Variable diffusion carburizing method

Abstract

A carburizing method of a powder metal part involves a plurality of carburizing steps. In the pre-forging carburizing step, a less than fully dense powder metal part is carburized to form a pre-forging carburized 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 into a forged carburizing profile. In a post-forging carburizing step after the forging step, the powder metal part is carburized again, resulting in further diffusion of carbon from the forged carburizing profile into the powder metal part and further introduction of carbon into the powder metal part at the surface of the powder metal part.

Description

[0001] This application is a divisional application of the invention patent application with international application number PCT / US2020 / 025005, international application date of March 26, 2020, Chinese national phase application number 202080043056.9, and invention title "Variable Diffusion Carburizing Method".

[0002] Cross-reference to related applications

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 833,407 entitled "Variable Diffusion Permeation Method", filed on April 12, 2019, the entire contents of which are incorporated herein by reference for all purposes.

[0004] Statement of federally funded research or development

[0005] none. Invention Field

[0006] The disclosure relates to a method for carburizing powder metal parts. background

[0007] Many high-stress iron-containing components are designed to have high strength near their surface and a ductile core region with greater toughness. For example, many conventional gears employ this design in their teeth to provide a robust tooth surface, and the tooth core has considerable ductility to provide an appropriate balance of mechanical and material properties according to the tooth function.

[0008] A common method for achieving material property differences between the surface and core of an iron-containing component is carburizing. During typical carburizing, the component is heated in the presence of a carbon-carrying material, introducing carbon into the component surface where it is absorbed. This carbon then diffuses inward into the volume of the component body. Typically, the component is heated in a carefully controlled carbon-containing gas environment, although historically, components may have been packaged to contact carbon-rich materials such as charcoal to provide carbon.

[0009] Carburized parts are typically then quenched to form a martensitic phase within the part. The formation of martensite increases the strength and hardness of the surface layer, but martensite is also very brittle. Further heat treatment, such as tempering, can be used to alleviate some of the brittleness in the part.

[0010] As a diffusion-based process, carburizing in this case is time- and temperature-dependent, and this dependence can have implications for microstructure, macrostructure, and economics. For example, due to the slow diffusion rate of steel, a significant and relatively abrupt transition in carbon content typically occurs between a surface layer with a high carbon content and the original base material, which may contain less carbon (if any). This means that while a prominent carburized layer helps support the applied load, it quickly transitions to a tough core, which provides little support for high loads relative to the carburized layer (although the core can provide the aforementioned toughness and ductility). Furthermore, since time and temperature are process variables, carburizing can be very expensive, requiring significant energy and high equipment throughput. When selecting carburizing process parameters for a specific component production line, the cost of such inputs and the desired profile of the result (e.g., depth, carbon content, etc.) should be considered. Summary of the Invention

[0011] This paper discloses an improved method for carburizing powder metal parts. These carburizing methods involve sequentially processing the powder metal parts as follows: carburizing the powder metal parts at a lower than full density (in some cases, as part of a sintering carburizing process), then forging the parts to increase the density of the powder metal parts, and then carburizing the powder metal parts again after forging. The pre-forging carburizing step can help carbon initially penetrate to a deeper depth than fully dense forged parts under similar time-temperature-atmosphere conditions, before the pores are closed by sintering and / or forging, by utilizing the ability of carbon-containing gases to transport carbon through the porous network of the powder metal parts early in the process. This allows the initial carbon profile to be more effectively "doped" into the portion that will become the outer surface of the part after the forging step, rather than relying solely on solid-state diffusion as a transport mechanism, as is necessarily the case in fully dense parts. After forging, the forged part can be carburized again at elevated temperatures; however, since some locked-in carbon has already formed at a certain depth on the outer surface of the forged powder metal part, this initially introduced carbon can now diffuse further into the part during heating, while introducing new carbon at the part surface, resulting in a higher carbon content at that surface. This multi-step process can produce more complex and progressive carbon gradients by varying different process parameters, which can be widely varied and designed to have the desired case profile.

[0012] In addition to providing a less abrupt transition between the hardened shell and the core, the forging step can be specifically designed to form an initial, locked carbon profile. For example, by creating variable flow of part material during the forging step (which can be achieved by designing the pre-forging preform to be disproportionate in size to the post-forging form), the cross-section of the initial carbon profile can be thickened and / or thinned, or stretched and / or compressed, to create an initial carbon layer of varying thickness prior to subsequent carburizing steps. In this way, fairly precise, custom carburizing profiles can be produced.

[0013] These improved carburizing methods offer numerous advantages relative to current technology. As an example, this sequence allows for the formation of variable shell depths in the resulting parts using conventional shell carburizing processes (i.e., sintering and gas carburizing on parts at full or near-full density), but with reduced time and cost compared to carburizing fully dense forged parts. Furthermore, this sequence allows for the formation of variable shell depths and geometries that cannot be produced using only existing sintering carburizing techniques.

[0014] According to one aspect, a method for carburizing powder metal parts is provided. In a pre-forging carburizing step, the powder metal part, which is smaller than fully dense, is carburized to form a pre-forging carburized profile. Then, after the pre-forging carburizing step, the powder metal part is forged to increase its density. This forging also transforms the pre-forging carburized profile into a forged carburized profile. In a post-forging carburizing step, the powder metal part is carburized after the forging step. This post-forging carburizing step causes carbon to diffuse further from the forged carburized profile into the powder metal part, and further introduces carbon (from the carburizing environment) into the powder metal part at its surface.

[0015] In some forms of this method, the powder metal part can be forged to its effective full density during the forging step. Those skilled in the art will recognize that even after forging, the effective full density of the powder metal part is less than the theoretical full density and involves eliminating virtually all internal porosity. In the case of ferrous parts, the effective full density is expected to be, for example, 98% or more of the theoretical full density. However, the effective full density of a given material can also be determined with reference to the standards of the Metal Powder Industries Federation (MPIF). The theoretical full density in this application refers to the true density of the material, which corresponds to a full density without any pores or voids.

[0016] In some forms, the density of a powder metal part prior to the forging step may be 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. However, it should be understood in any case that the density of the powder metal part after the first pre-forging carburizing step is less than the full density, and that, at least in some forms, there is a network of connecting pores to allow non-solid or gaseous migration of carbon within the powder metal part, making the carburizing depth not solely a primary function of time and temperature as in fully dense parts.

[0017] In some forms of this method, the method may further include sintering the powder metal part. When the method includes sintering, it is anticipated that the pre-forging carburizing step may occur during the sintering of the powder metal part (i.e., simultaneously or in parallel as in a sintering-carburizing process). In this way, it is anticipated that, when in green compact form, the powder metal part has a relatively low density relative to its theoretical full density, and that the remaining porous network of the green compact can at least temporarily be used to transport carbon from the atmosphere in a gaseous state into the compact when any binder or lubricant holding the powder metal particles is burned off. For example, the density of the powder metal part before sintering may, for example, be less than 90% of the theoretical full density of the powder metal part.

[0018] In some forms, the base carbon percentage of the powder metal part can be from 0.1 wt% to 0.3 wt% of the powder metal part before the pre-forging carburizing step. The "base carbon percentage" refers to the carbon content in the material during powder metal processing in the form of a compact or sintered metal, prior to any carburizing heat treatment. For such an initial base carbon percentage, the carbon percentage in the carburized region of the pre-forging carburized profile after the pre-forging carburizing step can be, for example, 0.5 wt% to 0.6 wt% carbon. After the forging and post-forging carburizing steps, the carbon percentage in the region closest to the surface of the powder metal part can be from 0.7 wt% to 0.9 wt% carbon.

[0019] In some forms of this method, converting a pre-forging carburized profile into a forged carburized profile may include starting with a pre-forging carburized profile having a uniform depth relative to the surface of the powder metal part, and then forging the powder metal part such that, after forging, the forged carburized profile has a different depth relative to the surface of the powder metal part. In other words, forging can cause geometric changes in the profile due to varying material flow during forging. This can be specifically designed into the process and is described, for example, in U.S. Patent No. 8,517,884 entitled "Powder Forged Differential Gear" issued by GKN SinterMetals, LLC, dated August 27, 2013, which is incorporated herein by reference in its entirety for all purposes.

[0020] In some forms, powder metal parts can be iron-containing, and specifically, the carburizing is carried out on iron-based powder metal particles.

[0021] In some forms of this method, the pre-forging carburizing step can result in a carburizing depth greater than that of a fully densified control powder metal part with a similar composition under similar carburizing time-temperature-atmosphere exposure conditions. This greater carburizing depth is because the porosity of the powder metal part prior to the forging step provides a non-solid diffusion pathway, allowing carbon to penetrate deeper into the volume of the powder metal part compared to a fully densified part.

[0022] In some forms, powder metal parts prepared by this method can be gears. Because forging is impossible in pin holes or splines, for example, when the surface already has significant outer hardening before forging, differential gears, especially small differential gears, may have characteristics that cannot be processed using conventional sintering carburizing techniques. In hypoid gears, root depth hardness may not be maintained due to material flow. In parallel-shaft gears with small gear modules, the carburized layer may be too deep relative to the tooth thickness.

[0023] In some forms, powder metal parts can have a final carburized profile during the post-forging carburizing step. The final carburized profile can have a carbon gradient from the surface to the volume of the powder metal part, which is gentler than the carburized gradient of a fully dense part carburized in a single step under similar time-temperature-atmosphere exposure conditions.

[0024] In some forms, the method may also include machining the powder metal part (e.g., soft turning) between the forging step and the post-forging carburizing step. At this point, although some carbon is added, but not all of it, the part is in its final or near-final geometry, and forging the powder metal part at this stage may be easier than doing so after the post-forging carburizing step.

[0025] However, powder metal parts produced by the various methods described herein are also considered to be within the scope of this invention.

[0026] These and other advantages of the invention will be understood from the following detailed description and accompanying drawings. The following description pertains only to some preferred embodiments of the invention. In order to assess the full scope of the invention, it should be understood from the claims that these preferred embodiments are not intended to be the only embodiments within the scope of the claims. Attached Figure Description

[0027] Figure 1 A and 1B provide shell profile images of forged parts produced using only conventional gas carburizing methods.

[0028] Figure 2 A and 2B provide shell profile images of powder metal parts produced using only conventional sintering and carburizing methods, where all carburizing occurs prior to the forging step.

[0029] Figure 3 A and 3B provide shell profile images of powder metal parts produced using a multi-stage carburizing process that includes sequential sintering carburizing, forging, and gas carburizing steps.

[0030] Figure 4 A schematic diagram of the stepwise sintering and carburizing of powder metal parts and the carbon content in each region is provided.

[0031] Figure 5 A and 5B provide images depicting the following items: Figure 5 A depicts the gear form before forging carburization and after forging, and... Figure 5 B depicts the form of the same gear after a further final carburizing step.

[0032] Figure 6 Provided similar Figure 4 The diagram shows the final gas carburizing step after the forging process. Detailed Implementation

[0033] To provide the best understanding of the disclosed method and its improvements over existing technologies, a brief comparison of conventional gas carburizing and conventional sintering carburizing will now be provided. Understanding these carburizing modes allows for the understanding of how the novel method achieves aspects of both modes (which can be slightly modified) through an intermediate forging step, thereby obtaining a unique carburized profile in powder metal parts and enabling cost reduction.

[0034] In conventional gas carburizing, a fully dense iron-containing product is placed in a heated environment (containing a carbon-containing gas) for a period of time under conditions that cause carbon from the environment to be absorbed into the iron-containing product. Increased heat leads to shorter cycles and increased carbon penetration depth per time interval, but may come at the cost of other undesirable metallurgical or dimensional effects (e.g., grain growth, sagging, or warping of the part). Therefore, conventional gas carburizing methods are limited by the operating heat (i.e., temperature) and the material diffusion profile at that temperature. For materials based on gas carburizing, the final result is therefore highly dependent on the time and diffusion profile at that temperature.

[0035] In fully dense forged materials, the carburized surface profile is typically described as an extremely high carbon region above an extremely low carbon region with a narrow transition zone. Due to the method of this process, the carburized zone of the part usually has a very uniform thickness across the entire exposed surface.

[0036] Figure 1 Images A and 1B depict the uniformity of the profile in two images of forged and gas-carburized gear teeth, where the housing profile has a very uniform thickness on the tooth surface. Figure 1 A is a photomicrograph showing the depth of the outer shell, with arrows indicating the uniform depth of the outer shell at different locations on the tooth surface. Figure 1 B shows the Vickers hardness values ​​of the teeth at different locations, and it can be seen that the outer surface layer of the teeth has a high hardness value (up to 786 HV), while the core has a significantly lower hardness value (only 435-500 HV). The transition between the outer layer or hardened shell and the core is very abrupt and sharp.

[0037] Commercially, most carburizing of iron-containing components is based on gas carburizing of forged or fully dense parts. This is largely because many components are fully dense prior to carburizing due to their manufacturing method (such as casting, extrusion, etc.).

[0038] However, it is worth noting that there is another, less common method for carburizing powder metal parts, called sintering carburizing or “sinta carb,” which can be used to develop a deeper carbon profile.

[0039] To understand how sintering and carburizing work, it is first necessary to understand how powder metal parts are typically manufactured. In the production of powder metal parts, powdered metal is compressed together with a binder, wax, and / or lubricant to form a green compact with a geometry very similar to the final desired product. The green compact is then sintered in a furnace at a temperature typically close to, but slightly below, the melting temperature of the powder metal. While some sintering processes involve a limited amount of the green compact becoming liquid (known as liquid-phase sintering), many sintering processes are based solely on solid-state diffusion, where the binder, wax, and / or lubricant are typically burned off as the powder metal particles neck each other to form a uniform sintered body. During the sintering process, the spaces between the particles provide a network of pores, although these pores decrease in size and close as sintering continues. However, even after sintering, a certain amount of porosity may still exist in the sintered body.

[0040] Sintering carburizing utilizes the presence of this porous network to introduce carbon during the sintering production step, when the part is already at a high temperature. Because a porous network exists between the powder metal particles at least at the beginning of the sintering step, carbon-containing gases can flow into these pores to transport the carbon-containing gaseous state deeper into the part body, reaching depths at least beyond the final outer surface of the sintered part, allowing carbon to be absorbed at these deeper depths. Since gaseous transport through the porous network is primarily based on gas flow rather than solid-state diffusion, this sintering carburizing technique can rapidly achieve deeper carburizing depths and gentler gradients than gas carburizing of fully densified parts. Therefore, sintering carburizing can be used for effective carburizing and increasing diffusion depth in low-density powder metal parts, typically performed before the final fully densified forging process.

[0041] refer to Figure 2 A and 2B show the relationship with Figure 1 The gear tooth profiles of A and 1B are similar, but... Figure 2 The gear teeth in A and 2B are produced by sintering carburizing rather than gas carburizing. It can be seen that, given the gaseous diffusion into the pores of the part, there is a wider transition zone from high-carbon to low-carbon areas. Variations in the density of the powder metal compact can lead to different carbon diffusion depths in different regions, and post-sintering carburizing forging can also cause variations in the shell thickness. From... Figure 2 Micrograph A shows that the carbon diffusion depth is greatest at the tip of the tooth and becomes shallower near the root. This is illustrated in the Vickers hardness values, which range from 334 HV to 714 HV. Figure 2 B shows that, with Figure 1 Compared to the parts in B, there is deeper carbon penetration into the teeth and a smoother transition from the core to the surface layer.

[0042] However, even within the sintering-carburizing process, there are certain conditions and products where the process does not yield ideal results. For example, in parts with thin cross-sections or low-profile rack / gear modules, effective carburizing is often impossible before reaching a fully hardened state due to the high diffusivity of the carbon material during the sintering-carburizing process. The end result can be a lack of ductility and toughness due to deep shell embrittlement, and based on these characteristics, the product may be unsuitable for metallurgy. Furthermore, products that do not meet process design requirements may also have a shallow shell depth due to excessive thinning of the material during forging operations, resulting in lower strength and / or impact resistance.

[0043] This paper discloses a method that combines two carburizing modes (i.e., sintering carburizing for powder metal parts with less than full density and gas carburizing for parts with full or near-full density) and an intermediate forging step. The method includes a sequential first carburizing step while the part is still below full density, a forging step to increase the part density, and a second carburizing step, which typically involves gas carburizing for parts near or fully dense.

[0044] At this stage, it should be noted that the first carburizing step can be sintering carburizing—performing both the sintering and first carburizing steps simultaneously may be the most economical—however, it is also conceivable that there may be methods to perform the sintering and first carburizing steps separately. However, this decoupling of the sintering and first carburizing steps may need to be carefully and thoughtfully executed, because once the pores in the part close (which typically occurs at the end of sintering), the mechanism by which carbon-containing gases can migrate into the part is compromised and weakened.

[0045] It should also be noted that while the first step may involve sintering carburizing on parts that are not fully dense, this sintering carburizing differs from known sintering carburizing methods because it is not performed to provide all the carbon in the final carbon profile. In other words, in this novel method, only a portion of the final shell profile is established during the sintering carburizing step, whereas in conventional sintering carburizing, sintering carburizing is the final and only step to provide carbon for carburizing.

[0046] The disclosed method, often referred to as variable diffusion carburizing (or VDCC), is an improvement on both conventional gas carburizing and sintering carburizing processes, addressing some of their respective shortcomings. By combining aspects of both processes, it is possible to produce customized deep shell depths with variable properties, while reducing costs compared to conventional gas carburizing processes.

[0047] Similarly, at a higher level, the method includes a pre-carburizing step for parts below fully dense, a forging step to increase the density of partially carburized parts and transform the pre-carburizing profile into a forged carburizing profile, and a post-carburizing step in which carbon from the forged carburizing profile further diffuses into the powder metal part and introduces additional carbon at the surface of the powder metal part. Each of these steps is now described in more detail below.

[0048] In the first step, for example, the method can begin by a mild carburizing step on a powder metal part or an iron-containing steel billet smaller than a fully dense powder metallurgy part. This smaller than fully dense powder metallurgy part can be a green compact compacted using conventional powder metallurgy compaction techniques, wherein the compacted part height is close to the final geometry of the final part, but slightly larger in size to account for shrinkage during sintering. It is conceivable that, as a green compact, the density of the powder metal part can be less than 90% of the theoretical full density.

[0049] The sintering of smaller than fully dense powder metal parts involves metallurgically binding the powder metal particles together, which may also involve burning off any wax, binder, or lubricant in the powder metal parts. As described above, a porous network is formed when the powder metal particles bind together and neck. During the sintering operation—which takes place at elevated temperatures—the powder metal parts can then readily accept carbon from the sintering atmosphere, assuming a sufficiently high carbon concentration, to thermodynamically drive the carbon into the iron-containing powder metal compact.

[0050] Adding carbon from the environment during sintering is a sintering carburizing process that produces a controlled carbon content and depth as an initial carburizing profile. This initial carburizing profile is also known as the pre-forging carburizing profile, given the subsequent forging steps. As described in the section above describing the sintering carburizing process, carbon readily penetrates the part due to the lower density of powder metal parts and their very high diffusion rate, resulting in a deep carbon profile.

[0051] A specific example of the first sintering and carburizing step is as follows: Figure 4 As shown. In Figure 4 In the diagram, the topmost or first box shows an iron-containing powder metal part starting with 0.10 wt% to 0.30 wt% carbon. The powder metal part contains a certain amount of initial carbon, albeit low, likely because the goal of the process is to create a carburized outer shell layer, and 0.10 wt% to 0.30 wt% carbon is typical for many applications; however, the specific amount of carbon ultimately depends on the desired hardness of the material's Jominy curve relative to different locations used in the final part production and the type of quenching method. After sintering and carburizing, a carbon-enriched layer of approximately 0.50 wt% to 0.60 wt% carbon can be formed, such as... Figure 4The central and second boxes are shown. The bottom and third boxes indicate that the carbon profile migrates considerably with additional carbon diffusion and time, and a large gradient forms between the outer surface layer and the core with higher carbon content (maintaining approximately 0.10 to 0.30 wt% carbon). It should be understood that these figures are merely exemplary and should not be considered limiting.

[0052] Similarly, it should be understood that in most forms of this method, the first carburizing step will occur simultaneously or in parallel with the sintering step. In this way, carbon can be easily introduced through a porous network smaller than that of a fully dense part. The combination of sintering and carburizing is economical, and the heating of the part used for sintering can also be used to promote carbon diffusion. However, it is also conceivable that the first carburizing step can be performed separately from sintering, although such separate carburizing is most effective when the particles are partially bonded and the pores are not yet closed. In other words, if the part is subsequently sintered and then carburized, a large number of pores may close during sintering, and the effectiveness of carburizing will be reduced unless the process is carefully controlled before carburizing to prevent pore closure.

[0053] Next, the powder metal part is forged to increase its density under suitable material flow conditions. In some forms, this may include increasing the density to an effective complete density, which may be, for example, the MPIF effective complete density. As noted in other parts of the invention, the effective complete density is less than the theoretical complete density, but is generally close to it. In some forms, this effective complete density may be higher than 98% of the theoretical complete density, and the density of the powder metal part before forging, which is less than fully dense, may be lower than 95% of the theoretical complete density. This forging step may result in a slightly higher carbon surface content in the powder metal part, a consequence of the forging process. Furthermore, the forged powder metal part may be placed under non-quenching conditions for cooling.

[0054] Many end uses of parts manufactured by these methods require products with the strength obtained through forging, where the density and strength of the parts are significantly increased.

[0055] It is worth noting that the forging process not only increases the density of the powder metal part, but can also transform the pre-forging carburized profile into a forged carburized profile. This transformation may simply involve making the part uniformly denser, and in the process, only slightly altering the dimensions and shell profile (if the pre-forging powder metal part has a pre-forging carburized profile of uniform depth, and the forged powder metal part has a forged carburized profile of equally uniform depth, even if there is a slight difference in depth between the pre-forging and forged parts due to forging and densification). However, in more complex or elaborate methods, the forging process can also thicken and / or thin the carburized profile segment due to variable material flow in the forging die. Similarly, how to use this variable forging to change the carburized layer or carburized layer transformation can be found in U.S. Patent No. 8,517,884 entitled "Powder Forged Differential Gear" issued by GKN Sinter Metals, LLC on August 27, 2013, which is incorporated herein by reference in its entirety for all purposes. For a direct example, the carburized profile can be achieved by forging a preform with a relatively uniform shell depth, such as... Figure 4 As shown in the center panel, the shell depth is thinned in the root section and thickened in the tip section. This involves engineering the preform (i.e., preformed powder metal part) and tooling and die assembly to achieve the final outer surface geometry and variable shell depth of the forged part.

[0056] It should also be noted that because only a portion of the carbon is added in the first carburizing step (with additional carbon applied after forging), powder metal parts may be easier to forge than parts with all the carbon added before forging, since heavy carburizing produces a hard, brittle outer shell layer that is, if not impossible to forge, very difficult to forge.

[0057] Furthermore, the forging step is described herein as increasing the density of the powder metal part; however, in most cases, this may involve forging the powder metal part to its effective full density (an effective state in which almost all pores are closed or eliminated during forging). Nevertheless, it is anticipated that in some cases, a powder metal part that is less than effectively fully dense but still densified may have sufficient closed pores to allow for a second carburizing step.

[0058] At this point, after the forged powder metal part has been forged and controlled-cooled, a second carburizing step can be performed on the forged powder metal part (now with increased density or effective full density) in the post-forging carburizing step. Now, as the powder metal part reaches increased density or effective full density, it can undergo gas carburizing with significantly higher efficiency due to the carbon introduced in the first carburizing step prior to forging. Therefore, during this second carburizing step, pre-existing or "doped" or "filled" carbon from earlier in the process can continue to diffuse into the powder metal part, and additional carbon can be absorbed into the surface of the powder metal part to create additional carbon-rich outer regions or shells. Because of the pre-existing carbon already present from the first carburizing step, the second gas carburizing step is not as strictly limited by time and temperature as pure gas carburizing of fully dense parts (where carbon is introduced in a single cycle / step), thus avoiding a large portion of the slow diffusion rate.

[0059] Figure 5 A, 5B, and 6 illustrate the benefits of this carbon doping or filling in forged parts. Figure 5 Panel A shows the outer surface profile of the gear tooth component and the underlying carbon layer. Similarly, in this panel, the powder metal part has been forged, so the carbon layer obtained from the first carburizing step has varying depths relative to the tooth surface, being thicker at the tooth tip and thinner at the tooth root. Figure 5 In step B, after further gas carburizing of the part in the second post-forging carburizing step, the original shell outline (now shown as a dashed line) has been further penetrated into the part body in a generally uniform manner. Figure 6 This is also illustrated schematically. In Figure 6 In the middle, the first three panels display information related to the above. Figure 4 The same process, followed by additional gas carburizing in the second post-forging carburizing step, forms the outline of the rightmost panel, wherein the surface carbon content can be increased to about 0.70% to 0.90% by weight of carbon, and transitions to another region with higher carbon content of 0.5% to 0.6% by weight of carbon before reaching the core.

[0060] Figure 3 Figures A and 3B also show the contours obtained by a method involving two carburizing steps (separated by a forging step), which provides a comparison with the above. Figure 1 Comparison images of images A, 1B, 2A, and 2B. Figure 3 As can be seen from A, especially with the passage Figure 1 The forging and gas carburizing processes shown in A and 1B, as well as Figure 2Compared to parts produced by sintering-infiltration and forging processes shown in A and 2B, a deep and variable shell depth has been achieved, with a considerable carbon gradient existing in the transition zone between the hardened outer surface and the core. Similarly, Figure 3 B shows the hardness distribution, which exhibits a large gradient and smooth transition.

[0061] In addition to the improved structure and shell profile, there are many other methodological benefits.

[0062] By using forged parts with "doped" or pre-carburized conditions, the surface carbon content of forged powder metal parts may increase by less than half compared to powder metal parts without pre-carburization. Therefore, the immersion time at high carbon levels during post-forging carburizing can be significantly shortened. Similarly, for powder metal parts with high surface areas—where the ability of the atmosphere or environment to provide carbon to the part surface quickly enough is a rate-limiting factor—these parts can be processed more rapidly because some of the carbon to be introduced into the part is already present in the part from the pre-forging carburizing step.

[0063] Furthermore, using this method, many shell depths already exist and are within the near-surface of the powder metal part, involving only a reduction in the time and energy required for further progress. This can produce results such as Figure 3 The smoother transition from the outer shell to the core shown in A and 3B (this also illustrates how variable forging can be used to produce different shell depths) ultimately reduces internal shear stress due to the progressive stress state that can be matched to the product's performance requirements.

[0064] Furthermore, this process can be customized for products with fewer modules and thinner design components, where thorough or complete hardening may need to be considered during the sintering carburizing / forging-only process. In this process, due to geometric conditions, standard sintering carburizing cannot achieve the final forging form (i.e., thin sintered carburized components cannot be forged without cracking). However, this two-stage carburizing process offers the ability to effectively forge parts pre-filled with a small amount of carbon (which can still be forged without cracking), and then further carburize the part after forging to obtain the additional carbon needed for the final desired hardness.

[0065] To understand the economic benefits of this novel method, compared to methods that only perform gas carburizing on powder metal parts without pre-doping or filling with carbon, this novel method—doping or filling carbon into the powder metal parts before forging—is estimated to reduce the cost of subsequent gas carburizing after forging by 40%. This 40% cost reduction in this example is based on increased throughput and reduced time requirements to achieve the same or deeper carbon penetration depth using post-forging gas carburizing as the sole carburizing mode. Individually, the sintering carburizing step of carburizing powder metal parts before forging does not involve many of the additional capital costs associated with its implementation. Compared to conventional sintering carburizing, which introduces all carbon into a single process, the first pre-forging carburizing step is not designed to impart all carbon in a single shot peening or implementation, thus resulting in lower implementation costs and higher throughput compared to conventional sintering carburizing processes.

[0066] Because the carburizing steps are separated—that is, before and after the forging steps—there is no choice or advantage to directly quenching and tempering from the forging press. Therefore, by definition, forged products are non-heat-treated products that can be soft-machined according to the final geometry. This can also lead to cost reductions on medium to high-machined products, as soft turning is generally cheaper than hard turning, while still allowing for additional cost reductions in gas carburizing operations.

[0067] While the methods described herein involve more processing steps than fully sintered carburizing alone, these novel methods do not involve more steps than conventional gas carburizing processes, which already include sintering, forging, and gas carburizing. Therefore, the methods described herein offer better economics while providing opportunities for highly engineered shell profiles (not yet found in single-step carburizing methods).

[0068] It should be understood that various other modifications and changes can be made to these preferred embodiments within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. For determining the full scope of the invention, reference should be made to the appended claims.

Claims

1. A method for carburizing powder metal parts, the method comprising the following steps: In the pre-forging carburizing step, carburizing is performed on powder metal parts smaller than fully dense to form a pre-forging carburized profile. Following the pre-forging carburizing step, the powder metal part is forged to increase its density and transform the pre-forging carburized profile into a forged carburized profile; and In the post-forging carburizing step, the powder metal part after the forging step is carburized, which causes carbon to diffuse further from the carburized profile of the forging into the powder metal part, and further introduces carbon into the powder metal part at the surface of the powder metal part.

2. The method of claim 1, wherein, During the step of forging powder metal parts after the pre-forging penetration step to increase the density of the powder metal parts, the powder metal parts are forged to the effective full density of the powder metal parts.

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

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

5. The method of claim 1, wherein, The method further includes sintering the powder metal parts, wherein the pre-forging carburizing step occurs during the sintering of the powder metal parts.

6. The method of claim 5, wherein, Before sintering, the density of powder metal parts is less than 90% of the theoretical full density of powder metal components.

7. The method of claim 1, wherein, Prior to the precalcination carburizing step, the base carbon percentage of the powder metal parts is 0.1% to 0.3% by weight of the powder metal parts.

8. The method of claim 7, wherein, After the pre-forging carburizing step, the carbon percentage in the carburized area of ​​the pre-forging carburizing profile is 0.5 wt% to 0.6 wt% carbon.

9. The method of claim 8, wherein, After the post-forging carburizing step, the carbon percentage in the area closest to the surface of the powder metal part is 0.7% to 0.9% by weight.

10. The method of claim 1, wherein, Converting a pre-forging carburizing profile into a forged carburizing profile involves starting with a pre-forging carburizing profile having a uniform depth relative to the surface of the powder metal part, and forging the powder metal part such that the forged carburizing profile has a different depth relative to the surface of the powder metal part.

11. The method of claim 1, wherein, Powder metal parts contain iron.

12. The method of claim 1, wherein, The pre-forging carburizing step results in a greater carburizing depth than a fully dense control powder metal part with a similar composition, because the porosity of the powder metal part before the forging step provides a non-diffusive pathway for carbon to penetrate deeper into the volume of the powder metal part.

13. The method of claim 1, wherein, Powder metal parts are gears.

14. The method of claim 1, wherein, After the post-forging carburizing step, the powder metal part has a final carburized profile with a carbon gradient from the surface of the powder metal part to the volume, which is gentler than the carburized gradient of a fully dense part that has been carburized in a single step.

15. The method of claim 1, further comprising: The powder metal parts are machined between the forging step and the post-forging carburizing step.

16. A sintered powder metal part prepared by the method of claim 1.

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