1、98FTM3 t Basic Studies on Fatigue Strength of Case-Hardened Gear Steel - Effects of Shot I Peening and/or Barrelling Processes by: S. Hoyashita, Saga University, M. Hashimoto, Sumitomo Heavy Industries, and K. Seto, Saga University TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, I
2、nc.Licensed by Information Handling ServicesO Basic Studies on Fatigue Strength of Case-Hardened Gear Steel - Effects of Shot Peening and/or Barrelling Processes . . S. Hoyashita, Saga University, M. Hashimoto, Sumitomo Heavy Industries, and K. Seto, Saga University The statements and opinions conta
3、ined herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract The shot peening process is frequently utilized in order to induce compressive residual stresses onbelow a surface, which will remarkably improve
4、the bending fatigue strength of a gear tooth. in this study, two kinds of case-hardened steels, namely the carburized and the carbo-nitrided steels were used. First, bending fatigue strength using a notched plate simulating a gear tooth was investigated. As expected, the bending fatigue strengths of
5、 two kinds of case-hardened steels are improved by the shot peening process. If the shot peening process is employed in order to improve the bending fatigue strength of a gear tooth, the surface durability of the shot peened surface may be reduced because of degradation of its surface roughness. On
6、the contrary, it is expected that the surface durability may be improved by compressive residual stresses induced by shot peening. To understand these factors, the surface durability of the shot peened or/an barrelled case- hardened steel rollers was investigated using a two roller contact fatigue t
7、esting machine. Key words: Gear, Case-Hardened Steel, Tribology, Fatigue Strength, Bending Fatigue Strength, Surface Durability, Residual Stress, Surface Roughness Copyright O 1998 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 223 14 . October. 1998 ISBN:
8、1-55589-721-5 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesBasic Studies on Fatigue Strength of Case-Hardened Gear Steel - Effects of Shot Peening and/or Barrelling Processes - Shigeru Hoyashita: Lecture, Saga University, Japan Munetoh Hashimoto: Se
9、nior Engineer, Sumitomo Heavy Industries, Ltd., Japan Kiyokazu Seto: Graduate Student, Saga University or not. The problem of contact should be solved not only by means of a material strength and an acting stress, but also as a problem of tribology in taking surface roughness Generally, a low hardne
10、ss steel is superior to a high and lubricating condition, etc. into consideration. In hardness steel in the dynamic load and bending stress almost researches the surface roughness, the Oil film conditions, because the steel is apt to become brittle with thickness and the relations between them Were
11、not made the increase of hardness. On the other hand, the latter is clear. superior to the former for a resistance of wear of the In contact fatigue tests conducted using the case- contact surface dudg a power transmission. Then a case- carburized and hardened steel rollers, the authors couldnt hard
12、ened steel has been developed as a material with both obtain the distinct result about whether Or not the Surface hardness and toughness, which is obtained by heat durability of rollers shot peened after grinding to about 3 treatment methods like carburizing, carbo-nitriding, and ,um% would be impro
13、ved 6. Scoring has often occurred induction heating, etc. and is often utilized as a gear at the early stage of running due to severe metallic material. When a gear with a high load carrying capacity contacts because the sum of a pair of surface roughnesses needs to be developed in order to down siz
14、e a speed is very much larger than the oil film thickness. Therefore. reducer, a shot peening process may be expected as a when a barrelling process was employed after shot method to improve the fatigue strength of case-hardened peening, the surface durability has remarkably increased. steel because
15、 a gear material has already achieved the It is difficult to improve the surface durability by only limit of endurance. compressive residual stress induced by the shot peening There are many reports that a shot peening process process when the surface roughness is very rough. In the has improved the
16、 bending fatigue strength of a gear tooth present report, the effects of the shot peening and 11-3. However, when the shot peening process is applied to the tooth root, the tooth surface for carrying a load may also be shot peened. Then it is interesting the surface durability of gear. Though to dat
17、e there are whether or not the shot peening process will influence some reports about the effects of the shot peening clarified whether the shot peening process is effective, 1. Introduction : Table 1 Chemical compositions of test rollers Chemical compositions (%) Material process pon the “Iface 415
18、13 it not SCM822H 0.222 0.320 0.790 0.023 0.070 1.040 0.350 0.100 -1- COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesbarrelling processes upon the surface durability will be reconfirmed from the viewpoint of surface roughness, interior stress and obse
19、rvation of the surface failures using two kinds of case-hardened steel rollers. 6oo . 2. Selection of Conditions of Shot Peening and Bending Fatigue Strength of Tooth : iCarbo-nitrided steel -Carburized steel First of ali, it is examined how much the shot peening process will improve the bending fat
20、igue strength molybdenum steel (SCM822H in JIS steel) and the chemical compositions are shown in Table 1. The depth of effective hardness over 513 HV is about 0.5 mm. The conditions of heat treatment are almost the same as those of rollers shown later, but the time of heat treatment is a tittle diff
21、erent. In this study, reasonable conditions of the shot peening process -hardness of shot material and pressure of shot, etc.- were selected so that excessive compressive residual stress onlbeow the surface would not be induced (referred to Table 2). When the carrying load acts on the tooth, a tensi
22、le stress u will be induced at the fillet of tooth root in the meshing side. The resultant stress ( ub+ uR) will act in the tangential direction at the fillet of the tooth root, where uR is the compressive residual stress induced by shot peening. Since tension is positive and compression is negative
23、, the resultant stress I u + o I will become small. On the other hand, the fillet of tooth root in non- meshing side will have a compressive stress ub. The resultant stress ( u b+ uR) may become very much large, because both u and uR are negative. Then yielding may occur. When uy is the yield stress
24、, the yielding condition is Ia,+u,/ 2 (1-1.15)uy, which is given by Trescas maximum shear stress theory or von Mises maximum strain theory. As it is estimated that the yield stress of the carburized or the carbo-nitrided and hardened steels must be over 1600 MPa because of their inherent - of the ge
25、ar tooth. The material is a chromium- / 1 O00 O.,- Carbo nitrided steel m CI with S.P.+Barreiling 1 L c L b L1 800 w ln hardness, and the compressive residual stress will be (iR = -1000 MPa, the allowable compressive stress due to loading is I ub I S 600-800 MPa. The examination of the bending fatig
26、ue strength was conducted using a notched plate simulating a gear tooth whose module is 3. The results are shown in Fig. 1. The bending fatigue limit of test pieces that were only case-carburized and hardened after machining was ow = 640 MPa. As a machining mark was found out at the arc of fillet, b
27、y polishing there using the emery paper, increased to about 760 MPa. Furthermore, the bending fatigue strength increased to aw=850 MPa by the shot peening process. It is confirmed that this condition of shot peening is proper to improve the bending fatigue strength. The bending fatigue strength of t
28、he carbo-nitrided and hardened steel was ow = 720 MPa, which was higher than that of the case-carburized steel. And, the bending fatigue strength of the carbo-nitrided steel was barely increased to u (y = 780 MPa by the shot peening and barrelling processes. e 3. Experimental Conditions : 3.1 Test R
29、ollers Figure 2 shows a pair of test rollers. The outside diameter is d=68 mm and the effective face width is B=10 mm. The material is a chromium-molybdenum steel (refer to Table 1). Figure 3 shows the conditions of the heat treatment of the carburized and the carbo-nitrided steels. The depths of ef
30、fective hardness of the carburized and the carbo-nitrided and hardened steels are about 2 mm and about 1.2 mm, respectively. (a) Driver (flat roller) m (b) Follower (chamfered roller) w Fig. 2 A pair of test rollers finished by barrelling process 2 COPYRIGHT American Gear Manufacturers Association,
31、Inc.Licensed by Information Handling ServicesThe contact surfaces of rollers were finished to 1- 5 PmRy (cut-off length; i. =0.8 mm) by a cylindrical grinding machine after heat treatment. Some of these rollers were processed by shot peening. Figure 4 shows distributions of residual stresses before/
32、after shot peening (see marks (3 and a). The compressive residual stress on the shot peened surface is about 800 MPa, and that near the depth of about 40 becomes to 1200 MPa. The residual stresses of the normalized and the hardened steels, which are coated using a carburizing inhibitor and hardened
33、by the heat pattern as shown in Fig. 3(a), are also shown in Fig. 4 for reference. When the normalized and the hardened steel rollers without case-hardening were shot peened by same conditions as Table 2 after grinding to about 3 p Oil quenching A.C.; Air cooling 1 930C Q, D.T.; Diffusion time Press
34、ure Size Hardness Arc height Coverage MPa mm HV mmA YO 0.3 d0.6 600 0.37 600 O.Q, C.N.T.; Carbo-nitriding time 160C Barrel material Type I Time (a) Carburizing and hardnening Speed of tub m/min “ Gyro (b) Carbo-nitriding and hardening Fig. 3 Heat treatment for case-hardening Alundum(#6- 10) 1 O0 30-
35、40 which is only ground after heat treatment, Group GS shot peened after grinding, Group GSB barrelled after shot peening, and Group GSG reground to about 3 ,um 2200 .- N + Group N-GSB tr Group GSI L I 2000 i . ,I I . ,111 1 Oj 1 o6 1 o7 Revolutions N, Fig. 6 S-N curves (pitting limit) Contact area
36、-+ Face width t. 1 mm U C O C O O .- c F i3 i i1 1 i. However, as a load applied in this research was very high, pitting limits up to lo7 revolutions were examined. Figure 6 shows S-N curves for pitting tests of cas hardened steels. The pitting limit of rollers in Groups GI and GSG only ground to ab
37、out 3 pmI$ was p,=2150 MPa. The pitting limit of Group GS shot peened after grinding may be a little higher than that of Group GI, if a scoring or mild-scoring wont occur. In Group GSB barrelled after shot peening, the pitting limit increased to pnlm = 2450 MPa. The pitting limit of carbo-nitrided s
38、teels in Group N-GSB shot peened and barrelled after grinding has come up to pn,m = 2600 MPa. Though the shot peening and barrelling processes did improve the bending fatigue strength of the carbo-nitrided steel somewhat as shown in Fig. 1, as above mentioned their processes could remarkably improve
39、 the surface durability. This cause is guessed as the following. As the hardness of nitride layer of carbo-nitrided steel is very hard, a notch sensibility is very high and relative hardness of the shot peening particle to the carbo-nitrided surface becomes smaller than the case of carburizing. Ther
40、efore, the bending fatigue strength of unground test pieces under an atmospheric condition couldnt be improved enough by the shot peening process. However, it is considered that the surface durability of ground roller whose contact surface is under a hydrostatic condition will effectively be improve
41、d by induced residual stress and retrained austenite related to inner toughness, etc. in carbo-nitride layer. 4.2 Shapes of Pits In experiments using rollers whose surface roughness are very large, pitting (spalling) is apt to occur. For example, the following roller in Group GSG had a large pit and
42、 two initial pits as shown in Fig. 7 up to 7.6 X lo6 revolutions at a Hertzian pressure of pmm 2250 MPa. The occurrence of the large pit is seen as originating from e. O Face width 1-1 Face width (a) Large pit (b) Initial pits (a) pmaX=2750 MPa (b) pmax=2800 MPa ( Follower ) ( Driver ) Fig. 7 Pits o
43、n follower after 7.6x lo6 revolutions (Group GSG; p,=2250 MPa) Fig. 8 Pits occurred on carbo-nitrided steel rollers 4 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services1 the surface near the center of contact face width. TWO initial pits have occurred at
44、 the side of face width. These pits will propagate all over the face width immediately (within about 2 X 10“ revolutions) because of a high contact pressure. Figure 8(a) shows a pit that occurred on the follower after 2.6 X106 revolutions at p, = 2750 MPa. This experiment was conducted using the bar
45、relled rollers .in Group N-GSB. The initial crack for the pit did not occur from the center, but it occurred at the side of face width. The pit is apt to occur from both sides of face width when the case-hardened steels with a smooth surface is tested. This reason is considered the following. When a
46、 pair of rollers is pressed with a high load, the high contact pressure will be produced at both sides of contact face width as shown in Fig. 9 li. Moreover, as the lubricant enclosed within the oil film between a pair of rollers will leak from both sides, the thickness of oil film at both sides bec
47、omes thinner than that near the center. Therefore, severe metallic contact will often occur at both sides and pitting will be apt to occur. Figure 8(b) shows a large pit e . Fig. 9 Distributions of actual Hertzian pressure - =-I Face width =-I Face width C O lu O O .- CI CI L w- (a) Large pit (b) In
48、itial pits Fig. 10 Pits occurred at p,=2100 MPa that occurred on the driver (faster roller) after 3.8X106 revolutions at p,i = 2800 MPa. Though the pitting limit of rollers in Group GI was about p,=2150 MPa, when the flat shape roller was mounted on the lower shaft, even at the low pressure of p, =
49、2100 MPa five pits occurred after 1.2 X IO revolutions as shown in Fig. 10. Figure 10(b) is two small pits, but these pits will immediateiy grow into a large pit (spalling) as shown in Fig. 10(a). In this combination of a pair of rollers, since a tensile stress acts in the axial direction near both sides of face width of the flat shape roller mounted on the lower shaft, the pitting limit will have decreased a little. This phenomenon was also seen in the case of austenitic ductile cast iron which may have a br
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