JP2008511759A - Optimization of steel metallurgy to improve broach tool life - Google Patents

Abstract

Steel for use as a broachable workpiece with a microstructure that is substantially free of pearlite and maximized the useful life of broach tools. The formation of pearlite is suppressed by alloy modifications that control microstructure and hardness levels; on-line processing; off-line processing; or a combination of these techniques. Workpieces are broached into the form of power transmission components such as gears or races, which are surface hardened by one of carburizing, nitriding or induction hardening depending on the carbon content of the steel after broaching Is done. Steel compositions and processing methods for producing steel, broachable work pieces and broached, surface hardened power transmission articles are disclosed.

Description

Cross-reference to related applications This application is a US application filed September 2, 2004 entitled "Optimization of Steel Metallurgical to Improve Brochure Tool Life". We claim the benefit of provisional application 60 / 606,816, which is incorporated herein by reference in its entirety.

Description of federally funded research Part of the research on this subject is partially funded by the US Department of Energy under contract number DE-FC36-991D13819.

  The present invention is generally used in the manufacture of articles such as power transmission gears, races, and similar components that are formed by iron metallurgy, and more particularly, hardened by carburization or induction hardening, for example by broaching. Related to the steel composition and microstructure. Even more particularly, the present invention relates to a technique for obtaining an optimized steel metallurgy microstructure that provides a steel workpiece material that significantly improves broach tool life, resulting in lower manufacturing costs per member. The present invention also relates to a method for obtaining the desired microstructure and properties in steel workpieces and finished products.

  Broaching is a machining technique commonly used to cut gear teeth or cam profiles for mass production of power transmission components such as automobile transmissions and the like. The part profile is formed in a single broaching operation in a minimum amount of time, making the broaching operation ideal for such cost sensitive applications. However, to perform a broaching operation at a single station, the broaching machine uses a single long high speed steel broaching tool that removes metal from the work piece in a single motion to provide the necessary part profile. The entire roughing, forming and finishing must be carried out. Broach tools are relatively expensive to manufacture and can only be reduced or sharpened a certain number of times before the tool can no longer be used. The cost and reduction cost of the original broach tool is an important part of the total production cost of the finished part. Precise broaching and tooling costs per manufactured part are highly dependent on the number of parts that can be manufactured during broach tool reduction. Tool cost to life of spiral broaching bars in the range of $ 50,000 to $ 80,000 and all manufactured with a single broaching bar currently in the range of 10,000-80,000 parts For goods, the cost per part is typically in the range of $ 0.60 to $ 5.00. Therefore, the broaching tool cost corresponds to about 15% to 50% of the total manufacturing cost for the finished part. Broaching therefore represents a time and plant space efficient method for cutting a profile into an annular steel part, but the tool cost to perform this operation represents a significant part of the total manufacturing cost .

  Previous attempts to improve broach tool life by modifying workpiece materials have included alternative material selection, changes to traditional heat treatment routes to improve machinability, or alloying components such as sulfur and calcium. Including addition. Producing optimized broaching performance has been hampered by previous material and heat treatment selection attempts made without full knowledge of the factors affecting broach tool life. Furthermore, only limited amounts of specific inclusion additives are acceptable in the steel grade of these conventionally used workpieces. These previous attempts have only provided incremental improvements in tool life that fall in the range of 25% to 100% improvement, but due to the fact that they have not created an optimal broaching microstructure and hardness combination And below the level of improvement provided by the present invention. Also, previous approaches have been to reduce the alloy level to reduce the hardness level and work steel friability; increase the tempering temperature to decrease the hardness level; and decrease the carbon level or lower carbon, lower Includes conversion of heat treatment method to allow the use of friable steel. All of these approaches have produced only the incremental improvements described above.

  Workpiece steel used to produce profiled parts for powertrain gears and races is carburized / nitrided or induction-baked, depending on the method used to harden the load bearing surface of the finished part. It can be roughly categorized as either a container type. Steel has historically been roughly selected based on the hardening method and subsequent carbon levels and heat-treatable requirements of the hardened surface in an effort to minimize the cost of manufacturing the part. It was. Conventional workpiece steels so selected are processed to produce ferrite / pearlite type microstructures for narrower purposes / ranges within the overall broached hardness range of typically 150-300 BHN. And / or have been heat treated. The resulting work piece steel selection, processing and heat treatment to achieve the microstructure and hardness have been shorter than the optimal broach tool life so far, but are acceptable based on historical data. Has been considered. This historical data is seldom questioned or improved in a systematic way due to the prohibitively high cost of testing the possibility of new grades of workpiece steel on production equipment It wasn't done.

  For power transmission components such as gears, it is desirable to harden the broached tooth surface for wear resistance while maintaining a low hardness level in the gear core for strength. As implied above, the main techniques for obtaining localized surface hardening are carburizing, nitriding and induction hardening, depending on the type of steel used to make the part. In general, steel grades having less than about 0.32 wt% C have insufficient carbon on the surface to provide thermal hardening. Thus, this steel type is usually carbonized (or carbon diffused) to diffuse a carbon (or nitrogen) rich layer in the load bearing surface of the part to allow subsequent thermal / hot hardening. Nitrided). On the other hand, steel having greater than 0.32 wt% C, such as grades having about 0.35 to 0.80 wt% C, can be surface hardened using radio frequency heating techniques. An inductive high frequency magnetic field heats the surface layer of the broached part in a matter of seconds to the desired austenitizing temperature of 1700-1800 ° F., and then the heated surface immediately becomes water or other Immediately hardened in a quenching agent. Since carburizing (and nitriding) is typically performed in a controlled atmosphere furnace for 5-10 hours, carburizing and nitriding is considerably more time consuming and expensive than induction hardening techniques. To date, higher carbon type work steels suitable for induction hardening tend to have shorter broach tool life than lower carbon carburization types, and broach tool life tends to decrease as the carbon content of the work steel increases. Accompanied by. Thus, the economic benefits enjoyed by fast induction hardening have been offset by increasing broach tool costs per part so far.

  Applicants are aware of conventional work in this industry where it has been attempted to provide broachable steel within the desired broaching hardness range of about 150-240 BHN. This steel has a non-pearlite microstructure obtained by off-line heat treatment that is implemented as a means of optimizing the surface hardening treatment reaction and not as a means of optimizing broach tool life. However, this steel has a carbon range of 0.25 to 0.31% by weight and is subjected to a nitriding heat treatment after broaching to achieve surface hardening for transmission gears.

  Applicants are further aware of conventional work involving broached power transmission components that require high core hardness prior to broaching in order to develop the desired mechanical properties of the component. The hardness range for these parts is typically 250-300 BHN or higher. Most steels cannot achieve this hardness range while maintaining the pearlite microstructure. As a result, the most common way to increase hardness to a level within this range is to perform off-line heat treatment (quenching and tempering) to produce a non-perlite bainite / martensite tempered structure. That is. The reason for obtaining this non-pearlite structure is to develop the core hardness range, not to optimize broaching. Indeed, at hardness levels of 250-300 BHN or higher, broach tool life is sacrificed for high hardness.

  The present invention solves the problems of the prior art by providing the workpiece with an optimized steel microstructure for the production of articles by broaching, resulting in a significantly improved broach tool life. This article is useful as an automobile, a truck, a tractor, and a power transmission system component such as a transmission system gear and a race. Furthermore, the present invention provides steels for workpieces including carburizing, nitriding, and induction hardening types that significantly increase the broach tool life for these steel types.

  Briefly stated, the present invention contemplates a steel metallurgy method optimized for the workpiece to be broached, the microstructure of which is selected from bainite, martensite, and / or ferrite. Substantially eliminating the presence of perlite. The present invention provides the following processing techniques: (a) change of steel alloy composition to suppress pearlite formation; (b) online thermal processing to suppress pearlite formation; (c) to suppress pearlite phase formation. And / or (d) one or more combinations of the above techniques or use of other techniques; at least one or more of: bainite and / or baking substantially free of pearlite phase. This optimized steel metallurgy method in a work piece to be broached to achieve the desired microstructure of return martensite is provided. A further aspect of the invention is a steel workpiece suitable for surface hardening by carburizing, nitriding or induction hardening after broaching, wherein the steel microstructure is one or both of on-line or off-line heat treatment. A steel workpiece substantially free of pearlite is provided by changing the combined steel alloy composition. A preferred hardness range for the workpiece material is about 160-250 BHN, more preferably about 170-245 BHN, and even more preferably about 180-240 BHN. The workpiece steel has a carbon content C of about 0.15 to 0.80% by weight, more preferably about 0.18 to 0.70% by weight and is therefore of the carburizing, nitriding and induction hardening type. Including steel. One aspect of the present invention is a carbon content of about 0.15 to 0.35 wt%, preferably about 0.20 to 0.32 wt% so that the broached workpiece can be carburized. Contemplates C. A further aspect of the present invention is about 0.32 to 0.80 wt% C, more preferably about 0.33 to 0.70 wt% so that the broached workpiece can be induction cured. C, and even more preferably, includes work steel for broaching having about 0.35 to 0.65 weight percent C. The present invention includes steel materials that make broached workpieces, broached steel workpieces, finished products made from steel workpieces, and methods of making them.

A laboratory broach tester has been devised and built to allow an economical and efficient test and comparison of the effect of metallurgical variables on broach tool life. The laboratory test utilized a reciprocating three-tooth high speed steel broach tool made from M4 tool steel. This broach tool quickly and efficiently cut the equivalent steel volume that a typical broach tool contained in a production broach bar would cut into many parts. Laboratory testing machines and procedures include broach tool material and its heat treatment, tool tooth design, cutting depth per tooth, cutting speed, cutting lubricant type and lubricant delivery system and tool wear criteria limits Designed to closely resemble the production broaching environment in all possible aspects. The broach tool utilizes an index table to place an appropriately circular test steel workpiece at each cut, and uses specific parameters established for each listed variable to create a ring-shaped annulus. It is drawn repeatedly through the inner diameter of the test steel workpiece. Broach tools are regularly measured for wear and, when previously established wear criteria are met, laboratory tests are considered complete and the number of cuts to broach tool defects for each workpiece material condition. Assigned.

  This laboratory broach test was performed on various steel types and conditions for each steel / condition relative to each other based on the number of cuts against a specific wear criteria limit. The database generated from this large scale test shown in Table I shows that optimal work steel broaching conditions or microstructure / hardness exist for all steels, and most steels are now broached in this condition. It has not been shown. This test also reveals the non-optimized workpiece steel state for broaching, and surprisingly most steels are now broached in its non-optimal state or a slightly modified version It revealed that. In addition, the steel alloy composition reported in Table II and the steel processing and heat treatment scheme reported in Table II allow for the development of optimized workpiece steel conditions for broaching over a wide range of overall parameters. Developed by the present invention.

The steels tested included the following compositional ranges in weight percent.
C- 0.18% to 0.62%
Mn- 0.55% to 1.60%
Si- 0.15% to 0.70%
S-0.010% -0.060%
Cr-0.10% -1.0%
Ni-0.05% -0.55%
Mo- 0.02% to 0.30%
V- 0.01% to 0.12%
Al- 0.002% to 0.035%
Fe- remaining amount, plus 0.05% trace impurities or additives, respectively. Steel conditions tested are: heat-processed, air-cooled; heat-processed, slowly air-cooled; fine and coarse particles Fines and coarse particles were tempered and tempered; tempered and tempered; quenched and tempered with interrupt; and including annealed states It was. Typical parameters for these states are as follows:

  The steel samples reported in Tables I-III (numbers 1-15, 30-33, and 40-44) were melted in a production electric furnace operation and ladle refined according to the composition detailed in Table II. Steel specimens are continuously cast in bloom, then heated to 2250 ° F., hot rolled to round slabs, then reheated to 2250 ° F., penetrated, and rolled into a tubular shape. Gave. This tube is then cut into a ring-shaped annular steel workpiece for further heat treatment according to Table III prior to testing or test broaching. The resulting microstructures for the various steel samples reported in Table I show that while varying the degree of carbide spheroidization in both the tempered and non-tempered versions, It contained a mixture of ferrite, pearlite, bainite and / or martensite. The hardness levels of the various steels tested ranged from 150 BHN to 330 BHN, and most of the hardness levels were within the broachable range of 160 BHN to 260 BHN. As shown in Table I, the preferred broachable hardness range here is about 150-250 BHN, more preferably 175-240 BHN. Analysis of a wide range of steel compositions shown in Table II, processing scheme (Table III), microstructure and hardness level (Table I) identifies key metallurgical variables for optimizing broach tool life in accordance with the present invention. Made possible.

  Historical broaching knowledge and expertise has so far shown that higher carbon content and hardness levels, together with traditional processing methods, negatively impact broach tool life. These results are the results of the broaching test data produced (Table I), where a number of conventional “baseline” workpiece steel grades (sample numbers 1-15) were 0.20 weight. Supported at carbon and hardness levels (shown as a function of FIG. 2) ranging from% to 0.60 wt% (FIG. 1).

More particularly, in FIG. 1 and Table I, a baseline work steel (sample number 1) having a carbon content of 0.20 wt% and a Brinell hardness of 210 BHN is 6,200 (number of cuts to the limit). It will be appreciated that it has resulted in a broach tool life. As the carbon content of the baseline steel increased to 0.6 wt% (Sample No. 15), the Brinell hardness increased to 252 BHN and the broach tool life decreased to 80 (number of cuts to the limit). The microstructure of these conventional baseline work steel grades was primarily ferrite and pearlite as shown in FIGS. In the micrographs of FIGS. 6 and 7, ferrite appears as bright areas and pearlite appears as darker areas. FIG. 6 shows the microstructure of baseline 5130 grade steel (Sample No. 2) with ferrite and pearlite in the as-rolled state, and FIG. 7 shows the ferrite and pearlite in the normalized and tempered state. The microstructure of the reference line 5150 grade (Sample No. 10) is depicted. These ferrite and pearlite microstructures are typically identified as broaching microstructures that are acceptable for most conventional broaching applications.

  In contrast, the present invention modifies certain stages of the metallurgical process to suppress or minimize the formation of pearlite microstructure at the same carbon and hardness levels shown in the baseline steels of Table I. Modifications include the discovery that broach tool life can be increased by 2-10 fold by forming a finer needle-like carbide microstructure, which preferably tends to spheronize. The preferred microstructure of the present invention is essentially free of pearlite and consists primarily of bainite and / or martensite and ferrite and can be tempered at higher carbon levels within the desired hardness range of 160-260 BHN. There may be some pearlite phase, up to about 20% by volume layered pearlite, preferably up to less than 10% by volume layered pearlite, and even more preferably less than 5% by volume layered pearlite. Ideally, virtually no layered pearlite is present in the microstructure. As used herein, the term “substantially free of pearlite” or “minimum amount of pearlite” unless otherwise specified, is a micro comprising up to about 20% by volume, more preferably 0% layered pearlite. Means structure.

  The desired acicular bainite microstructure is shown in FIG. 8, the desired spheroidized bainite microstructure is shown in FIG. 9, and the desired spheroidized martensite and ferrite microstructure is shown in FIG. 10. FIG. 8 is an alloy-modified 4030 grade (Sample No. 20) as it is rolled, and FIG. 9 is an online-quenched and tempered 5130 Grade (Sample No. 30). 10 is grade 5150 steel (sample number 33) that has been online hardened and tempered, all made according to the present invention.

  The substantially pearlite-free microstructure of the present invention comprising one or more bainite, martensite and ferrite with zero or minimal amount of pearlite is not necessarily limited to: (1) heat-processed, Change of alloy composition to suppress pearlite formation in air-cooled state (Sample Nos. 20 to 23) and / or (2) Online heat treatment to suppress pearlite formation during quenching from thermal processing temperature (Sample No. 30) ~ 33), and / or (3) off-line heat treatment to inhibit pearlite formation (sample numbers 40-44), or some combination of these techniques, or other techniques such as isothermal transformations below the pearlite formation temperature Formed by various techniques. The final bainite / martensite / ferrite substantially pearlite free microstructure is, of course, tempered to the desired broaching hardness range if desired.

  The effect resulting from the modification of the microstructure is similar to the previously described broach tool life trends for various carbon and hardness levels having a pearlite microstructure, with the same carbon level (FIG. 1) and hardness range (FIG. 2). ) In an optimized broach tool life result for a similar steel substantially free of pearlite. As is readily noticed, improvements in broach tool life are on the order of 200% to 1000% when comparing the same or similar initial carbon content and broach tool life, all within similar hardness ranges. For example, the broaching life of prior art baseline 5120 grade workpiece steel (Sample No. 1) with C0.20% with ferrite / pearlite microstructure results in a broaching life of 6200 (number of cuts to the limit) It was. This can also be compared to an off-line normalized 8620 grade (sample no. 40) with a C content of 0.20% but gives a broach tool life of 12,000 (number of cuts to the limit) The bainite / ferrite microstructure of the present invention gave approximately twice the broaching life of conventional workpiece steel. A similar improvement in broach tool life achieved with increasing carbon content can be seen graphically in FIG. This significant level of broach tool life improvement translates directly into a significant reduction in broach tool cost per part. More particularly, the present invention provides a 40% to 80% reduction in tool cost per part, resulting in a parts manufacturer realizing a significant overall reduction in part manufacturing costs. As implied above, this cost saving is of greatest importance in the manufacture of high volume precision automotive components.

  Obtaining the optimal steel workpiece state for broaching is not necessarily limited to the following, but (1) changing the alloy composition to suppress or minimize pearlite formation in the thermally processed and air-cooled state (2) on-line heat treatment design to suppress pearlite formation during quenching from hot working temperature and / or (3) off-line heat treatment to suppress pearlite formation, if necessary, followed by Any of the three previously described techniques including a tempering operation to be performed, or a combination thereof, can be realized by the present invention.

  Although other techniques for controlling the undesirable formation of pearlite microstructure may occur to those skilled in the art, these techniques will fall within the spirit and scope of the present invention. Examples of preferred techniques of the present invention are discussed below.

1. Alloy Modification Depending on the base steel composition, the alloy composition can be designed to inhibit pearlite formation by adding and adding individual or combinations of several possible chemical elements. An example of an alloy modification approach is the four carbon levels (C0.3%, C0.41%) with approximately 0.25 wt% Mo added to the base carbon and manganese composition to suppress pearlite formation. C0.51% and C0.62%) by demonstrating by melting a series of experimental vacuum induction melts. It will be understood that all percentages are by weight unless otherwise noted. More specifically, the alloy compositions for these sample numbers 20-23, grades 4030, 4040, 4050 and 4060 shown are shown in Table II with carbon levels ranging from 0.30 to 0.62% by weight, respectively. Have a composition. As seen in Table III, the ingot is hot rolled, air cooled, formed into test rings, and baked as necessary to achieve the desired broaching hardness range of about 180-240 BHN (Table I). Returned. The microstructure of these steels was mainly composed of fine bainite / ferrite. When the steel was tempered as in sample numbers 21-23, the microstructure tended to be spheroidized. An example of a spheroidized bainite microstructure is shown in FIG.

  FIG. 3 shows broaching test results for alloy modified steels of the present invention (Sample Nos. 20-23) and conventional baseline ferrites covering a similar carbon range of C 0.3-0.60%. / Graphically compared to pearlite-containing steel (samples 3, 5, 7, 11 and 14). Baseline steels range from 4600 at C0.30% (number of cuts to the limit) (sample number 3) to only 200 (number of cuts to the limit) at C0.60% as the carbon level increases ( A steady decrease in broach tool life on sample number 14) is shown. However, the alloy-modified steel of the present invention shows 9500 cut to the limit at C0.41% (Sample No. 21) and then about 4000 cuts to the limit at C0.62% (Sample No. 23). . The modified steel composition of the present invention exhibits an order of magnitude improvement in broach tool life on the broach over conventional baseline steel of comparable carbon levels. Therefore, the alloy modification approach of the present invention to optimize the microstructure and broach tool life of grades with different carbon levels should be successful and effective as evidenced by test data. Was supported.

The presently preferred modified steel composition of the present invention that inhibits or minimizes the formation of pearlite-containing microstructures in the hot-rolled and air-cooled (optionally tempered) state is shown in Table IV Listed below in weight percent.

  More particularly, and presently preferred alloy modified compositions to achieve the desired properties of the present invention are listed in Table II under sample numbers 20-23 (heading “Alloy Optimized”). Is explained in detail.

2. Online Processing A second technique for improving broach tool life according to the present invention involves online processing to achieve the desired microstructure. Online heat treatment uses a devised thermomechanical treatment scheme in which unwanted pearlite phases are suppressed during quenching from thermal processing such as hot rolling, without the need to modify the steel composition all or to some extent Includes thermal processing and quenching. As used herein, the term “on-line” optimization or processing refers to a final hot working operation (rolling, in which the pearlite phase is avoided or minimized and the bainite / martensite / ferrite phase is promoted. A thermomechanical steel processing scheme directly associated with forging, etc.).

  An example of this online technique is the austenitization (final as-processed microfabrication) of grade 5130 and 5046 steel tubes (sample numbers 30 and 32, respectively) at 1600 ° F. to various austenitic granular structures. Simulate the structural area), perform subsequent quenching in water to avoid pearlite formation, then 2 at 1225 ° F. to bring the steel to the desired hardness range (approximately 180-240 BHN) This was demonstrated by time tempering (sample number 30) and tempering at 1325 ° F. for 2 hours (sample number 32). Both grades of microstructure and hardness level consisted of tempered bainite that has a tendency to spheronize with a hardness in the range of 200-235 BHN. FIG. 4 shows the results of the broaching test performed on the on-line processed grade of the present invention for the various prior art shown previously in FIG. 3 (sample numbers 3, 5, 7, 11 and 14). It is shown in comparison with the ferrite / pearlite grade of the reference line. Broach tool life test results for online optimized grades 5130 and 5046 (sample numbers 30 and 32) are 2-6 times greater than the reported baseline level, which is carburized steel grade and induction hardened Illustrates the success of this approach to optimize broach tool life for both steel grades. Grade 5130 (Sample No. 30) contains 0.29 wt% C and is a steel carburized type, but Grade 5046 (Sample No. 32) contains C 0.47 wt% and is an induction hardening type of steel. Please keep in mind.

  Almost the same composition for baseline grade 5130 (sample numbers 2 and 3) and grade 5046 (sample numbers 6 and 7) with on-line optimized grade 5130 (sample numbers 30 and 31) and grade 5046 (sample number 32) Through direct comparison, attention is directed to Tables I, II and III. Has a lifetime of only 1300 cuts to the limit (sample number 2, as-rolled) and 4600 cuts to the limit (sample number 3, normalized) (both including 0.30 wt% C) Compared to baseline grade 5130, online optimized sample number 30 grade 5130 (C 0.29 wt%) steel has a broach tool life of 9600 cuts to the limit, sample number 31 to the limit Broach tool life of 8800 cuts. Also, the conventional grade 5046 (sample no. 6) with higher carbon levels in the normalized state, containing C 0.46% by weight, had a broach tool life of 600 cuts to the limit. In comparison, grade 5046 (sample no. 32) containing 0.46 wt% C, online processed, tempered and online optimized had a broach tool life of 7600 cuts to the limit. This represents an increase of more than 12 times in broach tool life.

3. Off-line processing A further inventive technique for obtaining the desired microstructure in broachable workpieces includes off-line processing. As used herein, the term “off-line” processing or optimization means a thermal processing method that is sometimes performed after thermal processing to substantially avoid the pearlite phase and promote the bainite / martensite / ferrite phase. . Off-line optimization schemes can be implemented for various steel types and cut surface dimensions to completely suppress or otherwise minimize pearlite formation. Such heat treatment is very often tempered to a suitable broaching hardness range (typically 180-240 BHN). Typically, the offline heat treatment according to the present invention comprises the following steps: austenitizing the test steel part at 1500 ° F. to 1750 ° F .; based on classical steel hardening calculations, for example water to avoid pearlite formation Includes complete or interrupted quenching of the steel part in a suitable quenching agent. After quenching, the desired microstructure (substantially free of pearlite phase) is formed and typically 1100 ° F to 1350 ° F for 1 to 4 hours to provide the desired hardness for broaching of about 180-240 BHN. Tempering (depending on steel type and temperature).

  Table I shows that off-line optimized sample number 40 (grade 8620) containing 0.20 wt% C and sample number 41 (grade 4027) containing 0.27 wt% C were normalized and tested. It shows the highest broach tool life of the samples, i.e. 12,000 cuts to the limit and 11,000 cuts to the limit, respectively. Normalizing involves heating the steel to a temperature above the transition zone and then quenching to room temperature with static air. The microstructure produced for sample numbers 40 and 41 was bainite / ferrite with no pearlite in the microstructure.

  Sample No. 43 (Grade 4150) containing higher carbon at 0.50 wt% C was also subjected to a normalization treatment, followed by tempering at 1340 ° F. for 3 hours, as well as a bainite / ferrite micro that did not contain pearlite. The structure is shown. The workpiece from sample number 43 is compared in a broaching test with a cut number 460 up to the limit of sample number 10 (conventional grade 5150 steel) with carbon content and hardness (Table I) equivalent to sample number 43. This resulted in a broach tool life of 5,800 cuts to the limit.

Offline optimized sample number 42 (grade 5046) containing C 0.47 wt% and sample number 44 (grade 1552) containing C 0.53% wt were austenitized,
Quenched with water, then tempered for 13 hours at 1325 ° F. for sample 42 and 4 hours at 1300 ° F. for sample number 44. Quenching and tempering produced a perlite-free tempered martensite microstructure according to the invention. The broaching life was 8,700 to the limit for sample number 42 and 6,200 to the limit for sample number 44. (This is a significant improvement in these high carbon induction hardened steel types.)
FIG. 5 shows the broach tool life for these respective steels relative to the baseline ferrite / pearlite grade, which is similar to the same trend as the carbon level and over the baseline grade compared to other techniques or Illustrates a greater level of improvement. Thus, it will be appreciated that the offline processing optimization approach represents another technique for optimizing broach tool life in accordance with the present invention.

4). Combinations of optimization techniques Two or more of the optimization techniques 1-3 discussed above can be combined to provide the desired microstructure and properties to the broachable workpiece according to the present invention.

For example, sample numbers 40 and 41 are off-line treated grades 8620 and 4027, discussed previously, and molybdenum additions: Mo 0.21 wt% (sample number 40) and Mo 0.27 wt% (sample number 41) was also subjected to alloy optimization (see Table II). These off-line normalized and alloy modified steels provide the best broach tool life of all the samples tested, which is the effect of the combined alloy and heat treatment optimization techniques of the present invention. Prove that.
Surface Hardening Treatment As discussed hereinabove, reduced hardness to provide internal strength to hardened outer surfaces and members for improved wear resistance of broached gear teeth Providing a core of this is beneficial in powertrain components. Known techniques for surface hardening of broached power transmission system components include carburizing, nitriding and induction hardening, the particular technique used depends on the carbon content of the steel. In general, steel containing less than about 0.32 wt% C has insufficient carbon levels for the heat-induced surface hardening treatment provided by induction hardening. Thus, these steels require additional carbon or other components to allow surface hardening treatments. Steel having a carbon content of less than about 0.32% by weight is typically carburized or nitrided to obtain an enhanced surface hardening treatment.

  The test results discussed previously clearly support that the present invention provides significant improvements in broach tool life for broaching various workpiece steels, and alloy modifications and materials in which these improvements are realized. Several aspects of the processing scheme have been described herein. The microstructure of the target broached workpiece has been shown to be a fine carbide bainite / martensite microstructure that is non-perlite, ideally prone to spheroidization. Alloys and processing schemes for developing target microstructure / hardness combinations in workpiece steel have been shown and described in relation to the sample steel. The data indicates that the present invention is applicable to a wide range of steel compositions and alternative processes, as the ultimate target metallurgical properties are achieved. In addition to the specific examples detailed above, alternating alloy modifications and / or that can be used to obtain a target non-perlite fine carbide microstructure, preferably one that is prone to spheroidization The existence of a steel processing scheme will occur to those skilled in the art. Of course, if these alternate and / or additional methods are also expected to achieve the beneficial broach tool life results detailed herein, such additional modifications will be It is construed to be within the spirit and scope.

Figure 3 is a graph of broach tool life versus carbon content for a baseline and an optimized steel workpiece material of the present invention. 2 is a graph of broach tool life versus hardness for the steel workpiece tested in FIG. Figure 7 is a graph of broach tool life versus carbon content for a modified alloy consisting primarily of fine bainite and a baseline alloy workpiece having a ferrite / pearlite microstructure. FIG. 6 is a graph of broach tool life versus carbon content for an on-line heat-treated workpiece composed of tempered bainite and a baseline alloy workpiece with a ferrite / pearlite microstructure. FIG. 4 is a graph of broach tool life versus carbon content for off-line heat treated workpieces composed of the microstructure of the present invention and baseline alloy workpieces having a ferrite / pearlite microstructure. FIG. 5 is a photomicrograph showing a typical microstructure comprising as-rolled baseline grade 5130 (Sample No. 2) conventional steel pearlite and ferrite. FIG. 5 is a photomicrograph showing a typical microstructure comprising tempered and tempered baseline grade 5150 (Sample No. 10) conventional steel perlite and ferrite. FIG. 2 is a photomicrograph of steel in the as-rolled condition (Sample No. 20), processed according to the present invention, with the alloy optimized and showing the microstructure of acicular bainite. FIG. 4 is a photomicrograph showing a microstructure comprising steel spheroidized bainite and ferrite under interrupted tempered conditions processed online according to the present invention. FIG. 4 is a photomicrograph showing a microstructure comprising spheroidized martensite of steel (sample no. 33) under tempered and tempered conditions processed online according to the present invention.

Claims (40)

  Steel suitable for making broachable workpieces having a substantially pearlite-free microstructure and a hardness of 150-250 BHN prior to broaching and subsequent surface hardening by one of carburizing or induction hardening .   The steel of claim 1, wherein the microstructure comprises one or more of bainite, martensite and ferrite phases and has a hardness of 160-240 BHN.   Steel according to claim 1, having a carbon content of 0.15 to 0.35% by weight and suitable for carburization after broaching.   The steel according to claim 1, having a carbon content of 0.32 to 0.80% by weight and suitable for induction hardening after broaching.   A steel workpiece for broaching having a microstructure substantially free of pearlite and a hardness of 150 to 250 BHN, which is suitable for surface hardening by one of carburizing or induction hardening after broaching .   The steel workpiece of claim 5, wherein the microstructure comprises one or more of bainite, martensite and ferrite phases and has a hardness of about 160-240 BHN.   6. A steel workpiece according to claim 5, wherein the steel is one of the carburized types having a carbon content of 0.15 to 0.35% by weight.   The steel workpiece according to claim 5, wherein the steel is of an induction hardening type having a carbon content of about 0.35 to 0.80 wt%.   A broached steel article suitable for use as a power transmission system component, having a microstructure that is substantially free of pearlite and a broached surface profile, said surface profile being carburized or high frequency Article having a hardened surface applied by one of quenching.   The article of claim 9, which is in the form of a gear or race.   The article of claim 9, wherein the microstructure comprises one or more of bainite, martensite and ferrite phases.   The article of claim 11, which is in the form of a gear or race.   The article of claim 9, wherein the steel has a carbon content of 0.20 to 0.32 wt% and is surface hardened by carburization.   14. The article of claim 13, which is in the form of a gear or race.   The article of claim 9, wherein the steel has a carbon content of about 0.33 to 0.70 wt% and is surface hardened by induction hardening.   The article of claim 15 which is in the form of a gear or race.   In weight%, C0.15-0.65%; Mn0.5-1.75%; S0.010-0.10%; Si0.10-0.70%; Cr0.1-0.50%; Ni0 0.01-1.0%; Mo0.10-0.50%; Al0.001-0.07%; V0.001-0.20%; B0.005-0.03%; Steel compositions that are suitable for broaching, each containing less than 0.05% of incidental additives and impurities.   C0.20 to 0.65%; Mn 0.7 to 1.5%; S 0.015 to 0.07%; Si 0.15 to 0.35%; Cr 0.03 to 0.35%; Ni 0.03 to 0 18. Steel according to claim 17, comprising 0.15 to 0.40% Mo; 0.01 to 0.05% Al; 0.01 to 0.10% V; and 0.007 to 0.025% B. .   C 0.3 to 0.6%; Mn 0.80 to 1.30%; S 0.018 to 0.030%; Si 0.20 to 0.30%; Cr 0.05 to 0.25%; Ni 0.05 to 0 18. Steel according to claim 17 comprising 20%; Mo 0.20-0.30%; Al 0.015-0.04%; V 0.01-0.03%; and B0.010-0.02%. .   C 0.3-0.6% and normal: Mn 1.0%; S 0.025%; Si 0.25%; Cr 0.1%; Ni 0.1%; Mo 0.25%; Al 0.03%; V 0.001% 18. The steel of claim 17 comprising B0.015.   Thermally processed or normalized, including bainite, martensite and / or ferrite, having a microstructure that is substantially free of pearlite and suitable for surface hardening by one of carburizing or induction hardening 18. Steel according to claim 17, which is one of the states. A process for producing a workpiece material that is subjected to surface hardening by one of broaching and subsequent carburizing or induction hardening,
(A) supplying steel;
(B) subjecting the steel to one or more treatments including alloy modification, on-line heat treatment and off-line heat treatment, thereby providing a steel having a microstructure substantially free of pearlite and a hardness of 150-250 BHN prior to broaching; Providing.   23. The method of claim 22, wherein the alloy modification treatment includes adding one or more alloying elements to the steel to inhibit pearlite formation in the steel.   24. The method of claim 23, wherein Mo is added to the steel in an amount of 0.15-0.40% by weight to suppress pearlite formation.   23. The method of claim 22, wherein the on-line heat treatment comprises an austenitizing step and an interrupted quenching step to produce a microstructure comprising bainite, martensite and / or ferrite.   26. The method of claim 25, comprising a tempering step after the quenching step.   27. The method of claim 26, wherein the tempering step is performed at a temperature of 1100 <0> F to 1340 <0> F (593 [deg.] C to 727 [deg.] C) for 1 to 4 hours.   23. The method of claim 22, wherein the off-line heat treatment comprises one of normalizing, normalizing and tempering, and austenitizing, quenching and tempering.   29. The method of claim 28, wherein the steel is one of being normalized to produce a microstructure comprising bainite and ferrite, or normalized and tempered.   29. The method of claim 28, wherein the steel is austenitized, quenched and tempered to produce a microstructure comprising tempered martensite.   29. The method of claim 28, wherein the tempering in the off-line heat treatment is performed at 1200 [deg.] F to 1340 [deg.] F (649 [deg.] C to 727 [deg.] C) for 1 to 4 hours.   23. The method of claim 22, comprising forming the steel into a tubular shape that is hot rolled to provide a workpiece for broaching the power transmission system gear and race.   23. The method of claim 22, wherein the steel is subjected to at least two of the treatments. (A) supplying molten steel having a carbon content of about 0.15 to 0.80 wt% C and optionally subjecting the molten steel to an alloy modification treatment to inhibit the formation of a pearlite phase; Process and;
(B) subjecting the steel to one or more treatments including on-line heat treatment and off-line heat treatment to inhibit the formation of pearlite phase;
(C) providing a steel workpiece for broaching having a microstructure substantially free of pearlite and a hardness of 150-250 BHN;
(D) broaching the steel workpiece to form a broached power transmission component;
(E) A method of manufacturing a power transmission system component including a step of surface hardening a power transmission system component that has been broached by selecting one of carburization or induction hardening.   35. The method of claim 34, wherein the steel comprises from 0.2 to about 0.32 wt% C and the selected surface hardening step is carburization.   36. The method of claim 35, wherein the steel comprises greater than C0.32 wt% to C0.65 wt%, and the selected surface hardening step is induction hardening.   35. The method of claim 34, wherein the optional alloy modification treatment in step (a) comprises adding 0.10 to 0.50 wt% Mo to the molten steel. (A) supplying molten steel having a carbon content of about 0.15 to 0.80 wt% C;
(B) subjecting the molten steel to an alloy modification treatment to inhibit the formation of a pearlite phase;
(C) subjecting the steel to one or more of an on-line heat treatment and an off-line heat treatment to inhibit the formation of a pearlite phase to provide a workpiece for broaching having a microstructure that is substantially free of pearlite;
(D) broaching the workpiece to form a broached power transmission component;
(E) A method of manufacturing a power transmission system component including a step of surface hardening the broached power transmission system component by one of carburizing, nitriding, or induction hardening.   40. The method of claim 38, wherein the steel comprises 0.2 to about 0.32 wt% C and the surface hardening step is one of carburizing or nitriding.   39. The method of claim 38, wherein the steel comprises C0.35-0.65% by weight and the selected surface hardening step is induction hardening.