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Effect of sursulf treatment on fatigue life of medium carbon steel spur gears
Mech. Mach. Theory Vol. 24. No. 6. pp, 493-498. 1989 Printed in Great Britain
EFFECT
0094-114X/89 $3.00 + 0.00 Pergamon Press plc
OF SURSULF
LIFE OF MEDIUM
TREATMENT
CARBON
ON
FATIGUE
STEEL SPUR
GEARS
S. KRISHNAMURTHY and A. RAMAMOHANA RAO Department of Mechanical Engineering, Indian Institute of Technology, Madras-600 036, India
Abstract--Medium carbon steel gears untreated as well as Sursulf treated were tested in a back-to-back gear test rig. The failure of the test gears was by pitting. The contact stress-pitting life curves have been established for both untceated and Sursulf treated conditions. Sursulf treated gears exhibited low wear compared to untreated. Wear particle analysis of lubricating oil was carried out to analyse the nature of failure. Cumulative wear particle concentration at pitting limit has been suggested as a basis for predicting the onset of the failure in the gears.
1. INTRODUCTION Gears are used for transmission of power, motion or both. Power rating of a gear set is essentially dependent on the surface durability of the gear teeth. The materials suggested by gear standards vary from plain low and medium carbon steels to a variety of alloy steels and cast irons. Surface durability is closely related to the ability of the teeth to resist pitting[l]. Increase in hardness improves considerably the surface durability resulting in higher power rating. Improvement in hardness of medium carbon steels can be achieved by imparting different heat treatments such as through hardening, induction hardening, gas nitriding, liquid nitriding (Sursulf and tufftride process) etc. This paper is concerned with surface durability studies on gears made from medium carbon steel and liquid nitrided by Sursulf treatment. Sursulf[2] is a surface hardening salt bath nitriding process developed by Hydromecanique et Frottement of France. It is useful in the treatment of all ferrous materials[3]. It is carried out in a molten salt bath which consists of alkaline cyanates and carbonates stabilized by the addition of lithium compounds. The bath is therefore free from cyanides when made up, and also specifically contains very small additions of sulphur compounds (normally of the order of a few ppm by weight). These sulphur compounds have two functions--to ensure the formation of iron sulphide at the surface of the treated parts and to see that as the operation proceeds the non-polluting character of the bath is maintained by oxidising, as they appear, any trace of cyanide which could possibly be formed by chemical reaction within the working bath. A treatment time of 90 min in a Sursulf bath at 565°C imparts on the treated components a deeper compound zone of about 10-20 pm which is porous at and near the surface and a diffusion zone of about 500/~m. The porous zone contains sulphur (Troilite FeS) which confers on the treated parts improved resistance to scuffing, fatigue and seizure. The deeper diffusion zone beneath the compound zone gives the treated parts better mechanical properties, including increase fatigue strength[4]. This method of treatment has several process and functional advantages--mainly short duration of the process time and negligible thei'mal distortion. The authors have made some investigations on Sursulf treated medium carbon steel gears to study the effect of this treatment on surface durability. The results of these investigations are presented in this paper. 2. EXPERIMENTAL DETAILS The material used for the gears is 0.35% carbon steel. The mechanical properties of this steel are given in Table I. The specification of the tested gears is shown in Table 2. The gear blanks were cut from bar stock and machined to standard tolerance limits. The spur gears were then hobbed on a gear hobbing machine and chamfered at the ends to reduce stress concentration effects. Six pairs of gears were subjected to Sursuif treatment for a period of 90 min and an equal number were left untreated. The maximum hardness values achieved before and after the treatment are indicated in Table 1. The Vickers Pyramid Hardness distribution of a Sursulf treated gear taken on a tooth v..nder a load of 0.5 N is shown in Fig. 1. 493
494
S. KRISHNAMURTHYand A. RAMAMOHANARAO
Table 1. Properties of gear material Property
Value
Ultimate tensile strength Yield strength Untreated hardness (HV) Hardness after Sursulf treatment (HV) Modulus of elasticity
700 MPa 500 M P a 215 650 2.1 x 10 ~ MPa
Table 2. Specifications of test pinions and gears Value of the variables used in experiment
Variable, symbol N u m b e r of teeth, Module, m (ram)
-, = 20
:
.-,
= 40
3
Pitch circle, dia, do (ram) Pressure angle, ~0 (deg) Face width, B (ram) Addendum circle dia, d, (mm) Root circle dia, dr (ram) Centre distance, ao (mm)
d,, = 60.0
do: = 120.0
20.0 B~ = 13.0 dul = 66.0 dr1 = 52.5
B: = 30.0 d,, = 126.0 dr. = 112.5 90.0
Subscript l--pinion, 2--gear
The tests were conducted using a back-to-back power recirculation gear test rig built on FZG principles[5]. The gear tests were performed at a pinion rotational speed of 1420 rev/min. Power for running the gears was provided by a 3.0 kW induction motor. The gears were lubricated with Hydrol 32 oil (viscosity of 0.028 Pa-S at 38°C) by drop feeding under gravity at a rate of 2.2 l/h. The test rig consists of two housings accommodating identical pairs of gears in them. These housings are connected by their input and output shafts which are directly coupled to form a closed circuit. Their ratios being equal, no auxiliary gearing is required. A special torsion coupling, used for loading, is such that its halves can be clamped together in any angular position. Thus with the clamping bolts released, one half is held stationary while the other half is acted upon by a lever and dead weights of desired value. The applied torque is estimated as the product of the lever arm and weight applied. Two sets of gears were tested in each experiment under approximately identical conditions. Experiments were conducted under different loads and the test programme followed is shown in Table 3. In all the experiments, a small torque was initially locked in the system and the gears were coo
700
600 ~o I
500
400
S
~oo
ZOO
~ . ~ = Core h=rdne.
1 O0
0
I ZOO
I I I I 400 600 B00 1000 Dis'canoe from s u r f a c e t v m )
I 1200
,J 4400
Fig. 1. Micro hardness distribution of Sursulf treated 0.35% carbon steel gears.
Sursulf treatment on steel spur gears
Experiment No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Treatment Untreated Untreated Untreated Untrea:ed Untreated Sursulf Sursulf Sursulf Sursulf Sursulf
495
Table 3. Test programme Torque applied on Contact stress at pinion (N. m) pitch point on (MPa) 59.8 936 32.8 704 24.9 603 13.3 440 !2.3 424 119.8 1325 59.8 936 32.8 704 29.8 660 28.0 640
Power transmitted
(kW)
8.90 4.87 3.68 1.99 1.82 17.80 8.90 4.87 4.43 4.16
run for about 20 rain. The load was then raised to give the pinion torque value corresponding to a particular experiment as given in Table 3, and the gears were run at the rated speed. During the tests, the rig was stopped at regular intervals and the teeth were inspected for any damage. In experiments l, 2, 3, 6, 7 and 8, the failure was due to pitting and the number of macropits were counted at regular intervals on selected teeth. In experiments 4, 5, 9 and 10, where no failure was observed, the tests were terminated after running for 5 million cycles. Oil samples were also collected in intervals of 0.3 million cycles for wear particle analysis.
3.
DETERMINATION
OF S T R E S S - L I F E
CURVES
As already said in experimental details, the surfacec of gear teeth were inspected at regular intervals. The number of macropits on the pinion have been counted at regular intervals of cycles and these were plotted on log-log coordinates. The points fell on a straight line and this line was extrapolated to yield the life to one pit formation which is considered as the pitting life[6]. This method was repeated to obtain the pitting life at all stress levels for which failure has taken place. In experiment numbers 4, 5, 9 and 10 of the test programme, micropits started appearing but they did not grow in size even though the gears were run upto 5 million cycles. On the contrary the pits disappeared. In these experiments, it is assumed that no failure has taken place. The contact stress (all) values and the corresponding pitting lives (N) have been plotted in Fig. 2. Stress levels at which no failure has taken place have also been indicated in the figure with cross arrows. The overall curve is similar to S - N curve in a traditional fatigue test. The curves show the marked improvement in the surface durability of Sursulf treated gears in relation to untreated. In the finite life region of the stress-life curve, where pitting life is influenced by stress, the relationship between contact stress and pitting life for the tested gears follows the equation. (aM)raN = constant where m is the slope of the a~c-N curve in log-log coordinates. The value of m is 2.75 for hobbed and Sursulf treated 0.35% carbon steel gears and 3.27 for untreated gears of the same material.
A o~ o
200C
n°
1000
0.33% carbon steer A Sursutf trea~ed 0 Untreated
~ ~
: 2.73
-
% 3.z7
zoo
10C 3 x 104
o
I 10 ~
I 106 Pittincj
I ~0 ~
Life N, c y c L e s ( Log )
Fig. 2. Contact stress-life curves for Sursulf treated and untreated 0.35% carbon steel gears.
S. KRISHNAMURTHYand A. R.AMAMOHANARAO
496
~ 160
Untreated
035*/°
carbon steGK gears
I= 2 0
Sursutf treata¢l 0.3R,°/e carbon
,,p 936 MPo
steel gears
~ i = 2.0
/
1525 MPo
o ~2o /e/° 80
936 MPo
~
603MPo
40
660 MPo (J
0
I
I
I
I
6
10
14
t8
I
22 2
I
I
6
10
.... l
14
I
J
18
22
CycLes run (N xlO 5) Fig. 3. Plot of cumulative wear loss versus cycles
run.
The pitting limit for Sursulf treated 0.35% carbon steel gears is 660 MPa and for untreated gears the value is 440 MPa. 4. CUMULATIVE WEAR LOSS In each experiment, the gears were removed at every 0.3 million cycles, cleaned thoroughly with acetone and weighed. The loss in weight was measured and this was plotted as cumulative wear loss against cycles run as shown in Fig, 3. It is evident from the figure that in experiments where contact stress was 1325 MPa and 936 MPa the wear loss was initially moderate but showed a substantial increase after subsequent running due to high contact stress present. It is also observed from the figure that the Sursulf treatment considerably reduces wear loss. 5. WEAR PARTICLE ANALYSIS Oil samples were collected from the outlet of the gear box at regular intervals and they were inspected for the concentration of large and small wear particles according to standard principles[7] by using a Direct Reading Ferrograph. The DR Ferrograph gives values of DE(large particles) and Ds (small particles) which can be processed to allow easy identification of abnormal wear modes by determining the Wear Particle Concentration, WPC = DE + Ds. Knowing the WPC values at any given stress level and at different cycles run (one WPC at each N), the Cumulative Wear Particle Concentration CWPC is obtained. The CWPC at any life N is the sum of WPC values at life N and all WPC values before N for which oil samples have been analysed. In a similar way CWPC t, which is Cumulative Wear Particle Concentration at pitting limit (corresponding to one million cycles) is also determined. A plot of CWPC versus cycles run for Sursulf treated gears is shown in Fig. 4. The plot reveals that the wear particle concentration is low at the pitting limit stress value of 660 MPa and moderate at the stress levels of 704 and 0.35 */, carbon steel, geors 250 F i= 20 936 M Pa
i
200
t
,,2oMo / °
a
~ I~° I
i'°° I ~
0
,
MOo
I
6 CycLes
I
18 run
I
18
I
24
( N x l O 51
Fig. 4. Plot o f cumulative wear particle concentration versus cycles run.
Sursulf treatment on steel spur gears
4')7
780 MPa. There is a rapid increase in wear particle concentration at stress levels of 936 and 1325 MPa indicating early failure. The mean value of CWPCt is CWPCt and is the mean of oil CWPC values taken at pitting limit, the corresponding standard deviation being st. Based on the present experimental investigation, it is suggested that failure initiation at any stress level would have taken place when the corresponding CWPC > (CWPC)t + st and when CWPC > (CWPC)t + 2sl failure is at an advanced stage and when CWPC > (CWPC)I + 3st there is a total failure of the gear. 6. SURFACE INTEGRITY To study the surface integrity of the 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 using a Perthometer. Plots of surface roughness (Rm.~value) versus cycles run at different stress levels for untreated condition (a) and Sursulf treated condition (b) are shown in Fig. 5. It is clear from the plots that the roughness value decreases during the running-in-period and increases thereafter. The increase in roughness is very hig!: at the stress levels of 1325 and 936 MPa for Sursulf treated gears and 936 and 704 MPa for untreated gears. A comparison of (a) and (b) reveals that the deterioration of the surface was very severe in case of untreated gears in comparison to Sursulf treated ones. Figure 6 gives SEM pictures of failed gear tooth. The pictures at (a) and (b) belong to untreated gear tooth worked at a stress level of 936 MPa and run upto !.5 million cycles. The pictures at (c) and (d) belong to Sursulf treated gear tooth worked at a stress level of 1325 MPa and run up to 1.5 million cycles. The picture at (a) shows a deep pit with several microcracks spreading from it on the surface. A cross sectional view of the pitted tooth zone is shown at (b). In this picture a large crack starting from the pitted surface (from top) and spreading towards the interior and several voids are seen. This crack was observed to propagate approximately at 45:C. Figure 6(c) shows the pitted and unpitted areas on the surface of the gear tooth with several microcracks which may develop into pits with further running. Figure 6(d) shows the cross sectional view of the pitted tooth zone. In this picture a spall is seen on the left and a large crack spreading from it towards the interior. Also several microcracks and voids are seen. 7. C O N C L U S I O N S Results of testing 0.35% carbon steel gears for surface durability are presented in this paper. Sursulf treatment has been found to improve the performance of the gears by substantially altering the contact stress versus life curves. The pitting limit increases from 440 to 660 MPa. Sursulf treatment also improves the wear resistance of the material. The wear loss for Sursulf treated gears
0.35 e/I carbonsteel gears 160 Untreated ;--20 . 936 MPa 140 -/ / 704MPo 120 v:L ~- 100 .c
~
8o
o ~
60
o ~
40
/
~ ~
VI 2O
_ 4
t a
SursuLftreated0.35=/ocarbonsteel
_
/
gears
1325MPa
/
/936
MPa
603 MPa
440
I IZ
I 16
I 20
I Z4
CycLes r u n
MP~,
0
I 4
I 8
I 12
( N x 10 5 )
Fig. 5. Variation of surface roughness against cycles run.
I
16
I
20
I
24
49~
S. KRISHNAMURTHYand A. RAMAMOHANARAO
~
~
" . /: ~:~-~
~._ ~ . ~ . ~ e ~ .
-
/ . 4
Fig. 6. SEM Photographs of failed tooth surfaces. is c o n s i d e r a b l y l o w e r t h a n for u n t r e a t e d gears• R e s u l t s o f the D R F e r r o g r a p h can be used to set guide-lines for p r e d i c t i n g failure o f gears. Acknowledgement--The authors wish to acknowledge with thanks the assistance rendered by Ms. Tribology India Private Limited, Smith Road, Madras 2, India for carrying out the Sursulf treatment.
REFERENCES I. D. W. Dudley. How increased hardness reduces the size of gear sets, Gear Des(~n and Applications (Edited by Nicholas P. Chironis). McGraw-Hill, New York, pp. 2-12 (1967). 2. Sursulf-Nitriding treatment in a non-polluting salt bath, Bulletin of Hydromecanique et Frottement, France. 3. A. Gaucher, G. Guilhot and C. Amsallem, Tribology Int. 9(3), 131-137 (1976). 4. D. Goldstraw, Int. Conf. Advances in Surface Coating Technology, London, 307-314 (1978). 5. Testing of Lubricants, Mechanical Testing of gear oils in the FZG gear test rig machine. DIN (Deutsches Institut ffir Normung) 51354 (1977). 6. R. A. Onions and J. F. Archard, Proc. Inst. Mech. Engnrs 188, 673~i82 (1975). 7. FOXBORO Analytical, Sample Processing with model 7067 Direct Reading Ferrogra~h.
EINFLUSS DER SURSULF-BEHANDLUNG AUF DIE ERMUDUNGSLEBENSDAUER VON STIRNRADER AUS MITTLEREN KOHLENSTOFF-STAHL Zusammenfassung--Stirnrfider aus mittlerem Kohlenstoff-Stahl ohne und mit Sursulf-Behandlung wurden auf einer Zahnrad-Verspannungs-Priifmaschine untersucht. Die R',ider fallen aus durch Griibchenbildung. Lebensdauerlinien ffir Grfibchenbildung durch Hertzsche Pressung sind aufgestellt worden f/ir die Zustblnde mit und ohne Sursulf-Behandlung. Behandehe R/ider weisen einen niedrigeren Verschleiss auf als unbehandelte. Die Verschleissart wurde durch die Analyse des Verschleissteilchen--Gehaltes im Schmier61 untersucht. Die kumulative Dichte der Verschleissteilchen bei der Grenz--Griibchen bildung wird als Mass f/Jr die Ausfallsvorhersage vorgeschlagen. Die Untersuchungen zeigen, dab die SursulfBehandlung die Leistung yon Stirnr/ider aus mittleren Kohlenstoff wesentlich verbessert.
EFFECT
0094-114X/89 $3.00 + 0.00 Pergamon Press plc
OF SURSULF
LIFE OF MEDIUM
TREATMENT
CARBON
ON
FATIGUE
STEEL SPUR
GEARS
S. KRISHNAMURTHY and A. RAMAMOHANA RAO Department of Mechanical Engineering, Indian Institute of Technology, Madras-600 036, India
Abstract--Medium carbon steel gears untreated as well as Sursulf treated were tested in a back-to-back gear test rig. The failure of the test gears was by pitting. The contact stress-pitting life curves have been established for both untceated and Sursulf treated conditions. Sursulf treated gears exhibited low wear compared to untreated. Wear particle analysis of lubricating oil was carried out to analyse the nature of failure. Cumulative wear particle concentration at pitting limit has been suggested as a basis for predicting the onset of the failure in the gears.
1. INTRODUCTION Gears are used for transmission of power, motion or both. Power rating of a gear set is essentially dependent on the surface durability of the gear teeth. The materials suggested by gear standards vary from plain low and medium carbon steels to a variety of alloy steels and cast irons. Surface durability is closely related to the ability of the teeth to resist pitting[l]. Increase in hardness improves considerably the surface durability resulting in higher power rating. Improvement in hardness of medium carbon steels can be achieved by imparting different heat treatments such as through hardening, induction hardening, gas nitriding, liquid nitriding (Sursulf and tufftride process) etc. This paper is concerned with surface durability studies on gears made from medium carbon steel and liquid nitrided by Sursulf treatment. Sursulf[2] is a surface hardening salt bath nitriding process developed by Hydromecanique et Frottement of France. It is useful in the treatment of all ferrous materials[3]. It is carried out in a molten salt bath which consists of alkaline cyanates and carbonates stabilized by the addition of lithium compounds. The bath is therefore free from cyanides when made up, and also specifically contains very small additions of sulphur compounds (normally of the order of a few ppm by weight). These sulphur compounds have two functions--to ensure the formation of iron sulphide at the surface of the treated parts and to see that as the operation proceeds the non-polluting character of the bath is maintained by oxidising, as they appear, any trace of cyanide which could possibly be formed by chemical reaction within the working bath. A treatment time of 90 min in a Sursulf bath at 565°C imparts on the treated components a deeper compound zone of about 10-20 pm which is porous at and near the surface and a diffusion zone of about 500/~m. The porous zone contains sulphur (Troilite FeS) which confers on the treated parts improved resistance to scuffing, fatigue and seizure. The deeper diffusion zone beneath the compound zone gives the treated parts better mechanical properties, including increase fatigue strength[4]. This method of treatment has several process and functional advantages--mainly short duration of the process time and negligible thei'mal distortion. The authors have made some investigations on Sursulf treated medium carbon steel gears to study the effect of this treatment on surface durability. The results of these investigations are presented in this paper. 2. EXPERIMENTAL DETAILS The material used for the gears is 0.35% carbon steel. The mechanical properties of this steel are given in Table I. The specification of the tested gears is shown in Table 2. The gear blanks were cut from bar stock and machined to standard tolerance limits. The spur gears were then hobbed on a gear hobbing machine and chamfered at the ends to reduce stress concentration effects. Six pairs of gears were subjected to Sursuif treatment for a period of 90 min and an equal number were left untreated. The maximum hardness values achieved before and after the treatment are indicated in Table 1. The Vickers Pyramid Hardness distribution of a Sursulf treated gear taken on a tooth v..nder a load of 0.5 N is shown in Fig. 1. 493
494
S. KRISHNAMURTHYand A. RAMAMOHANARAO
Table 1. Properties of gear material Property
Value
Ultimate tensile strength Yield strength Untreated hardness (HV) Hardness after Sursulf treatment (HV) Modulus of elasticity
700 MPa 500 M P a 215 650 2.1 x 10 ~ MPa
Table 2. Specifications of test pinions and gears Value of the variables used in experiment
Variable, symbol N u m b e r of teeth, Module, m (ram)
-, = 20
:
.-,
= 40
3
Pitch circle, dia, do (ram) Pressure angle, ~0 (deg) Face width, B (ram) Addendum circle dia, d, (mm) Root circle dia, dr (ram) Centre distance, ao (mm)
d,, = 60.0
do: = 120.0
20.0 B~ = 13.0 dul = 66.0 dr1 = 52.5
B: = 30.0 d,, = 126.0 dr. = 112.5 90.0
Subscript l--pinion, 2--gear
The tests were conducted using a back-to-back power recirculation gear test rig built on FZG principles[5]. The gear tests were performed at a pinion rotational speed of 1420 rev/min. Power for running the gears was provided by a 3.0 kW induction motor. The gears were lubricated with Hydrol 32 oil (viscosity of 0.028 Pa-S at 38°C) by drop feeding under gravity at a rate of 2.2 l/h. The test rig consists of two housings accommodating identical pairs of gears in them. These housings are connected by their input and output shafts which are directly coupled to form a closed circuit. Their ratios being equal, no auxiliary gearing is required. A special torsion coupling, used for loading, is such that its halves can be clamped together in any angular position. Thus with the clamping bolts released, one half is held stationary while the other half is acted upon by a lever and dead weights of desired value. The applied torque is estimated as the product of the lever arm and weight applied. Two sets of gears were tested in each experiment under approximately identical conditions. Experiments were conducted under different loads and the test programme followed is shown in Table 3. In all the experiments, a small torque was initially locked in the system and the gears were coo
700
600 ~o I
500
400
S
~oo
ZOO
~ . ~ = Core h=rdne.
1 O0
0
I ZOO
I I I I 400 600 B00 1000 Dis'canoe from s u r f a c e t v m )
I 1200
,J 4400
Fig. 1. Micro hardness distribution of Sursulf treated 0.35% carbon steel gears.
Sursulf treatment on steel spur gears
Experiment No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Treatment Untreated Untreated Untreated Untrea:ed Untreated Sursulf Sursulf Sursulf Sursulf Sursulf
495
Table 3. Test programme Torque applied on Contact stress at pinion (N. m) pitch point on (MPa) 59.8 936 32.8 704 24.9 603 13.3 440 !2.3 424 119.8 1325 59.8 936 32.8 704 29.8 660 28.0 640
Power transmitted
(kW)
8.90 4.87 3.68 1.99 1.82 17.80 8.90 4.87 4.43 4.16
run for about 20 rain. The load was then raised to give the pinion torque value corresponding to a particular experiment as given in Table 3, and the gears were run at the rated speed. During the tests, the rig was stopped at regular intervals and the teeth were inspected for any damage. In experiments l, 2, 3, 6, 7 and 8, the failure was due to pitting and the number of macropits were counted at regular intervals on selected teeth. In experiments 4, 5, 9 and 10, where no failure was observed, the tests were terminated after running for 5 million cycles. Oil samples were also collected in intervals of 0.3 million cycles for wear particle analysis.
3.
DETERMINATION
OF S T R E S S - L I F E
CURVES
As already said in experimental details, the surfacec of gear teeth were inspected at regular intervals. The number of macropits on the pinion have been counted at regular intervals of cycles and these were plotted on log-log coordinates. The points fell on a straight line and this line was extrapolated to yield the life to one pit formation which is considered as the pitting life[6]. This method was repeated to obtain the pitting life at all stress levels for which failure has taken place. In experiment numbers 4, 5, 9 and 10 of the test programme, micropits started appearing but they did not grow in size even though the gears were run upto 5 million cycles. On the contrary the pits disappeared. In these experiments, it is assumed that no failure has taken place. The contact stress (all) values and the corresponding pitting lives (N) have been plotted in Fig. 2. Stress levels at which no failure has taken place have also been indicated in the figure with cross arrows. The overall curve is similar to S - N curve in a traditional fatigue test. The curves show the marked improvement in the surface durability of Sursulf treated gears in relation to untreated. In the finite life region of the stress-life curve, where pitting life is influenced by stress, the relationship between contact stress and pitting life for the tested gears follows the equation. (aM)raN = constant where m is the slope of the a~c-N curve in log-log coordinates. The value of m is 2.75 for hobbed and Sursulf treated 0.35% carbon steel gears and 3.27 for untreated gears of the same material.
A o~ o
200C
n°
1000
0.33% carbon steer A Sursutf trea~ed 0 Untreated
~ ~
: 2.73
-
% 3.z7
zoo
10C 3 x 104
o
I 10 ~
I 106 Pittincj
I ~0 ~
Life N, c y c L e s ( Log )
Fig. 2. Contact stress-life curves for Sursulf treated and untreated 0.35% carbon steel gears.
S. KRISHNAMURTHYand A. R.AMAMOHANARAO
496
~ 160
Untreated
035*/°
carbon steGK gears
I= 2 0
Sursutf treata¢l 0.3R,°/e carbon
,,p 936 MPo
steel gears
~ i = 2.0
/
1525 MPo
o ~2o /e/° 80
936 MPo
~
603MPo
40
660 MPo (J
0
I
I
I
I
6
10
14
t8
I
22 2
I
I
6
10
.... l
14
I
J
18
22
CycLes run (N xlO 5) Fig. 3. Plot of cumulative wear loss versus cycles
run.
The pitting limit for Sursulf treated 0.35% carbon steel gears is 660 MPa and for untreated gears the value is 440 MPa. 4. CUMULATIVE WEAR LOSS In each experiment, the gears were removed at every 0.3 million cycles, cleaned thoroughly with acetone and weighed. The loss in weight was measured and this was plotted as cumulative wear loss against cycles run as shown in Fig, 3. It is evident from the figure that in experiments where contact stress was 1325 MPa and 936 MPa the wear loss was initially moderate but showed a substantial increase after subsequent running due to high contact stress present. It is also observed from the figure that the Sursulf treatment considerably reduces wear loss. 5. WEAR PARTICLE ANALYSIS Oil samples were collected from the outlet of the gear box at regular intervals and they were inspected for the concentration of large and small wear particles according to standard principles[7] by using a Direct Reading Ferrograph. The DR Ferrograph gives values of DE(large particles) and Ds (small particles) which can be processed to allow easy identification of abnormal wear modes by determining the Wear Particle Concentration, WPC = DE + Ds. Knowing the WPC values at any given stress level and at different cycles run (one WPC at each N), the Cumulative Wear Particle Concentration CWPC is obtained. The CWPC at any life N is the sum of WPC values at life N and all WPC values before N for which oil samples have been analysed. In a similar way CWPC t, which is Cumulative Wear Particle Concentration at pitting limit (corresponding to one million cycles) is also determined. A plot of CWPC versus cycles run for Sursulf treated gears is shown in Fig. 4. The plot reveals that the wear particle concentration is low at the pitting limit stress value of 660 MPa and moderate at the stress levels of 704 and 0.35 */, carbon steel, geors 250 F i= 20 936 M Pa
i
200
t
,,2oMo / °
a
~ I~° I
i'°° I ~
0
,
MOo
I
6 CycLes
I
18 run
I
18
I
24
( N x l O 51
Fig. 4. Plot o f cumulative wear particle concentration versus cycles run.
Sursulf treatment on steel spur gears
4')7
780 MPa. There is a rapid increase in wear particle concentration at stress levels of 936 and 1325 MPa indicating early failure. The mean value of CWPCt is CWPCt and is the mean of oil CWPC values taken at pitting limit, the corresponding standard deviation being st. Based on the present experimental investigation, it is suggested that failure initiation at any stress level would have taken place when the corresponding CWPC > (CWPC)t + st and when CWPC > (CWPC)t + 2sl failure is at an advanced stage and when CWPC > (CWPC)I + 3st there is a total failure of the gear. 6. SURFACE INTEGRITY To study the surface integrity of the 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 using a Perthometer. Plots of surface roughness (Rm.~value) versus cycles run at different stress levels for untreated condition (a) and Sursulf treated condition (b) are shown in Fig. 5. It is clear from the plots that the roughness value decreases during the running-in-period and increases thereafter. The increase in roughness is very hig!: at the stress levels of 1325 and 936 MPa for Sursulf treated gears and 936 and 704 MPa for untreated gears. A comparison of (a) and (b) reveals that the deterioration of the surface was very severe in case of untreated gears in comparison to Sursulf treated ones. Figure 6 gives SEM pictures of failed gear tooth. The pictures at (a) and (b) belong to untreated gear tooth worked at a stress level of 936 MPa and run upto !.5 million cycles. The pictures at (c) and (d) belong to Sursulf treated gear tooth worked at a stress level of 1325 MPa and run up to 1.5 million cycles. The picture at (a) shows a deep pit with several microcracks spreading from it on the surface. A cross sectional view of the pitted tooth zone is shown at (b). In this picture a large crack starting from the pitted surface (from top) and spreading towards the interior and several voids are seen. This crack was observed to propagate approximately at 45:C. Figure 6(c) shows the pitted and unpitted areas on the surface of the gear tooth with several microcracks which may develop into pits with further running. Figure 6(d) shows the cross sectional view of the pitted tooth zone. In this picture a spall is seen on the left and a large crack spreading from it towards the interior. Also several microcracks and voids are seen. 7. C O N C L U S I O N S Results of testing 0.35% carbon steel gears for surface durability are presented in this paper. Sursulf treatment has been found to improve the performance of the gears by substantially altering the contact stress versus life curves. The pitting limit increases from 440 to 660 MPa. Sursulf treatment also improves the wear resistance of the material. The wear loss for Sursulf treated gears
0.35 e/I carbonsteel gears 160 Untreated ;--20 . 936 MPa 140 -/ / 704MPo 120 v:L ~- 100 .c
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Fig. 6. SEM Photographs of failed tooth surfaces. is c o n s i d e r a b l y l o w e r t h a n for u n t r e a t e d gears• R e s u l t s o f the D R F e r r o g r a p h can be used to set guide-lines for p r e d i c t i n g failure o f gears. Acknowledgement--The authors wish to acknowledge with thanks the assistance rendered by Ms. Tribology India Private Limited, Smith Road, Madras 2, India for carrying out the Sursulf treatment.
REFERENCES I. D. W. Dudley. How increased hardness reduces the size of gear sets, Gear Des(~n and Applications (Edited by Nicholas P. Chironis). McGraw-Hill, New York, pp. 2-12 (1967). 2. Sursulf-Nitriding treatment in a non-polluting salt bath, Bulletin of Hydromecanique et Frottement, France. 3. A. Gaucher, G. Guilhot and C. Amsallem, Tribology Int. 9(3), 131-137 (1976). 4. D. Goldstraw, Int. Conf. Advances in Surface Coating Technology, London, 307-314 (1978). 5. Testing of Lubricants, Mechanical Testing of gear oils in the FZG gear test rig machine. DIN (Deutsches Institut ffir Normung) 51354 (1977). 6. R. A. Onions and J. F. Archard, Proc. Inst. Mech. Engnrs 188, 673~i82 (1975). 7. FOXBORO Analytical, Sample Processing with model 7067 Direct Reading Ferrogra~h.
EINFLUSS DER SURSULF-BEHANDLUNG AUF DIE ERMUDUNGSLEBENSDAUER VON STIRNRADER AUS MITTLEREN KOHLENSTOFF-STAHL Zusammenfassung--Stirnrfider aus mittlerem Kohlenstoff-Stahl ohne und mit Sursulf-Behandlung wurden auf einer Zahnrad-Verspannungs-Priifmaschine untersucht. Die R',ider fallen aus durch Griibchenbildung. Lebensdauerlinien ffir Grfibchenbildung durch Hertzsche Pressung sind aufgestellt worden f/ir die Zustblnde mit und ohne Sursulf-Behandlung. Behandehe R/ider weisen einen niedrigeren Verschleiss auf als unbehandelte. Die Verschleissart wurde durch die Analyse des Verschleissteilchen--Gehaltes im Schmier61 untersucht. Die kumulative Dichte der Verschleissteilchen bei der Grenz--Griibchen bildung wird als Mass f/Jr die Ausfallsvorhersage vorgeschlagen. Die Untersuchungen zeigen, dab die SursulfBehandlung die Leistung yon Stirnr/ider aus mittleren Kohlenstoff wesentlich verbessert.