JP2010537048A - Manufacturing method of forged carburized metal powder parts - Google Patents

Abstract

  The present invention provides a method for producing metal powder parts that are not selectively carburized. In this manufacturing method, a metal powder includes a plug having one or more first surfaces forming a hardened layer depth hardness surface and one or more second surfaces forming a carburized portion to be removed before forging. A compression step for compressing the preform, a carburizing step for sintering or carburizing the preform sequentially or simultaneously, a removal step for removing the carburized portion from the one or more second surfaces of the preform, and the preform One or more second surfaces having improved post-forging characteristics and surface hardness characteristics by sequentially performing a forging step for forging a forged part and a cooling step for cooling the forged part. A forged part having one or more first surfaces is obtained.

Description

  The present invention relates to a forged powder metal part, in particular a selectively non-carburized metal powder part, and a method of manufacturing the same.

  In producing parts, there has been a need for a manufacturing process that can reduce cost, time, and steps. The benefits brought about by the improved manufacturing process are sought by customer demand to first improve the product to have better dimensional, mechanical and / or performance characteristics. For example, a conventional differential side gear may have any or all of the following performance requirements: dimensional precision, high shear strength, and brinnelling. spline area that requires resistance; hub and thrust face that require dimensional accuracy, surface finish, and case compatibility; dimensional accuracy, surface finish, and Gear geometry requiring an optimized profile; and impact and wear resistance, spalling resistance, and tooth and core strength that may require different surface and core metallurgies . Different performance requirements of the same part or otherwise are achieved by different non-compatible manufacturing methods, ie casting, steel forging or metal powder forging.

  According to FIG. 1, in order to satisfy such performance requirements, the gear 10 forges the metal powder 14 and then carburizes the gear to obtain a substantially constant effective case depth 16. Is manufactured. For a certain effective hardened layer depth 16 for each (gear) tooth 12, see the partial cross-sectional view of FIG. The parameters to be controlled in order to carburize a substantially dense part (part) with specific hardness, hardened layer depth and carbon gradient almost constant are generally known. ing. However, a substantially constant hardened layer depth does not necessarily have a predetermined mechanical property and a predetermined machining property in a post-forged product. It would be advantageous to better balance these performance requirements in the final product, which saves time, processing, or costs from not being adversely affected by the manufacturing process.

  A manufacturing process for improving the performance requirements of metal powder parts in a process known as “sinter-carburization” is described in US Pat. No. 3,992,763 (Title: Metal (Production method of powder parts). According to this manufacturing process, a carburization process is performed after sintering and before forging to improve the hardened layer depth in the critical wall of the final forged product, thereby obtaining case hardness. The subsequent heat treatment step can be eliminated (omitted). U.S. Pat. No. 4,002,471 (invention name: method for producing scale-free forged metal powder product cured without heat treatment after forging) is subjected to further machining, surface treatment or heat treatment after quenching Without mentioning, a method for producing a wrought iron powder-based metal product having a high Rc hardness is described.

  U.S. Pat. No. 4,165,243 (Title for making selectively carburized forged metal powder parts) includes a step of masking parts prior to sintering, and between carburizing and forging. A manufacturing process is described that further includes removing the masking to provide a selected carburized surface on the part.

  However, the above-mentioned patent documents provide improved manufacturing methods, improved dimensional precision, or performance characteristics of the carburized and non-carburized surfaces of parts, but remove the carburized surface before forging. There is no description or suggestion of a process by which specific material requirements in the final forged product can be obtained. Further, the above-described patent document does not describe or suggest any advantageous and simple control obtained by the removal step.

  Accordingly, there has been a need for a manufacturing method that removes the carburized surface of the part prior to forging and selectively leaves the carburized and non-carburized surface in the final forged product. Also, strategically removing the carburized part of the part before forging and quenching, while retaining the advantageous forged carburized part of the part, while providing better post-forging operation for the non-carburized part of the part. For manufacturing methods that allow for improved tighter tolerance, improved spline classification, or improved shear resistance properties due to the non-carburized part of the part Needs existed. There has also been a need for parts with excellent dimensional precision or performance features.

US Pat. No. 3,992,763 U.S. Pat. No. 4,002,471 U.S. Pat. No. 4,165,243

  In response to the aforementioned needs, the present invention removes the carburized surface of the part prior to forging, selectively carburized parts with improved performance characteristics, improved tolerance and classification. And a method of manufacturing the same, characterized by leaving a selectively non-carburized surface with improved post forging properties.

  Specifically, the present invention provides a method for obtaining a metal powder part that is not selectively carburized. The method includes a compression step, a sintering step, a removal step, a forging step, and a cooling step. By compressing the metal powder, one or more first surface portions (where the forged portion needs to have a case hardness depth) and one or more second surface portions (where the carburized portion is A preform having a need to be removed before forging). The preform is subjected to a sintering and carburizing process. After carburizing, one or more second surface portions of the preform are removed, and forging and cooling are performed. The forged portion has one or more second surface portions having improved shear resistance properties and one or more first surface portions having surface hardness properties.

  The present invention provides a part manufactured according to the method of the present invention. This part is based on a softer noncarburized material portion, providing improved tolerances and dimensional control, while forging at less cost and time It includes a noncarburized portion that allows processing (post-forging treatment). This part also has improved wear resistance, load bearing, impact resistance, which can be obtained through a sint-carb forging manufacturing process with a net-finish finish. Alternatively, it includes a carburized portion having advantageous properties such as bending fatigue.

  According to the present invention, a carburized part (selectively carburized part) having improved performance characteristics (for example, wear resistance, load resistance, impact resistance, bending fatigue, etc.) and improved tolerance characteristics. In addition, it is possible to obtain a metal powder part (such as a gear) having grade characteristics and a non-carburized part (a part that is not selectively carburized).

FIG. 1 is a partial cross-sectional view of a case carburized gear. FIG. 2 is a partial cross-sectional view of a differential side gear having a variable hardened layer depth profile that is advantageously used in accordance with embodiments of the present invention. FIG. 3 shows the microstructure below the effective hardened layer depth of the gear of FIG. FIG. 4 shows the microstructure within the effective hardened layer depth of the gear of FIG. FIG. 5 is an isometric view of the sintered preform according to the present invention required to obtain the product of the present invention after forging. 6A is a partial cross-sectional view of the preform of FIG. 5 after carburizing, and FIG. 6B is a partial cross-sectional view of the carburized portion to be removed from the preform of FIG. 6A by a removal process prior to forging. FIG. 7 is an isometric view of a differential side gear made from the preform of FIG. 6B in accordance with an embodiment of the present invention. FIG. 8 schematically shows a manufacturing process of a variable hardened layer depth metal powder gear according to a specific example of the present invention. The example (manufacturing process) of this invention for manufacturing a forged carburized metal powder component is shown roughly.

  For better understanding of the present invention, specific examples of the present invention will now be described with reference to the accompanying drawings.

  In the drawings, the same reference numerals denote similar parts. Accordingly, the same reference numerals may be given in different drawings. In some examples, similar parts in different drawings may be numbered differently for reasons such as clarity.

  FIG. 2 is a partial cross-sectional view of a differential side gear 50 having a variable case depth profile 58 in accordance with the advantages of the present invention. FIG. 7 is an isometric view of a differential side gear 50 made from the preform of FIG. 6B in accordance with the present invention. Before discussing the features of the present invention to obtain selectively non-carburized metal powder parts, a gear 50 having a variable hardened layer depth profile 58 that is one of the features of the present invention will be described. To do.

  The differential side gear 50 includes a variable hardened layer depth profile 58 formed in the carburized portion of the gear 50 and a plurality of teeth 52. Each tooth of the plurality of teeth 52 has a first surface 54 and a tooth core or root 56. The differential side gear 50 has a rotation shaft 60. Here, the teeth 52 extend radially in the same direction as the rotation axis of the gear, but are inclined with respect to the rotation axis. The differential side gear 50 includes an axially splined internal section 62 that is axially aligned with the rotary shaft 60. This internal region 62 is formed in the non-carburized portion of the gear 50. The non-carburized portion is selectively obtained by removing the carburized portion of the preform before forging as described herein.

  The variable hardened layer depth profile 58 is formed on the plurality of teeth 52. The variable hardened layer depth profile 58 can provide a gear having better tooth wear resistance on the first surface 54 and better impact resistance on the root 56. This variable hardened layer depth profile 58 can be an effective hardened layer depth profile obtained after forging by carbon diffusion before forging (of the gear). Therefore, the variable hardened layer depth profile 58 can be obtained by the forging process described in this specification.

  Although the process will be described with respect to differential side gear 50, variable hardened layer depth profile 58 may be used for various gears including bevel gears, differential gears, or pinion gears (small gears). Alternatively, it can be obtained for parts.

  The differential side gear 50 may be constructed of a low alloy, fully compacted iron metal powder material. However, the gear can be composed of various types of forged metal powder steel.

  According to FIG. 2, the first surface 54 of each tooth in the differential side gear 50 includes a tip surface 64, a pitch line surface 66, a root fillet surface 68, And includes a root diameter or land surface 70. The variable hardened layer depth profile 58 has an effective hardened layer depth of 2.4 mm at the tip face 64, 1.9 mm at the pitch line surface 66, 0.4 mm at the root fillet surface 68, and 0.8 mm at the root land surface 70. (Effective case depth). It is due to carbon diffusion and subsequent preform forging. Although specific values are shown in this example, the variable hardened layer depth may have an effective hardened layer depth profile that is not constant on the cross section of a specific surface, and is limited to the specific profile shown here. Absent.

  The variable hardened layer depth profile 58 can be expressed as a hardened layer depth ratio. The effective hardened layer depth ratio may be determined by comparing the hardened layer depth measured at the tip face 64 with the hardened layer depth measured at the root fillet surface 68 or at the pitch line surface 66. The measured hardened layer depth is compared to the hardened layer depth measured at the root fillet surface 66 or the hardened layer depth measured at the root land surface 70 is measured at the root fillet surface 68. It can be obtained by comparing with the layer depth. For example, the variable hardened layer depth ratio of tip surface 64 to root fillet surface 68 is 6: 1, the variable hardened layer depth ratio of pitch line surface 66 to root fillet surface 68 is 19: 4, and The hardened layer depth ratio of the root land surface 70 to the root fillet surface 68 is 2: 1. A hardened layer depth ratio of approximately 1: 1 is considered to be within the effective range of a fixed hardened layer depth 16 of the gear 10 shown in FIG.

  Advantageously, the hardened layer depth ratio in the variable hardened layer depth profile 58 from the maximum depth to the shallower depth is 6: 1. This is because better mechanical properties such as tooth wear resistance and impact resistance can be obtained.

  The tooth root 56 of the gear 50 includes a mid-tooth section 74 having a hardness of about 43 HRC, a root section 76 having a hardness of about 31 HRC, and a core (central portion) having a hardness of about 32 HRC. ) (Core section) 78. These numerical values (hardness) are only given as examples of gears having improved mechanical properties, and the core hardness ratio is determined by the tooth intermediate part 74 and the root or core parts 76 and 78. Between, and is approximately 4: 3. A higher core hardness ratio means that the gear is superior in tooth impact resistance, ie, ductility. On the other hand, since the gear as shown in FIG. 1 has a core hardness ratio of 1: 1, it is inferior in flexibility (ductility).

  3 shows the microstructure below the effective hardened layer depth of the gear of FIG. 2, and FIG. 4 shows the microstructure within the effective hardened layer depth of the gear of FIG. Yes. The depth boundary is the point at which the effective carbon content of the material can be made approximately constant and enabled by the variable hardened layer depth profile 58.

  Returning to the method of manufacturing the metal powder gear with variable hardened layer depth, the process is shown in FIG. The process begins with mixing (step) 20 followed by filling (step) 22, compression (step) 24, sintering (step) 26, carburization (step) 28, preheating (step) 30, variable forging (variable). forging) 32 and cooling (step) 34. The post forging operation 36 may be a process for further improving the gear. Since these process steps are well known to those skilled in the art (technical field relating to the forging of metal powders), only some features of the present invention will be described later. From this point of view, material selection, temperature processing, and compression pressure are briefly mentioned.

  In the mixing step 20, a metal powder containing the required binder or lubricant (substantially uniform mixture obtained by mixing and suitable for filling into a compacting form in the filling step 22) is prepared. The compressing step 24 includes compressing the metal powder through a preform into a preform having a substantially uniform initial carbon content. This initial carbon content is obtained by mixing the metal powder with a required amount of graphite with the required binder or lubricant to form a preform. The preform has one or more cross-section surfaces. At this cross-sectional surface, the final forged part results in a variable hardened layer depth profile as described herein.

  The sintering step 26 and the carburizing step 28 can be performed simultaneously, or the carburizing step is completed after sintering the preform. The metal powder is bonded by sintering the preform. The carburization of the preform substantially increases the initial carbon content as the carbon gradient progresses from the preform surface into the core. The carbon gradient is obtained by providing a controlled carbon atmosphere and maintaining the preform for a predetermined time under a controlled atmosphere. In order to improve the critical flow of the metal during forging to obtain the desired variable hardened layer depth profile in the forged part, it is necessary to obtain a substantially constant carbon hardened layer depth. Of course, the density gradient, part geometry, and carburizing conditions are directly related to the uniformity of the carburizing step. The carbon hardened layer depth required for the preform is determined by the desired area of critical metal flow during forging and the geometry of the preform. In order to obtain the above-described variable depth profile of the gear 50 described above, the preform is carburized to a hardened layer depth corresponding to 1/4 of the tooth height. It can also be satisfied by carburizing to a hardened layer depth of up to 20 or 7/8. Providing the preform with too small a hardened layer depth will result in the formation of non-carburized portions. Also, providing a preform with a too large cured layer depth will result in a substantially constant cured layer depth profile. 6A shows a partial cross-sectional view of the carburized preform 85 of the preform 84 shown in FIG. 5 after the carburizing process. The preform 85 has a substantially constant deep carbon hardened layer depth 86 obtained after sintering and carburizing of the preform.

  The variable forging step 32 includes forging the carburized preform at a forging temperature and forging pressure to obtain a substantially dense, net shape part. The variable hardened layer depth profile of the gear produces a substantially symmetric profile for each tooth. It is due to the symmetrical features of the carburized preform and forging die. However, different carburization methods and forging steps can be used to obtain a wide variety of variable hardened layer depth profiles.

  The variable hardened layer depth profile can be obtained by using a set of forging dies in the forging process to improve the critical flow of the carburized metal part. At this time, a certain hardened layer depth of the carburized metal powder preform needs to be strategically compressed into the mold part. . Here, during forging, one part of the preform is stretched and thinned, and the other part of the preform is thickened and deepened with carburized metal powder. Too shallow hardened layer depth or too deep hardened layer depth in the carburized metal powder preform prior to forging will not provide a variable hardened layer depth profile in the final product.

  The cooling step 34 can provide the forged part with a specific metallurgy that produces a gear having the desired variable hardened layer depth profile. The forging part can be cooled by quenching in oil, water or air, or other methods suitable for metal powder forging.

  It is also conceivable to improve the characteristics by stabilizing the temperature of the material of the part by dwelling the forged part for a predetermined time before cooling.

  It is also conceivable to improve the desired metal flow during forging by preheating the preform to a pre-forge temperature before forging.

  Post-forging processing step 36 (optional process) includes product turning, facing, surface grinding, splining, and broaching based on final detailed requirements. May be included. This makes it ready for cleaning, packing or shipping.

  With the proper selection and combination of metal powder, compaction die, processing time, processing temperature, processing pressure, forging die, and cooling method, it is almost net shape with variable hardened layer depth profile A sufficiently dense product can be obtained, thereby minimizing the degree of need for machining, and promoting cost savings and performance (performance improvement).

  Returning to the method of manufacturing a forged carburized metal powder part of the present invention, a specific example of the process is shown in FIG. The process includes a mixing step 120, a filling step 122, a compression step 124, a sintering step 126, a carburizing step 128, a removing step 130, and a cooling step 134. The post-forging process 136 is used to further improve the gear. The compatible processing steps described above with respect to the process shown in FIG. 8 can be used to further improve the metal powder part. Since some of these processes are well known to those skilled in the art of metal powder forging, the features of the present invention are mainly described below.

  To assist in understanding the process of FIG. 9, the preform 184 of FIG. 6B will be briefly described. 6B is a partial cross-sectional view of the carburized portion 188 removed from the preform 85 of FIG. 6A by a removal process prior to forging. 6A is a partial cross-sectional view of the preform 84 of FIG. 5 after carburizing. The preform 184 has one or more first surfaces 185, a hardened layer depth 186, one or more second surfaces 187, and a carburized portion 188. The carburized portion 188 is selectively selected and removed from the preform prior to forging. By selectively removing the carburized part before forging, it provides excellent machinability for secondary operations, for example, higher spline classification with relatively soft non-carburized materials ) Or a final part with enhanced broaching tolerance. Other advantages are derived from the strategically improved carburized parts obtained from the sintering-carburizing and forging processes.

  Returning to the process of FIG. 9, metal powder parts that are not selectively carburized require the step of compressing the metal powder into a preform.

  Carrying out the sintering and carburizing of the preform at the same time, or carburizing and carburizing to obtain the preform 184 can be performed by the sintering and carburizing process described above.

  As is well known, typical sintering temperatures for steel metal powders are about 2000 to about 2100 degrees Fahrenheit. Here, the initial carbon content of the preform may be less than 0.22% by weight. By carburizing, the final carbon content at the one or more first surfaces can be between 0.22 wt% and 0.37 wt%.

  The step of removing the carburized portion from the one or more second surfaces of the preform is performed before the forging step. By removing one or more carburized portions, the carburized portion on one or more first surfaces having an advantageous case hardness while providing one or more second surfaces where the material is not case hardened. This produces a forged part with Such a removal step enables strategic removal of the carburized portion 188 of the second surface 187 from the sintered preform prior to forging. This removal process can be performed by well-known methods such as drilling, machining, or grinding. One advantage gained by removing the carburized portion is that it eliminates the need for complex masking or unmasking to control carbon diffusion during the sintering-carburizing process. Another advantage associated with removing carburized parts from sintered preforms (when relatively soft) is to increase tool life and higher product like spline classification. It is mentioned that performance can be obtained. Also, splining is usually completed when the part is in a hard state after forging, resulting in unnecessary tool wear, as well as reduced spline resistance or performance.

  Thereafter, when the forging and cooling steps are completed, the component comprises one or more first surfaces having surface hardness properties and one or more second surfaces having shear resistance properties. Is obtained. The forging and cooling steps can be performed based on methods known to those having ordinary knowledge in the field of metal powder forging, and in the case of steel powder, usually at a temperature of about 1600 to about 1800 degrees Fahrenheit. . It is desirable to obtain a substantially dense part having a theoretical density of 99.6% or more by applying a forging pressure of 50 to 70 tons per inch.

  If the final part is approximately net-shaped after forging, the material to be removed in the removal step is described by producing a suitable preform or an oversize preform in the compression step.

  It may be advantageous for the process shown in FIG. 9 to use a variable forging step as shown in the process of FIG. In particular, a variable hardened layer depth profile is obtained in the final part after forging the carburized portion on one or more first surfaces of the preform after the removal step. In this case, if the part is a gear manufactured in accordance with the present invention, it has the soft internal diameter required for better spline rating and shear strength, while 2 having a variable hardened layer depth profile as shown in FIG.

  The parts made based on the process of the present invention shown in FIG. 9 can include any parts having the required hard and soft surfaces. In particular, the inventive method shown in FIG. 9 is particularly advantageous for toothed gears.

  While the specification describes various process steps, they do not limit the scope of the invention as defined by the claims. Although the present invention has been described based on some specific examples, it should be understood that the present invention is not limited to these specific examples. Accordingly, the present invention includes modifications, variations, and equivalents included in the technical idea described in the following claims.

Claims (22)

A method for producing a metal powder part that is not selectively carburized,
Compressing metal powder into a preform comprising one or more first surfaces forming a hardened layer depth hardness surface and one or more second surfaces forming a carburized portion to be removed prior to forging. Steps,
Sintering and carburizing the preforms sequentially or simultaneously;
Removing the carburized portion from the one or more second surfaces of the preform;
Forging step of forging the preform to obtain a forged part;
Cooling the forged part to obtain a forged part having one or more second surfaces having improved post-forging properties and one or more first surfaces having surface hardness properties;
The manufacturing method characterized by performing sequentially.   The manufacturing method according to claim 1, wherein the forged part is cooled by quenching in an oil bath after forging and temperature stabilization.   The manufacturing method according to claim 1, wherein the sintering is performed at a temperature of about 2000 to about 2100 ° Fahrenheit.   The manufacturing method according to claim 1, wherein the preform obtained from the metal powder has a substantially uniform initial carbon content throughout.   Providing a controlled carbon atmosphere of a rich endothermic gas to maintain the preform for a predetermined time under the controlled carbon atmosphere so that the initial carbon content is higher than the initial carbon content. The manufacturing method according to claim 1, wherein the preform is carburized by obtaining a predetermined hardened layer depth on the surface to substantially increase a carbon content in the preform.   The initial carbon content of the preform is lower than 0.22 wt%, and the final carbon content of the first surface having the carburized portion is 0.22 wt% to 0.37 wt%. Item 6. The manufacturing method according to Item 5.   The manufacturing method according to claim 1, wherein the forging part is obtained by performing the forging step at a temperature of about 1600 ° F. to about 1800 ° F. until a theoretical density of 99.6% or more is reached.   The manufacturing method according to claim 1, wherein the step of removing the carburized portion from the one or more second surfaces of the preform is performed by soft turning or broaching.   The manufacturing method according to claim 1, wherein the preform is enlarged to supplement the material to be removed in the removing step to obtain a substantially net-shaped part after forging.   The manufacturing method according to claim 1, wherein the final forged part is substantially a net shape.   The manufacturing method according to claim 1, wherein cleaning is performed after the cooling.   The manufacturing method according to claim 1, wherein hard turning is performed after the cooling.   The manufacturing method of Claim 1 whose said metal powder is a low alloy iron metal powder. Forging the preform to form the forged part having a variable hardened layer depth profile on the one or more first surfaces; and
2. The variable hardened layer depth profile is obtained by variably improving the critical flow of the carburized portion of metal using the forging die set during forging of the forged part. The method described. A bevel gear manufactured by the manufacturing method according to claim 1, comprising a rotation shaft and a plurality of radial teeth,
The gear according to claim 1, wherein the radial teeth extend in substantially the same direction as the rotation axis of the bevel gear and are inclined with respect to the rotation axis of the bevel gear.   The bevel gear according to claim 15, wherein the bevel gear is a spline-type differential side gear. Compressing the metal powder into a preform having one or more first surfaces forming a hardened layer depth hardness surface and one or more second surfaces forming a carburized portion to be removed before forging;
Sintering or carburizing the preforms simultaneously or sequentially;
Removing carburized portions from the one or more second surfaces of the preform;
Forging the preform to obtain a forged part;
Cooling the forged part to obtain a forged part having one or more second surfaces having improved tolerance characteristics and one or more first surfaces having surface impact characteristics;
A part manufactured by a method of manufacturing a metal powder part that is not selectively carburized, characterized in that:   18. The component of claim 17, wherein the preform is forged to obtain the forged component having the one or more first surface variable hardened layer depth profiles. A compression step of compressing the metal powder into a preform comprising one or more gear surfaces forming a hardened layer depth hardness surface and one or more spline surfaces having a carburized portion to be removed before forging;
Sintering or carburizing the preforms simultaneously or sequentially;
Performing turning or broaching to remove the carburized portion from the one or more spline surfaces of the preform;
Forging the preform to form a forged part;
Cooling the forged part to obtain a forged part comprising one or more spline surfaces having shear resistance and one or more gear surfaces having surface hardness characteristics;
A method for manufacturing a gear having a metal powder portion that is not selectively carburized, characterized in that:   20. The method of claim 19, wherein the preform is forged to obtain a forged part having a variable hardened layer depth profile on the one or more gear surfaces.   A sintered forged metal powder component comprising a first surface carburized to a hardened layer depth and a second surface not carburized, wherein the first and second surfaces are the first and second before forging. A sintered forged metal powder part formed by forging a sintered metal powder preform carburized on the surface, and the carburized portion on the second surface was machined before forging.   The sintered forged metal powder part of claim 21, wherein the carburized hardened layer depth that is relatively constant throughout the first surface changes to variability through forging.

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