Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/06—Surface hardening
  • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
  • C21D1/10—Surface hardening by direct application of electrical or wave energy; by particle radiation by electric induction
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0257—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/25—Process efficiency

Definitions

• the present invention relates generally to ferrous metallurgy and, more particularly, to steel compositions and microstructures used for making articles such as, for example, powertrain gears, races and like parts that are formed by broaching and hardened by carburizing or induction hardening. Still more particularly, the invention is directed to techniques for obtaining an optimized steel metallurgy microstructure which provides a steel workpiece material that significantly improves broach tool life, thus lowering the manufacturing costs per part. The present invention also relates to methods for obtaining the desired microstructures and properties in the steel workpiece as well as to the finished article.
• Broaching is a machining technique commonly used to cut gear teeth or cam profiles for the high volume manufacture of powertrain parts, such as for automotive transmissions and the like.
• the part profiles can be formed in a single broaching operation with minimal overall time, making it ideal for such a cost-sensitive application.
• the broach machine in order to accomplish the broaching operation in a single station, the broach machine must perform the entire roughing, shaping and finishing of the desired part profile using one long, high-speed steel broach tool which removes metal from a workpiece in a single motion.
• the broach tool is relatively expensive to manufacture and can only be redressed or sharpened a fixed number of times before the tool is no longer usable.
• broaching and tooling cost per manufactured part is highly dependent upon the number of parts that can be manufactured between broach tool redressings.
• the cost per part is typically in the range of $0.60 to $5.00.
• the broach tooling cost represents around 15% to 50% of the total manufacturing cost for a finished part. Therefore, whereas broaching represents a time and plant space efficient method to cut profiles into annular steel parts, the tooling cost to perform this operation represents a significant portion of the total manufacturing cost.
• the prior approaches also include lowering of the alloy content to lower the hardness level and abrasiveness of the workpiece steel; increasing tempering temperature to lower the hardness level; and lowering carbon level or changing heat treat method to allow for the use of lower carbon, less abrasive steels.
• Workpiece steels used to manufacture profiled parts for powertrain gears and races can be broadly categorized as either carburizing/nitriding or induction hardening types, depending upon the method used to harden the load bearing surfaces of the finished part. Steels have historically been chosen broadly based upon the hardening method and subsequent carbon level, and heat treat hardenability requirements for the hardened surface in an effort to minimize cost to manufacture the part.
• the conventional workpiece steels so selected have been processed and/or heat treated to develop a ferrite/pearlite type microstructure, typically to a narrower aim/range within the overall broachable hardness range of 150 to 300 BHN.
• the main techniques for obtaining localized surface hardening are carburizing, nitriding and induction hardening, depending upon the type of steel used to make the part.
• steel grades having less than about 0.32 wt.% C possess insufficient carbon at the surface to provide thermal hardening. Accordingly, this steel type is usually carburized (or nitrided) to diffuse a carbon-enriched layer (or nitrogen) in the load bearing surface of the part to permit subsequent thermal/heat treat hardening.
• steels having greater than 0.32 wt.% C such as grades having about 0.35 to 0.80 wt.% C, may be surface hardened using the induction heating technique.
• the high frequency magnetic field of the inductor heats the surface layer of the broached part in a matter of mere seconds to a desired austenitizing temperature of, say, 1700-1800°F and the heated surface is then immediately quenched in water or other quench media. Since carburizing (and nitriding) usually is conducted in a controlled atmosphere furnace for 5- 10 hours, carburizing and nitriding are considerably more time-consuming and expensive than the induction hardening technique.
• Applicants are aware of prior work conducted in the industry to provide a broachable steel within the desirable broaching hardness range of about 150 to 240 BHN.
• This steel possesses a non-pearlitic microstructure obtained by off-line thermal processing performed as a means of optimizing the surface hardening response and not as a means of optimizing broach tool life.
• This steel has a carbon range of between 0.25 to 0.31 wt.% and is subjected to nitriding heat treatment after broaching to achieve surface hardening for transmission gears.
• Applicants are further aware of prior work involving broached powertrain parts that require a high core hardness prior to broaching to develop the desired mechanical properties for the part.
• the hardness range for these parts is typically from 250 to 300 BHN or higher. Most steels cannot achieve this hardness range while maintaining a pearlitic microstructure.
• the most common method to increase hardness to a level within this range is to perform an off-line heat treatment (quench and temper) to develop a non-pearlitic bainite/martensite tempered structure.
• the reason for obtaining this non-pearlitic structure is to develop the core hardness range and not to optimize broaching. Indeed, at these high hardness levels of 250 to 300 BHN or higher, broach tool life is sacrificed for high hardness.
• the present invention solves the problems of the prior art by providing optimized steel microstructures in a workpiece for the manufacture of articles by broaching, which yield greatly improved broach tool life.
• the articles are useful as automotive, truck, tractor and like powertrain parts such as, for example, transmission gears and races.
• the present invention provides a steel for a workpiece that includes carburizing, nitriding and induction hardening types which significantly increases broach tool life for these steel types.
• the present invention contemplates an optimized steel metallurgy for the workpiece to be broached, wherein the microstructure substantially eliminates the presence of pearlite in favor of bainite, martensite and/or ferrite.
• the present invention provides this optimal steel metallurgy in a workpiece for broaching by at least one or more of the following processing techniques: (a) modification of the steel alloy composition to suppress pearlite formation; (b) on-line thermal processing to suppress pearlite formation; (c) off-line heat treatment to suppress the formation of the pearlite phase; and/or (d) combinations of one or more of the above techniques or use of other techniques to achieve the desired microstructure comprising bainite and/or tempered martensite with substantially no pearlite phase.
• a further aspect of the invention provides a steel workpiece suitable for surface hardening subsequent to broaching by one of carburizing, nitriding or induction hardening wherein the steel microstructure is substantially free of pearlite by modification of the steel alloy composition coupled with one or both of on-line or off-line thermal processing.
• a preferred hardness range for the workpiece material is between about 160 to 250 BHN and, more preferably, between about 170 to 245 BHN and, still more preferably, between about 180 to 240 BHN.
• the workpiece steel has a carbon content of between about 0.15-0.80 wt.% and, more preferably, between about 0.18-0.70 wt.% C, which, thus, includes carburizing, nitriding and induction hardening types of steel.
• One aspect of the invention contemplates a carbon content of about 0.15-0.35 wt.% and preferably between about 0.20-0.32 wt.% C to permit the broached workpiece to be carburized.
• a still further aspect of the invention includes a workpiece steel for broaching having about 0.32-0.80 wt.% C, more preferably between about 0.33-0.70 wt.% C, and still more preferably between about 0.35-0.65 wt.% C to permit the broached workpiece to be induction hardened.
• the invention includes the steel material for making the workpiece to be broached, the steel workpiece to be broached, the finished article made from the steel workpiece, as well as the process for making same.
• Fig. 1 is a graph of broach tool life versus carbon content for baseline and optimized steel workpiece materials of the present invention
• Fig. 2 is a graph of broach tool life versus hardness for the steel workpieces tested in Fig. 1 ;
• Fig. 3 is a graph of broach tool life versus carbon content for modified alloys composed primarily of fine bainite and baseline alloy workpieces having a ferrite/pearlite microstructure;
• Fig. 4 is a graph of broach tool life versus carbon content for on-line thermally treated workpieces composed of tempered bainite and baseline alloy workpieces having ferrite/pearlite microstructures;
• Fig. 5 is a graph of broach life versus carbon content for off-line heat treated workpieces composed of microstructures of the present invention and baseline alloy workpieces having ferrite/pearlite microstructures;
• Fig. 6 is a photomicrograph of a conventional steel, baseline grade 5130 as rolled (Sample No. 2) exhibiting a typical microstructure containing pearlite and ferrite;
• Fig. 7 is a photomicrograph of a conventional steel, baseline grade 5150 normalized and tempered (Sample No. 10) exhibiting a typical microstructure containing pearlite and ferrite;
• Fig. 8 is a photomicrograph of a steel in an as-rolled condition (Sample No. 20) treated according to the invention, alloy optimized, exhibiting an acicular bainite microstructure;
• Fig. 9 is a photomicrograph of a steel in an interrupted and tempered condition (Sample No. 30) treated on-line according to the invention exhibiting a microstructure containing spheroidized bainite and ferrite;
• Fig. 10 is a photomicrograph of a steel in a quenched and tempered condition (Sample No. 33) treated on-line in accordance with the invention, exhibiting a microstructure containing spheroidized martensite.
• a laboratory broach testing machine was devised and built to enable economical and efficient testing and comparison of the effects of metallurgical variables on broach tool life.
• the laboratory test utilized a reciprocating 3-tooth, high-speed steel broach tool made from M4 tool steel. This broach tool rapidly and efficiently cut the equivalent steel volumes that a typical broach tool contained in a production broach bar would cut over many parts.
• the laboratory test machine and procedure were designed to closely mimic a production broaching environment in all possible aspects, including the broach tool material and its heat treatment, the tool tooth design, the cutting depth per tooth, the cutting speed, the cutting lubricant type and lubricant delivery system, as well as the tool wear criterion limits.
• the broach tool was repeatedly pulled through the inner diameter of a ring-shaped, annular test steel workpiece utilizing an indexing table to properly position the annular test workpiece for each cut, employing the specific parameters established for each of the listed variables.
• the broach tool was periodically measured for wear and the laboratory test was deemed to be complete when the previously established wear criterion was met, assigning a number of cuts to broach tool failure for each material workpiece condition.
• This laboratory broach test was performed on a large variety of steel types and conditions, with each steel/condition rated against one another based upon the number of cuts to the specific wear criterion limit.
• the database generated from this extensive testing reported in Table I indicates that an optimal workpiece steel broaching condition or microstructure/hardness exists for all steels, and that most steels are currently not being broached in this condition.
• the test also revealed the non-optimal workpiece steel condition for broaching and, surprisingly, that most steels are currently being broached in that non-optimal condition or in a slightly modified version of that condition.
• the steel alloy compositions reported in Table II, steel processing and heat treatment schemes reported in Table II have also been developed in accordance with the present invention that allow for development of optimized workpiece steel conditions for broaching over a wide range of overall parameters.
• V represents vanadium modified
• R represents resulfurized TABLE II COMPOSITIONS OF THE WORKPIECE STEELS BROACH TESTED
• V vanadium modified
• R represents resulfurized. Alloy optimized grade designations created by the inventors.
• Austenitizing performed at standard temperatures (based on carbon level) and times (based on section size), and air cooling for normalized entries. Hot rolling performed at standard temperatures, based on carbon level. Interrupt quench was sufficient to avoid pearlite formation, and varied in time based on grade and section size.
• the steel conditions tested included: as hot worked and air cooled; hot worked and slow cooled; fine and coarse grain normalized; fine and coarse grain normalized and tempered; quenched and tempered; interrupted quench and tempered; and annealed. Typical parameters for these conditions are as follows.
• the steel samples (Nos. 1-15, 30-33 and 40-44) reported in Tables I-III were melted in a production electric furnace operation and ladle refined in accordance with the compositions set forth in Table II.
• the steel samples were continuously cast into blooms and then heated to 2250 0 F, hot rolled into round billets, subsequently reheated to 2250 0 F and pierced to provide an as-rolled tubular shape.
• the tube was then cut to form a ring-shaped, annular steel workpiece for testing or for further thermal treatment in accordance with Table III prior to test broaching.
• the resultant microstructures for the various steel samples included a mixture of ferrite, pearlite, bainite and/or martensite in both tempered and non-tempered versions with varying degrees of carbide spheroidization.
• the hardness levels of the various steels tested ranged from 150 BHN to 330 BHN, with most of the hardness levels being in the broachable range from 160 BHN to 260 BHN.
• a presently preferred broachable hardness range is from about 150 to 250 BHN and, more preferably, between 175 to 240 BHN.
• a baseline workpiece steel (Sample No. 1) having a carbon content of 0.20 wt.% and a Brinell hardness of 210 BHN yielded a broach tool life of 6,200 (cuts to limit).
• the baseline steel carbon content increased to 0.6 wt.% (Sample No. 15)
• the Brinell hardness rose to 252 BHN
• the broach tool life decreased to 80 (cuts to limit).
• the microstructure of these conventional baseline workpiece steel grades was predominantly ferrite and pearlite, as shown in Fig. 6 and in Fig. 7. In the photomicrographs of Figs. 6 and 7, the ferrite appears as the light regions and the pearlite appears as the darker regions.
• Fig. 6 shows a baseline 5130 grade steel (Sample No. 2) microstructure containing ferrite and pearlite in the as rolled condition and Fig. 7 depicts a baseline 5150 grade (Sample No. 10) microstructure containing ferrite and pearlite in the normalized and tempered condition.
• Sample No. 10 shows a baseline 5130 grade steel (Sample No. 2) microstructure containing ferrite and pearlite in the as rolled condition
• Fig. 7 depicts a baseline 5150 grade (Sample No. 10) microstructure containing ferrite and pearlite in the normalized and tempered condition.
• These ferrite and pearlite microstructures are typically specified as an acceptable broaching microstructure for most conventional broaching applications.
• the present invention encompasses the discovery that at the same carbon and hardness levels as shown in the baseline steels in Table I, broach tool life can be increased by 2 to 10 times by altering some stage of the metallurgical processing to suppress or minimize formation of the pearlitic microstructure and, in its place, forming a finer acicular carbide microstructure which preferably may tend towards spheroidization.
• the preferred microstructure of the invention is substantially pearlite- free and consists of either predominantly bainite and/or martensite and ferrite, and may be tempered into a desired hardness range of 160 to 260 BHN at higher carbon levels.
• Some pearlite phase may be present, up to a maximum of about 20% lamellar pearlite by volume, and preferably no more than 10% lamellar pearlite by volume maximum, and still more preferably no more than 5% lamellar pearlite by volume.
• substantially no lamellar pearlite is present in the microstructure.
• substantially pearlite-free or minimal pearlite means a microstructure containing up to about 20% by volume lamellar pearlite and, more preferably, 0%, unless otherwise qualified.
• a desired acicular bainite microstructure is shown in Fig. 8; a desired spheroidized bainite microstructure is shown in Fig.
• Fig. 8 is an alloy modified 4030 grade (Sample No. 20) in the as rolled condition; Fig. 9 is an on-line quenched and tempered 5130 grade (Sample No. 30); and Fig. 10 is an on-line quenched and tempered grade 5150 steel (Sample No. 33), all produced according to the present invention.
• the substantially pearlite-free microstructures of the invention containing one or more of bainite, martensite, and ferrite with zero or minimal pearlite are formed by a variety of techniques including, but not necessarily limited to, (1) modification of the alloy makeup to suppress pearlite formation in the hot worked, air-cooled condition (Sample Nos. 20-23); and/or (2) on-line hot processing to suppress the pearlite formation on cooling from hot working temperatures (Sample Nos. 30-33); and/or (3) off ⁇ line heat treatment to suppress the pearlite formation (Sample Nos. 40-44); or by some combination of these techniques, or by other techniques such as by isothermal transformation below pearlite formation temperatures.
• the final bainite/martensite/ferrite, substantially pearlite-free microstructure can, of course, then be tempered into the desired broaching hardness range, if desired.
• the resultant effect of this microstructure modification can be observed by comparing the previously mentioned broach tool life trends for various carbon and hardness levels having a predominantly pearlitic microstructure with the optimized broach tool life results for similar steels substantially pearlite-free within the same carbon levels (Fig. 1) and hardness ranges (Fig. 2).
• the improvement in broach tool life is on the order of 200% to 1000%, comparing the same or similar initial carbon contents and broach tool life, all within similar hardness ranges.
• the broach tool life of prior art baseline 5120 grade workpiece steel (Sample No. 1) having 0.20% C with a ferrite/pearlite microstructure yielded a broach tool life of 6200 (cuts to limit).
• Obtaining the optimal steel workpiece condition for broaching can be realized in accordance with the invention by any individual or combination of the three aforementioned techniques including, but not necessarily limited to: (1) modification of the alloy makeup to suppress or minimize pearlite formation in the hot worked, air cooled condition; (2) designing the on-line hot processing to suppress the pearlite formation on cooling from the hot working temperature; and/or (3) off-line heat treatment to suppress the pearlite formation, followed by a temper operation, when necessary.
• Alloy compositions can be designed to suppress pearlite formation by the addition of several potential chemical elements and addition levels, individually or in combinations, depending upon the base steel composition.
• An example of the alloy modification approach was demonstrated by melting a series of laboratory vacuum induction melted heats at four carbon levels (0.3%C, 0.41 %C, 0.51 %C and 0.62%C), where approximately 0.25% Mo was added to a base carbon and manganese composition to suppress the pearlite formation. It will be understood, unless noted otherwise, that all percentages are in % by weight. More specifically, the alloy compositions for these sample Nos.
• Example Nos. 20-2 graphically depicts the broach test results for the alloy modified inventive steels (Sample Nos. 20-23) in comparison to a conventional baseline set of ferrite/pearlite containing steels (Samples 3, 5, 7, 11 and 14) covering a similar carbon range of 0.3 to 0.60% C.
• the baseline steels exhibit a steady decrease in broach tool life with increasing carbon level from 4600 cuts to limit at 0.30% C (Sample No. 3), down to only approximately 200 cuts to limit at 0.60% C (Sample No. 14).
• the alloy modified steels of the invention show 9500 cuts to limit at 0.41% C (Sample No. 21), and then nearly 4000 cuts to limit at 0.62% C (Sample No. 23).
• the modified steel composition of the present invention shows well in excess of an order of magnitude improvement in broach tool life over the conventional baseline steel at comparable carbon levels. Therefore, an alloying modification approach of the invention to optimize the microstructure and broach tool life of grades with varying carbon levels has been demonstrated to be successful and effective, as evidenced by the test data.
• the second technique for improving broach tool life according to the invention involves on-line processing to achieve the desired microstructure.
• On-line thermal treatment involves hot working and cooling, using thermomechanical processing schemes that can be devised whereby the unwanted pearlite phase is suppressed upon cooling from hot working, such as hot rolling, without having to modify the steel composition at all or to a lesser extent.
• the term "on-line" optimization or processing as used herein means a thermomechanical steel process scheme directly coupled with the final hot working operation (rolling, forging, etc.) whereby the pearlite phase is avoided or minimized and the bainite/martensite/ferrite phases are promoted.
• a further inventive technique for obtaining a desired microstructure in a broachable workpiece involves off-line processing.
• off-line processing or optimization as used herein means a thermal processing method performed at some time subsequent to hot working whereby the pearlite phase is substantially avoided and the bainite/martensite/ferrite phases are promoted.
• the off-line optimization scheme can be performed on a wide variety of steel types and section sizes to totally suppress or otherwise minimize pearlite formation. Such thermal treatments are most often followed by tempering into the appropriate broaching hardness range, which typically is 180 to 240 BHN.
• Off-line heat treatments usually involve the following steps: austenitizing the steel test section at 1500 0 F to 1750 0 F; fully or interrupted quenching of the steel section in the appropriate quench media, such as water, to avoid pearlite formation based on classical steel hardenability calculations. Quenching is normally followed by tempering at 1100° to 135O 0 F for 1 to 4 hours (depending on steel type and temperature) to form the desired microstructure (substantially free of pearlite phase) and to provide a desired hardness for broaching between about 180 - 240 BHN.
• Table I shows that off-line optimized Sample Nos. 40 (grade 8620) containing 0.20 wt.% C and 41 (grade 4027) containing 0.27 wt.% C were in the normalized condition and provided the highest broach tool life of any of the samples tested, viz., 12,000 cuts to limit and 11 ,000 cuts to limit, respectively. Normalizing involves heating the steel to a temperature above the transformation range and then cooling in still air at room temperature. The resultant microstructure for Sample Nos. 40 and 41 was a 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 normalizing treatment and subsequently tempered at 134O 0 F for 3 hours and, likewise, exhibited a bainite/ferrite microstructure, free of pearlite.
• the workpiece from Sample No. 43 yielded a broach tool life of 5,800 cuts to limit in the broaching test, compared to 460 cuts to limit for Sample No. 10 (conventional grade 5150 steel) of comparable carbon content and hardness (Table I) to Sample No. 43.
• Fig. 5 shows the broach tool life for each of these steels versus the baseline ferrite/pearlite grades, which illustrates the same trends with carbon level and similar or greater levels of improvement over the baseline grades, as compared to the other techniques. Accordingly, it will be understood that the off-line processing optimization approach represents another technique to optimize broach tool life according to the present invention.
• Two or more of the above-discussed optimization techniques 1-3 can be combined to provide the microstructure and properties desired in a broachable workpiece according to the present invention.
• Sample Nos. 40 and 41 were off-line treated grades 8620 and 4027, discussed above, and were also subjected to alloy optimization with Mo additions: 0.21 wt.% Mo (Sample No. 40) and 0.27 wt.% Mo (Sample No. 41), see Table II. These off-line normalized alloy modified steels provided the highest broach tool life of all samples tested, thus evidencing the effectiveness of the combined alloy and heat treatment optimization techniques of the invention.
• broached powertrain components it is beneficial in powertrain components to provide a hardened outer surface for improved wear resistance of the broached gear teeth and a lowered hardness core to provide internal toughness to the part.
• Known techniques for surface hardening of broached powertrain components include carburizing, nitriding and induction hardening, and the particular technique employed is dependent upon the carbon content of the steel.
• steels containing less than about 0.32 wt.% C have insufficient carbon levels for thermally induced surface hardening as provided by induction hardening. Thus, these steels require additional carbon or other constituent to permit surface hardening.
• Steels having carbon contents less than about 0.32 wt.% are generally carburized or nitrided to obtain enhanced surface hardening.
• the target broaching workpiece microstructure has been shown to be a non-pearlitic, fine carbide bainite/martensite microstructure ideally tending towards spheroidization. Alloy and process schemes to develop the target microstructure/hardness combinations in the workpiece steels have been shown and described in connection with the above sample steels.

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