Localized tempering of carburized steel

Abstract

A method for joining a carburized steel workpiece to a cast iron workpiece is provided that includes tempering a localized area of the carburized steel workpiece, machining the localized area to reduce carbon content, and welding the carburized steel workpiece to the cast iron workpiece at the localized area. The tempering may be performed by induction heating and results in a hardness profile of the localized area of less than 50 HRC.

Claims

1. A method of joining a carburized steel workpiece to a cast iron workpiece comprising: tempering a localized area of the carburized steel workpiece, wherein the carburized steel workpiece is a hypoid or spiral bevel ring gear and the cast iron workpiece is a differential case; machining the localized area to reduce carbon content after tempering the localized area; and welding the carburized steel workpiece to the cast iron workpiece at the localized area.

2. The method according to claim 1, wherein the tempering results in a hardness profile of the localized area of less than 50 HRC.

3. The method according to claim 1, wherein the machining is performed using carbide tooling.

4. The method according to claim 1, wherein the carbon content is reduced below about 0.25% by weight.

5. The method according to claim 1, wherein the tempering is performed by induction heating.

6. The method according to claim 5, wherein the induction heating comprises heating coils configured to match a profile of the localized area.

7. The method according to claim 1, wherein the tempering is between about 600 F. and about 1,200 F.

8. The method according to claim 1, wherein the carbon content is reduced without a carbon blocking paste or an alloy cap.

9. The method according to claim 1, wherein the welding is laser welding.

10. A method of joining a carburized steel workpiece to a cast iron workpiece comprising: induction tempering a localized area of the carburized steel workpiece such that a hardness profile of the localized area is less than 50 HRC; machining the localized area to reduce carbon content below about 0.25% by weight after induction tempering the localized area; and welding the carburized steel workpiece to the cast iron workpiece at the localized area.

11. The method according to claim 10, wherein the induction heating comprises heating coils configured to match a profile of the localized area.

12. The method according to claim 11, wherein the carburized steel workpiece is rotated within the heating coils.

13. The method according to claim 10, wherein the tempering is between about 600 F. and about 1,200 F.

Description

DRAWINGS

(1) In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

(2) FIG. 1 is a perspective side view of an exemplary gear to be prepared according to the teachings of the present disclosure;

(3) FIG. 2 is a side cross-sectional view of an exemplary gear to be prepared according to the teachings of the present disclosure;

(4) FIG. 3 is a side cross-sectional view depicting varying hardening depths of penetration of an exemplary gear prepared according to the prior art;

(5) FIG. 4 illustrates induction heating of an exemplary gear according to the teachings of the present disclosure;

(6) FIG. 5 is a flow diagram illustrating a method of joining a carburized steel workpiece to a cast iron workpiece according to the teachings of the present disclosure;

(7) FIG. 6 is a flow diagram illustrating another method of joining a carburized steel workpiece to a cast iron workpiece according to the teachings of the present disclosure; and

(8) FIG. 7 is a side view of an exemplary gear prepared according to the teachings of the present disclosure;

(9) The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

(10) The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

(11) Methods for improving machinability of carburized steel and decreasing hard machining cycle time is provided. The methods include high temperature induction tempering of specific regions (also referred to as localized areas) of carburized parts such that the hardness profile is reduced to less than 50 HRC. The hardness profile is reduced prior to hard machining thus allowing the use of standard carbide tooling which has a depth of cut greater than four (4) times CBN tooling. The larger depth of cut for standard carbide tooling reduces the hard machining cycle time and increases tool life.

(12) As set forth in greater detail below, these methods include an induction coil designed for the specific geometry of the zone to be induction tempered. Frequency modulation of the induction coil controls surface and sub-surface heating, thus reducing heat affected zones. Under these methods, the micro-structure becomes uniform and predictable with induction temperatures between about 600 F. and about 1,200 F. for improving hard turning parameters. In general, the methods according to the teachings of the present disclosure improve the machinability of carburized steel for weld joints between steel and cast iron.

(13) Referring to FIGS. 1 and 2, an exemplary gear 20 to which the teachings of the present disclosure are applied generally includes multiple gear teeth 22, a gear webbing 24 situated between successive gear teeth 22, a nose 26, an outside face 28, and an outside flank 30. The gear 20 also includes an outside radius 32, an inside face 34, an inside flank 36, and an inside radius 38. These areas are referred to herein as localized areas or a localized area and it should be understood that only these areas of the gear 20 are treated according to the methods of the present disclosure. Other areas of the gear 20 are not treated because they are not subsequently joined to a cast iron workpiece, and such treatment negatively affects the strength of the gear 20 in these other areas. Therefore, the teachings of the present disclosure are focused on a localized treatment of an area of the gear that is subsequently welded to a cast iron workpiece.

(14) Referring to FIG. 3, a gear prepared according to the prior art illustrates depth at 50 HRC at various locations 40, 42, 44, 46 and 48. Batches of gears were carburized and prepared to measure the hardness at locations 40 through 48 as shown in FIG. 3 and Table 1 below.

(15) TABLE-US-00001 TABLE 1 Batch # #1 #2 #3 #4 #5 Effective Case Depth Measurement @ HRC 50, In: Tooth Pitchline #1 0.076 0.085 0.069 0.081 0.075 0.064 #2 0.033 0.049 0.048 0.046 0.030 0.047 #3 0.042 0.051 0.048 0.048 0.037 0.052 #4 0.050 0.063 0.058 0.064 0.048 0.060 Rockwell Hardness @ .085 depth from surface: Tooth Core Hardness #1 47 47 46 47 46 36 #2 34 38 39 40 34 32 #3 38 39 40 42 36 32 #4 44 45 45 46 38 36

(16) Hardness measurements were taken at depths corresponding to an HRC of 50 and at 0.085 inches from the surface of the gear 20. As shown in Table 1, the hardness measurements vary in depth from surface and location on the gear 20. These hardness variations increase tooling wear and costs. More specifically, tool life issues were experienced due to the varied case depth position and thus how to predict when to adjust tool feed/speed.

(17) Referring to FIG. 4, an exemplary induction heater 10 of the present disclosure is provided. In this form, the induction heater 10 is a three-turn coil that includes induction heating coils 50, 52, and 54, which are configured to match a profile of the localized area 25. The induction coils 50 through 54 correspond to locations on the gear 20 where tempering is desired. Each induction coil 50 through 54 may be heated to a different temperature to vary the rate of tempering. In this form, induction coil 50 tempers regions inside face 34, inside flank 36, and inside radius 38, induction coil 52 tempers the nose 26, outside flank 30, and inside flank 36, and induction coil 54 tempers the outside face 28, outside flank 30, and outside radius 32.

(18) Referring now to FIG. 5, a method of joining a carburized steel workpiece to a cast iron workpiece according to the present disclosure is illustrated in a flow diagram and represented by reference numeral 100. Method 100 comprises tempering a localized area of the carburized steel workpiece in step 102. In step 104, the localized area is machined to reduce carbon content, and in step 106 the carburized steel workpiece is welded to the cast iron workpiece at the localized area.

(19) The tempering should result in a hardness profile of the localized area less than 50 HRC so that this area can be machined with less expensive tooling. More specifically, less expensive tooling may include carbide tooling. The inventors have discovered that machining the localized areas such that the carbon content is below about 0.25% by weight inhibits weld failures. As set forth above, the tempering may be performed by induction heating, wherein induction coils are configured to match the profile of the localized area. Further, for this specific application and material of the localized area, namely, carburized steel, the tempering is between about 600 F. and about 1,200 F. The workpiece having the localized area may include a hypoid ring gear or spiral bevel ring gear, where the cast iron workpiece is a differential case. It should be understood, however, that the methods of the present disclosure may be applied to other part/workpieces in a various applications and thus these power-train components are merely exemplary and should not be construed as limiting the scope of the present disclosure. Since the carbon content is reduced below about 0.25% for subsequent welding, the present disclosure does not require the use of a carbon blocking paste or an alloy cap, thus reducing costs and improving quality of the weld. In one form, laser welding is used to join the carburized steel workpiece to the cast iron workpiece, although other forms of welding may be employed while remaining within the scope of the present disclosure.

(20) Referring now to FIG. 6, another method of joining a carburized steel workpiece to a cast iron workpiece according to the present disclosure is illustrated in a flow diagram and represented by reference numeral 140. The method 140 comprises induction tempering a localized area of the carburized steel workpiece such that a hardness profile of the localized area is less than 50 HRC at step 142. In step 144, the localized area is machined to reduce carbon content below about 0.25% by weight. In step 146, the carburized steel workpiece is welded to the cast iron workpiece at the localized area.

(21) Testing

(22) According to the methods of the present disclosure, several workpieces were treated and tested to reduce HRC below 50 prior to welding. Results of testing are shown below in Tables 2 and 3, which show microhardness results at each of the locations indicated in FIG. 2:

(23) Referring to FIG. 7, a portion of a hypoid ring gear 20 is illustrated that has was prepared according to methods of present disclosure and locally tempered by induction heater 10. The gear 20 was subsequently machined proximate the localized area 25 to reduce the carbon content. This machining trial was successful with carbide tooling, and the cycle time was reduced by at least 60% when compared to prior art processes.

(24) In one form, the gear 20, upon reaching 600 to 1200 F. in the localized region, can be rapidly cooled with water to ambient temperature to allow for subsequent machining operations without necessitating slow air cooling or complex quenching operations.

(25) The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.