Patent application title: BAINITIC MICROALLOY STEEL WITH ENHANCED NITRIDING CHARACTERISTICS
Kenneth W. Burris (West Peoria, IL, US)
Arash Tahamtan (Peoria, IL, US)
Ryan Adam Dusheke (Peoria, IL, US)
IPC8 Class: AC23C826FI
Class name: Carburizing or nitriding using externally supplied carbon or nitrogen source iron(fe) or iron base alloy with working, machining, or cutting
Publication date: 2014-09-25
Patent application number: 20140283954
A forged, microalloyed, and nitrided steel part is disclosed to have a
composition including 0.20-0.40 wt. % C, 0.50-1.60 wt. % Mn, 0.40-1.50
wt. % Cr, 0.07-0.30 wt. % Al, 0.03-0.20 wt. % V, 0.10-0.40 wt. % Si, and
a balance of Fe and incidental impurities. The part may be produced by
heating the steel part to austenization temperature of approximately 1100
degrees C. to 1260 degrees C., hot forging the steel part, controlled air
cooling the steel part after hot forging at a rate falling approximately
in the range from 1 degree C. per second to 5 degrees C. per second as
the steel part cools from approximately 900 degrees C. to approximately
500 degrees C. to produce a predominantly bainitic microstructure of
greater than approximately 50% bainite. The steel part may then be
machined to a desired configuration, and nitrided by heating in an
atmosphere containing ammonia.
1. A forged steel part with a surface nitrided layer formed by nitriding
following hot forging and controlled air cooling without heat treatment
and having a composition comprising: C: 0.20-0.40 wt. %, Mn: 0.50-1.60
wt. %, Cr: 0.40-1.50 wt. %, Al: 0.07-0.30 wt. %, V: 0.03-0.20 wt. %, Si:
0.10-0.40 wt. %, and a balance of Fe and incidental impurities.
2. The forged steel part of claim 1, wherein the steel part is heated to approximately 1100 degrees C. to 1260 degrees C. before being hot forged.
3. The forged steel part of claim 1, wherein a microstructure of the forged steel part after the controlled air cooling is greater than 50% by volume bainite.
4. The forged steel part of claim 1, wherein a microstructure of the forged steel part after the controlled air cooling is greater than 70% by volume bainite.
5. The forged steel part of claim 1, wherein a microstructure of the forged steel part after the controlled air cooling is greater than 85% by volume bainite.
6. The forged steel part of claim 1, wherein a microstructure of the forged steel part after the controlled air cooling is greater than 85% by volume bainite at a surface of the part and greater than 85% by volume bainite at a core of the part.
7. The forged steel part of claim 1, wherein at least a surface of the forged steel part has a Rockwell C hardness (HRC) of at least 25 after the controlled air cooling.
8. The forged steel part of claim 1, wherein the hardness throughout the forged steel part falls within a range between approximately 25 HRC to 30 HRC.
9. The forged steel part of claim 1, wherein the controlled air cooling following forging is performed at a rate approximately within the range from 1 degree C. per second to 5 degrees C. per second between approximately 900 degrees C. and 500 degrees C.
10. A forged steel part manufactured to have a chemical composition comprising: C: 0.20-0.40 wt. %, Mn: 0.50-1.60 wt. %, Cr: 0.40-1.50 wt. %, Al: 0.07-0.30 wt. %, V: 0.03-0.20 wt. %, Si: 0.10-0.40 wt. %, and a balance of Fe and incidental impurities; and the forged steel part being manufactured by hot forging, controlled air cooling after the hot forging with no further heat treatment to produce a predominantly bainitic microstructure of greater than 50% bainite throughout the forged steel part, machining, and nitriding.
11. The forged steel part of claim 10, further including the controlled air cooling after hot forging including air cooling at a rate in the range from approximately 1 degree C. per second to 5 degrees C. per second as the forged steel part cools from a temperature of approximately 900 degrees C. to a temperature of approximately 500 degrees C.
12. The forged steel part of claim 10, further including the nitriding enhanced by V and Al left over after the controlled air cooling reacting with nitrogen (N) diffused into a surface of the part during the nitriding.
13. The forged steel part of claim 10, wherein a portion of the V and Al reacts with N that is dissolved into the steel to form fine precipitates of VN and AN that strengthen the steel, and a majority of the V and Al combines with N diffused into the surface of the part during the nitriding for further strengthening of the steel.
14. A method of producing a forged steel part, comprising: subjecting a steel billet to hot forging and then to nitriding without heat treatment after the hot forging, the steel having a composition comprising, on a weight basis: C: 0.20-0.40 wt. %, Mn: 0.50-1.60 wt. %, Cr: 0.40-1.50 wt. %, Al: 0.07-0.30 wt. %, V: 0.03-0.20 wt. %, Si: 0.10-0.40 wt. %, and a balance of Fe and incidental impurities.
15. The method of claim 14, further including heating the steel billet to an austenization temperature of approximately 1100 degrees C. to 1260 degrees C. before the hot forging.
16. The method of claim 14, further including: air cooling the forged steel part after the hot forging including controlling the rate of air cooling to fall within a range from approximately 1 degree C. per second to 5 degrees C. per second as the forged steel part cools between approximately 900 degrees C. and 500 degrees C.
17. The method of claim 16, wherein the air cooling occurs on a conveyor belt at ambient temperature.
18. The method of claim 16, wherein the air cooling is performed by the forced circulation of air over the forged steel part.
19. The method of claim 14, wherein the microstructure of the forged steel part after the air cooling is greater than 50% by volume bainite.
20. The method of claim 14, wherein the microstructure of the forged steel part after the air cooling is greater than 70% by volume bainite.
21. The method of claim 14, wherein the microstructure of the forged steel part after the air cooling is greater than 85% by volume bainite.
 The present disclosure relates generally to a bainitic microalloy steel and, more particularly, to a bainitic microalloy steel with enhanced nitriding characteristics.
 Drivetrain components such as shafts, couplings, gears, cams, and sprockets are frequently subjected to high pressures, torque loads, and impact loading. A nitrogen case hardening process which is termed "nitriding" typically consists of subjecting machined and heat-treated parts to the action of a nitrogenous medium, commonly ammonia gas, at a temperature of about 510 degrees C. (950 degrees F.) to 538 degrees C. (1,000 degrees F.). Nitriding increases surface hardness, wear resistance, and resistance to certain types of corrosion and surface stresses that improve the fatigue resistance of a nitrided part. Accordingly, nitrided alloy steel articles are often used for gears, couplings, shafts and other applications that require resistance to wear and high stress loading.
 A group of hardenable alloy steels that have been nitrided after heat treating are the AISI/SAE 4100 series alloy steel. In particular, AISI/SAE 4140H alloy steel has been found to be useful in the manufacture of various gears that require a combination of high surface hardness and core hardness. AISI/SAE 4140H alloy steel has a specified composition as follows: Carbon 0.37-0.44% by weight; Manganese 0.65-1.10% by weight; Silicon 0.15-0.35% by weight; Chromium 0.75-1.20% by weight; Molybdenum 0.15-0.25% by weight; Iron and acceptable trace elements making up the balance.
 Typically, parts having the above composition are first forged, or rolled from billets, and are quenched and tempered, then machined and nitrided. Although AISI/SAE 4140H alloy steel has been useful in certain nitriding applications, it also has some disadvantages. For example, this steel contains molybdenum, an expensive alloying element. Further, it has been found that articles having the AISI/SAE 4140H composition are prone to quench cracking and therefore generally require an oil quench. Still further, the nitrided case hardness of AISI/SAE 4140H is generally limited to about Rockwell C Hardness (HRC) 55 or less.
 An alloy steel composition for the manufacture of non-hardened parts by a process called nitempering has been disclosed for producing low-distortion steel parts such as gears and other transmission components. Core hardness is achieved by alloy addition rather than by heat treatment. After machining from an as-rolled steel bar or forging, workpieces formed of the alloy steel are nitempered. Nitempering, also known in Japan as "soft nitriding", is faster than conventional nitriding and develops an extremely hard skin on steels and cast irons. In nitempering, parts are treated at 566 degree C. (1050 degree F.) for two to six hours in an atmosphere comprising equal parts of endogas (a reducing gas mixture such as carbon monoxide and hydrogen) and ammonia. The hard case that results contains a complex iron-carbon-nitrogen compound. However, the case produced is thinner than that obtained by nitriding, and the increase in toughness resulting from a pre-nitriding heat treatment of the workpiece is not obtained. Further, the increased cost of alloy additions to achieve a core hardness comparable to quenched and tempered steel is economically undesirable.
 One attempt to produce an economical, through hardening nitriding grade alloy steel is described in U.S. Pat. No. 4,853,049 to Calvin Loyd ("the '049 patent") that issued on Aug. 1, 1989. The '049 patent discloses a through hardening nitriding grade alloy steel that is economical because it has eliminated expensive alloys such as molybdenum. The steel disclosed in the '049 patent achieves desirable characteristics including hardenability, resistance to loss of hardness during tempering, and greatly enhanced response to nitriding through the use of small, carefully controlled amounts of Aluminum (Al) and Vanadium (V).
 Although the alloy steel disclosed in the '049 patent reduces the costs of the alloy steel and provides the beneficial characteristics described above, still further improvements in costs may be possible. In particular, the '049 patent describes multiple heat treatment processes after the initial hot forging of products produced from the disclosed alloy steel. These heat treatment processes include hardening the forged products by heating to a temperature of about 870 degree C. (1600 degree F.) for a period of about one hour and then quenching in either water or oil to complete transformation of the ferrite and pearlite microstructure to martensite. After tempering to precipitate and agglomerate the carbide particles and thereby provide improved toughness, the articles, if required, are machined to a desired final dimension and then nitrided.
 The bainitic microalloyed steel produced in accordance with the chemistry and processes of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.
 In one aspect, the present disclosure is directed to a forged steel part with a surface nitrided layer formed by nitriding following forging and controlled air cooling without heat treatment and having a composition comprising:
 C: 0.20-0.40 wt. %,
 Mn: 0.50-1.60 wt. %,
 Cr: 0.40-1.50 wt. %,
 Al: 0.07-0.30 wt. %,
 V: 0.03-0.20 wt. %,
 Si: 0.10-0.40 wt. %, and
 a balance of Fe and incidental impurities.
 In a further aspect, the present disclosure is directed to a forged steel part manufactured to have a chemical composition comprising:
 C: 0.20-0.40 wt. %,
 Mn: 0.50-1.60 wt. %,
 Cr: 0.40-1.50 wt. %,
 Al: 0.07-0.30 wt. %,
 V: 0.03-0.20 wt. %,
 Si: 0.100.40 wt. %, and
 a balance of Fe and incidental impurities; and the forged steel part being manufactured by hot forging, controlled air cooling after the hot forging with no further heat treatment to produce a predominantly bainitic microstructure of greater than 50% bainite throughout the forged steel part, machining, and nitriding.
 In yet another aspect, the present disclosure is directed to a method of producing a forged steel part. The method may include subjecting a steel billet to hot forging and then to nitriding without heat treatment after the hot forging, the steel having a composition comprising, on a weight basis:
 C: 0.20-0.40 wt. %,
 Mn: 0.50-1.60 wt. %,
 Cr: 0.40-1.50 wt. %,
 Al: 0.07-0.30 wt. %,
 V: 0.03-0.20 wt. %,
 Si: 0.10-0.40 wt. %, and
 a balance of Fe and incidental impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a schematic diagram of an exemplary disclosed process eliminating typical heat treating steps;
 FIG. 2 is a continuous cooling transformation (CCT) diagram for producing the microstructure of an exemplary embodiment of the disclosure; and
 FIG. 3 is a flowchart depicting an exemplary disclosed method that may be used to produce an exemplary microalloyed steel with enhanced nitriding characteristics.
 A microalloyed, air-hardenable, predominantly bainitic steel with enhanced nitriding characteristics is disclosed. The microalloyed, bainitic steel may be economically produced without requiring many of the heat treatment processes previously thought necessary to achieve desired hardness, toughness, and strength characteristics prior to nitriding for increased surface hardness. As shown in FIG. 1, heat treatment processes following hot forging of a steel part may include cooling, reheating to austenization temperature, quenching, and tempering. These heat treatment steps may be required with conventional hot forging processes in order to obtain desired strength and toughness characteristics, while at the same time ending up with a reasonably free-machining part that is not too hard for machining. Significant cost savings may be achieved if at least some of these intermediate heat treatment processes can be eliminated. Capital investments for heat treatment capacity, and maintenance costs on the furnaces and other equipment may be reduced.
 A predominantly bainitic microstructure according to various implementations of this disclosure is a microstructure that consists of at least 50% by volume of a bainitic microstructure. Certain embodiments may have at least 70% by volume of a bainitic microstructure. Other embodiments may have at least 85% by volume of a bainitic microstructure. Bainite is a microstructure that forms in steels at temperatures of approximately 250-550 ° C. (depending on alloy content). Bainite is one of the decomposition products that may form when austenite (the face centered cubic crystal structure of iron) is cooled past a critical temperature of 727 ° C. (1340 ° F.). A bainitic microstructure may be similar in appearance and hardness characteristics to tempered martensite.
 A fine, non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder than it normally would be. As shown in the continuous cooling transformation (CCT) diagram of FIG. 2, the temperature range for transformation to bainite (250-550 ° C.) is between those for pearlite and martensite. When formed during continuous cooling, the cooling rate to form bainite is more rapid than that required to form pearlite, but less rapid than is required to form martensite (in steels of the same composition). In accordance with various implementations of this disclosure, a microalloyed steel having the chemistry discussed in more detail below may be initially heated to austenization temperatures of approximately 1100-1250° C. or greater. The steel may then be hot forged into the desired shape, and control cooled from the forging temperature to achieve a bainitic microstructure. For the cooling after hot forging, atmospheric cooling or forced air cooling using a blower may be conducted. In various alternative implementations, the steel may be cooled rapidly down to about the eutectoid transformation temperature, and then cooled slowly over a range from about 900 to 500° C. In still further alternative implementations, the steel may be cooled quickly to about 500 to 300° C. after hot forging, and may be kept at an equilibrium temperature somewhere in the range from about 500 to 300° C. to promote bainite transformation.
 The cooling rate may be determined by reference to a CCT diagram, to know the range for cooling rates passing through the bainite transformation region and, thereby, controlling to the determined cooling rate range. The CCT diagram may have been previously prepared, stored in a database, or otherwise made available for control of the cooling process. The forged product may be air cooled using fans or other means of circulating the cooling air to achieve a cooling rate that falls approximately within the range from 1 to 5° C. per second, or 60 to 300° C. per minute, when cooling between approximately 900° C. and 500° C. Most alloying elements will lower the temperature required for the maximum rate of formation of bainite, though carbon is the most effective in doing so. Bainite generally has a hardness that is greater than the typical hardness of pearlite and less than the hardness of martensite. Pearlite in the microstructure may contribute to reduced toughness. The composition and processing of the microalloyed steel according to various embodiments of this disclosure are selected to avoid or at least minimize the amount of pearlite present. In commercial practice a small amount of pearlite, such as less than 2 percent by volume, may unavoidably be present, particularly in the center of large sections, but care is taken to minimize its presence and effects.
 The bainite microstructure essentially has a two-phase microstructure composed of ferrite and iron carbide. Depending on the composition of the austenite during the hot forging process, and the cooling rate after hot forging, there is a variation in the morphology of the resulting bainite. The resulting microstructures are referred to as upper bainite or lower bainite. Upper bainite can be described as aggregates of ferrite laths that usually are found in parallel groups to form plate-shaped regions. The carbide phase associated with upper bainite is precipitated at the prior austenite grain boundaries (interlath regions), and depending on the carbon content, these carbides can form nearly complete carbide films between the lath boundaries. Lower bainite also consists of an aggregate of ferrite and carbides. The carbides precipitate inside of the ferrite plates. The carbide precipitates are on a very fine scale and in general have the shape of rods or blades. For this reason, the bainitic microstructure becomes useful in that no additional heat treatments are required after initial cooling to achieve a hardness value between that of pearlitic and martensitic steels. The material characteristics of the microalloyed and forged steel can vary over a large range depending on the particular types and quantities of alloying elements included in the composition. The composition of alloying elements included in accordance with various embodiments of this disclosure results in a steel part having the strength, hardness, and toughness characteristics previously only achieved by including the intermediate heat treatment steps following hot forging of reheating to austenization temperature, quenching, and tempering.
 The advantageous material characteristics discussed above are found to be achieved to a greater extent as the percentage by volume of bainitic microstructure is increased. Accordingly, a part that is 70% by volume bainitic microstructure may exhibit greater strength, hardness, and toughness characteristics than a part that is 50% by volume bainitic microstructure. Additionally, a part that is 85% or greater by volume bainitic microstructure may exhibit even further enhanced characteristics of strength, hardness, and toughness than the part that is 70% by volume bainitic microstructure. As shown in FIG. 1, intermediate heat treatment steps of reheating to austenization temperature, quenching, tempering, reheating a second time, quenching a second time, and tempering a second time, may be eliminated before the final machining and nitriding of a forged product in accordance with various implementations of this disclosure. The alloying elements that are added to the composition in accordance with various embodiments of this disclosure may also be selected to obtain the desired volume percentages of bainitic microstructure throughout the part, regardless of the different cooling rates that may be experienced in different sections or portions of the part having different thicknesses.
 As discussed in more detail below, two key elements in a microalloyed steel composition according to various implementations of this disclosure are Aluminum (Al) and Vanadium (V). Al and V improve the nitrideability of the alloy during a nitriding process. During the controlled cooling process implemented in this disclosure to achieve a bainitic microstructure, a small amount of the V and Al provided in the microalloyed steel contributes to strengthening of the steel by reacting with the small amounts (on the order of 150 parts per million) of Nitrogen (N) that have dissolved into the steel as it solidifies. The reaction of V and Al with dissolved N forms fine particles or precipitates of Vanadium Nitrides (VN) and Aluminum Nitrides (A1N), which contribute to strengthening of the lattice of the microalloyed steel. Because the amounts of dissolved N are so small, unreacted V and Al is left over after the cooling process, and is available to combine with N diffused into the surface of the part after machining during the nitriding process.
 Various methods of nitriding may be employed. One commonly used method of nitriding is gas nitriding. Alternative methods may include salt bath nitriding and plasma nitriding. In gas nitriding the donor is a nitrogen rich gas, usually ammonia (NH3), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the material creating a nitride layer. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required.
 It has been discovered in various implementations of this disclosure that the V and Al left over after the cooling process following hot forging enhance the nitriding characteristics of the microalloyed steel, thereby improving wear resistance, and strengthening the machined part. The bainitic microstructure with V and Al after controlled air-cooling may also exhibit the same or similar hardness and strength characteristics as were previously achieved by following hot forging with quenching, reheating, quenching again, and tempering. The controlled air-cooled microalloyed steel according to this disclosure does not have the body-centered tetragonal structure of the martensitic microstructure resulting from rapid quenching after hot forging. A microalloyed steel may exhibit a martensitic microstructure after rapid cooling from hot forging temperatures through quenching in oil or water. The martensitic microstructure may have a Rockwell C hardness (HRC) of 50 after quenching. Typical methods of processing this martensitic microstructure steel may then include reheating back up to austenitic temperatures of approximately 870° C., quenching again, and then tempering by reheating again to approximately 550° C.-590° C. in order to soften the steel to approximately HRC 30. The controlled air cooling process for producing a predominantly bainitic microstructure according to various implementations of this disclosure may result in the same hardness of HRC 30 without all of the quenching, reheating, quenching and tempering steps previously required. As mentioned above, the predominantly bainitic microstructure may contain greater than 50% by volume of bainitic microstructure. The hardness after air cooling in accordance with this disclosure may fall within the range from approximately 35-45 HRC.
 The microalloyed steel according to various implementations of this disclosure may have a chemical composition, by weight, as listed in Table 1:
TABLE-US-00001 TABLE 1 Composition of microalloyed steel in weight percent. Constituents Concentration by weight (%) Carbon 0.20-0.40 Manganese 0.50-1.60 Chromium 0.40-1.50 Aluminum 0.07-0.30 Vanadium 0.03-0.20 Silicon 0.10-0.40 Iron and other residual elements Balance
 Carbon contributes to the attainable hardness level, as well as the depth of hardening. In accordance with various implementations of this disclosure, the carbon content is at least 0.20% by weight to maintain adequate core hardness after tempering and is no more than about 0.40% by weight to assure resistance to quench cracking and an adequate response to nitriding. It has been found that if the carbon content is more than about 0.34% by weight, water quenching may cause cracking or distortion in complex-shaped articles and, in such cases, a less drastic quench medium such as oil may be required. The microalloyed, bainitic steel according to various implementations of this disclosure may be air cooled in accordance with select cooling curves on the CCT diagram of FIG. 2.
 Manganese contributes to the deep hardenability and is therefore present in all hardenable alloy steel grades. The disclosed alloy steel contains manganese in an amount of at least 0.50% by weight to assure adequate core hardness and contains no more than about 1.60% to prevent cracking. In addition to the permissible broad range of 0.50% to 1.60% by weight, a narrower range of manganese from 1.00% to 1.30% is advantageous to maintain uniformity of response to heat treatment.
 Chromium contributes to the hardenability of the present steel alloy and is also an excellent nitride former thereby enhancing nitriding characteristics. To realize these effects a minimum of 0.40% chromium is required, and advantageously at least 0.90% chromium should be present. To avoid embrittlement, the amount of chromium should be limited to a maximum of 1.50%, and preferably no more than about 1.20%.
 Aluminum contributes to hardenability and is a good nitrider former. Aluminum should be present in an amount of at least 0.07%, and preferably at least 0.10%. If aluminum is present in an amount less than about 0.07%, not only is there little observable improvement in either hardenability or nitride response, but also, the benefits are inconsistent. It has also been found that while aluminum in amounts greater than 0.30% is beneficial to nitrideability, the tendency for case embrittlement also increases. Accordingly, it is desirable to maintain an upper limit of no more than 0.30% aluminum and advantageously no more than about 0.20%. It has been discovered that the present alloy steel having aluminum in the designated range consistently improves hardenability.
 Vanadium is also an ingredient in the present alloy steel composition, and must be present in an amount of at least 0.03% to realize a consistently measurable enhancement of case and core hardness. Vanadium, in amounts greater than 0.20% does not significantly enhance the nitride response or the hardenability of the material. For these reasons, the limits of vanadium are at least 0.03% and no more than 0.20%; and advantageously from 0.05% to 0.10% to make the best economic use of this ingredient.
 It has been found that the unique combination of aluminum and vanadium, within the specified ranges, greatly contributes to improved nitride response, thereby decreasing required nitriding time and increasing case hardness and depth. Further, the unique combination of aluminum and vanadium, within the specified ranges, contributes to hardenability and temper resistance.
 The remainder of the alloy steel composition is essentially iron except for nonessential or residual amounts of elements which may be present in small amounts. For example silicon (Si) in the recognized commercially specified amounts is used for deoxidation of the molten steel, and may also contribute to forming a carbide-free bainite with improved toughness. For this purpose silicon may be present in an amount of at least 0.10%. Titanium (Ti) may also be provided in amounts approximately between 0.02-0.06% to prevent grain coarsening before and after forging. Sulphur (S), which in small amounts may be beneficial in that it promotes machining, is allowable in an amount of no more than about 0.10%, and preferably no more than 0.04% to avoid loss of ductility. Phosphorus (P) in an amount over 0.05% may cause embrittlement, and preferably the upper limit should not exceed 0.035%. Other elements generally regarded as incidental impurities may be present within commercially recognized allowable amounts.
 Manufactured articles, such as shafts, couplings and gears, having the above stated composition, are advantageously initially formed to a desired shape by forging or rolling after heating up the microalloyed steel to austenization temperatures of approximately 1100-1250° C. The formed articles are then controlled cooled as described above to produce a predominantly bainitic microstructure, machined to a desired final dimension, and then nitrided.
 FIG. 3 illustrates an exemplary method that may be used to produce a predominantly bainitic microalloyed and nitrided steel part in accordance with various implementations of this disclosure. FIG. 3 will be discussed in more detail in the following section to further illustrate the disclosed concepts.
 The steel, and method of making the steel in accordance with various implementations of the present disclosure may reduce costs by eliminating heat treatment steps typically performed after hot forging. The disclosed microalloyed, forged, air-hardenable, and nitrided steel parts may provide similar hardness and strength to previously hot forged and heat treated steel parts. The microalloying elements of Vanadium and Aluminum have been added to produce a predominantly bainitic microstructure after controlled air cooling from hot forging temperatures, and to enhance the nitriding heat treatment process following machining.
 As shown in FIG. 3, at step 320 a microalloyed steel having the composition shown above in Table 1 may be heated to austenization temperatures of approximately 1100 degrees C. to 1260 degrees C. Exemplary types of parts being manufactured in accordance with various implementations of this disclosure may include transmission ring gears, engine gears, hubs, shafts, and other driveline components for various machines. The size of the parts determines the size of a steel billet that is initially heated to austenization temperatures in accordance with step 320.
 At step 322, the heated billet may be hot forged to a desired configuration. After the hot forging, step 324 may include air cooling the hot forged product at a cooling rate that results in the formation of a predominantly bainitic microstructure throughout the hot forged part. As shown by the CCT diagram of FIG. 2, the cooling rate may be chosen to avoid the formation of a martensitic microstructure or a predominantly ferrite and pearlite microstructure. In various implementations of this disclosure, the hot forged steel may be cooled at a rate that falls approximately in the range from 1 to 5 degrees C. per second as the steel cools from approximately 900 degrees C. to approximately 500 degrees C. The predominantly bainitic microstructure may be a microstructure with greater than 50% bainite, or more advantageously greater than 70% bainite, or still more advantageously greater than 85% bainite throughout the hot forged steel part.
 At step 326, the microalloying elements of Vanadium (V) and Aluminum (Al) may react with Nitrogen dissolved in the steel during solidification to form fine precipitates or particles that strengthen the crystal lattice of the steel microstructure. The amount of Nitrogen (N) is typically quite small, and may be on the order of 150 parts per million (ppm). As a result, the majority of the V and Al remains free to combine with more N that may be introduced at a later stage after machining during a nitriding process.
 At step 328, after the steel part has been air cooled, it may be machined using conventional machining techniques. After machining, at step 330, the machined part may be nitrided using techniques that may include heating the machined part in an atmosphere with Nitrogen rich gas such as ammonia (NH3). Nitriding is a heat treating process that diffuses Nitrogen into the surface of the part to create a case-hardened surface. The V and Al left over after some of the V and Al has reacted with N dissolved in the steel during solidification enhances the nitriding process by reacting with the N provided during the nitriding process.
 It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed microalloyed steel and method of forming the steel into a finished part without departing from the scope of the disclosure. Alternative implementations will be apparent to those skilled in the art from consideration of the specification and practice disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.
Patent applications by Caterpiller Inc.
Patent applications in class With working, machining, or cutting
Patent applications in all subclasses With working, machining, or cutting