Induction surface hardening of hard coated steels

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1 Induction surface hardening of hard coated steels Abstract The properties of hard coatings deposited using CVD processes are usually excellent. However, high deposition temperatures negatively influence the substrate properties, especially in the case of low alloyed steels. Therefore, a subsequent heat treatment is necessary to restore the properties of steel substrates. Here, induction surface hardening is used as a method of heat treatment after the deposition of TiN hard coatings on AISI 4140 (DIN4CrMo4) substrates. The influences of the heat treatment on both the coating and the substrate properties are discussed in relation to the parameters of induction heating. Thereby, the heating time, heating atmosphere and the power input into the coating substrate compounds are varied. As a result of induction surface hardening, the properties of the substrates are improved without losing good coating properties. High hardness values in the substrate near the interface allow the AISI 4140 substrates to support TiN hard coatings very well. Consequently, higher critical loads are measured in scratch tests after the heat treatment. Also, compressive residual stresses in the substrate are generated. In addition, only a very low distortion appears Elsevier Science S.A. All rights reserved. Keywords: AISI 4140; Hard coated steels; Induction surface hardening; TiN 1. Introduction deposition. Furthermore, in the case of hard coated steels, remarkable distortion has to be avoided because The deposition of hard coatings with chemical vapour after-machining of the parts is not possible due to the deposition (CVD) processes is commonly used to coating. These demands are fulfilled, for example, by improve the wear resistance of tool steels in forming the process of induction surface hardening as a beneficial operations and of cemented carbides in cutting opera- method to improve the mechanical properties of steels tions. The advantages of CVD are undisputed (high to a large extent [ 4]. deposition rates with simple equipment, excellent coating In the present paper, the influence of several parameters (power supply, heating time and atmosphere) of the properties). Although high deposition temperatures used for CVD favour a good adhesion of CVD coatings on induction process on the properties of both TiN hard the substrates, there is one major disadvantage of these coatings and AISI 4140 steel substrates is discussed. high temperatures. Because of the relatively low cooling rates from high deposition temperatures, the properties of steel substrates are negatively influenced. Therefore, a subsequent heat treatment of the coated steels is necessary to restore the properties ready for operation.. Experimental Such duplex treatments as combinations of deposition and heat treating processes can be carried out in different.1. Coating substrate system ways [1] with the aim to summarize the advantages of single processes. Thus, excellent coating properties genwas coated with titanium nitride using HT (high temper- The substrate material AISI 4140 (DIN4CrMo4) erally obtained due to CVD processes have to remain unaffected during further heat treatments after the ature)-cvd at deposition temperatures of 950 C for 5 h in a Bernex equipment. The TiN coatings were stoichiometric and about 5 mm thick. Fig. 1 shows the geometry of the samples. They are cylindrical (diameter 50 mm, height 4 mm) with four plane chamfers ( lateral dimen-

2 496 K. Pantleon et al. / Surface and Coatings Technology (1999) Fig. 1. Geometry of the samples. sion 16 mm) arranged symmetrically at the shell of the cylinder... Induction surface hardening Fig.. Scheme of the experimental arrangement for induction surface hardening. For the post-deposition heat treatment by means of induction surface hardening, a special experimental or inert gas (nitrogen); heating times: 1, 4, 7, 30 s. arrangement was developed, as shown schematically in After induction heating, the samples were immediately Fig.. Both the sample and the inductor remain station- oil-quenched. ary during heating. The induction process is performed within a chamber that can be permanently filled with.3. Methods of investigation inert gas during the heating period. Alternatively, it can be operated with air. The coated steel sample is posi- All experimental investigations were carried out on tioned inside the inductor by a rod. The rod itself is the chamfers before and after the induction process, i.e. fixed on the top by a magnetic switch. By opening the as deposited and surface-hardened. The microstructure shutter at the bottom of the chamber after induction of the substrates was characterized using optical microheating, the magnetic switch is opened, and the rod with scopy. Chemical compositions were investigated by glow the sample falls into the quenching bath (oil ). discharge optical emission spectroscopy (GDOES). To obtain optimal results, the induction coil has to Microhardness measurements were carried out using be adjusted to the geometry of the samples. Therefore, the Vickers method. On longitudinal sections of the a special inductor was manufactured. The induction coil samples, the substrate hardness was measured in relation is round and consists of four turns. Only the shell of to the distance from the surface with loads of 9.81 N the cylindric samples is inductively heated, and the ends ( HV1). For measurements of the film hardness, a special (top and bottom) of the samples remain cold. The method was used, as described in Refs. [5,6]. Due to distance between the sample and inductor is 5 mm. The the low film thickness, measured hardness values repreinduction coil is operated using a HF generator. sent only a compound hardness, H, which is influenced According to working frequencies of about 00 khz, c by both the hardnesses of the film, H, and the substrate, the penetration depth of the induced current is due to f H. By subjecting the TiN coatings with different loads, the skin effect in the order of magnitude of only several s several compound hardnesses, HV0.01, HV0.05, millimeters [3]. HV0.05 and HV0.1, are measured. Then, the real film To investigate the effect of induction surface harden- hardness is calculated according to: ing on both the coating and the substrate properties, the following parameters were used; power supply of H =H + k dh s cp. the HF generator: 6, 8, 10 kw; heating atmosphere: air f s 3 dc c=0 (1)

3 497 The value k summarizes the information about the s substrate (hardness H and Young s modulus s E, k #.3 E /H ) and the value c, is defined by the s s s s film thickness, d, and the penetration depth of the indenter, t: c=d/t. The scratch test, with continuously increasing loads up to 100 N, was used to investigate the adhesion of coatings on the substrates. In addition to the determination of critical loads by recording the accoustic emission (L ) and the tangential force (L ), failure modes were c1 c also verified using optical microscopy. Before and after the induction surface hardening, distortion measurements were carried out using an auto- matic coordinate-measuring machine to determine devi- ations from an ideal cylindrical shape as a result of the heat treatment. Residual stresses of coatings and substrates were measured in axial directions using X-ray diffraction. Depth-dependent measurements were carried out with stepwise removal of the samples by means of local re-sputtering using the GDOES equipment. 3. Results and discussion 3.1. TiN AISI 4140 as deposited Fig. 3. Microstructure of TiN AISI 4140 compounds after the CVD process. Despite the excellent coating properties, the poor quality of the AISI 4140 substrates would prevent industrial applications where the coating substrate compound has to withstand high loads. 3.. Results of induction surface hardening Microstructure and properties Influence of heating atmosphere After induction surface hardening of the TiN AISI 4140 com- pounds, no changes in chemical composition are detected either in the coating or in the substrate compared with the as-deposited samples (see Fig. 4). Slight temper colours after induction heating in air indicate chemical reactions with oxygen, but with GDOES, no increased amounts of oxygen are found in the coatings. The oxidation resistance of TiN coatings seems to be good, even at high temperatures for short periods. However, further investigations are necessary. Nitrogen, present in the protective atmosphere, does not affect the stoichiometry of the TiN coatings. Most of the modifications of AISI 4140 substrates, due to induction hardening, are independent of the heating atmosphere. It will be noted in the following sections when there are differences between heating in air versus inert gas. Otherwise, the results described below are valid for both heating atmospheres. The low cooling rate after the CVD process (about 100 min for cooling from 950 to 00 C) results in a ferritic pearlitic microstructure of AISI 4140, as shown in Fig. 3. According to this, the substrate hardness is only 50 HV1. Low tensile residual stresses of 50 MPa maximum near the interface are measured for AISI In contrast to inadequate substrate properties, the TiN coatings show desirable properties. The microhard- ness is about 3000 HV. High compressive residual stresses of about 500 MPa are almost constant throughout the whole film thickness Influence of heating time Only the outer regions of samples are affected by the induction process: consequently, the temperatures decrease with increasing distance from the surface. The core of the substrate remains completly cold. If the heating times (<1 s) are too short, only local temperature maxima are generated, e.g. the temperature of the surface regions changes periodically between high and low, according to the spacing of the turns of the induction coil. With increas- ing induction heating time, the maximum temperature rises, and the temperature distribution becomes more uniform over the shell of the sample. During the induction process, the surface regions of the samples are heated up for several seconds to about 900 C. Temperature gradients finaly result in microstructure gradients. Near the interface, the microstructure consists of martensite (see Fig. 5), but the core of the substrate still shows a ferritic pearlitic microstructure with no differencestothesamplesasdeposited.fromamacroetching of a longitudinal section, as shown in Fig. 6, it can be concluded that the depth of the hardened zone is uniform over most of the sample. Only near the ends of the cylinder there is a deeper hardened zone. This may be caused by overheating, which generally occurs at sharp edges during induction processes [,3]. All the properties discussed below were measured in the middle zones of constant hardening depth. With increasing heating time, the width of the martensitic layer is enlarged.

4 498 Fig. 4. Chemical composition of TiN AISI 4140 compounds: (a) as deposited; (b) after CVD+induction hardening. This can also be deduced from hardness profiles of induction heating (compare Fig. 7(a) (c); see also the substrate. Plots of hardness gradients, as shown in Table 1). Fig. 7, are typical for surface hardening processes. The In summary, the hardness of the AISI 4140 substrates maximum hardness values near the interface are independent of the heating time (see Table 1), but the depth to support the coatings very well, as indicated by scratch seems to be sufficiently high after induction hardening of hardened layers is enlarged with increasing time of tests. Fig. 8 shows critical loads, L, for the delamina- c tion of TiN coatings from AISI 4140 substrates versus the depth of the hardened layer (corresponding to the time of induction heating). Due to the induction process, the coatings can withstand much higher loads before delaminating from the substrate. No delaminations up to 100 N are observed in the case of induction heating in nitrogen for 30 s. The critical loads, L, for crack c1 formation in the coatings are almost unaffected after the heat treatment. As shown in Table 1, both the hardness and compressive residual stresses of TiN coatings are lower after induction hardening. While the reduction of hardness is more pronounced for heating in air, residual stresses are more influenced in the case of heating in nitrogen. Considering the mean values of compressive residual stresses over the whole thickness of the TiN coating Fig. 5. Microstructure of TiN AISI 4140 after CVD+induction without taking into account the presence of slight stress hardening. gradients, residual stresses decrease with increasing heating time compared with the as-deposited samples (see Table 1). Nevertheless, gradients with slightly increasing compressive residual stresses from the surface to the interface are generated in TiN coatings due to induction hardening. Much more important than changes of residual stresses in the coatings may be the positive influences of the induction process on residual stresses in the substrate. There are changes in the residual stresses from tensile to compressive in the martensitic layers of the AISI 4140 substrates. Fig. 9 shows an example for the depth dependence of residual stresses due to induction surface hardening. Fig. 6. Macrograph of a longitudinal section of the induction hardened layer in the substrate Influence of power supply A reduction in the power supply from 10 to 8 and 6 kw, respectively, does

5 499 Fig. 7. Hardness gradients after CVD+induction hardening: (a) 1 s, 10 kw, air; (b) 7 s, 10 kw, air; (c) 30 s, 10 kw, air; (d) 30 s, 6 kw, air. not influence the maximal hardness values in the substrate near the interface. As described above, they are equal for different heating times, and the same values of about 680 HV1 are also obtained for different power supplies (see Table 1). The reduction in power supply for constant heating times reduces the depth of the hardened layer in the substrate. This is shown in Fig. 7(d) for the use of 6 kw with a heating time of 30 s, as compared to 10 kw in Fig. 7(c). It is very important that induction heating with low power supply of only 6 kw does not influence residual stresses in TiN coatings, even after high heating times. Thus, the decrease in compressive residual stresses, which is observed in the case of a high power supply of 10 kw, can be prevented (compare Table 1). In general, it can be assumed that a decrease in the power supply results in the same properties obtained for higher power supplies with lower heating times. However, this trend is limited, because if the power supply is too low, the core of the substrate is heated by thermal conductivity Distortion In order to show the advantages of induction surface hardening, the results of distortion measurements are compared with those of bulk hardening. From Fig. 10, it can be concluded that cylindrical samples show only very small deviations from ideal cylinders in the case of induction hardening. In contrast, worse shape values are obtained after bulk hardening. Here, only changes in the cylinder shape are considered. For further investigations concerning distortions due to induction and bulk hardening, see Ref. [9]. 4. Conclusions Fig. 8. Critical loads measured in scratch test in relation to the depth of the hardened layers. The combination of the CVD process with a subsequent induction surface hardening is very useful in

6 500 Table 1 Properties of TiN AISI 4140 after post-deposition heat treatment Heat treatment AISI 4140 TiN coating Hardnessa (HV1) Layer depthb (mm) Hardness (HV ) Residual stressesc (MPa) As deposited ±70 40 After CVD+induction surface hardening 1 s, 10 kw, air ± s, 10 kw, air ± s, 10 kw, air ± s, 10 kw, air ± s, 6 kw, air ± s, 10 kw, N ± s, 10 kw, N ± s, 10 kw, N ± s, 10 kw, N ± s, 6 kw, N ±70 10 a Maximum hardness values of the substrate near the interface. b Depth of the hardened layer in the substrate: 80% of the maximum hardness at the interface. c Measured on the surface without considering gradients of residual stresses. Fig. 9. Depth dependence of residual stresses after CVD+induction hardening. Fig. 10. Distortion of CVD+induction hardening (10 kw, air) compared with CVD+bulk hardening.

7 improving the properties of the steel substrates after Acknowledgement deposition without damaging the coating and losing good coating properties. With the help of several induc- The authors wish to thank the Deutsche tion parameters (heating time, heating atmosphere, Forschungsgemeinschaft for financial support of this power supply), the induction process can be optimized. work (project Ke616/3-1). This will be the aim of further investigations. Due to induction surface hardening, microstructural gradients are generated in AISI 4140 substrates. As a References result, hardened layers whose depths depend on the [1] O.H. Kessler, F.T. Hoffmann, P. Mayr, Surf. Coat. Technol. parameters of induction heating, are obtained, while the (1998) 11. core of the substrate remains tough. As described in [] S. Zinn, S.L. Semiatin, Elements of Induction Heating: Design, Control Refs. [7,8] and confirmed in own investigations, also and Applications, ASM International, Metals Park, OH, in the case of bulk hardening as a post-deposition [3] G. Benkowsky, Induktionserwärmung,, Verlag Technik, Berlin, treatment, an excellent load support of the coatings can [4] S.L. Semiatin, D.E. Stutz, Induction Heat Treatment of Steels, be achieved. American Society for Metals, Metals Park, OH, Nevertheless, induction hardening has many more [5] R. Wiedemann, T. Schulz-Krönert, H. Oettel, Prakt. Metallog. 34 (1997) 496. advantages due to the generation of compressive residual [6] K. Fischer, Freiberger Forschungshefte B 88 (1998) 1. stresses in the substrate and only very low distortions [7] O. Keßler, F. Hoffmann, P. Mayr, Härtereitechn. Mitteil. 49 resulting from the surface treatment. This makes the (1994) 48. process of induction surface hardening very useful com- [8] O. Keßler, F. Hoffmann, P. Mayr, Härtereitechn. Mitteil. 49 (1994) 191. pared with bulk hardening as a method of subsequent [9] K. Pantleon, O. Keßler, F. Hoffmann, P. Mayr, Härtereitechn. heat treatment of CVD-coated steels. Mitteil. 3 (1999)