Patent Application
20080145264
1. Field of the Invention
The present invention relates to both low carbon carburizing (surface hardening) and higher carbon through hardening steels primarily containing molybdenum, vanadium and nickel and, to a lesser amount, chromium used for rolling contact bearings, gears and other similar applications where high hardness at elevated temperatures is required.
2. Description of Related Art
Piston driven aircraft engines and early jet engines used through hardening 52100 alloy steel or carburizing grades of alloy steels for most bearing applications, see Table 1. However, over the years, as power requirements rose, operating temperatures increased. Thus, alloys with more heat resistance were necessary. Minor improvements in the performance of 52100 alloy steel were achieved by adding small amounts of extra alloying elements. One of the most commonly added elements to enhance high temperature performance was silicon. TSB-600 is a typical example of such a steel, Table 1. However, these incremental changes were not sufficient to keep pace with the increased operating temperatures of more powerful turbine jet engines. In the 1950's in Europe, T-1 and M-2 high speed steels (HSS) were used for some jet engine applications. T-1, also referred to as 18-4-1, contains tungsten, chromium and vanadium as the primary alloy elements, while M-2, referred to as 6-6-2, contains chromium, tungsten, vanadium and molybdenum, Table 1. In the United States, M50 HSS, containing chromium, vanadium and molybdenum, was selected as the alloy of choice for bearing applications.
These three grades of tool steels exhibited similar metallurgical properties. In particular, tool steels display the phenomena known as secondary hardening. Standard alloy steels display what is termed Class 1 tempering behavior, i.e., they soften as the tempering temperature is raised. However, tool steels exhibit Class 3 tempering behavior. As the tempering temperature is increased, the hardness remains constant or only slightly decreases. As the tempering temperature approaches approximately 500° C., a slight increase in hardness occurs. Then, as the tempering temperature increases beyond 600° C., the hardness rapidly decreases,
One advantage possessed by M50 is that the melting point and the range of hot working temperatures are lower than for the other grades of HSS. After processing, due to the relatively large percent of alloy carbides in these grades, microstructural banding can be prevalent.
Another difference between the microstructure of M50 HSS compared to other grades is the presence of large carbide plates; the plates appear as “rods” or “sticks” on a polished cross section. Since these types of microstructural features can be detrimental to performance, one of the challenges in manufacturing these alloys is to minimize banding and to have enough mechanical deformation in the forging and rolling practices to break up these large carbides.
At The Timken Company (assignee of the present invention) in the 1990's, a patented laser glazing process, disclosed in U.S. Pat. No. 5,861,067 to Hetzner, was developed to simultaneously remove the effects of microstructural banding and greatly decrease the size of carbides in wrought M50 and other grades of high speed steels. Bearings manufactured by the laser glazing process exhibited an L10% fatigue life approximately 10 times greater than identical bearings manufactured from wrought M50 HSS.
The laser glazing process greatly changes the microstructure of the M50 alloy whereby the tempered martensitic matrix previously containing banding and large carbides is replaced by a very small cellular dendritic solidification structure. The removal of the plate-like carbides and banding greatly reduces material factors that can initiate cracks and assist in crack propagation. Thus, a large increase in fatigue life results.
In all applications, carburized bearings possess one beneficial factor not realized by through hardened bearings: the carburized bearings have a compressive residual stress on and near the surfaces of the bearing races. In addition, bearings manufactured from carburized steels generally have small carbides in the microstructure. To utilize these beneficial properties, a carburizable version of M50 was developed in the early 1980's and as disclosed in U.S. Pat. No. 4,659,241 to Bamberger et al., and Table 1. The alloy known as M50-NiI contained nickel and only about 0.10% carbon but otherwise was nearly identical to wrought M50. While this steel addressed the detrimental effects of carbide banding and the presence of large carbides found in wrought M50, it is difficult to process. Generally, prior to carburizing, a preoxidation treatment is necessary. If the preoxidation process is not properly performed, a nonuniform carburized case often results. Thus, numerous quality control processes are required to assure the uniformity of bearings manufactured from this alloy.
The difficulties in carburizing high speed steels have been overcome by reducing the chromium content of these materials from 4% to approximately 1%, as disclosed in U.S. Pat. No. 6,702,981 to Hetzner. A material based on the initial composition of M50 but having a nominal composition of 1.2% Cr, 5.75% Mo, 1.2% V and 2.8% Ni has been shown to be quite easy to carburize using conventional processing. In addition, the microstructure of the outer case after carburizing and heat treating does not contain any microscopically large carbides. Bearings manufactured from this steel exhibit an L10% fatigue life approximately 20 times greater than that of conventional M50 steel.
The heat treatments used for high speed steels are different from the heat treatments used for alloy steels. For example, a typical heat treating cycle for a bearing steel such as 52100 would be to austenitize the material at 830° C. (1525° F.). After quenching, a tempering temperature of approximately 177° C. (350° F.) would be used. Low temperature tempering would be used for any bearing fabricated from an alloy steel such as 52100. This would ensure that the resulting component would have the highest hardness possible. Tempering temperatures exceeding 177° C. (350° F.) will lower the hardness of bearings made from alloy steels. For all alloy steels, after being austenitized and then oil quenched, increasing the tempering temperature is found to decrease the alloy's hardness. Steels having this type of tempering response are referred to as “class 1” types of steels,
The heat treating procedures used for high speed steels begin with a preheat of approximately 788° C. (1450° F.) to 843° C. (1550° F.). Components fabricated from HSS are equilibrated at the preheating temperature for at least one hour. Following the preheat, high speed steel alloys are then quickly placed in an austenitizing furnace that is at a higher temperature. Depending on the alloy, the high austenitizing temperature may range from 1090° C. (2000° F.) to 1220° C. (2225° F.). The components are only held at the austenitizing temperature for a brief amount of time—say, 3 to 10 minutes. Following austenitization, the material is quenched into a salt bath at 538° C. (1000° F.). After equilibrating in the salt bath, the components are allowed to air cool to at least 66° C. (150° F.). If an oil quench is employed, the material should be removed when it reaches 480° C. (900° F.), after which cooling to 66° C. (150° F.) in still air is recommended.
Following quenching, high speed steel alloys contain untempered martensite, alloy carbides and retained austenite. Tempering HSS must accomplish two things. The martensite needs to be tempered, and the retained austenite as to be transformed to martensite. The general procedure employed for tempering high speed steels is to heat the alloys to approximately 538° C. (1000° F.) for two hours and then air cool to room temperature. The cycle is then repeated one more time. Most high speed steels show “class 3” tempering response,
The material and chemical transformations occurring during the heat treating of high speed steels are much more complex than the transformations that occur in alloy steels. A typical high speed steel alloy contains from 0.80% to 1.40% carbon. In addition, up to 25% alloy elements may be present. The primary alloying elements are typically a combination of Cr, Mo, V and W. Lesser amounts of Co, Si and Cb may occasionally be present. Columbium is added to alloy steels and tool steels for grain refinement and to enhance toughness, see U.S. Pat. No. 3,966,423 to Mal et al., U.S. Defensive Publication No. T964, 003 to Rehrer, and U.S. Pat. No. 6,863,749 to Leap. After these alloys are cast, hot rolled and then annealed, the microstructure consists of low carbon iron, ferrite and a large volume fraction of alloy carbides.
The alloy carbides in high speed steels are generally composed of a combination of alloy elements and carbon; hence, the designation MxCy is used. M represents a metal atom, and C designates carbon. X corresponds to the number of metal atoms in the carbide, and Y is the number of carbon atoms, respectively. Typical carbides in the annealed high speed steels would be MC, M6C and M23C6.
When the annealed alloy is preheated to 843° C. (1550° F.), the ferrite transforms to austenite, and some alloy carbide may dissolve. When the steel is placed in the austenitizing furnace where the temperature is 1120° C. (2050° F.) or greater, all the M23C6 dissolves. As much as 50% of the M6C and the MC may dissolve at the high austenitizing temperature. As the carbides dissolve, the carbon is dispersed in the austenite matrix. When the alloy is quenched and then cooled to 66° C. (150° F.), most of the high carbon austenite transforms to martensite. Some of the austenite is retained, and the carbides that did not dissolve remain. The carbides present are types MC and M6C. At this stage in heat treating, the hardness of the alloy is high. Depending on the total alloy content, the hardness often exceeds 60 HRC (732 KHN).
Tempering high speed steels to temperatures up to 427° C. (800° F.) may slightly decrease the hardness of the alloy. However, tempering temperatures near 538° C. (1000° F.) increase the hardness of these steels,
The high hardness of high speed steels as well as their resistance to softening at elevated temperatures is primarily due to the phenomena of secondary hardening. The formation of the small alloy carbides is primarily responsible for the excellent hot hardness these alloys exhibit.
Work to date clearly demonstrated that the removal of carbide banding and refining the size distribution could both be beneficial in creating enhanced performance alloys. Laser glazing does not change the composition of the steels, but the process refines the carbide distribution. The carbides in both M50-NiI and CHS-50 are very small. M50 NiI contains approximately 4% Cr, Mo and V. Similarly, CHS-50 contains Mo, V and approximately 0.1% Cr. Neither of these alloys contains tungsten (W). The carburized version of M1 HSS is referred to as CHS-1, Table 1. This steel contains 1% Cr, Mo, V and W. Compared to CHS-50 and M50-NiI, the case in carburized CHS-1 contains many carbides that are easily resolved at 500× magnification but well dispersed throughout the martensitic matrix.
It is generally thought by those skilled in the art that for HSS alloys, based on atomic weights, 1% Mo can be substituted for approximately 2% W. While this may be true with regard to properties such as hardness, it is obviously not true when the carbide size distributions are considered. The addition of tungsten to the low chromium carburizing alloys greatly increases the size of the carbides in these steels.
Many air hardening steels contain relatively large amounts of chromium. As an example, A2 die steel contains approximately 5% Cr, and D2 die steel contains approximately 12% Cr. For these types of alloys, as the chromium content increases, the size of the carbides and the degree of carbide banding increases. Chromium is the primary alloying element in these alloys for two reasons. First, chromium is an excellent choice for increasing the hardenability of alloy steels. This promotes the air hardenability of these alloys. Second, the carbides contained in these steels are primarily M7C3 and M23C6. In both cases, the M in the alloy carbides is primarily chromium. Since M23C6 dissolves at temperatures of 1010° C. (1850° F.) or less, high austenitizing temperatures are not required to dissolve the carbides. This makes heat treating easier to perform, and the steels still display the secondary hardening phenomena.
Silicon can have different effects on through hardening high carbon steels and carburizing steels. As disclosed in a paper by Jatczak, silicon may be added to 1% carbon TBS-600, and 0.20% carbon to CBS-600 to increase the elevated temperature performance of this steel, Table 1, see: C. F. Jatczak, “Specialty Carburizing Steels for Elevated Temperature Service”, Metal Progress, April 1978. Similarly, Si has been added to M42 HSS, containing 1.08% C, Table 1, to allow the steel to be used at higher rates of material removal in machining operations. However, the addition of Si has been found to lower M42's maximum austenitizing temperature because carbide melting may occur, as disclosed in a paper by Hetzner et al., entitled “Effect of Austenitizing Temperature on the Carbide Distributions in M42 Tool Steel,” Microstructural Science, Vol. 17, Image Analysis and Metallography, P. J. Kenny et al., ASM 1989, p. 91.
The effect of silicon on the activity of carbon in austenite iron at elevated temperatures is well documented. Darken, in 1948, showed that carbon will diffuse away from high silicon austenite to low silicon austenite, see: L. S. Darken, “Diffusion of Carbon in Austenite with a Discontinuity in Composition,” TAIME, 180, 1949, pp. 430-438. This fact clearly indicates that it is easier to carburize steels containing low silicon contents compared to steels containing higher silicon contents. This was later demonstrated in U.S. Pat. No. 4,921,025 to Tipton et al. in 1990. Thus, while U.S. Pat. No. 4,157,258 to Philip et al., U.S. Pat. No. 5,518,685 to Sakamoto et al., U.S. Pat. No. 6,808,571 to Tanaka et al., and U.S. Pat. No. 6,699,333 to Dubois use silicon additions to enhance the elevated performance of carburizing steels, from a practical point of view, the increased silicon content makes carburizing more difficult based on the physical phenomena shown by Darken. Thus, the same element can be beneficial to the final properties of an alloy but detrimental to the ability to process the alloy to achieve these properties.
Hot hardness is an easily measurable property that can be used to relate to the performance of a steel alloy at elevated temperature. As would be expected, as the test temperature increases, the hot hardness decreases. For alloy steels such as 4340 or 52100, as the temperature increases beyond approximately 150° C., the hardness continuously decreases,
The present invention relates to moderately high alloy steels that contain molybdenum, vanadium and nickel as the primary alloying elements. It will be understood in the following description that percentages are expressed in weight %, unless otherwise stated. The maximum manganese content of these inventive steels can be as high as 4.0%. The steels can contain up to 1.25% chromium. The steels of the present invention can have carbon levels ranging from approximately 0.05% through 1.25%. The lower carbon alloys are to be regarded as carburizing steels (surface hardening). The carburizing steels of the invention preferably contain from 0.05% C to 0.40% C. The higher carbon alloys of the invention are to be regarded as through hardening steels, also sometimes referred to herein as high carbon steel; these alloys can contain from 0.4% C to 1.25% C. Preferably, the through hardening steels contain from 0.80% C to 1.00% C. For both types of steels, the silicon content is intentionally kept as low as practically feasible. For the carburizing grades, the lower silicon content enhances diffusion of carbon from the carburizing furnace atmosphere into the steel. As discussed, for the through hardening grades, the low silicon content prevents the formation of carbides having low melting points during the solidification of the liquid steel. High hot hardness is maintained in these steels by two mechanisms. First, low melting point alloy carbides do not form in the steels, particularly the steels containing 1% carbon. Second, the alloy carbides that form during heat treating are primarily MO2C, VC or V7C8. These carbides have very high dissolution temperatures and melting points compared to other alloy carbides such as M23C6. In order to achieve the properties described herein, the sum of (Mo+V+Ni+Cr) is at least 4%. However, the total aggregate amount of these alloys generally does not exceed 8%. The amount of carbide forming elements is purposely kept lower than that generally found in HSS alloys so as to minimize the total amount and size of carbides in the bearing alloys. In addition, tungsten is intentionally not added to these steels to minimize the number and size of large alloy carbides that can be formed during solidification or after heat treating.
The selection of alloying alloy elements and their effect on properties are listed below.
The present invention is also directed to a method for making high alloy steels possessing a room temperature surface hardness of at least 60 HRC and containing less than 10% retained austenite. When subjected to elevated temperatures, a minimum hardness of 58 HRC is maintained to temperatures of at least 302° C. (575° F.) for the carburizing steels, and as high as 524° C. (975° F.) for the through hardening steels. The method of achieving these properties comprises the steps of:
Providing an alloy consisting essentially of in % by weight less than 1.25% Cr, preferably 0.75% to 1.25% Cr, about: 0.40% Mn≦4%, 0≦Mo≦4.00, preferably 1.0% to 3.00% Mo, 0%≦V≦2.0%, preferably 0.75% to 1.25% V, 1.0%≦Ni≦3.0%, less than 0.20% Si, a carbon content selected from one of about 0.05%≦C≦0.40% defining a carburizing steel or 0.40%<C≦1.25% defining a high carbon steel, and the balance iron plus incidental impurities. Tungsten (W) is not purposely added to these alloys. The amount of tungsten is ideally zero, or as low as commercially feasible and practical, preferably no more than 0.20% W. In general, the total amount of (Mo+V) will be less than 4% and the amount of (Mo+V+Ni+Cr) will preferably be 4% to 8%. The uniqueness of these alloys is that they achieve excellent hot hardness with a minimum of the recited alloy additions, and contain manganese (Mn) to enhance carbon diffusion and increase hardenability. This is unlike normal high speed steels that achieve good hot hardness with large amounts of alloying elements. Elements such as nitrogen (N), niobium (Nb) or even titanium (Ti) and other similar elements may be added in small amounts up to about 0.05% each for grain refinement and enhanced toughness purposes.
(b) Performing a step selected from: subjecting the carburizing steel to a carburizing treatment at approximately 960° C. without any oxidation treatment or heat treatment prior to the carburizing treatment to provide a carburized steel, or subjecting the high carbon steel to hot working to provide a wrought high carbon steel;
(c) quenching the carburized steel to provide a quenched carburized steel;
(d) preheating the wrought steel or the quenched carburized steel to 870° C., and then austenitizing said steel at temperatures ranging from 1125° C. through 1225° C. to provide an austenitized steel;
(e) quenching the austenitized steel by either gas or oil to about 65° C. to provide a quenched steel; and
(f) tempering the quenched steel, preferably twice, at temperatures up to 550° C. followed by air cooling to about 65° C. after each tempering treatment.
The following detailed description shows how some selected alloy compositions within the scope of the invention respond to carburizing and/or heat treating. The accompanying data exemplify the superior hot hardness properties achieved thereby.
The present invention relates to moderately high alloy steels that contain molybdenum, vanadium and nickel as the primary alloying elements. As stated above, percentages are expressed in weight %, unless otherwise specified. The maximum manganese content of these steels can be as high as 4.0%. The steels can contain up to 1.25% chromium. The steels described in this invention can have carbon levels ranging from approximately 0.05% through 1.25%. The lower carbon alloys are to be regarded as carburizing steels. The carburizing steels preferably contain from 0.05% C to 0.40% C. The higher carbon alloys are to be regarded as through hardening steels; these alloys can contain from 0.4% C to 1.25% C. Preferably, the through hardening alloys contain from 0.80% C to 1.00% C. For both types of steels, the silicon (Si) content is intentionally kept as low as practically feasible. For the carburizing, low carbon grades, the low silicon content enhances diffusion of carbon from the carburizing furnace atmosphere into the steel. As discussed, for the through hardening, high carbon grades, the low silicon content prevents the formation of carbides having low melting points during the solidification of the liquid steel. High hot hardness is maintained in these inventive steels by two mechanisms. First, low melting point alloy carbides do not form in the steels, particularly the high carbon steels containing 1% carbon. Second, the alloy carbides that form during heat treating are primarily MO2C, VC or V7C8. These carbides have very high dissolution temperatures and melting points compared to other alloy carbides such as M23C6. In order to achieve the properties described herein, the sum of (Mo+V+Ni+Cr) is at least 4%. However, the total aggregate amount of (Mo+V+Ni+Cr) generally does not exceed 8%. The amount of carbide-forming elements is purposely kept lower than that generally found in HSS alloys so as to minimize the total amount and size of carbides, particularly in the bearing alloys. In addition, tungsten (W) is intentionally not added to these steels to minimize the number and size of large alloy carbides that can be formed during solidification or after heat treating. If tungsten is present, the W content should be no more than 0.20 W.
The present invention is also directed to a method for making high alloy steels possessing a room temperature surface hardness of at least 60 HRC and containing less than 5% retained austenite. When subjected to elevated temperatures, a minimum hardness of 58 RC can be maintained to temperatures as high as 540° C. for the carburizing, low carbon steels, as well as for the through hardening, high carbon steels. The method of achieving these properties comprises the steps of:
(a) Providing an alloy of either the low carbon or high carbon type discussed above, consisting essentially of in% by weight less than or equal to 1.25% Cr, approximately 0.40% Mn≦4%, 0≦Mo≦4.0%, 0%≦V≦2.0%, 1.0%≦Ni≦3.0%, less than 0.20% Si, and the balance iron plus incidental impurities. Tungsten is not purposely added to these alloys. The amount of tungsten is ideally zero, or as low as commercially feasible and practical, and preferably no more than 0.20 W. In general, the total amount of (Mo+V) will be less than 4%. The (Mo+V+Ni+Cr) content is preferably 4% to 8%. The uniqueness of these alloys is that they achieve excellent hot hardness with a minimum of the proper alloy additions and contain manganese to enhance carbon diffusion and increase hardenability. This is unlike conventional high speed steels (HSS) that achieve good hot hardness with large amounts of alloying elements. The low amounts of alloy elements necessary to achieve high hot hardness in the alloys of the present invention also minimize the presence of microscopically large carbides in the microstructure of the steels of the present invention. The removal of large carbides improves bearing life and enhances fracture toughness. Elements such as nitrogen (N), niobium (Nb) or titanium (Ti) and other similar elements, such as zirconium (Zr), hafnium (Hf) and tantalum (Ta) can be added in small amounts up to about 0.05% each for grain refinement and possibly enhanced toughness.
(b) Subjecting the low carbon steel to a carburizing treatment at approximately 960° C. without any oxidation treatment or heat treatment prior to the carburizing treatment and then quenching to provide a carburized steel, or subjecting the high carbon steel to hot working to provide a wrought high carbon steel;
(c) for both the carburized steel or the wrought high carbon steels, preheating the steels up to 870° C. and then austenitizing the steel at temperatures ranging from 1125° C. through 1225° C. to provide an austenitized steel;
(d) quenching the austenitized steel by either gas or oil to approximately 65° C.; and
(e) tempering the quenched steel, preferably twice, at temperatures up to 550° C. followed by air cooling after each tempering treatment.
The following detailed description shows how some selected alloy compositions within the scope of the invention respond to carburizing and/or heat treating. The accompanying data exemplify the superior hot hardness properties achieved thereby.
The initial research pertaining to the present invention examined several alloy steels presently used in various applications and determined how they could be carburized with respect to a laboratory alloy containing just 0.20% carbon and the balance iron. Specimens of the iron −0.20% C alloy (Heat 2128) and laboratory heats of 8119 (Heat 2132), CBS 400 (Heat 2124) and CBS 600 (Heat 2126) were carburized using normal production cycles, Table 3. As indicated by measuring the carbon content of the specimens at different depths, the results for 8119 and the 0.20% carbon alloy (Heat 2128) were similar,
It is important to note, to a first approximation, the hardness of a quenched and tempered alloy steel is directly proportional to the carbon content of the steel. Thus, as illustrated by this experiment, carburized steels containing reasonable levels of molybdenum will be harder at any given depth below the surface of a component than a steel containing little or no molybdenum. This implies that there are several beneficial impacts made by adding molybdenum to these steels: (1) the carbon at any point below the surface is higher in these alloys as compared to steels containing lesser amounts of molybdenum; (2) the increased carbon content increases the hardness of the steels at the same depth below the surface; (3) molybdenum increases the hardenability of these steels and assures the highest hardness is achieved at any particular depth below the surface; and (4) increased steel hardness directly correlates to increased bearing fatigue life.
Based on the preliminary results, Mo was discovered to be very beneficial in enhancing carbon diffusion. Similarly, Mo improves both the hot hardness and wear resistance of high speed steels. In addition, Mo assists in improving the secondary hardening of high speed steels. Hence, Mo was selected as one of the primary alloying elements in the formulation of the alloys of this invention.
Nickel was included in the preliminary study because Ni extends the austenite range at elevated temperatures and improves the hardenability of alloy steels. Nickel is important in allowing the alloy to achieve the maximum possible hardness in the quenched and tempered condition.
Manganese was not included in any of the design matrices. This is because manganese primarily enhances hardenability. In addition, as shown previously, manganese appears to promote the diffusion of carbon into carburized components, and this will assist in increasing hardness. However, manganese does not enhance hot hardness, nor does manganese contribute to secondary hardening to any significant amount. Thus, while manganese is not included in the experimental development to the alloy systems, the initial testing indicates that manganese levels up to 4% can improve the room temperature hardness and, secondarily, the softening resistance of steels. Thus, for alloys of the present invention, the manganese content can range from 0% through 4%, i.e., 0%≦Mn≦4%.
The first design matrix, referred to as Series 1, used for alloy development was composed of the following elements: Approximately 1% Cr was included in each laboratory alloy. Chromium increases the hardenability of alloy steels, and at the 1% level, it has been shown not to be detrimental to the carburizing process. Approximately 0.40% Mn was included in each alloy. This level was chosen for the improvement in hardenability given by Mn and by its enhancement of the carburizing process. However, the level of Mn was not made larger because, as mentioned above, Mn does not significantly improve the hot hardness of alloy steels. Less than 0.20% Si was included in the experimental steels so as to avoid detrimental effects on carbon diffusion in austenite. A low silicon content was also desirable in preventing the formation of low melting point carbides in the test alloys. Various amounts of Mo, V and Ni were initially evaluated as indicated in Tables 4 and 5. The steels in Table 4 containing 1.0% carbon are considered high carbon or through hardening steels. The steels in Table 5 containing 0.20% carbon are considered low carbon or carburizing steels.
Based on the results obtained from the alloys in Series 1, two additional groups of alloys were melted, processed and evaluated. As with Series 1, the additional heats of steel (Series 2) were split into two groups. For the steels of Series 2, the nominal carbon contents were 1.25% (through hardening) and 0.20% (carburizing). The molybdenum content of these steels ranged from 2.9% through 6.8%, Tables 6 and 7. For the steels of Series 3, the nominal carbon contents were 1.0% (through hardening) and 0.20% (carburizing), and the nominal molybdenum content was 3%. The vanadium content of these alloys ranged from 0% through 1%, and the Ni content ranged from 1% through 3%, Tables 8 and 9.
For the purposes of statistical analysis, the results from Series 1, Series 2 adn Series 3 were grouped together. For the 29 heats of 1% carbon alloys contained in Tables 4, 6 and 8, a Y-Hat statistical analysis was performed on the steels. The first analysis was for an austenitizing temperature of 1190° C. as used for bearings manufactured from high speed steels. The primary variables used in the analysis were C, Mo, V and Ni, Tables 10.
Several important conclusions may be drawn from the above analysis. With regard to hardness, for vanadium, a maximum appears to exist near 1.05% V as indicated in a surface plot of hardness as a function of Mo and V for Ni=2% and C=1%.
A similar data analysis was performed for the combined set of high carbon alloys that were given a conventional alloy steel bearing heat treatment. That is, austenitized at 830° C. and tempered at 175° C. for 2 hours, Table 11.
When these alloys are heat treated using the conventional thermal cycles for standard alloy steels, molybdenum maximum appears in the surface plot of hardness as a function of vanadium and molybdenum with constant carbon of 1% and nickel of 2%,
Series 1 Alloys. The hot hardness properties of the alloys from Series 1 were investigated next. The hot hardness test is conducted by heating an appropriate test specimen in an inert atmosphere chamber to various temperatures above room temperature and then measuring the hardness of the steel at each elevated temperature by using a Rockwell tester. For these tests, the Rockwell A scale was used. The resulting data was converted to Rockwell C values. For each temperature, five hardness measurements were used to obtain an average hardness. The temperatures used ranged from room temperature through 600° C. For most steels, the hardness is found to significantly decrease as temperature is increased. However, for the alloys of the present invention, in certain alloy combinations, only a very small decrease in hardness occurred for temperatures as high as 545° C. when the steels were austenitized at 1190° C. and double tempered at 540° C.,
For each specimen tested, a linear or second degree polynomial equation indicating the relationship between hardness and temperature was determined by least squares analysis. This equation was used to predict the temperature at which the material hardness was 58 HRC, Table 12. In Table 12 is listed the temperature at which the specimen hardness dropped to 58 HRC. In many cases, this temperature is below room temperature. This is because the temperatures were determined by mathematically extrapolating the hardness, temperature curves to the temperature that would produce a hardness of 58 HRC. In general, the temperatures below room temperature are meaningless. However, they are useful and necessary in performing the mathematical analysis that follows. In addition, from a casual inspection of the data contained in Tables 4, 6 and 8, respectively, alloys containing in excess of 3% Mo and heat treated at 1190° C. are found to have the highest hot hardnesses. For the alloys heat treated at 830° C., high Mo contents did not significantly improve the hot hardness properties of these alloys. In contrast, the highest hot hardness values obtained for specimens heat treated at 830° C. appear to be for alloys with intermediate Mo contents, Table 12.
Series 2 and 3. Hot hardness tests were performed on the 1% carbon and 0.2% carbon alloys from Series 2 and 3. The procedures used for the evaluation were identical to those described for Series 1. The results of the tests are contained in Tables 13 and 14.
Combined Series 1 and 2. The data from alloys from Series 1 and 2 was grouped together, Table 14.
Statistical analysis, similar to that discussed for room temperature hardness, was performed on each set of grouped data for both heat treatments used for the hot hardness tests. A good correlation coefficient for the 1% carbon steels austenitized at 1190° C. was found when Mo, V and Ni were used as the primary variables, Table 15.
Using the transfer function established from Table 15, several important facts can be observed and several different conclusions can be made. The plot of molybdenum vs. vanadium for 1% carbon and 2% nickel indicates the existence of a relative maximum,
A similar analysis was performed on the steels contained in Table 14 that were austenitized at 830° C. and tempered at 175° C., Table 16.
When given this heat treatment, no relative maximum exists between vanadium and molybdenum,
Hot hardness tests were performed on the carburized steel alloys from Series 1 and 2. The procedures used for the tests were identical to those described for the alloys previously described. The combined results are contained in Table 17.
Statistical analysis of the combined data for alloys from Series 1 and 2 contained in Table 18 was performed. For the specimens austenitized at 1190° C. and double tempered at 540° C. for 2+2 hours, a good statistical correlation was found, Table 18.
When processed by these methods, the analysis indicates that by increasing either vanadium or molybdenum, at constant nickel, the temperature where the alloys become softer than 58 HRC increases,
Similarly, a plot of Ni vs. V for a Mo content of 4% reveals a saddle point,
Using the transfer function contained in Table 18, a relationship between alloy composition and softening temperature can be derived. As an example, consider alloys containing 3% Ni. For various combinations of vanadium and molybdenum, alloys within the shaded region of
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
