1、i i 98FIlV4 1 Effect of Uncontrolled Heat Treat IDistortion on the Pitting Life of Ground, Carburized and Hardened Gears by: A.K. Rakhit, Solar Turbines Incorporated TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesEffect of Uncontrolled
2、 Heat Treat Distortion on the Pitting Life of Ground, Carburized and Hardened Gears A.K. Rakhit, Solar Turbines Incorporated The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Associatio
3、n. Abstract Heat treatment causes distortion of gear tooth geometry. To improve tooth quality, particularly after carburizing and hardening, gears are generally ground. For low distortion, stock removal is small up to a maximum of 0.005 inch from each tooth flank. This allows minimum surface hardnes
4、s reduction with no possible detrimental effect on gear pitting life. Uncontrolled distortion, on the other hand, results in an undue amount of stock removal with a significant loss of tooth surface hardness. Lower surface hardness accelerates contact-stress-induced tooth pitting that reduces gear l
5、ife. For heavily distoeed gears, the pitting life may be reduced as much as 30 percent after grinding. For a realistic evaluation of the pitting life of ground, carburized and hardened gears with uncontrolled distortion, derating factors are necessary and established for a number of gear materials.
6、Copyright O 1998 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 22314 October, 1998 ISBN: 1-55589-722-3 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesEFFECT OF UNCONTROLLED HEAT TREAT DISTORTION ON THE PITTI
7、NG LIFE OF GROUND, CARBURIZED AND HARDENED GEARS A. K. Rakhit Solar Turbines Incorporated, San Diego, California Introduction All heat-treated gears experience distortion. Of the various hardening processes, carburizing and quenching causes the highest distortion of the pre-heat treat tooth geomey.
8、The mechanics of this distortion phenomenon are quite complex. Its in-depth analysis is beyond the scope of this paper. But the fact is that all carburized and hardened gears distort. For most applications, these gears need to be finish-machined for improved tooth geometry. Presently, ginding is Con
9、sidered to be the most economical process to achieve this qualiy. However, the higher the distortion, the more stock needs to be removed from the tooth surface. This not only reduces case thickness but also lowers the surface hardness, detrimentally affecting gear pitting life. For a gear to meet ex
10、pected pitting life, it is thus necessary that stock removed by pnding be minimized. To establish minimum grind stock, distortion data of a gear after heat treatment are essential for which a clear definition of distortion is necessary. In general, a change in any one or more of the following dimens
11、ions are considered to define gear distortion and its severity Flatness ofrim and hub Profile, lead and spacing of teeth Pitch diameter (PD) growth and runout In a well controlled carburizing and hardening process, distortions could be held to within one American Gear Manufacturers Association (AGMA
12、) class below the pre-heat treat quality for gears made of commonly used materials in industnal application, such as 4320H, 8620H and 93 10H. Such a process allows the establishment of a minimum grind stock on a tooth surface and the maintenance of the minimum surface hardness required after grindin
13、g. This satisfies the design requirement for minimum gear pitting life in most applications. Unfortunately, the state-of-the-art carburizing and quenching technology is unable to assure low gear distortion consistently in a production mode of operation. Sometimes, distortions within the same batch o
14、f gears are found to vary widely. Hence, it is very likely to find some gears after heat treatment with distortion far more than others. Gnnding of gears with unpredictable distortion necessitates inspection of every gear in a batch for distortion severity. Certainly, this increases the cost of gear
15、s. Furthermore, tooth surface hardness and effective case depth may not be acceptable after grinding. Besides grinding, surface hardness reduction of carburized and hardened teeth is also influenced by the slope of hardness vs case depth gradient of gear steel. Each carburizing grade steel has its o
16、wn unique slope and is primarily controlled by its alloying elements. The steeper the slope, the greater the hardness reduction on tooth surface after grinding. However, some special carburizing process or cycles such as boost-diffuse may produce a flatter slope of this gradient that allows larger s
17、tock removal without any appreciable loss of surface hardness. Unfortunately, at present these processes, which are complicated and expensive, have limited use in industry and are not considered in this investigation. In addition to the slope of hardness gradient, grinding machine Set-up plays an im
18、portant role in the amount of equal stock removal from each flank of the gear teeth, and its associated hardness reduction. COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services0.020 - 14 16 18 20 22 24 26 28 PD OVER THE PINS, Inch 972262-001M Figure 1 : He
19、at Treat Growth of Experimental Gears 5 B 3 4 - o OUT-OF-ROUNDNESS, Inch 972282.002M Figure 2: Out-of-Roundness of Gears after Heat Treatment 12 11 10 B 8 7 6 5 4 3 2 1 O FLATNESS DISTOFTION AT RIM, Inch + Grindina Stock To establish an ideal grind stock, detailed distortion characteristics and grow
20、th data of gears are beneficial. These are best obtained from a pre-production processing of gears using production equipment and facilities. Results of a typical processing for a family of gears made of AIS1 8620H are shown in Figures 1 through 3 that depict the average heat treat growths, out of r
21、oundnesses and rim flatness dimensions i. Figure 4 shows the configuration of one such gear. In this investigation, even though an improved carburizing and quenchmg system was utilized and gears were normalized for uniform grain structure prior to heat treat, distortion and growth varied from one ge
22、ar to another. The data so collected were still quite useful in determining the optimum grind stock for these gears. For a great majority of gears that are designed for carburizing and hardening, this type of investigation may not be eumomicaiiyjustified. In such cases, distortion data of similar ge
23、ars are sometimes useful. For gears without any pre-production or historical data, general knowledge on heat treat response of the gear material heipsio establish effective gnnd stock. 2.500 I-+- GEAR DATA NDP NPA: No. OF TEEM: HEUX ANGLE: 5 20 77 17.5 Figure 4: Basic Dimensions of an Experimental G
24、ear Grindina of Distorted Gears With the pd stock so determined either by a pre-production investigation or from data of similar gears, grinding of carburized and hardened gears is performed by properly positioning the grinding wheel in the tooth space. In modem gear grinding machmes with advanced m
25、easurement systems, a probe measures the helix (lead) deviation on a pre-determined number of right and left flanks of teeth at different locations that include teeth at maximum PD runout. The measured values not only contain the O 07pBz-W3k1 effect of all helix deviations and PD runouts, but also t
26、he effect of profile and cumiilative pitch variations. The grinding machine control computer calculates the optimum angular position of the gear before gnnding starts. This method ensures that the gnnding Figure 3: Flatness Distortion at Rim After Heat Treatment COPYRIGHT American Gear Manufacturers
27、 Association, Inc.Licensed by Information Handling Services* wheel will be positioned at the center between two flanks allowing equal stock removal fiom each flank Starting with teeth at the maximum PD runout. The method works well as long as the distortions are comparatively small and their variati
28、ons are distributed within a narrow band - one AGMA class below the pre-heat treat quality. 30 For gears made of high alloy steels such as AISI 93 1 O that exhibit uncontrolled distortion Material: r O 0.015 0.030 0.045 0.060 0.075 DEPTH BELOW SURFACE, Inch Figure 6: Hardness Gradient of a Carburize
29、d and Hardened Gear tooth; Material: AiSI 8620H 70 I Minimum Hardies ment 20 I I I l I I I I O 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 DEF“ BELOW SURFACE, inch o- Figure 7: Hardness Gradient of a Carburized and Hardened Gear Tooth; Material: AiSI 931 OH a oe5 1 I I t I O 0,010 0.020 0.030 0.040 0.06
30、0 DEPTH BELOW SURFACE, Inch Figure 8: Hardness Gradient of a Carburized and Hardened Gear Tooth; Material: HP 94-30 17CrNiMoG COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesIt is clear from the hardness gradients of various alloy steels that stock rem
31、oval over 0.005 inch fiom the flanks of distorted carburized and hardened gear teeth may result in lower than the minimum surface hardness required for the expected pitting life. Because of the steeper slope, a usual characteristic of low-alloy steels (Figure 6), the surface hardness reduction of ge
32、ar teeth made of these materials is generally higher than those made of high-alloy steels. On the other hand, distortion associated with low-alloy steel gears is comparatively less and hence these warrant smaller stock removal during grinding. Thus, the actual tooth surface hardness reduction after
33、grinding is almost of the same magnitude for both low-and high-alloy steel gears. This allows the consideration of similar amounts of grind stock for all gear materials. Furthermore, grind stock required on each tooth flank for both low and high alloy steel gears with controlled heat treat distortio
34、n is found not to exceed an average of 0.005 inch. Removal of stock to this depth does not seem to reduce tooth surface hardness below the minimum considered in design with carburized and hardened gears. In a production-type carburizing and hardening operation which is frequently associated with unc
35、ontrolled distortion, stock over 0.005 inch is very ofien removed from teeth located at the area of maximum distortion. Such stock removal in some gears may result in surface hardness reduction as high as two points on the RC scale, particularly for materials with a steep hardness gradient slope. As
36、 an example, in gears made of i 7CrNiMo the hardness reduction for an additionai 0.002 inch stock removal over planned 0.005 inch is approximately one point in RC scale. This is a significant hardness reduction below the minimum considered during design. Lower suface hardness will definitely reduce
37、pitting life. The new pitting life may be calculated from the equation derived from the pitting life vs contact stress relationship. New Pittina Life The equation for pitting life in terms of stress cycles (L,) of a gear at any power level can be written as where Sca, - sc - a- allowable contact str
38、ess for the gear material at minimum hardness actual contact stress due to applied load slope of S-N curve for gear durability = 17.84 3 Similady, the new pitting life (L,) for a lower allowable contact stress (scat) due to reduced tooth surface hardness of the same gear and at the same power levei
39、is L, = 107 (-) Sca, ! Equations (i) and (2) yield Sca, a L2 = (4 From equation (3), the pitting life of a gear can be calculated at any allowable contact stress that corresponds to a specific tooth hardness. For example, if due to additional stock removal (0.002 inch over the allowed 0.005 inch) th
40、e surface hardness of teeth reduces from RC 58 (minimum design requirement) to RC 57 (Figure 5), the allowable contact stress (Scq) for AGMA grade 1 material (Figure 9) becomes Sca, = 26,000 + 327 BHN; where BHN = Brinell Hardness Number of tooth surface - 214,000 psi Figure 9 (modified from AGMA St
41、andard) represents fairly weil an acceptable relationship between allowable contact stress and Brinell hardness for any type of steel gears. Metalluiglcal and CtuaMy n- Control Plocedores Requi .- 200 -S=27,000+364BHN O t-x Maximum for Grada 2 Maximum for Grada 1 (AGMA) Mated 75 I I I I I I I 250300
42、 350 400 450 500 550 600 650 BRINELL HARDNESS, BHN Q22dgQoy Figure 9: Allowable Contact Stress Number for Steel Gears The allowble contact stress (Sca,) at minimum hardness (RC 58) per design is 2 18,000 psi. Using equation (3) the new pitting life is L, = 0.72 L, (4) For gears with hgh distortion,
43、it is possible that an additional stock removal may be necessary for the desired tooth geometry reducing the pitting life further. Fortunately, Uiis type of severe distortion is uncommon with frequently used gear materials. For such materials, 0.002 inch stock over the allocated (0.005 inch) is cons
44、idered adequate for gear tooth gnnding. Even then, the reduction of pitting life is signifcant as indicated by equation (4) for 17CrNiMo material. COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesTABLE 1 - DISTORTION DERATING FACTORS Gear Material Hardn
45、ess vs. Case Depth Gradient Distortion Severity Hardness Drop less than half a Air Melt Vacuum Melt point RC for every 0.002 in. Stock over Low High Low High allocated Alloy Alloy Alloy Alloy (0.005 in.) 8620H 9310H d 93 1 OH d 4320 4320 HP 94-30 c/ 4330M c/ 17CrNMo6 Hardness Drop more than half a L
46、ow High point RC for (within one (over one every 0.002 in. AGMA Class AGMA Class Distortion Stock over below pre- below pre- Derating allocated heat treat heat treat Factor (0.005 in.) quality) quality) (DF) c/ d o 80 d 0.80 d O 85 d d O 80 c/ d o 85 4 0.90 c/ O 80 4 d O 72 To determine the true pit
47、ting life of carburized, hardened and ground gears it is thus essential to consider heat treat distortion. Certainly, this necessitates the availablity of heat treat distortion data for all newly designed gears - a difficult task to accomplisl. One possible solution to the problem is to develop dist
48、ortion derating factors for various gear materials fiom known heat treat data and apply them to derate pitting life. Distortion Deratina Factor The distortion derating factor (DDF) is defued as the ratio of actual pitting life to the required pitting life of gears. For realistic values of distortion
49、 derating factors, it requires comprehensive knowledge on composite distortion characteristics that include all possible gear materials, configuration and size of gears, gear diametral pitch and helix angle, heat treat process and equipment, and hardness gradient of each material. Such a task is beyond the scope of this investigation. Nevertheless, it is endeavored to establish derating factors for some commonly used materials from available gear distortion characteristics as given in Table 1. These are based on an additional 0.002 inch stock removal above 0.00
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