- Email: [email protected]
The effect of combined induction heating and shot-peening on the properties of oil-pump arms
ELSEVIER
J. Mater. Process. Technol.,42 (1994) 311-318
Journal of Materials Processing Technology
The effect of combined induction heating and shot-peening on the properties of oil-pump arms H u a n g Jinliang*, Li Yanxiang, Z h u Y a o m i n e Department of Material Engineering, Luoyang Institute of Technology, Luoyang, Henan, China
(Received February 13, 1993; accepted August 11, 1993)
Industrial Summary A new compound technique of the combination of medium-frequency induction quickpenetration quenching and tempering with surface shot-peening was used in the heat treating of oil-pump arms, a thorough comparison between the new technique and conventional electric-furnace heat-treating being made. The results show that the new compound technique can not only endow the oil-pump arms with finer grains, an ideal microstructure and higher mechanical properties, special tensile fatigue life, but can also enhance the production efficiency by a factor of six and economize on the electric power by 1.2 GJ (for an output of 1.5 x 10 6 m). The practical usage of the oil-pump arms heat treated by the new compound technique has shown double the service life of that of conventional electric-furnace treated oil-pump arms.
1. Introduction The range of application of induction heating technology has been enlarged greatly since its inception in the surface heating of metals in the 1940s. At the present time it is a type of well-accepted quick-heating technology, not only for surface heating but also for penetration heating, because of its obvious industrial advantages such as high production efficiency, low energy consumption, lesser pollution, and lesser oxidation and decarbonization, as compared with conventional electric-furnace heat-treating. The surface shot-peening of metals after conventional heat-treating or chemicothermal treatment can enhance the bending and contact fatigue strength remarkably [1,2]. However, study of the compound technique of combined shot-peening with induction penetration quenching and tempering to improve the tensile fatigue strength of metals is inadequate, without mentioning consideration of its industrial application.
*Corresponding author. 0924-0136/94/$07.00 © 1994ElsevierScience B.V. All rights reserved SSDI 0924-0136(93)E0096-Y
312
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
860 + 5 °C
G
air cooling
560 °C I--
Time (rain) Fig. 1. Conventional heat-treating technique.
An oil-pump arm is one of the most critical components of crude oil extraction in an oil field. In usage, an oil-pump arm bears some cyclic tensile-tensile load and usually breaks due to tensile fatigue. It is reported that the number of oil-well repairments caused by oil-pump arm fatigued fracture is about 18% of that of all repairments [3], such repairment not only giving rise to great waste of manpower and material resources, but also seriously affecting the output of crude oil. Therefore, improving the application performance of oil-pump arms in the oil industry is an important problem demanding a prompt solution. The conventional heat-treating technique for an oil-pump arm is a typical normalizing process performed in an electric heat-treating furnace, the heat-treating technique being illustrated in Fig. 1. The unavoidable defects of this technique are a long heating cycle, a high power consumption, and serious oxidation and decarbonization. In view of these problems, a new compound heat-treating technique, i.e. the combination of medium-frequency induction quick penetration quenching and tempering with surface shot-peening was used to treat the oil-pump arms, the mechanical properties and the tensile fatigue life being measured and the microstructures being observed. On the basis of the results, the effects of the new technique on the properties were discussed. Finally, the authors briefly assess the industrial significance of the new technique.
2. Experimental procedure 2.1. Tested material
A type of special oil-pump arm steel, YG35D, in the as-rolled condition selected as the test material, was steel having the following chemical composition (wt.%): 0.32 C, 1.02 Cr, 0.30 Mo, 0.60 Mn, 0.25 Si, 0.03 S, 0.03 P and balance Fe.
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318 I~
313
8000
Fig. 2. Schematic of the oil-pump arm (dimensions : ram).
All of the test specimens were cut from a CYG20/8000 type oil-pump arm, the shape and dimensions of which are illustrated schematically in Fig. 2.
2.2. Heat-treating technique and equipment Technique I: A technique as shown in Fig. 1 was used to normalize and temper the oil-pump arm in a chain-transmitted 450 kW electric heat-treating furnace having an accuracy of _ 5 °C. Technique II: Medium-frequency induction quick penetration quenching and tempering. A ZP-III 250 type silicon-controlled rectifier medium-frequency induction machine was employed as the heating equipment. The technique parameters are: frequency 2500Hz, quenching temperature 890 __+5°C, tempering temperature 680 __+5 °C, oil-pump arm moving speed 40 mm/min, working voltage 500 V, current 350 A, working power 150-160 kW, power factor (cos q~) 0.9. The inductors were wound using square-sectioned brass tube, the inductor for quenching having the dimensions diameter (D) x length (L) = 75 x 1500 (mm), and the inductor for tempering having the dimensions D x L = 75 x 600 (mm). The water-spray quenching ring has the dimensions D × L = 95 x 280 (mm) with water-spraying holes of ~b2.2 mm. The water pressure employed was 12 MPa. 2.3. Shot-peening process The oil-pump arms heat treated by technique II was shot-peened in a PW-40 type forceful shot-peening machine, where during the shot-peening operation the oil-pump arms were rotated at 20 rev/min. The shot-peening parameters are listed in Table 1. The residual stress was measured using a MSF-2M type X-ray stress apparatus.
2.4. Determination of properties The values of strength and plasticity were determined using WE-60 type universal material tester. Specimens of 420 x 500 (mm) were cut from the differently-technique treated oil-pump arms whilst retaining their original surface conditions. The notchimpact testpiece, which was the German DVM specimen 10 x 10 x 53 (mm), was tested in a pendulum-type impact testing machine, whilst the cyclic tensile fatigue test with a specimen of ~b20x 500 (mm) was performed in a LOS testing machine, the stress
314
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311 318
Table 1 Shot-peening parameters Material
Diameter (mm)
Hardness (HRc)
Flow (kg/min)
Pressure (kPa)
Time (min)
Cover rate (%)
Steel shot
0.8
48 55
80
116
12
100
Table 2 Results of measured propertiesa Technique Technique I Technique II Technique II plus shot-peening
ab (MPa)
O's (MPa)
fi (%)
(%)
ak (J/cm)
Fatigue life (N × 10 6)
844 1120 1260
762 920 1178
12 12 11.4
54 53 53
80.4 156 163.5
0.280 0.521 1.132
~
aAll data are the mean values for three specimens.
range being Aa = 320 M P a (R = 1) and the cyclic frequency being 800 cycles per minute. Microstructure analyses were carried out using a H-800 transmission electron microscope.
3. Results 3.1. Mechanical and fatigue properties
The results obtained in the mechanical and tensile fatigue tests are summarized in Table 2, which latter shows clearly the effects of induction quick penetration heating and surface shot-peening on the properties of the oil-pump arm, especially on the strength, the impact toughness and the tensile fatigue lives. It is found that the strength and the impact toughness as well as tensile fatigue life of the oil-pump arms treated by technique II are much improved compared with those treated by technique I, whilst possessing identical plastic behaviour. The compound heat-treating of technique II and surface shot-peening results in a much greater improvement of the properties, especially of the tensile life, which is 4.8 times that of technique I and twice that of technique II alone. 3.2. Microstrueture
Fig. 3 shows the microstructures of differently heat-treated oil-pump arms. The conventional technique I usually causes coarse grains of the order of 30-40 lam and a severely decarbonized layer about 0.20-0.25 m m thickness due to its slower heating
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
315
Fig. 3. Microstructuresshowing:(a) conventionalheat-treatingperlite and ferrite;(b) induction-quenched lath rnartensite;(c) induction-temperedmorphology;(d) refined shot-peeningstructure.
rate, the final microstructure being lamellar perlite and blocky ferrite, as seen in Fig. 3(a). While using technique II, the much quicker induction heating rate firstly prevents the growth of austenite and secondly gives insufficient time for carbon atoms to diffuse a long distance, thus leading to very fine grains, of the order of 6-8 ~tm, a thinner decarbonized layer of less than 0.05 mm and a microstructure with more refined lath martensite when water quenched, as shown in Fig. 3(b). Similarly, the fast induction tempering postpones the decomposition of lath martensite and the aggregation of carbide, which results in a final microstructure of tempered lath martensite with fine carbide precipitated at boundaries in the interior of the original quenched lath martensite, as seen in Fig. 3 (c). Shown in Fig. 3 (d) is the influence of shot-peening on the surface microstructure. It is obvious that the microstructure is much more refined and that the distribution and shape of carbides are changed also.
316
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311 318 400 te~~'~cx~e\
200
_
~ec~~a~e\\
200
-400
t
"~-
- 600 o
- 800
-
-
~
/..J
1(~0
~vo~-¢~
. I ~ "
teC~6~e \,,
2()0
300
Depth of surface (l~m) Fig. 4. The changes of residual stress vs. depth.
3.3. The change of residual stress Fig. 4 illustrates the change of residual tensile stress vs. layer depth before and after shot-peening. A small amount of tensile stress is observed at the surface of the oil-pump arm after heat treating by technique I, whilst a small residual compression stress is observed after heat treating by technique II. However, a quite high residual compression stress, not only on the surface but also down to a certain depth, is present after shot-peening. At the same time it is noted that after a particular number of cycles of tensile fatigue the residual compressive stress still retains quite a large value although it decreases to some extent, which suggests that the residual compressive stress arising from shot-peening is uneasy to reduce with the microstructure obtained by technique II.
4. Discussion
4.1. The effect of induction quick penetration quenching and tempering on the properties of the oil-pump arm It is well known that finer grain can simultaneously enhance both the strength and the toughness of metals. Furthermore, finer grain can also effectively increase the fatigue strength: it has been shown that the fatigue strength of a medium-carbon low-alloy steel may be increased by a factor of two if its grain size is reduced by a factor of two [4]. Therefore, the much finer grain size produced by technique II is one contribution towards the securing of improved properties of the oil-pump arm.
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
317
The tempered lath martensite has a much better combination of strength and toughness than that of perlite and ferrite, which is another contribution to the improvement of properties. Further, the better surface condition of the oil-pump arm treated by technique II (less decarburization) is the third contribution to property enhancement, especially to the increase of tensile fatigue life. 4.2. The effect of surface shot-peening on properties
Investigation [5] has shown that a metal subjected to tensile-tensile fatigue usually follows a fatigue mechanism of surface crack initiation leading to the final failure of the material. Thus, the remarkable improvement of the properties, especially the longer tensile fatigue life, obtained by shot-peening should be attributed to the effect of the shot-peening on surface or sub-surface cracking. As has been mentioned, shot-peening can not only refine the microstructure but can also produce quite large residual compressive stress on the surface and in the sub-surface (see Fig. 3(d) and Fig. 4). The finer microstructure increases the yield strength as, the increase of as suggesting that crystal slip deformation in the shot-peened surface layer is more difficult, which is unfavourable to fatigue-crack initiation. The existence of residual compression stress on the surface and in the sub-surface can effectively counteract part of the externally-added tensile stress and yet decrease the real cyclic tensile stress exerted on the oil-pump arm. Therefore, the initiation of a fatigue crack is postponed. If a fatigue crack once initiates, the enhanced yield stress gained by shot-peening naturally provides the surface and the sub-surface of the oil-pump arm with a high deformation resistance, which not only decreases the fatigue propagation but also delays the decline of the residual compressive stress. On the other hand, the residual compressive stress can also cancel part of the tensile stress in the stress field at the crack tip, thus hindering the propagation of the fatigue crack. In short, the effect of surface shot-peening on the tensile fatigue life of an oil-pump arm depends to a large degree on the remarkable increase of yield strength and on the large residual compressive stress duo to shot-peening. 4.3. The industrial significance of the present work
All of the above-mentioned experimental results and discussion have shown the outstanding improvement in the properties of oil-pump arms when utilizing the present new compound technique, i.e. combined medium-frequency induction quick penetration quenching and tempering with surface shot-peening. The authors assess the industrial significance of the new compound-technique compared with the conventional treating technique (as seen in Fig. 1) as follows. (1) The limited surface-decarbonized layer formed in induction heating is eliminated completely by the subsequent shot-peening. (2) The new compound technique increases the production efficiency of the oilpump arm by a factor of six. (3) Electric power is greatly economized. A factory with an annual output of 1.5 × 106 m oil-pump arms can save 1.2 GJ electric power per year.
318
Huang Jinl&ng et al./J. Mater. Process. Technol., 42 (1994) 311-318
(4) The new compound technique realizes higher automation, the intensity of labour is reduced and the working environment is improved. (5) The practical application of the oil-pump arms treated by the new compound technique has shown twice the service life of that of conventionally treated oil-pump arms.
5. Conclusions
(1) The medium-frequency induction quick penetration heating technology used to quench and temper the oil-pump arms can not only endow them with an ideal microstructure and with improved properties, but can also economize on electrical power, can enhance production efficiency and can decrease surface decarbonization. (2) The mechanical properties, especially the tensile fatigue life, can be enhanced further by shot-peening after induction quick heating (quenching and tempering). The new compound technique of combined medium-frequency penetrated quenching and tempering with surface shot-peening ensures that the oil-pump arms have twice the service life in practical application compared with that of conventional normalizing oil-pump arms.
References
[1] [2] [3] [4] [5]
Chen Qing, Mech. Desi#n, 4 (1986) 17-27. Xiao Hongbin,Chen Qing and Shao Eryu, Wear, 151 (1991) 77 80. Yan Shen, Oil Mech., 9 (1988) 40-42. Yuo Jahong and Zhu Weiping,Heat Treat. Met., 8 (1992) 45-46. Wang Renzhi, Aero. Mater., 3 (1979) 18 26.
J. Mater. Process. Technol.,42 (1994) 311-318
Journal of Materials Processing Technology
The effect of combined induction heating and shot-peening on the properties of oil-pump arms H u a n g Jinliang*, Li Yanxiang, Z h u Y a o m i n e Department of Material Engineering, Luoyang Institute of Technology, Luoyang, Henan, China
(Received February 13, 1993; accepted August 11, 1993)
Industrial Summary A new compound technique of the combination of medium-frequency induction quickpenetration quenching and tempering with surface shot-peening was used in the heat treating of oil-pump arms, a thorough comparison between the new technique and conventional electric-furnace heat-treating being made. The results show that the new compound technique can not only endow the oil-pump arms with finer grains, an ideal microstructure and higher mechanical properties, special tensile fatigue life, but can also enhance the production efficiency by a factor of six and economize on the electric power by 1.2 GJ (for an output of 1.5 x 10 6 m). The practical usage of the oil-pump arms heat treated by the new compound technique has shown double the service life of that of conventional electric-furnace treated oil-pump arms.
1. Introduction The range of application of induction heating technology has been enlarged greatly since its inception in the surface heating of metals in the 1940s. At the present time it is a type of well-accepted quick-heating technology, not only for surface heating but also for penetration heating, because of its obvious industrial advantages such as high production efficiency, low energy consumption, lesser pollution, and lesser oxidation and decarbonization, as compared with conventional electric-furnace heat-treating. The surface shot-peening of metals after conventional heat-treating or chemicothermal treatment can enhance the bending and contact fatigue strength remarkably [1,2]. However, study of the compound technique of combined shot-peening with induction penetration quenching and tempering to improve the tensile fatigue strength of metals is inadequate, without mentioning consideration of its industrial application.
*Corresponding author. 0924-0136/94/$07.00 © 1994ElsevierScience B.V. All rights reserved SSDI 0924-0136(93)E0096-Y
312
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
860 + 5 °C
G
air cooling
560 °C I--
Time (rain) Fig. 1. Conventional heat-treating technique.
An oil-pump arm is one of the most critical components of crude oil extraction in an oil field. In usage, an oil-pump arm bears some cyclic tensile-tensile load and usually breaks due to tensile fatigue. It is reported that the number of oil-well repairments caused by oil-pump arm fatigued fracture is about 18% of that of all repairments [3], such repairment not only giving rise to great waste of manpower and material resources, but also seriously affecting the output of crude oil. Therefore, improving the application performance of oil-pump arms in the oil industry is an important problem demanding a prompt solution. The conventional heat-treating technique for an oil-pump arm is a typical normalizing process performed in an electric heat-treating furnace, the heat-treating technique being illustrated in Fig. 1. The unavoidable defects of this technique are a long heating cycle, a high power consumption, and serious oxidation and decarbonization. In view of these problems, a new compound heat-treating technique, i.e. the combination of medium-frequency induction quick penetration quenching and tempering with surface shot-peening was used to treat the oil-pump arms, the mechanical properties and the tensile fatigue life being measured and the microstructures being observed. On the basis of the results, the effects of the new technique on the properties were discussed. Finally, the authors briefly assess the industrial significance of the new technique.
2. Experimental procedure 2.1. Tested material
A type of special oil-pump arm steel, YG35D, in the as-rolled condition selected as the test material, was steel having the following chemical composition (wt.%): 0.32 C, 1.02 Cr, 0.30 Mo, 0.60 Mn, 0.25 Si, 0.03 S, 0.03 P and balance Fe.
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318 I~
313
8000
Fig. 2. Schematic of the oil-pump arm (dimensions : ram).
All of the test specimens were cut from a CYG20/8000 type oil-pump arm, the shape and dimensions of which are illustrated schematically in Fig. 2.
2.2. Heat-treating technique and equipment Technique I: A technique as shown in Fig. 1 was used to normalize and temper the oil-pump arm in a chain-transmitted 450 kW electric heat-treating furnace having an accuracy of _ 5 °C. Technique II: Medium-frequency induction quick penetration quenching and tempering. A ZP-III 250 type silicon-controlled rectifier medium-frequency induction machine was employed as the heating equipment. The technique parameters are: frequency 2500Hz, quenching temperature 890 __+5°C, tempering temperature 680 __+5 °C, oil-pump arm moving speed 40 mm/min, working voltage 500 V, current 350 A, working power 150-160 kW, power factor (cos q~) 0.9. The inductors were wound using square-sectioned brass tube, the inductor for quenching having the dimensions diameter (D) x length (L) = 75 x 1500 (mm), and the inductor for tempering having the dimensions D x L = 75 x 600 (mm). The water-spray quenching ring has the dimensions D × L = 95 x 280 (mm) with water-spraying holes of ~b2.2 mm. The water pressure employed was 12 MPa. 2.3. Shot-peening process The oil-pump arms heat treated by technique II was shot-peened in a PW-40 type forceful shot-peening machine, where during the shot-peening operation the oil-pump arms were rotated at 20 rev/min. The shot-peening parameters are listed in Table 1. The residual stress was measured using a MSF-2M type X-ray stress apparatus.
2.4. Determination of properties The values of strength and plasticity were determined using WE-60 type universal material tester. Specimens of 420 x 500 (mm) were cut from the differently-technique treated oil-pump arms whilst retaining their original surface conditions. The notchimpact testpiece, which was the German DVM specimen 10 x 10 x 53 (mm), was tested in a pendulum-type impact testing machine, whilst the cyclic tensile fatigue test with a specimen of ~b20x 500 (mm) was performed in a LOS testing machine, the stress
314
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311 318
Table 1 Shot-peening parameters Material
Diameter (mm)
Hardness (HRc)
Flow (kg/min)
Pressure (kPa)
Time (min)
Cover rate (%)
Steel shot
0.8
48 55
80
116
12
100
Table 2 Results of measured propertiesa Technique Technique I Technique II Technique II plus shot-peening
ab (MPa)
O's (MPa)
fi (%)
(%)
ak (J/cm)
Fatigue life (N × 10 6)
844 1120 1260
762 920 1178
12 12 11.4
54 53 53
80.4 156 163.5
0.280 0.521 1.132
~
aAll data are the mean values for three specimens.
range being Aa = 320 M P a (R = 1) and the cyclic frequency being 800 cycles per minute. Microstructure analyses were carried out using a H-800 transmission electron microscope.
3. Results 3.1. Mechanical and fatigue properties
The results obtained in the mechanical and tensile fatigue tests are summarized in Table 2, which latter shows clearly the effects of induction quick penetration heating and surface shot-peening on the properties of the oil-pump arm, especially on the strength, the impact toughness and the tensile fatigue lives. It is found that the strength and the impact toughness as well as tensile fatigue life of the oil-pump arms treated by technique II are much improved compared with those treated by technique I, whilst possessing identical plastic behaviour. The compound heat-treating of technique II and surface shot-peening results in a much greater improvement of the properties, especially of the tensile life, which is 4.8 times that of technique I and twice that of technique II alone. 3.2. Microstrueture
Fig. 3 shows the microstructures of differently heat-treated oil-pump arms. The conventional technique I usually causes coarse grains of the order of 30-40 lam and a severely decarbonized layer about 0.20-0.25 m m thickness due to its slower heating
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
315
Fig. 3. Microstructuresshowing:(a) conventionalheat-treatingperlite and ferrite;(b) induction-quenched lath rnartensite;(c) induction-temperedmorphology;(d) refined shot-peeningstructure.
rate, the final microstructure being lamellar perlite and blocky ferrite, as seen in Fig. 3(a). While using technique II, the much quicker induction heating rate firstly prevents the growth of austenite and secondly gives insufficient time for carbon atoms to diffuse a long distance, thus leading to very fine grains, of the order of 6-8 ~tm, a thinner decarbonized layer of less than 0.05 mm and a microstructure with more refined lath martensite when water quenched, as shown in Fig. 3(b). Similarly, the fast induction tempering postpones the decomposition of lath martensite and the aggregation of carbide, which results in a final microstructure of tempered lath martensite with fine carbide precipitated at boundaries in the interior of the original quenched lath martensite, as seen in Fig. 3 (c). Shown in Fig. 3 (d) is the influence of shot-peening on the surface microstructure. It is obvious that the microstructure is much more refined and that the distribution and shape of carbides are changed also.
316
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311 318 400 te~~'~cx~e\
200
_
~ec~~a~e\\
200
-400
t
"~-
- 600 o
- 800
-
-
~
/..J
1(~0
~vo~-¢~
. I ~ "
teC~6~e \,,
2()0
300
Depth of surface (l~m) Fig. 4. The changes of residual stress vs. depth.
3.3. The change of residual stress Fig. 4 illustrates the change of residual tensile stress vs. layer depth before and after shot-peening. A small amount of tensile stress is observed at the surface of the oil-pump arm after heat treating by technique I, whilst a small residual compression stress is observed after heat treating by technique II. However, a quite high residual compression stress, not only on the surface but also down to a certain depth, is present after shot-peening. At the same time it is noted that after a particular number of cycles of tensile fatigue the residual compressive stress still retains quite a large value although it decreases to some extent, which suggests that the residual compressive stress arising from shot-peening is uneasy to reduce with the microstructure obtained by technique II.
4. Discussion
4.1. The effect of induction quick penetration quenching and tempering on the properties of the oil-pump arm It is well known that finer grain can simultaneously enhance both the strength and the toughness of metals. Furthermore, finer grain can also effectively increase the fatigue strength: it has been shown that the fatigue strength of a medium-carbon low-alloy steel may be increased by a factor of two if its grain size is reduced by a factor of two [4]. Therefore, the much finer grain size produced by technique II is one contribution towards the securing of improved properties of the oil-pump arm.
Huang Jinliang et al./J. Mater. Process. Technol., 42 (1994) 311-318
317
The tempered lath martensite has a much better combination of strength and toughness than that of perlite and ferrite, which is another contribution to the improvement of properties. Further, the better surface condition of the oil-pump arm treated by technique II (less decarburization) is the third contribution to property enhancement, especially to the increase of tensile fatigue life. 4.2. The effect of surface shot-peening on properties
Investigation [5] has shown that a metal subjected to tensile-tensile fatigue usually follows a fatigue mechanism of surface crack initiation leading to the final failure of the material. Thus, the remarkable improvement of the properties, especially the longer tensile fatigue life, obtained by shot-peening should be attributed to the effect of the shot-peening on surface or sub-surface cracking. As has been mentioned, shot-peening can not only refine the microstructure but can also produce quite large residual compressive stress on the surface and in the sub-surface (see Fig. 3(d) and Fig. 4). The finer microstructure increases the yield strength as, the increase of as suggesting that crystal slip deformation in the shot-peened surface layer is more difficult, which is unfavourable to fatigue-crack initiation. The existence of residual compression stress on the surface and in the sub-surface can effectively counteract part of the externally-added tensile stress and yet decrease the real cyclic tensile stress exerted on the oil-pump arm. Therefore, the initiation of a fatigue crack is postponed. If a fatigue crack once initiates, the enhanced yield stress gained by shot-peening naturally provides the surface and the sub-surface of the oil-pump arm with a high deformation resistance, which not only decreases the fatigue propagation but also delays the decline of the residual compressive stress. On the other hand, the residual compressive stress can also cancel part of the tensile stress in the stress field at the crack tip, thus hindering the propagation of the fatigue crack. In short, the effect of surface shot-peening on the tensile fatigue life of an oil-pump arm depends to a large degree on the remarkable increase of yield strength and on the large residual compressive stress duo to shot-peening. 4.3. The industrial significance of the present work
All of the above-mentioned experimental results and discussion have shown the outstanding improvement in the properties of oil-pump arms when utilizing the present new compound technique, i.e. combined medium-frequency induction quick penetration quenching and tempering with surface shot-peening. The authors assess the industrial significance of the new compound-technique compared with the conventional treating technique (as seen in Fig. 1) as follows. (1) The limited surface-decarbonized layer formed in induction heating is eliminated completely by the subsequent shot-peening. (2) The new compound technique increases the production efficiency of the oilpump arm by a factor of six. (3) Electric power is greatly economized. A factory with an annual output of 1.5 × 106 m oil-pump arms can save 1.2 GJ electric power per year.
318
Huang Jinl&ng et al./J. Mater. Process. Technol., 42 (1994) 311-318
(4) The new compound technique realizes higher automation, the intensity of labour is reduced and the working environment is improved. (5) The practical application of the oil-pump arms treated by the new compound technique has shown twice the service life of that of conventionally treated oil-pump arms.
5. Conclusions
(1) The medium-frequency induction quick penetration heating technology used to quench and temper the oil-pump arms can not only endow them with an ideal microstructure and with improved properties, but can also economize on electrical power, can enhance production efficiency and can decrease surface decarbonization. (2) The mechanical properties, especially the tensile fatigue life, can be enhanced further by shot-peening after induction quick heating (quenching and tempering). The new compound technique of combined medium-frequency penetrated quenching and tempering with surface shot-peening ensures that the oil-pump arms have twice the service life in practical application compared with that of conventional normalizing oil-pump arms.
References
[1] [2] [3] [4] [5]
Chen Qing, Mech. Desi#n, 4 (1986) 17-27. Xiao Hongbin,Chen Qing and Shao Eryu, Wear, 151 (1991) 77 80. Yan Shen, Oil Mech., 9 (1988) 40-42. Yuo Jahong and Zhu Weiping,Heat Treat. Met., 8 (1992) 45-46. Wang Renzhi, Aero. Mater., 3 (1979) 18 26.