Materials Chemistry and Physics 112 (2008) 1099–1105
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of forging temperature on homogeneity of microstructure and hardness of precision forged steel spur gear M. Irani 1 , A. Karimi Taheri ∗ Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11365-9466, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 31 October 2007 Received in revised form 26 May 2008 Accepted 8 July 2008 Keywords: Hot working Microstructure Hardness Metals
a b s t r a c t Precision forging is a suitable process to produce spur gears due to its advantages such as reduction in machining time and production cost. The homogeneity in microstructure and mechanical properties of precision forging products can highly affect the performance of the gears during their service. In this research the effect of precision forging temperature on homogeneity of microstructure and hardness of forged gears of low carbon steel is studied. The microstructure and hardness map of the gears forged at a temperature range of 750–1150 ◦ C revealed that the forging temperature of 950 ◦ C is an optimum temperature to produce a spur five teeth gear with minimum inhomogeneity in the microstructure and hardness distribution. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among the many types of machine parts produced today, gears are the most commonly used elements [1]. Two groups of manufacturing methods for gears, i.e., cutting and noncutting are available [2]. As a noncutting method the precision forging process has been developed because of its advantages [3]. Wasting little material, saving cost and time due to the reduced number of after machining processes and improving strength and tolerances of the surface are some advantages offered by the precision forging process [4–9], so that in most cases the precision forged gears are ready for immediate heat treating and finished machining [10]. The accuracy of forging process is affected by many variables among which the workpiece temperature is probably the most complex and important one. It has been reported [11] that although the preheat temperature can be controlled accurately but the work of deformation causes substantial heating of the workpiece interior while the surface is chilled due to contact with the die. The temperature during forging also affects the flow stress of the material, and therefore, the required forging load and energy. A lower forging temperature results in a more elastic deformation of the dies and the forging machine due to the increase of forging load. Moreover, an increased scale formation and decarburization of workpiece are related to the increase in forging temperature.
∗ Corresponding author. Tel.: +982166165220; fax: +982166005717. E-mail addresses:
[email protected] (M. Irani),
[email protected] (A.K. Taheri). 1 M.Sc. graduate. 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.044
In the forging industry, the tooling cost can constitute up to 50% of the component total cost. With regard to this proportion, it is obvious that the component cost reduction requires an optimization increasing the performance and service life of dies [12,13]. The maximum temperature as well as the temperature distribution in the hot forging tool has a significant influence on its life. This is because the maximum temperature determines the hardness of the tool surface. The die surface is heated partly by conduction of heat from the hot workpiece and partly by the friction between the tool and the workpiece. The maximum temperature of the tool surface layer is highly influenced by workpiece temperature, workpiece-tool contact time, lubricant, and the tool base temperature [14]. Moreover, the quality of gear, its strength, uniformity and homogeneity of microstructure are the important parameters affected directly by the forging temperature [15,16]. Therefore, an optimum temperature should be selected in order to achieve the best condition of microstructure and hardness homogeneity and prolonging the service life of the die. To the knowledge of the authors, the effect of precision forging temperature on the microstructure homogeneity and hardness distribution of spur gears has not been assessed yet. Therefore, in this research, regarding the properties discussed above the optimum temperature for the precision forging of a low carbon steel spur gear is proposed. 2. Experimental In this work a set of die/punch was designed and made to forge a number of five teeth steel gear. Fig. 1 shows the gear die cavity made by Spark W-EDM (Wire - Electrical Discharge Machine) from H13 steel billet. The forging punch (the top
1100
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
Fig. 1. Precision forging die of spur gear with five teeth.
Fig. 3. The precision forged gear and the appropriate billet.
Fig. 2. The schematic presentation of the precision forging of spur gear. Fig. 4. The sample on which the hardness map tests were performed: (a) radial section, (b) vertical section.
Table 1 Specification of spur gear dies No. of teeth Module Pressure angle Modification coefficient Standard pitch circle diameter (mm) Base circle diameter (mm)
5 2.25 20◦ 0.0 12.1 11.36
Table 2 Chemical composition of low carbon steel billets used in this work (in wt%) C%
Mn%
Si%
Ni%
Mo%
P%
S%
Cr%
Cu%
0.17
0.54
0.01
0.04
0.01
0.019
0.018
0.01
0.01
Fig. 5. Presenting the position of metallography examination on a tooth of steel spur gear manufactured in this research.
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
1101
Fig. 6. The microstructures of the steel precision forged gears produced at 750 ◦ C: (a) surface, (b) depth of 600 m, (c) depth of 1200 m, (d) depth of 1800 m.
die) was of the same size and the same numbers of teeth as that of the female die (the bottom die). The female die corresponded to the shape and size of the gear to be forged. Suitable heat treatment was carried out on the die/punch set to achieve proper combination of hardness and toughness. The schematic diagrams of the top and bottom dies are shown in Fig. 2.
The spur gear specification and chemical composition of the testing steel are shown in Tables 1 and 2, respectively. The steel billets were received in the form of 6 mm diameter rod. Each billet was austenitized at 1150 ◦ C for 3 min in an electrical furnace followed by forging in a 100 t mechanical press at a temperature within the range of 750–1150 ◦ C. The
Fig. 7. The microstructures of the steel precision forged gears produced at 950 ◦ C: (a) surface, (b) depth of 600 m, (c) depth of 1200 m, (d) depth of 1800 m.
1102
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
Fig. 8. The microstructures of the steel precision forged gears produced at 1050 ◦ C: (a) surface, (b) depth of 600 m, (c) depth of 1200 m, (d) depth of 1800 m.
die was preheated to 200 ◦ C before the forging test. A sample of billet and precision forged gear is shown in Fig. 3. The forged gears were quenched immediately in water from the forging temperature to assess the microstructure of the gears by metallography examination. Due to the high speed of mechanical press used in this work, each billet was forged in less than one second. Therefore, the deformation during the precision forging process was assumed to be isothermal. To investigate the hardness distribution, an innovative surface hardness mapping technique, which is developed in the Department of Ferrous Metallurgy of RWTH Aachen University, was used. In this technique, the surface of the sample is scanned using an indenter which exerts a 0.8 g force to the surface of the sample. The hardness of the points is recorded in Vickers scale and the surface hardness map of the sample is plotted. The intervals between hardness test points are 0.3 mm in X and Y directions. Considering the fact that it is time consuming and tedious to carry out metallography test all over a section, this technique gives the best physical understanding of microstructure homogeneity of the forged gear. Fig. 4 shows the typical radial and vertical sections of a gear tooth on which the hardness map test was carried out. In order to achieve more precise information about the mechanical properties of the gears, a special region on the radial section was selected to perform the metallography examination. Referring to Fig. 5, the dotted area is the zone of maximum compressive stress during the service. Gears are frequently worn at this area to a limit where they begin to run rough leading to the gear failure [17]. Thus, the metallography examination was carried out along line AB which is vertical to profile curve at point A.
3. Results and discussion 3.1. Microstructural homogeneity At temperature where thermally activated deformation and restoration processes occur, the microstructural evolution is dependent on the deformation temperature, strain and strain rate fields [18]. In other words, the restoration phenomena occurring during the hot precision forging process has a significant effect on the inhomogeneity of microstructure and mechanical properties. Thus, an inhomogeneity in the microstructure and hardness distribution is expected to occur at different forging conditions. On this basis, in present work the forged gears were quenched in water in order to access the microstructures developed at high temperatures.
Fig. 9. Oxidation layer on surface of steel precision forged gear at process temperature of: (a) 950 ◦ C (b) 1050 ◦ C.
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
The microstructure of forged gears at 750 ◦ C is shown in Fig. 6. As it seen the microstructure contains elongated ferrite grains from the surface to the depth of 1800 m indicating an intensive strain on the surface of the gear and lack of equiaxed grains in this region being due to the low temperature of precision forging at 750 ◦ C. Fig. 7 represents the microstructure of forged gears at 950 ◦ C. As it is observed the microstructure contains both elongated and recrystallized equiaxed ferrite grains on the surface of the tooth while a large number of equiaxed ferrite grains have been formed by increasing the distance from the surface. Comparing these microstructures with those of 750 ◦ C, it is revealed that although the grain size changes from the surface to center, the microstructure of forged gear is more homogeneous at this temperature. Increasing the size of ferrite grains from the surface to center is due to strain field variation from the surface to center of the tooth. Due to the high strain exerted on the surface grains, their initial size is less than that of central grains [19]. Also, the temperature gradient between the surface and center of tooth and the heat of deformation may contribute to the growth of the central grains. The microstructure of forged gear at 1050 ◦ C is shown in Fig. 8. Referring to the figure, increasing the forging temperature from 950 to 1050 ◦ C decreases the inhomogeneity of microstructure from the surface to the center. Although, the increase in forging temperature results in reduction of inhomogeneity of microstructure, however,
1103
some inappropriate effects such as scale formation and decarburization may occur which can increase the surface roughness of the forged gear. It should be noted that tooth surface smoothness is an important parameter in service since a precision forged gear is used without any machining process on its tooth profiles. Fig. 9 represents the oxidation layers on the surface of the steel precision forged gears at 950 and 1050 ◦ C. Comparing Fig. 10a with Fig. 10b the inappropriate effect of increase in forging temperature is clearly observed. In fact increasing the forging temperature has increased the amount of scale formed during heating. Furthermore, increasing the forging temperature enhances the temperature of die/billet interface so that the service life of die decreases due to the die material softening [20]. 3.2. Hardness homogeneity The hardness maps of tooth region of precision forged gears are illustrated in Fig. 10. According to these maps, hardness distribution varies based on the forging temperature. In forged gears at 750, 850 and 950 ◦ C the surface hardness is in the range of 350–400 Hv. With increasing the forging temperature to 1050 ◦ C the surface hardness decreases considerably. Comparing the maps a, b and c in Fig. 10 suggests 950 ◦ C as the optimum forging temperature. Although the distributions of hardness are approximately the same in these three temperatures but the hardened zone in forged gear at 950 ◦ C is
Fig. 10. The hardness maps of tooth region of precision forged gears at (a) 750 ◦ C (b) 850 ◦ C (c) 950 ◦ C (d) 1050 ◦ C.
1104
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
Fig. 11. The hardness maps on CDEF plane, for precision forged gears at (a) 750 ◦ C (b) 850 ◦ C (c) 950 ◦ C (d) 1050 ◦ C.
expanded to the depth of the tooth leading to a more homogeneity in hardness distribution. The hardness maps of precision forged gears at different temperatures on vertical section shown as CDEF plane in Fig. 5 are represented in Fig. 11. By moving from the tooth tip to the gear center, the hardness decreases in all four hardness maps, but the most irregularities in the hardness distribution are seen in Fig. 11d which is in accord with the hardness map on radial section in Fig. 10d. As it is observed the hardness in some region which is not too far from the line of maximum compressive stress on the surface of tooth is less than 160 Hv. This makes an unreliable service condition for the gears forged at this temperature. Also the hardness of regions which had been in contact with the lower part of the die during the precision forging process has increased because of a more heat exchange between the workpiece and die at this region. Considering this point that the contact time between the lower part of die and the workpiece is longer than that between the punch and the workpiece, the formation of a region with higher hardness at lower contact surface is reasonable. Regarding the importance of line AB presented in Fig. 5, the microhardness and grain size curves are plotted along this line to compare the homogeneity of gears produced at different temperatures. Microhardness variation from the surface to the center of forged gears at different temperatures is shown in Fig. 12a. As it is seen by increasing the forging temperature the hardness values at surface and center are totally reduced and the hardness homogeneity increases across the tooth. The grains size variations from the surface to the center of the gears can be considered as a good criterion for comparing the inhomogeneity of microstructure. Based on this criterion and comparing
Fig. 12. (a) Hardness variation, (b) grain size variation, from the surface to center of the precision forged gears produced at different process temperatures.
M. Irani, A.K. Taheri / Materials Chemistry and Physics 112 (2008) 1099–1105
the curves plotted in Fig. 12b the homogeneity in microstructure of precision forged gear at 950 ◦ C is more acceptable. 4. Conclusion In this research the effect of forging temperature on the microstructure and hardness homogeneities of five teeth steel gear was studied. The following conclusions can be made from the results: 950 ◦ C is the optimum forging temperature for precision forging of the steel gear produced in this work, because the minimum inhomogeneity of microstructure and hardness in the gear is achieved at this temperature. Although forging above 1000 ◦ C decreases the hardness and grain size difference between the surface and the center of the gear and leads to a microstructure with more homogeneity but this increase results in scale formation and reduction in the surface smoothness and quality of the gear. Also assessing the hardness maps confirms that the 950 ◦ C is the optimum forging temperature for production the steel gear of high homogeneity. Acknowledgements The authors would like to thank the Research Board of Sharif University of Technology, Tehran, Iran for the financial support and
1105
provision of the research facilities used in this work. The help of Mr. Naderi at Department of Ferrous Metallurgy of RWTH Aachen University, Germany, for performing the hardness tests is highly appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
J.C. Choi, Y. Choi, Int. J. Mach. Tool. Manuf. 38 (1998) 1193–1208. J. Choi, H.Y. Cho, C.Y. Jo, J. Mater. Process. Technol. 104 (2000) 67–73. N.R. Chitkara, M.A. Bhutta, Int. J. Mech. Sci. 38 (1996) 891–916. J. Choi, H. Cho, J. Mater. Process. Technol. 103 (2000) 347–352. N.R. Chitkara, Y. Kim, Int. J. Mech. Sci. 38 (1996) 791–803. N.A. Abdul, T. Dean, Int. J. Mach. Tool. Des. Res. 26 (1986) 113–123. N.R. Chitkara, Y. Kim, Int. J. Mech. Sci. 38 (1996) 777–789. H.H.A. Hsu, Int. J. Mech. Sci. 44 (2002) 1543–1558. T. ALtan, Computer Simulation to Predict Load, Stress and Metal Flow in An Aximetric Closed-Die Forging in Metal Forming, Plenner Press, 1971. N.R. Chitkara, M.A. Bhutta, Int. J. Mech. Sci. 37 (1995) 1247–1268. R. Douglas, D. Kuhlmann, J. Mater. Process. Technol. 103 (2000) 182–188. O. Barrau, C. Boher, R. Gras, F. Rezaie-Aria, Wear 255 (2003). O. Brucelle, G. Bernhart, J. Mater. Process. Technol. 87 (1999) 237–246. P. Panjan, I. Urankar, B. Navinsek, Surf. Coat. Technol. 151–152 (2002) 505. D.K. Matlock, G. Krauss, J.G. Speer, J. Mater. Process. Technol. 117 (2001) 324. M. Jahazi, B. Eghbali, J. Mater. Process. Technol. 113 (2001) 594–598. D.W. Duddley, Practical Gear Design, McGraw Hill, New York, 1954. F.J. Humpherys, M. Hatherly, Recrystalizaion and Related Annealing Phenomena, Elsevier Science Ltd., 1995. P. Cotterill, P.R. Mould, Recrystallization and Grain Growth in Metals, Surrey University Press, 1976. E.T. George, A. Maurice, H. Howes, Steel Heat Treatment Handbook, Marcel Decker Inc., 1997.