefully balanced to ensure the desired essentially ferrite-free composition which is particularly important in the core of a case hardenable alloy. Nitrogen is also a powerful austenite former and can be substituted on a 1-for-1 basis up to 0.04 w/o for the larger amounts of carbon contemplated herein. Preferably, nitrogen is kept low in this alloy and no more than 0.002 w/o is present. Chromium contributes to the corrosion resistance of this alloy which is particularly advantageous in the case hardened portion of the alloy. For that purpose, at least 11 w/o, better yet at least 12 w/o or preferably at least 12.75 w/o chromium is present. Excessive chromium results in the presence of objectionable free ferrite and retained austenite, and detracts from the wear resistance of the alloy. Therefore, no more than 15 w/o chromium is present. Better yet no more than 14w/o, preferably no more than 13.5 w/o chromium is present. Nickel is a strong austenite former, though not as powerful as carbon, or nitrogen, and works to stabilize the alloy against the formation of undesired ferrite. Nickel also contributes to lowering the ductile/brittle transformation temperature and, by reducing the required amount of other austenite-forming elements, e.g., carbon, effectively increases the toughness of the alloy. Nickel also works to permit the addition of larger amounts of ferrite forming elements, such as chromium, molybdenum, and vanadium without upsetting the desired martensitic balance of the alloy. At least 1,5 w/o, better yet at least 2.0 w/o and preferably at least 2.25 w/o nickel is present. Increasing nickel above 3.5 w/o objectionably detracts from the mechanical properties of the alloy and unduly depresses the M s temperature of the alloy thereby undesirably stabilizing austenite and adversely affecting the formation of the martensitic structure in the alloy. Better yet no more than 3.0 w/o, preferably no more than 2,75 w/o nickel is present in this alloy. Molybdenum enhances the heat and temper resistance of this alloy as well as its corrosion resistance, all of which are important characteristics of this case hardenable alloy. Therefore, the alloy contains a minimum of 1 w/o, better yet 1.5 w/o, molybdenum. Above 3 w/o, molybdenum increases the amount of delta ferrite to an extent as to outweigh its benefits. Better yet, molybdenum is limited to no more than 2.5 w/o and no more than 2.0 w/o is preferred. Cobalt, like nickel, is also an austenite former and helps counterbalance the ferrite forming elements present to reduce the tendency to form ferrite in the alloy so that larger amounts of the ferrite forming elements chromium, molybdenum and vanadium can be present than would otherwise be tolerable. Cobalt also serves to raise the M s temperature of the alloy thereby reducing the likelihood of retained austenite in the alloy which is particularly disadvantageous when the alloy is case hardened. Thus a minimum of 3 w/o cobalt, better yet 4w/o and preferably 4.75 w/o is present. Because cobalt is a strategic material the cost of which has varied widely in the past, no more than necessary is present. Thus, though more may be useful, no more than 8 w/o, better yet no more than 7 w/o and preferably no more than 5.75 w/o is present. Though vanadium is a ferrite former and forms vanadium carbides, both of which tend to increase the ferrite forming potential of the alloy, vanadium is present because it contributes to the fine grain structure and wear resistance of alloy. For these reasons a minimum of about 0.1. w/o, better yet about 0.4 w/o and preferably about 0.5 w/o is present. Because excessive vanadium leads to ferrite being present and to the tieing up of too much carbon, vanadium is limited to no more than about 1 w/o, better yet no more than about 0.8 w/o and preferably no more than about 0.7 w/o vanadium is present. Though manganese is optional it is usually present in this alloy because it contributes to its austenitic balance. Manganese also depresses the M s temperature of the alloy. Therefore, no more than about 1.5 w/o may be present in the alloy. A useful range for manganese is about 0.25-1.25 w/o and the preferred composition contains about 0.5-1.0 w/o manganese. Silicon contributes to the tempered hardness of the alloy but because it is a strong ferrite former must be carefully controlled so as not to upset the balance of the alloy. Therefore, up to about 1 w/o may be present. Though optional, about 0.1-0.7 w/o, preferably about 0.2-0.6 w/o silicon is present. When making this alloy it is desirable, as is usually the case, to avoid using the minimum amount of austenite-forming elements with the maximum amount of ferrite-forming elements and vice-versa. Best results can be facilitated by using the following equation: This alloy is readily prepared by means of conventional, well-known techniques including powder metallurgy. Preferably, for best results, vacuum induction melting (VIM), followed by vacuum arc remelting (VAR) for further alloy refinement is used. After the VIM step the electrode can be stress-relieved at about 1100-1250F (about 595-675C) for 4-16h, preferably at about 1200F (about 650C) for 12h. After the VAR step the ingot is preferably stress-relieved in the same manner. The alloy is forged from a soak temperature of about 1800-2200F (about 980-1200C), preferably about 2050F (about 1120C), then slowly cooled to room temperature at a rate consistent with the thickness of the material. Preferably annealing should be subcritical, that is, carried out at a temperature below the critical temperature (A cl ) of the alloy. The critical temperature of the alloy is readily determinable by well-known techniques. When case-hardening is desired, the alloy may be nitrided or carbonitrided, but is preferably carburized using well-known methods for a time sufficient to secure the desired case depth and hardness. Articles made of the alloy can be hardened by austenitizing at about 1875-1925F (about 1025-1050C), and preferably at about 1900F (about 1040C), for about 0.25h plus about an additional 0.1h for each inch of thickness of the article. Following the austenitizing treatment, the article is quenched. Generally, oil quenching to room temperature is sufficient. However, to prevent the possibility of cracking in intricate articles, such articles are preferably quenched in salt kept at about 800°F (about 425C), allowed to equalize with the salt temperature, and then air cooled to room temperature. Following quenching, the article is deep frozen at about -100F (about -73C) for about 0.5-1h to ensure the transformation of essentially all austenite to martensite. The article is then warmed in air to room temperature. For maximum impact toughness, fracture toughness, and corrosion resistance, the article is preferably tempered at about 600F (about 315C) for about 2h, air cooled to room temperature, reheated to about 600F (about 315C), maintained at that temperature for about 2h, then air cooled to room temperature. It has been found that the present alloy, when case hardened and heat treated using conventional techniques, exhibits a unique combination of case and core properties including excellent high temperature capability, toughness, and corrosion resistance and a case having high hardness and containing essentially no retained austenite and a very ductile core containing essentially no free ferrite. The alloy may be produced in various forms including billet, bar, rod, and wire. Additionally, the alloy lends itself to use in the fabrication of a wide variety of articles of manufacture including bearings for spacecraft and aircraft engines, bearings and gears for aircraft transmissions, bearings for instruments, and knives requiring a combination of outstanding toughness and corrosion resistance. EXAMPLE 1 As an example of the present invention, a 400 1b (about 181kg) heat was vacuum induction melted and cast into a 6in (about 15cm) diameter electrode, stress relieved at 1300F (about 705C) for 6h, and then vacuum arc remelted to form an 8in (about 20cm) diameter ingot having the nominal and specific compositions listed in Table II. With respect to Example 1 (Ex. 1) the balance (bal.) was iron except for incidental impurities which included less than 0.005 w/o phosphorus, 0.002 w/o sulfur and 0.001 w/o nitrogen. To facilitate comparison, the nominal compositions of AISI 4340, 8620, and 440C steel alloys, used for comparison testing as described hereinbelow, are also given in Table 11. The resulting ingot of Ex. 1 was stress relieved, cut to provide a 12in (about 30cm) piece which was forged from a soak temperature of 2050F (about 1120C), to a 3.5in x 4.25in (about 8.9cm x 10.8cm) rectangular bar and then cooled in vermiculite to room temperature. The forged bar was annealed at 1200F (about 650C) for 56h, then air cooled. The hardness of the annealed bar was found to be HRC 25.0. Here and throughout this application, unless otherwise indicated all hardnesses are in the Rockwell C hardness scale (HRC). Case hardened and heat treated specimens of Example 1 were prepared for surface hardness testing; the specimens were taken from a portion of the annealed bar, heated at 1900F (about 1040C) for 1h in an air furnace to oxidize the specimen surface, then air cooled to room temperature. These specimens were case hardened by carburizing; the specimens were heated at 1700F (about 925C) for 7h at a carbon potential of 1.2 w/o then air cooled to room temperature. The carburized specimens were hardened by austenitizing at 1900F (about 1040C) in salt for 0.25h, oil quenching, deep freezing at -100F (about -73C) for 0.5h, then air cooling to room temperature. Following hardening, the specimens were tempered at 600F (about 315C) for two successive periods of 2h each, with air cooling to room temperature after each of the heating periods. The tempered surface hardness of each specimen was determined to be HRC 62. The case wear resistance of Example 1 was tested using a crossed cylinder wear test (ASTM G 83). In this test, duplicate 0.5in (1.27cm) diameter bar specimens of Example 1 (Table II) were carburized and heat treated as described in connection with the hardness specimens. The case of each bar was found to have a surface hardness of HRC 61. Each bar was placed in contact with, and in a perpendicular relationship relative to, a 0.5in (1.27cm) diameter bar of commercially purchased standard AISI 4340 steel, having the nominal composition indicated in Table II, hardened to HRC 31. The specimens of Example 1 served as the rotating member, while the AISI 4340 bar served as the stationary member in duplicate tests run at a speed of 600 revolutions per minute (rpm) for 40,000 revolutions with a load of 161bs (about 7.3kg). The test results, listed in Table III, compare the wear resistance of the present alloy/AISI 4340 system against that of an identically tested system in which the rotating member was a 0.5in (1.27cm) diameter carburized bar of commercially purchased standard AISI 8620 steel hardened to HRC 61. Because of the use of carburized AISI 8620 bearings and AISI 4340 bearing cages in some current engine designs, the 8620/4340 system was used for wear resistance comparisons. As seen in Table III, the total loss data indicate that the alloy of the present invention exhibits significantly better wear resistance than carburized AISI 8620 steel. Duplicate specimens of both Example 1, and AISI Type 440C, having the compositions listed in Table II, were comparison tested for pitting potential in 100 parts per million (ppm) NaCl solution. The specimens of Example 1 were carburized and heat treated as described in connection with the cross cylinder wear tests, while the Type 440C specimens were heat treated as indicated below in Table IV. Since positive numbers between about 0 and + 1.000 millivolt (mV) indicate comparable pitting resistance in this test, the results listed in Table IV show that the present alloy resists pitting similarly to Type 440C. Specimens of both the present alloy and commercially purchased standard AISI Type 440C, processed as described hereinabove, were subjected to humidity corrosion testing wherein 1 in (about 2.5cm) diameter bar specimens were machined to a 45° cone at one end and were subjected to 95% humidity at 95F (about 35C) for 200h. The present alloy resisted corrosion very well, having corroded similarly to Type 440C; both alloys exhibited approximately 5% surface corrosion at the conclusion of the test. In testing the properties of the core analysis, specimens for the various tests were cut from the annealed bar and then pseudocarburized, that is, subjected to the carburizing treatment except that an inert cover gas (N₂ or Ar) was substituted for the carburizing gas thereby replicating as closely as possible the processing conditions to which the carburized specimen cores were subjected. Standard (ASTM E 21) tensile strength test specimens were cut from the annealed bar of Example 1 and rough machined to 0.005in (about 0.013cm) oversize in the gage section. The specimens were cut such that their longitudinal axes were oriented either along or transverse to the direction of metal flow during forging of the bar. The specimens were pseudocarburized at 1700F (about 925C) for 7h, air cooled to room temperature, austenitized 0.25h in salt at 1900F (about 1040C), oil quenched, deep frozen at -100F (about -73C) for 0.5h, and tempered at 600F (about 315C) using a two-step cycle (2h/air cooled to room temperature/2h/air cooled to room temperature). The specimen gage sections were then polished to 0.252in (about 0.640cm) in diameter before testing. Results from duplicate room temperature tensile tests are shown in Table V, including yield point (Y.P.) and ultimate tensile strength (U.T.S.), both given in thousands of pounds per square inch (ksi) and in megaPascals (MPa), as well as the percent elongation (% El.) and the percent reduction in cross-sectional area (% R.A.). Duplicate standard (ASTM E 399) 1.25in (about 3.18cm) compact tension specimens for fracture toughness testing were cut from the annealed bar such that their longitudinal axes were oriented along the direction of metal flow during forging, and rough machined 0.010in (about 0.025cm) oversize. The specimens were then pseudocarburized and heat treated as described hereinabove for tensile strength test specimens and then finish machined to standard size. Valid K 1c results could not be determined since the specimens were too small for such a ductile alloy. However, the K Q results listed in Table VI were determined, showing very good core fracture toughness. The terms K 1c and K Q are used herein as defined in ASTM E 399. Longitudinal (L) and transverse (T) standard (ASTM E 23) Charpy V-notch (CVN) specimens of Example 1 were rough machined 0.020in (about 0.051cm) oversize and then heat treated and pseudocarburized in the manner hereinbefore described for tensile strength test specimens. After heat treating, the specimens were finish machined, notched and tested for impact toughness at 32F (0C), 78F (about 25C), and 212F (100C) yielding the results shown in Table VII. These results indicate that the present invention exhibits excellent core impact toughness, especially along the direction of metal flow during forging. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described, or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims: A case hardenable, corrosion resistant, martensitic steel alloy containing, in weight percent, the balance iron apart from incidental impurities, wherein nitrogen when present is substituted on a 1 for 1 basis for amounts of carbon in excess of 0.05 w/o and wherein 3(w/o Cr) 6(w/o Si) + 2.5(w/o Mo) + 6(w/o V) - 25(w/o C+N) - 6(w/o Ni) - 2(w/o Mn) - 2(w/o Co) - 21 ≦ 0. An alloy as claimed in claim 1 containing at least 0.06 w/o carbon and no more than 0.002 w/o nitrogen. An alloy as claimed in claim 2 containing no more than 0.09 w/o carbon. An alloy as claimed in claim 2 or 3 containing no more than 0.7 w/o silicon. An alloy as claimed in any of claims 2 to 4 containing no more than 3.0 w/o nickel. An alloy as claimed in any of claims 2 to 5 containing no more than 7 w/o cobalt. An alloy as claimed in any of claims 2 to 6 containing no more than 0.8 w/o vanadium. An alloy as claimed in claim 1 containing An alloy as claimed in any of claims 1 to 8 containing no more than 0.08 w/o carbon. An alloy as claimed in any of claims 1 to 7 containing A worked and heat-treated article formed from an alloy as claimed in any of claims 1 to 10. A case-hardened article, formed from an alloy as claimed in any of claims 1 to 10, said article having a core and a case, the core at room temperature being essentially ferrite-free and having a hardness of at least HRC 35, a longitudinal Charpy V-notch impact toughness of at least 60 ft-lb (81J), and a fracture toughness of at least 80 ksi in (88MPa √m), and the case having at room temperature a hardness of at least HRC 60, a hot hardness at 400°F (205°C) of at least HRC 60, good metal-to-metal wear resistance and essentially no retained austenite.

Inventors: HEMPHILL RAYMOND M, WERT DAVID E, HEMPHILL, RAYMOND M., WERT, DAVID E.
Applicants: CRS HOLDINGS INC, CRS HOLDINGS, INC.
Application year: 1990
Main classification: C22C38
IPC codes: C22C38