Variable diffusion carburizing process

Abstract

A method for carburizing a powder metal part includes two or more carburizing steps. In the pre-forging carburizing step, a less-than-full-density powder metal part is carburized to establish a pre-forging carburizing profile. After the pre-forging carburizing step, the powder metal part is forged to increase the density of the powder metal part and convert the pre-forging carburizing profile to an as-forged carburizing profile. In a post-forging carburizing step following the forging step, the powder metal part is again carburized, thereby both further diffusing carbon into the powder metal part from the as-forged carburizing profile and further incorporating carbon into the powder metal part at the surface of the powder metal part.

Description

[Technical field]

[0001] <CROSS REFERENCE TO RELATED APPLICATIONS> This application claims the benefit of U.S. Provisional Patent Application No. 62 / 833,407, filed April 12, 2019, entitled "Variable Diffusion Carburizing Process," which is incorporated herein by reference for all purposes as if fully incorporated herein.

[0002] <STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT> Not applicable. [Background technology]

[0003] The present disclosure relates to a method for carburizing a powder metal part.

[0004] Many highly stressed ferrous components are designed to have high strength near their surfaces and higher toughness in the ductile core region. For example, many conventional gears utilize such a design for their teeth, which allows for a strong tooth surface and an equally ductile core to be configured to provide the right balance between mechanical material properties based on the tooth's function.

[0005] A common method for obtaining such differences in material properties between the surface and core of a ferrous component is by carburizing the component. During a typical carburization, the component is heat treated in the presence of a carbon bearing material, and carbon is introduced and absorbed at the surface of the component. This carbon then diffuses inwardly into the body volume of the component. Typically, the component is heat treated in a carefully controlled carbon-containing gas atmosphere, although the component may have previously been packed in contact with a carbon-rich material, such as charcoal, to provide the carbon.

[0006] The carburized part is then often rapidly cooled to form the martensite phase in the part. The formation of martensite increases the strength and hardness of the surface layer, but martensite is also very brittle. Further heat treatments such as tempering can be used to reduce some of the brittleness in the part.

[0007] Such carburizing is a diffusion-based process and is therefore both time and temperature dependent, which can have microstructural, macrostructural and economic impacts. For example, the slow diffusion rates in steels often result in a distinct and relatively sharp transition in carbon content (i.e., material properties) between a high carbon-containing surface layer and the original base material, which contains little, if any, carbon. This means that while a distinct carburized layer can help support applied loads, there is a rapid transition to a ductile core that is unlikely to support high loads at the carburized layer (although the core can provide the toughness and ductility mentioned above). Furthermore, with both time and temperature as process variables, carburizing can potentially be very expensive, requiring both significant energy and equipment throughput. The cost of such inputs is considered along with the desired result profile (e.g., depth, carbon content, etc.) when selecting the process parameters for carburizing a particular part line. Summary of the Invention

[0008] Disclosed herein are improved methods for carburizing powder metal parts. These carburizing methods include sequential processing of powder metal parts by carburizing a powder metal part less than full density (sometimes as part of a sinter-carburizing process), then forging the part to increase the density of the powder metal part, and then carburizing the powder metal part again. The pre-forging carburizing step can help carbon penetrate to greater depths initially compared to a fully dense forged part under similar time, temperature, and atmospheric conditions by taking advantage of the ability of carbon-containing gas to transport carbon through the pore network of the powder metal part early in processing before the pores are closed by sintering and / or forging. This effectively "dopes" the initial carbon profile relatively deep into what will become the outer surface of the part after the forging step, without relying solely on solid-state diffusion as a transport mechanism as may be necessary for a fully dense part. After forging, the forged part can be carburized again at high temperature, and since some trapped carbon is already pre-established at a depth from the outer surface of the forged powder metal part, this initially introduced carbon can diffuse further into the part during heat treatment, while new carbon is introduced to the surface of the part, thereby creating a higher carbon content at this surface. This multi-step process can create more complex and gradual carbon gradients, which can be widely modified and designed to have the desired case profile by varying different process parameters.

[0009] The forging step can be specifically designed to shape the initially locked-in carbon profile in addition to providing a non-sharp transition between the hardened case and the core. For example, by creating a variable flow of material in the part during the forging step (which can be achieved by designing the pre-forged preform to have dimensions that are not proportional to the forged form), the initial carbon profile can be thickened and / or thinned or stretched and / or compressed in cross section, resulting in a variation in thickness of the initial carbon layer prior to the subsequent carburization step. In this way, fairly sophisticated and tailored carburization profiles can be produced.

[0010] These improved carburizing methods can provide many advantages over conventional methods. As one example, the above sequence allows for the development of variable case depths in the resulting parts using conventional case carburizing processes (i.e., sinter carburizing and gas carburizing on full or near full density parts), but with time and cost savings compared to carburizing full density forged parts. Additionally, the sequence allows for the development of variable case depths and geometries that cannot be produced using existing sinter carburizing techniques alone.

[0011] According to one aspect, a method is provided for carburizing a powder metal part. In a pre-forged carburization step, the powder metal part is carburized below full density to establish a pre-forged carburization profile. The powder metal part is then forged after the pre-forged carburization step to increase the density of the powder metal part. The forging converts the as-forged carburization profile to an as-forged carburization profile. In a post-forged carburization step, the powder metal part is carburized after the forging step. The post-forged carburization step provides both further diffusion of carbon into the powder metal part from the as-forged carburization profile and further introduction of carbon (from the carburizing atmosphere) into the powder metal part at the surface of the powder metal part.

[0012] In some forms of the method, the forging step forges the powder metal part to bring the powder metal part to an effective full density. Those skilled in the art will appreciate that the effective full density of a powder metal part, even after forging, is less than the theoretical full density and includes the elimination of substantially all of the internal porosity. In the context of ferrous parts, it is contemplated that the effective full density may be, for example, 98% or more of the theoretical full density. However, such effective full density may also be determined by reference to the Metal Powder Industry Federation (MPIF) standard for a particular material. In the context of this application, theoretical full density refers to the true density of a material, which corresponds to a completely filled density (full density) with no pores or zero porosity.

[0013] In some embodiments, the density of the less than full density powder metal part prior to the forging step is less than 95% of the theoretical full density. Again, this percentage is merely an example and the specific percentage may depend on the sinterability of the particular material composition, but in any event, the powder metal part that is subjected to the first pre-forging carburization step is less than full density and, at least in some embodiments, has a network of connected pores to allow non-solid or gaseous transport of carbon to occur within the powder metal part, such that the depth of carburization is not simply primarily a function of time and temperature as in the case of a fully dense part.

[0014] In some forms of the method, the method further includes performing a sintering process on the powder metal part. When the method includes performing a sintering process, it is contemplated that the pre-forging carburizing step is performed during the sintering process of the powder metal part (i.e., performed simultaneously or contemporaneously with the sintering process, such as in a sinter-carburizing process). In this manner, the powder metal part, now in the form of a green compact, may have a density relatively lower than the theoretical full density, and it is contemplated that once the binder or lubricant holding the powder metal particles together is combusted, the remaining pore network of the green compact may be available, at least temporarily, for gas transport of carbon from the atmosphere into the compact. For example, the density of the powder metal part prior to the sintering process may be less than 90% of the theoretical full density of the powder metal part.

[0015] In some forms, the base carbon percentage of the powder metal part may be 0.1 to 0.3 weight percent of the powder metal part prior to the pre-forging carburizing step. "Base carbon percentage" refers to the amount of carbon in the material prior to the carburizing heat treatment during processing of the powder metal in compact or sintered form. Starting from such a base carbon percentage, the percentage of carbon in the carburized region of the pre-forging carburizing profile may be 0.5 to 0.6 weight percent carbon after the pre-forging carburizing step. The percentage of carbon in the region closest to the surface of the powder metal part after the forging and post-forging carburizing steps may be 0.7 to 0.9 weight percent carbon.

[0016] In some forms of the method, converting the pre-forged carburized profile to an as-forged carburized profile may include starting with a pre-forged carburized profile having a uniform depth relative to the surface of the powder metal part, and then forging the powder metal part so that the as-forged carburized profile has a variable depth relative to the surface of the powder metal part after forging. In other words, forging can cause changes in the geometry of the profile during the forging process due to changes in material flow. This can be specifically designed into the process, as described, for example, in U.S. Patent No. 8,517,884, entitled "Powder Forged Differential Gears," issued Aug. 27, 2013 by GKN Sinter Metals, LLC, which is incorporated by reference in its entirety for all purposes.

[0017] In some configurations, the powder metal part may include iron, and in effect, such carburization is performed on iron-based powder metal particles.

[0018] In some forms of the above method, the pre-forging carburization step allows carburization to occur to a greater depth under comparable time, temperature, and atmospheric exposure conditions as compared to a comparable powder metal part of similar composition that is fully dense. This greater depth of carburization can occur because the porosity of the powder metal part prior to the forging step provides a non-diffusion based path for deeper penetration of carbon into the volume of the powder metal part as compared to a fully dense part.

[0019] In some configurations, the powder metal parts produced by the above method may be gears. In the case of differential gears, especially small differential gears, may have features that cannot be processed using traditional sinter carburizing methods due to the inability to forge into bore pinholes or splines when significant case hardening is achieved at the surface prior to forging. In hypoid ring gears, material flow may not allow hardness to be maintained deep at the base of the gear. In small tooth module parallel axis gears, the case may be too deep for the tooth thickness.

[0020] In some configurations, after the post-forging carburization step, the powder metal part may have a final carburization profile that has a less steep carbon gradient from the surface into the volume of the powder metal part than would be achieved by carburizing a fully dense part that is carburized in a single step under similar time, temperature, and air exposure conditions.

[0021] In some embodiments, the method may further include machining (e.g., soft tuning) the powder metal part between the forging step and the post-forging carburizing step. At this point, the part has a final or near-final shape, although some but not all of the carbon has been added, and it may be easier to forge the powder metal part at this stage than after the post-forging carburizing step.

[0022] Additionally, powder metal parts produced from the various methods described herein are also considered within the scope of this disclosure.

[0023] The above and other advantages of the present invention will become apparent from the detailed description and drawings. The following description is merely a description of some preferred embodiments of the present invention. These preferred embodiments are not intended to be the only embodiments of the claims, and therefore reference should be made to the claims to assess the full scope of the present invention. [Brief description of the drawings]

[0024] [Figure 1] (A) and (B) are images of the case profile of a forged part produced using only the conventional gas carburizing process. [Diagram 2] (A) and (B) are images of the case profile of a powder metal part produced using only the conventional sinter-carburization process where all carburization occurs before the forging step. [Diagram 3] (A) and (B) are images of the case profile of a powder metal part produced using a multi-step carburization process that includes a series of sinter-carburization, forging and gas carburization steps. [Figure 4] FIG. 1 is a schematic diagram illustrating the step-wise sinter carburization of a powder metal part and the amount of carbon in each of the various regions. [Diagram 5] (A) is an image depicting the gear geometry after pre-forging carburization and forging has been performed, and (B) is an image depicting the same gear geometry after a further final carburization step. [Figure 6] FIG. 5 is a schematic diagram similar to that of FIG. 4, but illustrating the final gas carburizing step after the forging step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] To best understand the disclosed method and its improvements over the prior art, a brief comparison of conventional gas carburizing and conventional sinter carburizing is provided. Understanding these modes of carburizing makes it possible to understand how the novel method includes aspects of both modes (which can be slightly modified) and an intermediate forging step to arrive at a unique carburization profile in a powder metal part that can be made at low cost.

[0026] In conventional gas carburizing, a fully dense ferrous product is placed in a heated atmosphere containing a carbon-containing gas for a period of time under conditions that result in the absorption of carbon from the atmosphere into the ferrous product. Increasing the temperature will result in shorter cycles and greater carbon penetration depth per length of time, but may result in certain other undesirable metallurgical or dimensional results (e.g., grain growth or sagging or warping of parts). Thus, conventional gas carburizing is limited by the thermal budget (i.e., temperature) of the operation and the diffusion curve of the material at that temperature. The end result is therefore highly dependent on the material being gas carburized and its diffusion curve and the time at temperature.

[0027] In full density wrought material, the carburized surface profile is typically described as having a very high carbon zone outside a very low carbon zone with a narrow transition region. This processing method typically results in the carburized zone in the part being very uniform in thickness across the entire exposed surface.

[0028] Figures 1A and 1B show the uniformity of such a profile in two images of a forged and gas carburized gear tooth, where the case profile is of very uniform thickness across the surface of the tooth. Figure 1A is a photomicrograph showing the case depth, with the arrows representing the uniform case depth at various locations on the tooth surface. Figure 1B shows the Vickers hardness numbers of the tooth at various locations, from which it can be seen that the tooth has a high hardness number (up to 786 HV) in the outer surface layer, but a much lower hardness number (only 435-500 HV) in the core. The transition between the outer layer or hardened case and the core is quite dramatic and sharp.

[0029] The majority of commercial carburizing of ferrous parts is based on gas carburizing of forged or full density parts, since for the most part, many parts will be full density due to the mode of manufacture prior to carburizing, such as casting or extrusion.

[0030] It should be noted, however, that there is another, less common method of type of carburization for powder metal parts, called sinter-carburization or "sinta carb," which can be used to create a deeper carbon profile.

[0031] To understand how sinter carburization works, one must first understand how powder metal parts are made in a general sense. In the manufacture of powder metal parts, powder metal powders are compressed with binders, waxes and / or lubricants to form a green compact having a shape very similar to the final desired product. This green compact is then sintered in a furnace, typically at a temperature close to but slightly below the melting temperature of the powder metal. While there are some sintering processes (known as liquid phase sintering) where a limited amount of the green compact becomes liquid phase, many sintering processes are based solely on solid-state diffusion, usually involving binders, waxes and / or lubricants that are burned off while the powder metal particles are necked together to form a cohesive sintered body. During the sintering process, interparticle spaces may provide a pore network, but such pores decrease in size and close as sintering continues. However, some porosity may remain in the sintered body as it is sintered.

[0032] The sinter-carburizing process takes advantage of the presence of this pore network to introduce carbon while the part is already at high temperature during the sintering manufacturing step. Due to the presence of the pore network between the powder metal particles at least at the beginning of the sintering process step, it is possible to flow a carbon-containing gas into these pores to provide gas transport of the carbon-containing gas deeper into the body of the part, at least to depths beyond those that will ultimately form the outer surface of the part after the sintering process, so that carbon can be absorbed at these deeper depths. Since gas transport through the pore network is mainly based on gas flow rather than solid-state diffusion, it is possible to quickly obtain deeper carburization depths and more gradual gradients with this sinter-carburizing technique than with gas carburizing of fully dense parts. Thus, sinter-carburizing can be used to efficiently carburize low-density powder metal parts with improved diffusion depths, often before the final fully densified forging process.

[0033] 2A and 2B show a comparable carburization profile to FIG. 1A and 1B for a gear tooth, but the gear tooth in FIG. 2A and 2B was produced by sinter carburization rather than gas carburization. Considering gas diffusion into the pores of the part, it can be seen that there is a much wider transition zone from high carbon areas to low carbon areas. Density variations in the powder metal compact can result in different depths of carbon diffusion in different regions, and forging after sinter carburization can also result in case thickness variations. In the micrograph of FIG. 2A, it can be seen that the carbon diffusion depth is deepest at the tip and becomes shallower as it approaches the base end of the tooth. In FIG. 2B, which shows Vickers hardness numbers from 334HV to 714HV, it can be seen that the carbon penetrates deeper into the tooth than the part in FIG. 1B, and the transition from core to surface is more gradual.

[0034] However, even with sinter carburizing, there are certain conditions and products where the sinter carburizing process produces less than ideal results. For example, parts with thin cross sections or parts with low spline / gear height modules often cannot be effectively carburized without achieving a fully (or sufficiently) hardened state due to the high diffusion rate of carbon material during the sinter carburizing process. The end result can be products that lack ductility and toughness due to case depth embrittlement, and such products may not be metallurgically suitable due to the brittleness of such features. In addition, products that do not meet the process design requirements may have a thin case depth due to excessive material thinning during the forging operation, resulting in reduced strength and / or reduced resistance to impact forces.

[0035] Disclosed herein is a method that incorporates aspects of both modes of carburizing (i.e., sinter carburizing of less than fully dense powder metal parts and gas carburizing of fully or near fully dense parts) with an intermediate forging step. The method includes, in sequence, a first carburizing step while the part is still less than fully dense, a forging step to increase the density of the part, and a second carburizing step that typically includes gas carburizing of the near fully or near fully dense part.

[0036] It should be noted that at this stage, the first carburizing step may be sinter-carburizing (and it is likely to be most economical to perform the sintering and first carburizing steps together), however, it is conceivable that the sintering and first carburizing steps could be performed separately, however this may need to be done carefully and thoughtfully as the mechanisms by which carbon-containing gases can be transported into the part are compromised and weakened once the pores of the part have closed, as often happens at the end of the sintering process.

[0037] Also, it is noted that while the first step may include sinter-carburizing on a less than fully dense part, this sinter-carburizing differs from known sinter-carburizing modes in that it is not performed to impart all of the carbon in the final carbon profile. In other words, in this new method, only a small portion of the final case profile is established during the sinter-carburizing step, whereas in conventional sinter-carburizing, sinter-carburizing is the final and only step that provides the carbon for carburizing.

[0038] The disclosed method, sometimes commonly referred to as variable diffusion carburizing (or VDCC), is an improvement over the separate conventional gas carburizing and sinter carburizing processes, and addresses many of their separate shortcomings. By incorporating aspects of both processes, it is possible to produce deep case depths of tailored and variable nature, while at the same time reducing cost inputs compared to the conventional gas carburizing processes.

[0039] Again, generally speaking, the method includes a pre-forge carburization step for a less than fully dense part, a forging step that increases the density of the partially carburized part and converts the pre-forge carburization profile to an as-forged carburization profile, and a post-forge carburization step that results in further diffusion of carbon from the as-forged carburization profile into the powder metal part and the introduction of additional carbon to the surface of the powder metal part. Each of these steps will be described in further detail.

[0040] The method may begin by performing a mild carburization step on a less than full density powder metal part or billet, e.g., iron, as a first step. The less than full density powder metal part may be a green compact formed using conventional powder metallurgy compaction techniques, where the part as formed closely approximates the final geometry of the final part, but is slightly larger in size to account for shrinkage from sintering the part. It is contemplated that the powder metal part as a green compact may have a density of less than 90% of theoretical full density, for example.

[0041] This less than full density powder metal part is sintered to metallurgically bond the powder metal particles together, which may include burning off any wax, binders, or lubricants in the powder metal part. As explained above, a pore network is formed when the powder metal particles bond together to form necks. Thereafter, during the sintering operation, which is performed at high temperatures, the powder metal part can readily accept carbon from the sintering atmosphere, assuming that the carbon concentration in the sintering atmosphere is high enough to thermodynamically transport carbon into the iron powder metal compact.

[0042] The addition of carbon from the atmosphere during the sintering process creates a controlled carbon content and depth as the initial carburization profile, also referred to as the pre-forging carburization profile in view of the subsequent forging step. As explained above for the sintering carburization process, due to the less than full density nature of the powder metal part, the diffusion rate is very high and therefore carbon can easily penetrate into the part, resulting in a deep carbon profile.

[0043] One specific example of the first sinter-carburization step is shown in FIG. 4. FIG. 4 shows starting with an iron powder metal part with 0.10% to 0.30% carbon by weight, as shown in the first block at the top. Some initial carbon content in the powder metal part is suitable, albeit at a low level, because the goal of the process is to produce a hardened carburized layer, and 0.10% to 0.30% carbon by weight is typical for many applications. However, the specific amount of carbon ultimately depends on the Jominy curves at various locations of the material versus the desired hardness and the type of quenching method used to produce the final part. After sinter-carburization, a concentrated carbon layer is produced containing about 0.50% to about 0.60% carbon by weight, as shown in the second block in the center of FIG. 4. The third block at the bottom shows that additional carbon diffusion and time can shift the carbon profile over a significant range, and create a wide enough gradient between the carbon-rich outer surface layer and the core (which remains at about 0.10% to about 0.30% carbon by weight). It should be understood that these figures are illustrative only and should not be considered limiting.

[0044] It will again be understood that in most forms of the method, the first carburizing step is performed simultaneously or contemporaneously with the sintering step. In this way, carbon incorporation through the pore network of a less than fully dense part can be most easily promoted. There is some economy in combining the sintering process with the carburizing, and the heat treatment of the part for the sintering process can also be used to promote carbon diffusion. It is contemplated, however, that the first carburizing step may be performed separately from the sintering process, however this may be considered where such a separate carburizing would be most effective when the grains are partially bonded and the pores are not yet closed. In other words, if the part is sintered and then subsequently carburized, it is possible that most of the pores will close during the sintering process, rendering the carburizing ineffective, unless the process is carefully controlled to prevent the pores from closing before carburizing.

[0045] The powder metal part is then forged to increase the density under suitable material flow conditions. In some embodiments, this may include increasing the density to an effective full density, which may be, for example, the effective full density of the MPIF standard. As described elsewhere in this disclosure, the effective full density is less than the theoretical full density, but is often close to it. In some forms, this effective full density may be greater than 98% of the theoretical full density, and the density of the powder metal part that is less than full density before forging may be less than 95% of the theoretical full density. This forging step results in a slightly higher surface carbon content of the powder metal part due to the forging process. It is further contemplated that the as-forged (immediately after forging) powder metal part may be subjected to non-quenching conditions to cool.

[0046] Many of the end uses for parts produced by these methods require the product to have the strength achieved by forging, such that the density and therefore strength of the part is significantly increased through forging.

[0047] In particular, the forging process can not only increase the density of the powder metal part, but also convert the pre-forged carburized profile to an as-forged carburized profile, which simply involves uniformly densifying the part, with only minor changes to the dimensions and case profile (this is true when the pre-forged powder metal part has a pre-forged carburized profile of uniform depth and the pre-forged powder metal part has an as-forged carburized profile of uniform depth, even if the pre-forged and forged part have slightly different depths as a result of forging and densification). However, in more involved or complex ways, the forging process may result in thicker and / or thinner cross sections of the carburized profile due to variable material flow in the forging die. How such variable forging can be employed to modify or transform the case depth can be found in U.S. Patent No. 8,517,884, issued Aug. 27, 2013 by GKN Sinter Metals, LLC, entitled "Powder Forged Differential Gears," the entire contents of which are incorporated herein as if set forth herein for all purposes. As a recent example, a carburization profile such as that shown in the center panel of FIG. 4 can be achieved by forging a preform with a relatively uniform case depth to produce a thinner case depth at the base end and a thicker case depth at the tip end. This involves engineering the preform (i.e., a pre-made powder metal part) and setting the tool and die to achieve the final outer surface shape and variable case depth of the part as forged.

[0048] Also, because only a portion of the carbon is added in the first carburizing step (additional carbon is added after forging), the powder metal part can be forged more easily than a part in which all of the carbon is introduced before forging, because heavy carburization creates a hard, brittle case that makes forging difficult, if not impossible.

[0049] Additionally, although the forging step is described herein as increasing the density of the powder metal part, in most cases this may involve forging the powder metal part to its effective full density (which is essentially a condition in which nearly all porosity is closed or eliminated during forging). Nonetheless, it is believed that there may be situations in which a less than full density, but still densified, powder metal part has enough closed porosity to proceed to the second carburizing step.

[0050] At this point, and after the forging and controlled cooling of the as-cast powder metal part, a second carburization may be applied to the as-forged powder metal part (now with an increased density or effective full density) in a post-forging carburization step. At this point, with the powder metal part having an increased density or effective full density, the carbon introduced in the first carburization step prior to forging allows the powder metal part to be subjected to a more efficient gas carburization. Thus, during this second carburization step, both the existing or "doped" or "charged" carbon from earlier in the process can continue to diffuse into the powder metal part and additional carbon can be absorbed at the surface of the powder metal part to create a more carbon-rich outer region or case. With the pre-existing carbon already there from the first carburization step, the second gas carburization step is not as severely limited by time and temperature as a pure gas carburization of a full density part where carbon is introduced in a single cycle / step, thus avoiding a significant portion of the slow diffusion rates.

[0051] The benefits of this doping or charging of the as-forged part with carbon are shown in Figures 5A and 5B and 6. In Figure 5A, the profile of the outer surface of a gear tooth part is shown with the carbon layer underneath. Again, in this panel, the powder metal part is after forging, so the carbon layer applied in the first carburizing step has a varying depth to the tooth surface, i.e., a thicker depth at the tip of the tooth and a thinner depth at the base of the tooth. Figure 5B shows that further gas carburizing of this part in a second post-forging carburizing step has further advanced the original case profile (now shown by the dashed line) more or less uniformly into the body of the part. Figure 6 also shows this diagrammatically. In FIG. 6, the first three panels show a similar process to that described above with respect to FIG. 4, followed by additional gas carburization in a second post-forge carburization step to produce the profile in the rightmost panel, in which the surface carbon content can increase from about 0.70 wt. % to about 0.90 wt. % carbon, with a transition to another region having higher carbon, such as 0.5 wt. % to 0.6 wt. % carbon, before reaching the core.

[0052] The profile obtained by the process including two carburizing steps separated by a forging step is also shown in Figures 3A and 3B, which provide comparative images to those from Figures 1A and 1B and 2A and 2B. In Figure 3A, it can be seen that a deep and variable case depth has been produced, and there is a significant carbon gradient in the transition zone between the hardened outer surface and the core, especially compared to the parts shown as produced by the forging and gas carburizing processes in Figures 1A and 1B, and compared to the sinter-carburizing and forging processes in Figures 2A and 2B. Similarly, Figure 3B shows the hardness distribution, which shows a good gradient and a smooth transition.

[0053] Apart from the improved structure and case profile, there are also many processing advantages.

[0054] By using forged parts in a "doped" or pre-carburized state, the as-forged (immediately after forging) powder metal part may have an increased surface carbon content of less than half the carbon content of a powder metal part that has not been pre-carburized. Thus, the soak time at high carbon levels during post-forging carburization can be significantly reduced. Similarly, for powder metal parts with large surface areas, where the rate limiting factor is the ability of the air or atmosphere to provide carbon to the surface of the part quickly enough, such parts can be processed more quickly because some of the carbon introduced is already present in the part from the pre-forging carburization step.

[0055] Additionally, using this method reduces the amount of time and energy required for further advancement since the majority of the case depth is already present and near the surface of the powder metal part. This allows for a smoother transition from case to core as shown in Figures 3A and 3B (which also shows how variable forging can be used to create variable case depths). Ultimately, this smoothed transition from case to core reduces internal shear stresses due to progressive stress states that can match product performance needs.

[0056] Additionally, this process can be adapted for lower module, thinner design element type products where total or sufficient hardening may be a concern with sinter carburize / forge only processes. In such processes, the standard form of sinter carburize to final forging is not possible due to geometry constraints (i.e., sinter carburized thin elements cannot be forged without fracturing). However, this two-step carburizing process provides the ability to efficiently forge parts that are pre-charged with a small amount of carbon (which can still be forged without fracturing) and then further carburize the part after forging to provide the additional carbon for the final desired hardness.

[0057] To understand the economic benefits of this new method, it is estimated that this new method (doping or charging the powder metal part with carbon before forging) allows for a 40% reduction in the cost of subsequent gas carburization after forging compared to a method that does not pre-dope or charge the powder metal part with carbon and includes only gas carburization of the powder metal part. This exemplary 40% cost reduction is based on the increased throughput and reduced time required to fill the same or deeper carbon penetration depth using only post-forging gas carburization as the mode for carburization. Apart from this, it is also recognized that the sinter-carburization step to carburize the powder metal part before forging does not involve many additional capital costs to execute. Compared to conventional sinter-carburization, i.e., a method in which all the carbon is introduced in that single process, the first pre-forging carburization step is not designed to impart all the carbon in a single shot or dose, and therefore has lower execution costs and in most cases higher throughput than conventional sinter-carburization processes.

[0058] Due to the separation of the carburizing step (i.e. before and after the forging step), there is no option or advantage to quenching and tempering directly from the forging process. Therefore, a forged product is, by definition, a non-heat treated product that can be soft machined as dictated by the final geometry. For moderately to highly machined products, soft tuning is often less expensive than hard tuning, thus reducing costs and at the same time realizing further cost savings in the gas carburizing operation.

[0059] While the method described herein involves more processing steps than full sinter carburization alone, this new method does not involve more steps than the conventional gas carburization process since the conventional gas carburization process already involves sintering, forging and gas carburization. Thus, the method described herein offers better economics while at the same time providing the opportunity to obtain highly engineered case profiles not previously seen in single step carburization processes.

[0060] It is to be understood that various other modifications and variations to the preferred embodiments may be made within the spirit and scope of the present invention. Accordingly, the present invention should not be limited to the described embodiments. Reference should be made to the following claims to ascertain the full scope of the present invention.

Claims

1. performing carburizing on the less than fully dense powder metal part to establish a pre-forging carburization profile in a pre-forging carburizing step; forging the powder metal part after the pre-forging carburizing step to increase density of the powder metal part and convert the pre-forging carburized profile to an as-forged carburized profile; a post-forging carburizing step, carburizing the powder metal part after the forging step, which results in both further diffusion of carbon into the powder metal part from the as-forged carburizing profile and further incorporation of carbon into the powder metal part at the surface of the powder metal part; A method of carburizing a powder metal part, wherein the pre-forging carburization step results in carburization to a greater depth than a comparable powder metal part of similar composition having full density because the porosity of the powder metal part prior to the forging step provides a non-diffusion based path for deeper penetration of carbon into the interior of the powder metal part.

2. 2. The method of claim 1, wherein a step of forging the powder metal part configured to increase the density of the powder metal part after the pre-forging carburizing step comprises forging the powder metal part to an effective full density of the powder metal part.

3. 3. The method of claim 2, wherein the effective full density is greater than 98% of the theoretical full density.

4. 2. The method of claim 1, wherein the density of the less than full density powder metal part prior to the forging step is less than 95% of theoretical full density.

5. sintering the powder metal part; 2. The carburizing method of claim 1, wherein said pre-forging carburizing step is performed during said sintering process of said powder metal part.

6. 6. The carburizing method of claim 5, wherein the density of the powder metal part prior to the sintering process is less than 90% of the theoretical full density of the powder metal part.

7. 2. The carburizing method of claim 1, wherein the base carbon percentage of the powder metal part is from 0.1 to 0.3 weight percent of the powder metal part prior to the pre-forging carburizing step.

8. 8. The carburizing method of claim 7, wherein the percentage of carbon in the carburized regions in the pre-forge carburization profile is 0.5 to 0.6 weight percent carbon after the pre-forge carburization step.

9. 9. The carburizing method of claim 8, wherein after the post-forging carburizing step, the percentage of carbon in the region of the powder metal part closest to the surface is between 0.7 and 0.9 weight percent carbon.

10. Converting the pre-forging carburization profile to an as-forging carburization profile comprises:

2. The method of claim 1 including starting with a pre-forged carburization profile having a uniform depth relative to the surface of the powder metal part and forging the powder metal part such that the as-forged carburization profile has a varying depth relative to the surface of the powder metal part.

11. The method of claim 1 , wherein the powder metal part comprises iron.

12. 2. The method of claim 1, wherein the powder metal part is a gear.

13. 10. The method of claim 1 further comprising machining said powder metal part between said forging and said post-forging carburizing steps.

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