Surface integrity studies on shot-peened thermal-treated En 24 steel spur gears

WEAR ELSEVIER

Wear 193 ( 1996) 242-247

Surface integrity studies on shot-peened thermal-treated En 24 steel spur gears D.V. Girish, M.M. Mayuram *, S. Krishnamurthy Muchine

Elements Laboratory,

Department

of‘Mechanicct1

Engineering,

Indian

Institute

Received 13 July 1995; accepted 27 September

of Technology.

Mudras

600 036, India

1995

Abstract En 24 steel spur gears in untreated and thermal-treated conditions with and without shot peening were tested in a back-to-back gear test rig. Surface finish at the pitch region and wear loss were monitored. Surface integrity studies and ferrographic analysis of wear debris enabled the interpretation of failure. The surface deterioration was less pronounced in the case of shot-peened gears and further they exhibited good steady-state wear characteristics compared to unpeened gears. Keywords:

Spur gears; Shot peening; Surface integrity

1. Introduction Spur gears find a wide range of application for transmitting motion and power. Carburizing and nitriding are heat treatments traditionally used to improve the surface integrity of spur gears. Shot peening is often used in gears along with the above-mentioned treatments as a method of improving the wear resistance characteristic of gears. This paper presents some experimental investigations on the surface integrity studies of spur gears, made of high-strength En 24 steel under untreated and thermal-treated conditions, with and without shot peening.

2. Test specimens and the shot peening process The material used for the test was En 24 steel with percentage of carbon 0.37, yield strength 600 MPa, and hardness 240 BHN. The test specimens were gears of 3 mm module made with involute profile 20” pressure angle. Other specifications are given in Table 1. Shot peening [ l] may be defined as the process of cold working the surface of structural or machine parts by means of a stream of high velocity shots. Relatively hard particles, usually steel shots, are entrained in an air jet and blown towards the part to be peened at a high velocity. The intensity of peening depends on variables such as shot size, shot velocity, coverage, angle of impingement and duration of peening. * Corresponding

author.

0043.1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06791-4

The test specimens prepared as per the above specifications were divided into four groups. The first group was left without peening. The second group was shot peened to a maximum permissible Almen intensity of 0.22 mm A-strip. The third and fourth groups were thermal treated in a traditional way. The thermal-treated fourth group was shot peened to a maximum permissible Almen intensity of 0.38 A. Shot peening to intensities of 0.22 mm A-strip and 0.38 mm A-strip was done by using SAE-230 shots under controlled processing. The setting values of the process parameters arc summarized in Table 2. The equipment used for shot peening was a cabinet-type abrasive blast cleaning unit incorporating auxiliary attachments necessary for controlled processing. 3. Test procedure The tests were conducted using a back-to-back gear test rig. This test rig consisted of two identical gear housings, each one accommodating a gear pair. The two housings were connected by their input and output shafts. The loading was accomplished in a static condition by a lever loaded with dead weights and a special torsional coupling. The gear tests were performed at a pinion pitch line velocity of 7.5 m so ‘. The power for running the gears was provided by a 3.0 kW induction motor. The gears were lubricated with SAE-40 oil (viscosity 0.138 Pa s at 38 “C) by drop feeding under gravity. Two sets of gears were tested in each experiment under identical conditions. The experiments were conducted under

D.V. Cirish

Table 1 Specification

et al. /Wear 193 (1996) 242-247

243

of test pinions and gears Symbol

Variable

Module Number of teeth Pitch circle diameter Centre distance Addendum circle diameter Root circle diameter Face width Number of teeth to be measured Tooth distance for inspection

Unit

Values of variables used in the experiments

mm mm mm mm mm mm

for inspection mm

Table 2 Shot peening parameters Parameter

Unit

Value

Air pressure Distance of tooth flank from the nozzle Number of runs Rotational speed Shot size Nozzle size

MPa mm

0.28-0.30 120.0 3 30 SAE-230 4.0

different loads and the test programme Table 3.

rev/min mm

followed is shown in

Pinions

Gears

3.0 18 54.0 90.0 60.0 46.5 10.0 2 14.04

3.0 42 126.0 90.0 132.0 118.5 30.0 5 41.61

exhibit rapid wear. This can be attributed to substrate yielding and consequent delamination associated with wear of gears during the early wear period, influenced by intrinsic material properties. Further stressing will result in cumulative yielding and wear could be resisted only by superposition of stressed layers such as those due to peening or any other mechanical treatments. The effect of thermomechanical treatment on gear wear character is clearly illustrated in Fig. 1 (b) . Unlike the case of untreated gears the appreciable difference in the wear character of thermomechanically treated gears can be attributed to the effect of possible surface defects associated with heat treatments, such as quenching cracks, hot tears, non-uniform oxidized layers and so on. The healing of such cracks in

4. Results and discussions 4.1. Wear characteristics 4.1. I. Cumulative wear loss Prior to the beginning of each experiment, the test pinions were thoroughly washed, weighed, and then assembled in the test rig. Throughout the tests the gears were lubricated as mentioned earlier. In all the experiments a small torque was initially locked in the system and the gears were run for about 20 minutes. The loads were then raised to the required pinion torque values and the gears were run at rated speeds. During the tests the rig was stopped at regular intervals of 0.2 million cycles and the pinions were taken out for inspection. These specimens after being washed with acetone and detergent solution to remove any oil present were weighed after drying to determine the material loss due to wear. The measured loss in weight was plotted as cumulative wear loss by weight against the number of cycles in Fig. 1. The graph at (a) is for untreated specimens and at (b) is for thermal treated specimens, each graph being made for two different stress levels. Referring to the figures, it is seen that in the early phase of running, i.e. up to around 8 X lo5 cycles, there is only a marginal difference in the wear character of both unpeened and shot-peened gears. Beyond this cycle the unpeened gears

??

A

UNPEENED SHOT PEENED

0-

(b)

‘O ”

‘*

22XlD5

10

14

t8

22

CYCLES

RUN,N

’

’

2

8

(a)

Fig. 1. Cumulative treated.

CYCLES RUN, N

weight loss versus cycles run; (a) untreated

, (b) thermal

244

D.V. Girish et al. /Wear

193 (1996) 242-247

Table 3 Test programme Contact stress oH ( MPa)

Torque applied (Nm)

Experiments

conducted

under the following conditions

Untreated

1230 1110 1069 8.50 790 760 711 680 640 630 610 570 490 460

67.7 55.6 51.1 32.5 27.8 25.8 22.6 20.6 18.3 17.7 16.6 14.5 10.7 9.5

Thermal treated

Unpeened

Shot peened

Unpeened

Shot peened

X” Y X Y X X Y X X X X Y Y Y

Yb Y X Y X X X Y Y X Y X X X

Y X Y Y X X X Y X Y Y X X X

Y X Y Y Y Y X X X X X X X X

a Tests conducted at the stress level indicated. b Tests not conducted at this stress level.

Table 4 Comparison

of wear loss at a contact stress level of 850 MPa at lo6 cycles Untreated

Cumulative wear loss (mg) Relative wear loss ratio

Thermal treated

Unpeened

Shot peened

Unpeened

Shot peened

174.60 1.0

128.20 0.73

91.80 0.52

28.80 0.16

surface quality consequent to shot peening has resulted in improved wear performance. For comparison a relative wear loss, i.e. wear loss expressed in terms of wear loss of untreated unpeened gears at lo6 cycles, was also estimated and these values are given in Table 4. 4.1.2. Cumulative wear particle analysis As indicated earlier, oil samples were collected from the outlet of the gear box at regular intervals of 0.2 million cycles and they were analysed using a direct reading (DR) ferrograph [ 2-61. From the large particle (Q) and small particle (Ds) readings obtained from the DR ferrograph, severity index (Is), cumulative wear particle concentration (CWPC) , and cumulative size distribution (CSD), were calculated. The severity index helps to identify the wear mode. Fig. 2 plots the severity index against the number of cycles run at a stress level of 850 MPa, under unpeened and shot-peened conditions. The severity index shows a fluctuating trend as could be expected in any normal wear process. It can be observed that the relative magnitude of the severity index of shot-peened gears both in untreated and thermal-treated conditions is lower compared to unpeened gears. However, with shot peen-

ing the severity index is observed to resist an increasing trend, possibly due to the occurrence of predominantly large size delaminates or flakes owing to cumulative stressing. Fig. 3 plots CWPC, c( D,_ + D,) against the number of cycles run and CSD, C(&, - D,) against the number of cycles, for a contact stress level of 850 MPa. Referring to Fig. 2(a) and Fig. 3(a), it can be seen that an increasing trend of wear severity index is supplemented with a relatively small difference between D, and D,. The occurrence of inflexion around 16 X lo5 cycles is also seen in Fig. 2(b) , indicating increasing order in the severity index, and in Fig. 3 (b) around the same cycle as above there can be seen a sudden change in slope in the case of peened gears, confirming that the gear was approaching a condition of surface distress. 4.2. Surface integrity and failure analysis 4.2.1.

Sugace

integrity

To study the surface integrity of gears, a particular tooth was identified in each gear and the roughness of the surface was measured for a sampling length of 5 mm repeatedly at regular intervals of 0.2 million cycles using a Perth-o-meter. Graphs of surface roughness (R,) against number of cycles

D.V. Girish et al. /Wear

193 (1996) 242-247

”

245

”

. SHOT PEENED

b /’

f

t

01

.

UNPEENED

.

SHOT PEENED

I

I

I

17 CYCLES

102X60

‘THERMAL 950

so-

zap,

I] 19x105

w I

RUN,N

~

2

6

(

*

,

14 10 CYCLES RUN, N

16

22x105

TREATED. MPa

In --

40-

??

2

.

f

30-

2 0: ,”

zo-

%

lo-

UNPEENED SHOT PEENED

THERMAL TREATED-

A

2 0 (b)

’

Fig. 2. Severity thermal treated.

4

9 CYCLES

index versus number

” RUN,N

l6

of cycles run; (a) untreated,

(b)

UNPEENED SHOT PEENED

??

9 *0

I

I

(b) ’

6

C::LES

Fig. 4. Surface roughness thermal treated.

I

I 18

1x105

R:N, N

versus number of cycles run; (a) untreated,

(b)

UNTREATED’ 8 50 MPa a”

I

---

600-

CWPC CSD UNPEENED A SHOT PEENED ??

0” G D 2 e

400-

;3 *

zoo-

0” ~

0

2

I

I

6

10 CYCLES

(a)

. .

6 ’

1 19

CSD UNPEENED SHOT PEENED

‘---

(b)

, 14 RUN,N

10 CYCLES

14 RUN,N

10

Fig. 3. Cumulative wear particle concentration and cumulative size distribution versus number of cycles run; (a) untreated, (b) thermal treated.

run at a stress level of 850 MPa for test conditions are shown in Fig. 4. It is clear from the figures that the roughness decreased during the running-in period and increased thereafter. A comparison of the graphs in this figure reveals that

Fig. 5. Micrographs

of the gear teeth; (a) machined,

(b) shot peened.

the deterioration of the surface was very severe in the case of unpeened gears as compared to peened gears. Referring to Fig. 1 (a), Fig. 1 (b), and Fig. 4, it is seen that the gear exhibited a running period of around 6-10X lo5

246

Table 5 Comparison

D. V. Girish et kzl. / Wear 193 (I 996) 242-247

of R, and R,,,

machined surface has cutting marks due to hobbing with cross surface lay along the face width of the gear, these marks have completely vanished on the peened flanks.

before and after shot peening

4 Before SP After SP

1.82 1.42

1.92

2.02

1.82

1.26

2.23 2.04

2.37 1.26

R max Before SP After SP

8.64 7.36

16.9 12.7

17.3 6.10

21.1 15.2

29.9 9.65

cycles. During this period there was an improvement in finish and the gear exhibited minimum wear associated with the severity index. Table 5 gives a comparison of the roughness value R, and RIIUX, measured before and just after shot peening. It can be observed from the table that surface roughness decreases a little after peening, probably due to flattening of peaks as the impinging balls strike the surface while plastically deforming it. This fact can be further substantiated by the optical photomicrographs of the gear teeth in the machined and peened conditions shown in Fig. 5. It can be observed that while the

Fig. 6. Pit formation peened.

for various test conditions:

(a) untreated, unpeened;

4.2.2. Failure appearance The failure of both unpeened and shot-peened gears was by pitting only. Fig. 6 shows photographs of pit formation for the various test conditions. The photographs were taken at a contact stress level of 850 MPa, after running for IO6 cycles. The average length and breadth of the pits observed in the case of peened and unpeened conditions were measured and are presented in Table 6. The experimental evidence suggests that the failure in both the unpeened and shot-peened gears was of surface origin type. The pits appeared below the pitch line and further running of the gears under the load resulted in the spreading of the pits on the tooth flank. Fig. 7 shows a typical fractograph of the failed untreated shot-peened gear tooth at a contact stress level of 850 MPa and run up to 20 X lo5 cycles. Here a big bell-mouth crack is visible even though the magnifica-

(b) untreated, shot peened; (c) thermal treated, unpeened;

(d) thermal treated, shot

Table 6 Average length and breadth of pits Thermal treated

Untreated

Length ( rm) Breadth (pm)

Unpcened

Shot peened

Unpeened

Shot peened

220-550 140-240

100430 80-230

100-160 80-120

80-140 80-l 10

D. V. Girish et al. / Wear 193 II 996) 242-247

Fig. 7. Cross-sectional

fractocraph

(Tested and failed).

tion is low. The occurrence of bell-mouth opening is normally associated with intergranular cracking (fatigue failure) with cracking being initiated at the surface. The position of the pit is indicated by a circle mark and the primary crack by an arrow mark. It can be seen that the cracks originated from the surface and propagated into the subsurface region. The voids seen in the picture may be due to stress corrosion cracking, since the test gears were in lubricating oil for a considerable time under load during the test.

5. Conclusions

Results of experimental observations on gears made of En 24 steel under untreated and thermal-treated conditions with and without shot peening for surface integrity studies are reported in this paper.

241

1. From observations made it can be seen that shot peening exerts a decisive and considerable influence on treated gears compared to untreated gears. 2. Regarding the severity index, though, shot peening results in a reduced severity index. With cumulative running the severity index seems to be increased in the case of shotpeened gears. This may be attributed to the presence of laminated wear. 3. Overall running of shot-peened gears was observed to be better than that of unpeened gears as far as running wear is concerned. 4. The comparison of surface roughness values at different cycles under similar stress conditions reveals that the deterioration in surface roughness is much reduced in the case of shot-peened gears compared to unpeened gears. 5. The comparison of failure due to pit formation at lo6 cycles reveals that the surface origin of failure and size of pits are smaller in the case of shot-peened gears compared to unpeened gears. References [ll Shot peening, in Metals Handbook, Vol. 2, ASM. Metals Park, OH, 1964, pp. 398-405. 121 R. Bowen, D. Scott, W. Seifert and V.C. Westcott, Ferrography, Tribal. Inr., 9 (1976) 109-l 1.5. 131 W. Seifert and V.C. Westcott, A method for the study of wear particles in lubricating oil, Wear, 21 (1972) 27112. [4] G. Pocock, Machinery health monitoring and particle size distribution, J. Mech. Eng. Sci.. 42 (1978) 97-105. [5] P.B. Senholzi, Oil analysis/wear particle analysis, J. Me&. Eng. Sci., 43(1978)113-118. [6] W. Holzharer and S.F. Murray, Continuous wear measurement by online ferrography, Wear, 90 ( 1983) 1 l-19.