1、STD-AGMA 74FTM4-ENGL 1994 m b87575 17004558 b5b m 94FTM4 Load Carrying Capacity of Nitrided Gears by: L. Albertin and R.L. Frolich, Westinghouse H. Winter, B.-R. Hhn and K. Michaelis, FZG, Germany American Gear TECHNICAL PAPER STD-AGHA 74FTH4-ENGL 1974 D Ob87575 0004557 572 D Load Carrying Capacity
2、of Nitrided Gears L. Albertin and R.L. Frolich, Westinghouse H. Winter, B.-R. Hhn and K. Michaelis, FZG, Germany The statements and opinions contained herein are those of the author and should not be construed as an oficial action or opinion of the American Gear Manufacturers Association. ABSTRACT:
3、The pitting and bending strength of gas nitrided steel gears made of modified 39CrMoV13.9 (a 3% CrMoV type doy) were investigated. Characteristics of the compound layer and the diffusion zone are examined. Residual stresses in the nitridedcase are shown after long nitriding times. Bending fatigue an
4、d contact stress limits were very high, in the same order as case carburizing steels. Forbending smngth, additionaidamage lineinvestigations were performed. From these results a comparatively poor overload tolerance has to be stated. The load carrying capacity of the modified 39CrMoV13.9 steel is di
5、scussed and compared with other carburized, gas, and ion nimded gears, Copyright O 1994 American Gear Manufacturers Association 1500 King Street, Suite 201 Aiexandria, Virginia, 223 14 October, 1994 ISBN: 1-55589-638-3 STD-ALMA 94FTM4-ENGL 1774 b87575 00045b 204 LOAD CARRYING CAPACITY OF NITRIDED ST
6、EEL GEARS L. Albertin; Westinghouse Electric Corporation, USA R. L. Frohlich; formally Westinghouse Electric Corporation, now Pullbrite Incorporated, USA H. Winter, B.-R. Hhn and K. Michaelis; Gear Research Centre (FZG), Germany ABSTRACT The pitting and bending strength of gas nitrided steel gears m
7、ade of modified 39CrMoV13.9 (a 3% CrMoV type alloy) were investigated. Characteristics of the compound layer and the diffusion zone are examined. Residual stresses in the nitrided case are shown after long nitriding times. Bending fatigue and contact stress limits were very high, in the same order a
8、s case carburizing steels. For bending strength, additional damage line investigations were performed. The load carrying capacity of the modified 39CrMoV13.9 steel is discussed and compared with other carburized, gas, and ion nitrided gears. I NTR OD UCTI ON Higher power density requirements often d
9、emand surface hardened gears While carburizing is the most common and effective surface hardening method used to boost power ratings, the technique has shown substantial difficulties in the production of large gears. The quench hardening of hobbed gear rims from a high austenitizing temperature ofte
10、n results in unpredictable levels of tooth deflection, helix angle change, and overall distortion. Carburizing, therefore, is usually limited to smaller, solid pinions, while larger gears are often nitrided. Nitriding large gears has certain advantages. The low temperature used in nitriding and the
11、absence of a quenching requirement assure minimal distortion. A post hardening treatment is not needed, and although profile grinding is required for better accuracy, metal removal is negligible compared to carburized or induction hardened gears. The high surface hardness of nitrided gears makes it
12、possible to reduce diameter and center distance of mating gear pairs (and thus weight) resulting in a higher specific power density. Large gears demand materials of high hardenability, and in the case of nitrided gears, materials with a good nitriding response. One such material is the German steel
13、39CrMoV13.9 (which is similar to a type 3% CrMoV steel). The steels high alloy content assures good core hardenability as well as hardness in the nitrided case. To assess the load carrying capacity of the modified (Mod.) 39CrMoV13.9 steel, a gear test program was conducted at the Gear Research Centr
14、e of the Technical University of Munich (FZG). The aim of the program was to generate fatigue data on the gear tooth bending strength and gear durability (pitting resistance) of this steel and to compare the results with other nitriding and carburizing steels. The overload tolerance of this steel in
15、 bending was characterized by damage line investigations. Further objectives of the program were to characterize the compound layer and diffusion- zone of the steel and to measure the residual stresses in the nitrided case after long nitriding times. EXPERIMENTAL PROCEDURE AND RESULTS MATERIAL AND G
16、EAR CONFIGURATIONS - The test gears were made from Mod. 39CrMoV13.9 steel forged bars. The chemical composition of the steel is shown in Table 1. The bars were cut and upset forged into blanks. The blanks were then normalized and hardened to two strength levels. The hardness and core tensile strengt
17、hs (based on hardness) of the test gears after forging and heat treatment are shown in Table 2. Gear configuration and design parameters for the bending and pitting gears are listed in Table 3. The machined gears were nitrided by the ammonia gas nitriding process. Bending and pitting type test gears
18、 were nitrided separately at different times and to different case depths. The bending gears were nitrided to a minimum case depth of 0.94 mm (0.037 inches) to HRC 40, while the pitting gears were nitrided to a depth of (0.027 inches). A nitriding time of 240 hours or more at a nitriding temperature
19、 of 540C (1 004OF) was required to achieve the case depth of 0.94 mm. About 150 hours were needed to produce a case depth of 0.69 mm. The hardness gradient of the nitrided case in two bending gears of different core hardness is shown in Figure 1. A typical microstructure of the nitrided case is show
20、n in Figure 2, and details of the white layer are depicted in Figure 3. Precipitation reactions in the nitrided case after long nitriding times caused preferential carbide formation along prior austenite grain boundaries. X-ray crystallography showed the compound layer to be y (Fe,N) with no &-phase
21、 (Fe,N) present. Microprobe analysis showed the porous white layer to be rich in oxygen (oxides). The layer was quite soft (DPH 296- 318) in comparison to the adjacent diffusion zone (DPH 772-802). Using x-ray diffraction techniques, the residual stress distribution in the nitrided case with increas
22、ing depth was determined in a gear tooth test coupon. The residual stress distribution had the form as shown in Figure 4. The surface exhibited moderate residual compressive stresses near the pitch diameter location and a relatively high residual stress at the root fillet. Low residual compressive s
23、tresses at the surface of nitrided Cr-Mo-V steels are apparently common after long nitriding times because of precipitation reactions occurring in the diffusion zone as was shown by Mittemeijerl. High compressive residual stresses were found below the surface with maximum values being at a depth of
24、about 0.4 mm (0.015 in.). The flanks of the test gears were ground after nitriding. The tooth root area was left as heat treated. BENDING FATIGUE TESTS - The bending fatigue tests were carried out on a mechanical resonance pulsator of 200 kN capacity. The frequency was 50 Hz. The gear teeth were cla
25、mped and loaded in such a way that the load direction was tangent to the base circle. The bending fatigue gear test set up is shown in Figure 6 . A preload of 5 to 7 kN was used. The fatigue equipment is set up to make twelve separate tests on each gear. The endurance strength in bending was calcula
26、ted by the DIN 39902 method. The bending fatigue S-N curves for 50% failure probability of the Mod. 39CMoV13.9 gears with two different core strengths are given in Figures 6 and 7. The curves are practically identical, with the lower core strength material showing a slightly higher endurance limit v
27、alue at 5 x lo6 load cycles (oFo of 1020 MPa or 148 ksi versus 1008 MPa or 146 ksi, respectively). The fatigue endurance limit for this material is rather high when compared to other nitriding steels tested at FZG. The carbide banding in the diffusion zone and the presence of the white layer in the
28、root area apparently did not have an adverse affect on the bending strength of the test gears. The rather shallow slope of the S-N curve in the low cyclic regime and the presence of a noticeable “knee“ in the S-N curve after a relatively short cyclic life (lo5 cycles) suggest a material which is pro
29、ne to overload damage. Tests have shown that through- hardened, carburized, and induction hardened gears are relatively immune to overload effects. Their damage lines according to FrenchS are generally close to and parallel to the S-N curves. Nitrided gears, on the other hand, have damage lines whic
30、h typically deviate substantially from their S-N curves. Therefore, the materials performance under occasional overloads was investigated. The damage line in bending fatigue of gears is obtained by initially determining the S-N curve and the endurance stress at 5 x lo6 cycles for the alloy and nitri
31、ding condition and then subjecting a number of test gears to an initial overstress in the low cycle life regime (like 1000 to 3000 cycles) and then testing the gears at about 90% of the endurance stress to 5 x lo6 cycles to determine whether or not damage by overstress occurred. If failure does not
32、occur for this number of cycles, the overstress level in a new gear is raised for a predetermined number of cycles, and the gear is then again tested below the endurance stress. This process is repeated until failure occurs at less than 5 x lo6 cycles. A series of such tests were performed on Mod. 3
33、9CrMoV13.9 bending fatigue gears at FZG. The results are shown in Figure 8. The squares in this figure represent the overstress and number of load cycles used in each test. The points connected with dotted lines to the same gear teeth indicate that failure occurs before 5 x lo6 cycles when tested sl
34、ightly below the endurance limit. The significance of the damage line for nitrided gear is this: Any combination of load and cycles within the field between the damage line and the S-N curve causes a decrease in the endurance limit. In the case of Mod. 31CrMoV13.9 steel gears any load higher than th
35、e endurance limit load causes a reduction of the endurance limit. The overload tolerance of the steel in bending, therefore, is very low. DURABILITY TESTS (PITTING) - Durability or pitting endurance tests were performed on FZG gear test rigs shown in Figure 9. The gear center distance was 91.5 mm (3
36、.6 inches). A detailed description of the test rigs is given by Winter and Michaelis4. The gears were lubricated by refined mineral oil IS1 VG100 (viscosity v = 100 mm2/s at 4OoC (100OF) with a 4% sulfur-phosphorus additive. The pitting test gears were surface ground and were paired with carburized
37、AIS1 9310 gears. The nitrided gears meshed as pinions with the carburized gears. The gears were loaded to various Hertz stresses until failure occurred. An endurance limit was considered reached when the gears ran for 5 x 10 cycles without damage. A gear was assumed damaged when pits covered 1% of t
38、he total active surface of the STD.AGMA 94FTM4-ENGL 1774 W b87575 UUU45b2 087 gear or when pits covered 4% of the total active surface of any tooth. The applied contact pressure and Hertz stresses were calculated by the DIN 39902 method. The S-N curve for pitting based on 50% failure probability is
39、shown in Figure 10. The obtained endurance contact stress was oHO = 1420 MPa (206 ksi). SUMMARY AND CONCLUSIONS The gear test results obtained in this investigation showed that nitrided Mod. 39CrMoV13.9 steel gears can tolerate high pitting and bending stresses. The bending endurance strength of oF0
40、 = 1008 MPa (146 ksi) and the contact limit stress of oHO = 1420 MPa (200 ksi) obtained are comparable to bending the pitting fatigue strengths of carburized and other nitrided gears. For instance, tests carried out at FZG on carburized AISI 931 OH gears showed a bending strength endurance limit of
41、oF0 = 912 MPa (132 ksi) and a contact fatigue stress limit of oHO = 1572 MPa (228 ksi). Shot peening improved the bending endurance limit of the carburized gears to uF0 = 1161 MPa (168 ksi). In a similar gear test program using the same test gear design conducted at the Technical University of Aache
42、n, Weck6 examined the performance of gas-nitrided gears made of alloy 39CrMoV13.9 with gears of other nitriding alloys, notably the steel 14C4MoV6.9. The bending fatigue strength endurance limit obtained in the 39CrMoV13.9 set of gears was oF0 = 1100 MPa (160 ksi) for the 14C4MoV6.9 gears. The corre
43、sponding pitting stress limits were oHO = 1530 MPa (222 ksi) and 1550 MPa (225 ksi). Plasma nitrided gears made of a variety of nitriding steels, among them 3lCrMoV9 and 42CrMo4 tested by Weck and Schltermann6 showed the effect of alloying on test results. While the bending endurance strength in the
44、 higher alloyed 31 CrMoV9 steel gears was about oF0 = 935 MPa (136 ksi), it was only o, = 750 MPa (109 ksi) for the low alloy steel 42CrMo4. The corresponding pitting stress limits for the two steels were oHO = 1642 MPa (238 ksi) for 31CrMoV9 and oHO = 1200 MPa (174 ksi) for 42CrMo4. A comparison of
45、 these results is shown in a bar chart in Figure 11. Differences in core hardness in the Mod. 39CrMoV13.9 bending test gears did not produce differences in the fatigue endurance strength. Gears heat treated to core tensile strengths of 1027 MPa (149 ksi) and 1178 MPa (171 ksi) had about the same end
46、urance strengths in bending. The high bending strength values were achieved despite the presence of a white layer and massive carbide bending along prior austenite grain boundaries, both of which are generally not conductive to good fatigue strength. High compressive stresses at the tooth root, howe
47、ver, could have contributed to the high bending strength. Compared to DIN 3990 standards for nitriding steels, the tests results on Mod. 39CrMoV13.9 gears showed a white layer and massive carbide bending along prior austenite grain boundaries, both of which are generally not conductive to good fatig
48、ue strength. High compressive stresses at the tooth root, however, could have contributed to the high bending strength. Compared to DIN 3990 standards for nitriding steels, the test results on Mod. 39CrMoV13.9 gears showed a tooth root bending strength fatigue limit in the upper bound of the DINAS0
49、allowables field for good quality nitrided gears (Figure 12). As for the surface contact (pitting) fatigue limit, Mod. 39CrMoV13.9 steel gears fell also into the upper bound of the DINAS0 allowables field (Figure 13). The advantage of changing from through- hardened gears to nitrided gears becomes obvious from simple gear allowable stress or strength calculations. The ratio, R, of a potential torque increase in bending when changing over from through-hardened gears to surface nitrided gears can be estimated from the hardness of the two material conditions and
