effect of shot peening old paper by mistubhishi

8/12/2019 Effect of Shot Peening Old Paper by Mistubhishi

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EFFE T OF SHOT PEENING ON THE PITTING F TIGUESTRENGTH O C RBURIZED GE RS

Motokazu Kobayashi a n d Katsutoshi Hasegawa,M i t s u b i s h i Motors Corporation, Japan

ABSTRACTThere ar e many re po rt s t o ind ica te tha t s ho t peen ing s a valid means to improvethe bending s t r eng th of gear tee th , b u t th ere ar e only a l im ited number ofreports on t s e f f e c t on p i t t i n g f a t i g u e s t r e n g t h and i t s mechanism s y e t t o b eunders tood c lear ly .The au tho rs inv est ig ate d th e cond it io ns under which p it t in g of truck and b u st r ans mis sion gea r s occu r s and conducted a ro l l e r p i t t in g f a t ig ue t e s t and a gearp i t t i n g f a t i g u e t e s t us ing spur gears i n o r d e r t o e v a l u a t e t h e e f f e c t s o f s h o tpeening on the p i t t in g fa t ig ue s t reng th of carbur ized gear s .The f in din gs obta ined f rom the t e s t s a r e l i s t e d below:1 ) P i t t in g of ca rbu r ized gea r s o r ig in a tes from the in te rg ra nu la r oxida t ion a r ea

on t he su rf ac e produced b y c a r b u r i z i n g .2 ) Shot-peened gears excel i n both fa t igue l m t and f a ti g u e l i f e .3 ) Elec t ron mic roscopy of the s l i d in g s u r f aces ind ica te d tha t the r e s i dua l

compressive s t r e s s , which develops a s a r e s u l t of sho t peening, works t osuppress opening (cra ckin g) of the in t erg ran ula r oxidat io n l aye r underthe Her t z s con tac t pressure and consequent ly improves the p i t t in g fa t i gu es t r e n g t h .

s ho t p ee ni ng , c a r bu r iz e d g e a r , r e s i d u a l s t r e s s , p i t t i n g f a t i g u e ,


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The transmission gears for trucks and buses are now required to have higherstrength in order to cope with increasingly higher engine outputs and to meetrequirements for longer life.Usually, the transmission gears for trucks and buses are made of Cr-Mo steel,Cr steel or some other carburized steel and are left as carburized aftercutting shaving) for cost and productivity considerations.In the case of specific gear combinations used mostly due to specialconditions in foreign countries, however, the bending fatigue fracture of gearteeth, pitting and spalling hereinafter referred to collectively as pitting)fatique or other problems sometimes occur with such gears.There are many reports 1) 6) to indicate that shot peening is a validmeans to improve the bending fatigue strength of gear teeth and this isachieved by selecting an optimum peening condition to give a high residualcompressive stress to the tooth face.There are only a limited number of reports 7) on the effect of shot peeningon the pitting fatigue strength of carburized gears and its mechanism 8 ) , 9 )is yet to be understood clearly.The authors investigated the conditions under which pitting of truck and bustransmission gears occurs and conducted a roller pitting fatigue test andgear pitting fatigue test using spur gears in order to evaluate the effects ofshot peening on the pitting fatigue strength of carburized gears. The reportof the tests and their results follows.TEST METHODInvestigation of Field Durability Test GearsFigure shows a cross section of a typical truck and bus transmissioncomponents.Generally, the 2nd and 3rd gears shown in Figure suffer bending fatiguefracture of the gear teeth most often, while the overdrive gear ndconstant-speed gear suffer pitting fatigue most often.

4th Gear 2nd Gear~~~~G~~~ 3r d Gear st Gear

Reverse Gear

L a rg e Ro l le r T e s ~ ~ e c en

Small Roller Tes: tece,Constant Gear 3rd Gear Idler Gearst Gear

4rh Gear 2nd GearFig.2 Roller Pitting Fatigue Test Apparatus

Fig. Cross Section Showing Transmission Components


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The sliding surfaces and cross sections of the carburized gears that developedduring the field durability test were observed through an electron

Roller Pitting Fatigue TestTest Apparatus. The test machine used in this test is a roller pitting fatiguetester that operates using sliding contact of a roller-shaped test piece undera high contact pressure. Figure 2 shows the apparatus.The small roller test piece was held constant at 1000 r/min and the specific~l id ing as set at 40 , which is equivalent to the maximum specific sliding ofan ordinary transmission gear.For the detection of pitting, an automatic pitting detector, employing anoptical fiber to measure the change in the reflectivity of the test piecesliding surface, and a vibrometer mounted to a loading lever to measure thechange in the vibration were used.Test Pieces. Figure 3 shows the shape of the test pieces used. They weremade of Cr-Mo carburized steel, the chemical composition is listed in Table 1and the heat treatment conditions are shown in Figure 4

Large Roller Test Piece

\ ~ r i n d i n gSmall Roller Test Piece Grinding 7

Fig.3 Te st Piece Designfor Rol ler Pit t ing F at igue Tes t

Tab 1 Chemical CompositionMaterial 1 S i 1 Mn 1 S i r Mo CuCr Mo S te el 0 24 0 24 1 0 89 0 015 i 0 008 1 0 1 1 I 1 13 I 0 21 0 12

Oil QuenchC a rb u ri zi ng 1 4 0 ~ 1 7 0 C

Fig .4 H ea t Tr ea t m en t C ond i t i ons

Half of the identically-prepared test pieces were shot peened under conditionsthat are feasible in commercial producticn. The shot peening conditions arelisted in Table 2.Table 3 lists the properties of \both the carburized test pieces and theshot peenedtest pieces after carburization (hereinafter referred to as theshot-peened test pieces).Residual stress distribution of the sliding surfaces of the test pieces,before and after testing, were compared using the X-ray diffraction method.As for the pitting fatigue test pieces, pitting on their surfaces and crosssections was also observed through an electron microscope.


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The lubricant was applied between the test pieces under pressure at a rate of0.5 liter/min, the oil temperature being 8 1 3 C.T ab .2 S h o t P e en in g S p e c i f i c a t i o n s T ab .3 P r o p e r t i e s o f S m al l T e s t P i e ce s f o r R o l l e r P i t t i n g F a t i g u e T e s t

1 S p e c i f i c a t i o n pA rc Height(A1men S t r i p A 0.7mmA Surface HardnessE ff e ct iv e Case Deoth fHV550)- 0.7mmoveraoe- 2Shot S izeHardness of Peening MediaP r o j e c t i o n V e r o s i ty- , . .$ 0 . 6 - 0 . 8HRC53 -5568m/s ec (E s t i ma t i on )

Gear Pitting Fatigue Test

Core HardnessD e p th o f I n t e r g r a n u l a r O x i d a t i o nS u r f ac e Res idua l S t re s sMax Res idua l Compresive Str ess

Test Apparatus. The apparatus used for the tests was a gear pitting fatiguetester using power circulation (10) as outlined in Figure 5, with the larnegear running at a constant speed of 1500 r/min (the small gear at 1890 r/min)rAfter a run-in period, the tester was operated continuously with a constantload. During the run-in period, the load was increased in steps from the 1ststage to one stage before the test stage according to the load stages in Table(the Hertz s contact pressure shown in this paper does not take dynamic loadinto account). Each load stage was held for 15 minutes. Then, at the testload stage, operation was maintained until serious surface damage was causedby pitting.As for the gear life assesment, after a given period of operation the testgear was removed from the tester and its weight lost through wear wasmeasured. Gear life was determined to be the total revolutions of the smallgear before the amount of wear due to pitting started to increase sharply.

HRC 3522 u m+160 MPa-180 MPa

Small INE Gear Small Test Gear~\

LQJo r

HRC 352 a m-480 MPa-1340 MPa

Large rwe Gear Large jest GearF ig .5 Gear P i t t i n g Fa t igue Tes t Appara tus

Tab.4 Load Stageso rque o f He r t z ' s Con tac t P res s u re

1 P i t c h P o i n t

Test. ears. The gears tested were spur gears of module m=3 (small gears withnumber of teeth Z1 = 27, large gears with number of teeth Z2 = 34). Table5 lists their dimensions.In order to increase the contact pressure without loss of gear tooth strength,the large gears had a face width of b = 20mm and the small gears had neffective contact width of b = 5 mm with 0.3 mm level difference in the toothtrace direction. The tooth shape of the small gears is shown in Figure 6.

Fig.6 T o o t h P r o f i l e o f S m a l l T e s t G e ar

Tab.5 Dimens ions o f Tes t GearsType of GearT oo th P r o f i l eModuleCu t t e r P res s u re A ng l eNumber of TeethAmount o f Addendum Modi f i ca t ionD i a m et e r o f G e n e r a t i n g P i t c h C i r c l e mmD i a m e t e r o f T i p C i r c l e mmCrowningAccuracyS u r f a c e F i n i s h i n g P ro ce ssCenter Dis tanc e mm

Larg e Gear Small Gear.Spur GearFu l l Dep th Too th .32034 70102 81 -ID8 87 .0JS 4 GradeShav ing91.5 .-


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The test gears were of the same chemical composition and had the same heattreatment conditions as the test pieces in the roller pitting fatigure test,Half of the identically-prepared gears were shot peened under the sameconditions (Table 2) as the test pieces used in the roller pitting fatiguetest.Table 6 lists the properties of both the carburized test gears and theshot-peened test gears.S with the roller pitting fatigue test pieces, the residual stressdistribution of the test gears before and after testing were compared usingthe X-ray diffraction method. As for the pitting test gears, pitting on theirsurfaces and cross sections was also observed through an electron microscope.

Tab.6 Properties of Small Gears for Gear Pitting Fatigue Test.Carburized Gear Shot Peened Gear.]

Surface Hardness HV 752 HV 835Effective Case Depth HY55O J K K ~ ~- 0. 7mmHRC 35 HRC 35ore Hardness C 1Depth o f Intergra? ar 0xidatio".j 1 5 ~ ~ 1sSurface Residual Stress -270MPa

~ax. Residual Comoresive Stress 490 M 330MPa

Lubricant. The lubricant used in the tests was equivalent to GL3 gear oilwith a kinematic viscosity of 198 x 1 0 - ~ m ~ / s t 40 ~ and 18.1 x10-6m2/s at 100 ~.The lubricant was sprayed under pressure at the meshing point at a rate of 2.4liters/min with an oil temperature of 6023 ~.TEST RESULTS AND DISCUSSIONField Durability GearsThe carburizing process currently applied in commercial production uses acarburizing gas with trace amounts of H2 and C02, which causes oxidationof the steel surface during carburization.When oxygen enters the steel, its diffusion rate is generally higher in thegrain boundary than in the transgranular structure. Therefore, as oxygenenters the steel from the surface, Cr, Mn and Si move toward the grainboundary faster than other elements present in the matrix in the vicinity ofthe grain boundary. Then, the elements combine with the oxygen that hasdiffused and reached the boundary. This phenomenon is called intergranularOxidation.Once this phenomenon occurs, the reduction in the amount of Cr amd Mn causes aloss of hardenability. As a result, in the vicinity of the area whereintergrariular oxidation is caused, martensitic transformation does not occur,but bainite and troostite are produced.This phenomenon is unavoidable as long as ordinary gas carburizing is used,which always causes a thick intergranular oxidation layer of about 5 to 30 pmto be produced on the surface (the longer the carburizing time, the thickerthis layer becomes).The presence of an intergranular oxidation and structural anormalies wereConfirmed by electron microscopy of the tooth sections of a new, carburizedSear as shown in Photo 1.


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seen, pitting occurs typically from around the pitch circle of the teeth tthe tooth roots.

The result of observation by electron microscope of these pitted areas on thesurface and cross section is shown in Photo 3. It can be seen that plasticflow developed under the surface from around the pitch circle to the toothroots and that the pitting originated from intergranular oxidation, asdiscussed earlier.Some theorize that pitting originates from immediately below the surface wheremaximum Hertz s stress occurs, but from the results presented above, it isbelieved that in the case of carburized gears, the intergranular oxidation othe surface is the origin of crack, which then propagates to cause pitting.

Non S l i d ing ZoneSl id ing Zone

Pi t t ing Area

ross Se c r ~ on

N o n S l ld ~ n g o ne S l ~ d ~ n go n e S l ~ d l n gZ o ne M a g n ~ f ~ c a t ~ o n ) P l tt l ng A re a

Photo.3 Observation by SEM of To oth Surface after Durab il i ty Test

Roller Pitting Fatigue TestFatigue Diagram. The results of the roller pitting fatigue tests of t h ecarburized pieces and the shot-peened pieces, conducted under the above testconditions, are plotted with the tooth strength PH on the ordinate and thelife N to generation of pitting on the abscissa (semi-logarithmic scale)(Figure 7 . It can be seen from the diagram that the shot-peened pieceshaveabout 1.15-times higher fatigue limit at 2 x l o 7 cycles and about 10-timeslonger life under 3200 MPa Hertz s contact pressure.


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Observation of Pitting. Photo 4 shows the typical pitting produced in thetests.

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Photo. P i t ted Sta te a f ter Ro l le r P i t t ing Fat igue TestPiloto 5 shows the surface and cross section of the pitted portions of thecarburized test pieces and Photo 6 shows that of the shot-~eened test piecesas observed by electrcn microscope.

Sliding Surface itting reaSectton

Photo.5 Observat ion by SEM of Carburized Test Piece Surface after Test

Sliding Surface Sliding SurfacePhoto. 6 Observat ion b y SEM of Shot Peened Test Piece Surface after Test

From these photos, plastic flow is observed on the topmost sliding surfaces ofthe carburized pieces as in the case of the field test gears. It can also beseen that the pitting originated from the intergranular oxidation. On thesliding surfaces, many open cracks were observed.On the low surface pressure portions of the contact surface side sections,many such shallow cracks were also observed, but separation originating fromthem was not observed. On the contact surface center increasing contactpressure) side, the cracks were smaller in number and pitting occurred in thecenter of the contact surface where the contact surface pressure was thehighestFrom these observations, it is believed that cracking originating from theintergranular oxidation starts in the early stages of cycling, stops for sometime due to the internal residual compressive stress and then propagates tofinal destruction in subsequent cycling.


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The shot-peened pieces also exhibit similar plastic flow and the pittingdevelops from the intergranular oxidation area with only the topmost. surfaceplastically deformed by shot peening.The cracks observed on the sliding surface, however, are closed in contrast tothe open cracks seen in the carburized pieces. This is probably because theresidual compressive stress stops the growth or propagation of cracking.As can be seen from the fatigue diagram, the shot-peened pieces have a longerfatigue life probably because of this effect of the surface residualcompressive stress that suppresses cracking and delays crack growth.

Gear Pitting Fatigue TestFatigue Diagram. Figure 9 shows the relationship between the tooth facestrength PH (Hertz s contact pressure at pitch point) of the carburized gearsand the shot-peened gears tested under the above conditions and the life N1to pitting (total revolutions of the small gear). The straight lines shown inthe figure are the linear regression curves obtained by the least squaring ofthe plotted data, except the large separated plot.It can be seen that the shot-peened gears have about 1.35-times higher fatiguelimit at 2x10 cycles and about 28-times longer life under 3200 MPa Hertz scontact pressure.The shot-peened gear data tends to have a larger scatter than that of thecarburized gears.The problem, therefore, will be how to achieve more uniform shot peening.Residual Stress. Figure 10 shows the residual stress distribution of thesliding surface of the shot-peened gears before and after testing asdetermined by using the X-ray diffraction method.

Total Number of Revolution of Small Gear N1

Fig 9 Results of Gear Pit t ing Fat igue Test

200 Shot Peened Test Gear

600?800- \ efore Test2 1000

12001400

0 100 200Distance from Surface , urn

Fig 10 Residual Stress Dist r ibut ionof Pit t in g Fat igue Test Gear

n the roller pitting fatigue test pi residual compressive stresswas produced on the surface using shot peening, with a residual compressivestress of about 1000 IdPa occurring at a depth of about 20 pm beneath theSurface. The test gears were subjected to shot peening under the sameconditions as the roller pitting fatigue test pieces and we thought that thedifference in the resultant residual stress level was due to the differencesn the test piece shapes.


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A residual stress distribution pattern after the test similar to one obtainedafter the roller pitting fatigue test was obtained.Observation of Pitting. Photos 7 and show the typical pitting produced inthe tests, along with enlarged views.Photo 9 shows the surface and cross section of pitted portions as observed yelectron microscope.From these photos, it is believed that cracking originated from t hintergranular oxidation layer and separation occurred therefrom, leading tpitting as in the roller pitting fatigue tests.The cracks observed on the sliding surface are closed by the residualcompressive stress of shot peening.

Photo . 7 Pi t ted S ta te a f te r 9Gear P i t t i nq Fat i sue

Surface ff ross surface cross AArea a f te r Gear P i t t i ng ~ect lon sectFatigue TestPhoto . Observat ion by SEM of Shot Peened TestGear Surface after Test

CONCLUS OhThe test results and observations may be summarized as follows:(1) Pitting of the carburized gears originated from the intergranular

oxidation area on the surface produced by carburizing.(2) The results of the roller pitting fatigue tests indicate that the

shot-peened test pieces had about 1.15-times higher fatigue limit at2xl0cycles and about 10-times longer life under 3200 MPa Hertz s contactpressure than the carburized test pieces.

(3) The results of the gear pitting fatigue tests indicate that theshot-peened gears had about 1.35-times higher fatigue limit at 2x10cycles and about 28-times longer life under 3200 MPa Hertz s contactpressure than the carburized gears.

4 ) The electron microscopy of the sliding surfaces indicated that theresidual compressive stress developing as a result of shot peening worksto suppress opening (cracking) of the intergranular oxidation layer underthe Hertz s contact pressure and consequently improves the pitting fatiguestrength.

ACKNOWLEDGEMENTThe authors extend their gratitude to all those who cooperated in conductingthis study.


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REFERENCES

N. Kenneth Burrell, Controlled Shot-Peening of Automotive Components .SAE Paper 850365 (1985)Sadaoki Hisamatsu and Takashi Kanazawa, Improvement of Carburized GearStrength by Shot Peening . Journal of the Society of Automotive Engineersof Japan, Vol. 41 No. 7 722-728 (1987)Katumi Inoue, Toshiyuki Maehara, Masashi yamanaka and Masana Kato, TheEffect of Shot Peening on the Bending Strength of Carburized Spur GearTeeth . Transaction of the Japan Society of Mechanical Engineers,54-502(C) 1331-1337 (1988)Yoshihiko Kojima, Yoshihisa Miwa, Shinya Shibata and Yukio Arimi, NewProcesses for Manufacturing High Strength Transmission Gears . Journal ofthe Society of Automotive Engineers of Japan, Vol. 42 No.6 755-760 (1988)Masaru Izumine, Kenichi Asano and Hideo Ihara, Application of ShotPeening to Engine Parts'. Journal of the Society of Automotive Engineersof Japan, Vo1.43 No.6 93-99 (1989)Munetoh Hashimoto, Masaki Shiratori and Shinichi Nagashima, ResidualStress Distribution and Fatigue Strnegth of Carburized Gear Steel .Transaction of the Japan Society of Mechanical Engineers, 55-520(C)3034-3038 (1989)Dennis P. Townsend and Erwin V. Zaretsky, Effect of Shot Peening onSurface Fatigue Life of Carburized and Hardened AISI 9310 Spur Gears S EPaper 881291 (1988)H.S. Cheng, L.M. Keer and T Mura, Analytical Modelling of SurfacePitting in Simulated Gear-Teeth Contacts . SAE Paper 841086 (1984)S.T.S. AL-Hassani, The Shot Peening of Metals Mechanics andStructures SAE Paper 821452 (1982)

[lo] Shoji Haizuka, Chotaro Naruse, Ryozo Nemoto and Kazuo Nojiri, Studies onthe Tooth Surface Strnegth of Spur Gears . Transaction of the JapanSociety of Mechanical Engineers, 53-487(C) 855-862 (1987)

effect of shot peening old paper by mistubhishi

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