Materials and Design 24 (2003) 25–30 The effects of induction hardening on wear properties of AISI 4140 steel in dry sliding conditions Y. Totik1, R. Sadeler, H. Altun, M. Gavgali Department of Mechanical Engineering, Ataturk University, 25240, Erzurum, Turkey Received 2 July 2002; accepted 11 October 2002 Abstract Wear behaviour of induction hardened AISI 4140 steel was evaluated under dry sliding conditions. Specimens were induction hardened at 1000 Hz for 6, 10, 14, 18, 27 s, respectively, in the inductor which was a three-turn coil with a coupling distance of 2.8 mm. Normalised and induction hardened specimens were fully characterised before and after the wear testing using hardness, profilometer, scanning electron microscopy and X-ray diffraction. The wear tests using a pin-on-disc machine showed that the induction hardening treatments improved the wear behaviour of AISI 4140 steel specimens compared to normalised AISI 4140 steel as a result of residual stresses and hardened surfaces. The wear coefficients in normalised specimens are greater than that in the induction hardened samples. The lowest coefficient of the friction was obtained in specimens induction-hardened at 875 8C for 27 s. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Wear; Induction hardening; Dry sliding; AISI 4140 steel; Pin-on-disc 1. Introduction depends of hardening depth and the magnitude and distribution of residual compressive stress in the surface Many mechanical parts, such as shafts, gears, springs, layer w4–6x. etc. are subjected to surface treatments, before the By most recent estimates, improved attention to fric- delivering, in order to improve wear behaviour. The tion and wear would save developed countries up to effectiveness of these treatments depends both on sur- 1.6% of their gross national product, or over $100 face materials properties modification and on the intro- billion annually in the USA alone w7x. The magnitude duction of residual stresses. Among these treatments, of the financial loss associated with friction and wear induction hardening is one of the most widely employed arises from the fact that entire mechanical systems, be to improve component durability w1–3x. It determines they automobiles, are frequently scrapped whenever only in the workpiece a tough core with tensile residual a few of their parts are badly worn. In the case of an stresses and a hard surface layer with compressive automobile, the energy consumed in its manufacture is stresses, which have proved to be very effective in equivalent to that consumed in 100 000 miles of opera- extending the component fatigue life and wear tion w8x. resistance. The object of the present work is to study the role of Induction surface hardened low alloyed medium car- the induction hardening on the wear behaviour of the bon steels are widely used for critical automotive and AISI 4140 steel specimens when a harder material, such machine applications which require high wear resistance. as WC-%6Co, is used as a counterpart. Wear resistance behaviour of induction hardened parts 2. Experimental procedure *Corresponding author. Tel.: q90-442-2312381; fax: q90-442- 2.1. Specimens preparation and hardness measurement 2360957. E-mail address: firstname.lastname@example.org (Y. Totik). Since the aim of the induction hardening is to improve 1 email@example.com the surface hardness, the carbon content in steel to be 0261-3069/03/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 0 2 . 0 0 0 9 9 - 7 26 Y. Totik et al. / Materials and Design 24 (2003) 25–30 Table 1 Chemical composition of AISI 4140 steel C Si Mn P S Cr Mo Ni (wt.%) AISI 4140 0.3954 0.225 0.785 0.0187 0.0070 0.847 0.166 0.0795 Table 2 Induction hardening parameters Specimens Frequency Heating time Temperature Cooling Microstructure (Hz) (s) (8C) 1 – – – – Normalised 2 1000 6 815 Water Normalised 3 1000 10 830 Water Normalised 4 1000 14 845 Water Normalised 5 1000 18 860 Water Normalised 6 1000 27 875 Water Normalised important process parameters that control both the hard- ness value and hardness depth. These parameters can be easily controlled during the induction hardening. In the tests, the specimens were induction hardened for differ- ent times at a frequency with 1000 Hz, and later quenched in water. The inductor was a three-turn coil with a coupling distance of 2.8 mm. Table 2 shows the Fig. 1. Schematic representation of the pin-on disc test configuration. selected induction hardening parameters. The hardness of the AISI 4140 steel with normalised Table 3 and induction hardening were measured as Vickers The test conditions from pin-on-disc tribo test hardness using a PC Controlled Buehler–Omnimet tes- ter. A standard microhardness tester, equipped with a Parameters Experimental conditions Vickers indenter and a 500-g indention load was used for surface and subsurface hardness measurement. Applied load (N) 10 Velocity (msy1) 0.08 Scar diameter (mm) 10 2.2. Wear tests Environment Air Temperature (8C) ;20 Duration (s) 900 The wear tests of the AISI 4140 steels with normali- Roughness (Ra, mm) 0.16 sed and induction hardening used in this work were Test ball diameter (mm) 5 mm performed in Teer POD2-Pin-on-disc tribometer, using the test configuration shown in Fig. 1. WC-%6Co ball induction hardened should be between 0.3 and 0.5%. was used with a diameter of 5 mm as counterpart. After Thus, the AISI 4140 (42CrMo4) steel used in the tests samples were cleaned by ultrasound in acetone for 5 was normalised prior to the induction hardening. The min, a pair of pins and disc were inserted into the investigation was performed on cylinder specimens machine. A particular load was applied and the rotation made of AISI 4140 (42CrMo4) steel, which is especially disc started. Experiments were carried out at a linear adaptable for machining and where surface hardness is speed of 0.08 msy1 and at a load of 10 N. The desirable. The chemical composition is indicated in atmosphere used was air with a relative humidity of Table 1. ;45% and temperature of ;20 8C. Experimental data Specimens were prepared normalised (hardness of were recorded continuously during the wear tests: coef- 307 HV). The cylinders (length of 120 mm) were ficient of friction and time. The wear friction test machined to a diameter of 20 mm prior to induction conditions are given in Table 3. hardening. After 2000 cycles of rotation in the case of 0.08 Induction hardening is a process where steel is hard- msy1 speed, which resulted in a distance of 50 m, the ened by means of induction heating and later quenching experiment was stopped, wear specimens were removed in water. The hardening temperature and time are most and the worm surfaces were examined using profilo- Y. Totik et al. / Materials and Design 24 (2003) 25–30 27 Fig. 2. (a) SEM image of the wear track after 3000 cycles; (b) the surface profile, the wear track and their superposition used in calculating the wear volume. 2.3. Microstructural analysis Microstructures of the AISI 4140 steels with normal- ised and induction hardening were characterised using a Nikkon Epithot 105 optic microscope, a Jeol 6400 Scanning Electron Microscope (SEM), a Rigaku Dmax 2200 X-Ray diffractometer with a CuKa radiation source. 3. Results and discussions Surface hardness, as well as subsurface hardness, Fig. 3. Microhardness distribution for induction hardening treatment. distributions were obtained with induction hardening is shown in Fig. 3. The main factor that controls the hardness was the heating time as the power output was metry. The arithmetic mean roughness value (Ra) of the the same at all the processes at induction hardened surfaces was evaluated by using a Mitutuyo profilometer. specimens with a frequency of 1000 Hz. The hardness The wear coefficient (k) was calculated using the values increased from surface of the specimen toward equation for wear which takes the form ksVyNL. Where its centre, depending on the increase of the induction V is wear volume, N is the load and L is the sliding temperature and time, as shown in Fig. 3. The hardness distance. To calculate the wear volume, the profiles were depths increased more considerably at longer induction recorded by a Mitutuyo profilometer before and after times, as the times supplied were those requested for induction hardening. Then, from the superimposed pro- carbide to be dissolved. files, the wear volume was calculated, as shown in Fig. Because the wear resistance of a material is related 2. to its microstructure and because changes in microstruc- Fig. 4. Microstructures of the AISI 4140 steel with (a) normalised and (b) induction hardening 27 s at 875 8C. 28 Y. Totik et al. / Materials and Design 24 (2003) 25–30 Fig. 5. XRD patterns of the AISI 4140 steels with normalised and induction hardening. ture may take place during the wear process, it seems transient period, the friction coefficient reaches a maxi- that in wear research emphasis should be placed on mum value. This value is the result of the formation of microstructure w9–11x. The microstructures of AISI 4140 large quantities of debris generated during wear. This steel normalised and induction hardened for 27 s contain debris had formed from fragmentation under high con- ferrite, perlite and metallic carbides, as shown in Fig. 4. tact pressure at the normalised steel surface. The highest In addition, the induction hardening also caused an friction coefficient was found as 0.76 for normalised increase at the amounts of the carbides and the marten- sample and this value is maintained until the end of the site in steel. test. In the case of low sliding speed (0.08 msy1) the The X-ray patterns of the phases existing in the coefficient of friction has a relatively constant value structure are presented in Fig. 5. It was found that the during the test and lies between 0.76 and 0.41. It is density of the phases with increasing induction time obvious that increasing the induction time caused an increased significantly. Increasing the density of the decrease in the coefficient of friction, attaining the phases is expected to improve the wear behaviour of minimum value at the highest time of 27 s. The AISI 4140 steel. reduction of the friction coefficient with increasing the Fig. 6 reveals the coefficient of friction for the AISI induction time is expected to be a result of increasing 4140 steels with normalised and induction hardening. the hardness and particularly compressive residual Results on the frictional behaviour of the mating pair stresses. substrate-ball are also included for comparison purposes. Fig. 7 shows the wear coefficients (rates) for the For the normalised samples, as seen in Fig. 6, the steady induction times under 10 N load and at sliding speed of state is achieved after approximately 1000 cycles. In the 0.08 msy1. It can be seen that changing the induction Fig. 6. The variations in the coefficients of the friction with the revolution for the AISI 4140 steels with normalised and induction hardening. Y. Totik et al. / Materials and Design 24 (2003) 25–30 29 es both the width of the wear track and the amount of the debris removed. The wear phenomenon occurred uniformly at the induction hardened specimens (Fig. 8c,d). It can be seen that the worn surfaces are compar- atively smooth with little plowing and some very small debris. 4. Conclusions The following conclusions are established for AISI 4140 steel in induction hardening conditions: Fig. 7. Wear coefficients vs. the induction times. ● It was observed that the induction hardening treatment improved the wear behaviour by depending on the time also causes change in the contact conditions con- process parameters selected in this work. This was firmed by the coefficient of friction. It can be seen that attributed to process parameters of the induction induction treatments cause a slight reduction in wear hardening influencing the residual stress-state of hard- coefficients as compared to normalised specimens. ened parts, to a great extent due to different temper- Studies have been made of the relationship between ature distributions in the specimens during heating. typical microstructures and wear behaviour of materials These compressive residual stresses were expected to during sliding wear. But micromechanisms of the wear delay the formations of the microcracks generated as of various microstructures are still not clear because the a result of wear loading on the wear surface. On the dynamic changes taking place during sliding have not other hand, the microstructure was altered with the been satisfactorily considered w12x. Surface topography induction hardening treatment, and it was observed analysis of the worn surface was performed in a SEM that the martensites in the structure dispersed finely to determine the influence of the induction hardening with the increasing of the induction time. on the wear properties. Worn surfaces of the AISI 4140 ● The induction hardening caused a decrease in the steels with normalised and induction hardening are coefficient of the friction after attaining the steady shown in Fig. 8. A severe adhesive wear was observed state, exceeding the transient periods depending on on the normalised specimens as revealed in Fig. 8a,b. induction time. The lowest coefficient of the friction Consequently, the amount of the debris removed from was obtained in specimens induction hardened at 875 the normalised specimens toward the sides of the wear 8C for 27 s. track was much more than that in induction hardening ● The wear coefficients in normalised specimens are specimens. Besides, the increasing induction time reduc- greater than that in the induction hardened specimens. Fig. 8. SEM images of the wear traces of AISI 4140 steel (a) and (b) normalised; (c) and (d) induction hardened for 27 s. 30 Y. Totik et al. / Materials and Design 24 (2003) 25–30 On the other hand, the wear coefficients are decreased w3x Melander M. Theoretical and experimental study of station- with increasing of the induction time. ary and progressive induction hardening. J Heat Treating 1985;2:145 –165. ● The increasing induction time reduces both the width w4x Xu D-H, Kuang Z-B. A study on the distribution of residual of the wear track and the amount of the debris stress due to surface induction hardening. J Eng Materials Tech removed and the wear phenomenon occurred uni- (Trans ASME) 1996;118:571 –575. formly at the induction hardened specimens. Besides, w5x Semiatin SL, Stutz DE. Induction Heat Treatment of Steel it was found that the density of the phases increased ASM. 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