In-situ heat treatment system for die steels using YAG laser with a machining center

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1 Precision Engineering Journal of the International Societies for Precision Engineering and Nanotechnology 25 (2001) In-situ heat treatment system for die steels using YAG laser with a machining center Toshiki Hirogaki a, *, Heisaburo Nakagawa a, Masato Hayamizu b, Yoshihiro Kita c, Yoshiaki Kakino d a School of Engineering, The University of Shiga Prefecture, Hassaka, Hikone, Shiga, Japan b Graduate Student, The University of Shiga Prefecture, Hassaka, Hikone, Shiga, Japan c Faculty of Engineering, Osaka Institute of Technology, Omiya, Asahi-ku, Osaka, Japan d Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto, Japan Received 21 August 1900; received in revised form 2 November 1900; accepted 27 November 1900 Abstract We propose a laser heat treatment system for die steels using a YAG laser on the machine tool table. Optical fiber is used to transmit the laser light from a source to the machine tool table in this system, which makes it possible to perform the cutting processes, the heat treatment and the grinding processes with a machining center. In the present report, the experiments of laser heat treatment were done in order to research suitable die steels for this system. Additionally, the temperatures based on a theoretical model were investigated during the laser irradiation. As a result, it is clear that the martensite start temperature (Ms point) of materials is an important factor to estimate the application of this heat treatment Elsevier Science Inc. All rights reserved. Keywords; In-situ Heat Treatment, Die, Machining center, Optical fiber, YAG laser 1. Introduction A new type of flexible and multifunctional machine tools has been developed for integrated manufacturing processes. Machining centers with high motion accuracy and high spindle speed have made it possible to perform grinding and honing processes [1][2]. Such manufacturing processes as cutting, drilling, grinding and honing can be done using the above mentioned type of machining center. The next requirement for multifunctional machine tools is the ability to provide heat treatment on the machine tool table. A laser light therefore has attracted attention as a heat source to achieve this heat treatment. To date, much work has been done to treat the surface using a laser source [3] [7]. Many studies have been conducted using a CO 2 laser source. However, a problem has emerged in that the freedom of the optical path is insufficient to transmit a laser source to the machine tool table * Corresponding author. The University of Shiga Prefecture, School of Engineering, Hassaka, Hikone Shiga Japan. Fax: hirogaki@mech.usp.ac.jp (T. Hirogaki). because mirrors must be used. Moreover, an absorptive material must be coated on the surface because the absorption of steels is insufficient. It is also difficult to control the coating thickness. On the other hand, high power has been achieved in the YAG laser field recently. With the laser under study, an optical fiber can be used to transmit a laser source to the machine tool table. Moreover, it seems that heat treatment is feasible without coating the absorption material on the surface because the absorption of YAG laser light is sufficient for steels. We, therefore, attempt to develop a laser heat treatment system using a YAG laser on the machine tool table. Generally, machining centers have frequently been used for manufacturing dies because it involves multiple smalllot production. Many kinds of materials have been used for various dies; a mold, a press, a forging and so on. Therefore, we must research what kinds of materials are suitable for this heat treatment. In the present report, the hardness after this heat treatment was evaluated for various die steels. On the other hand, the temperature in the workpiece during laser irradiation was calculated in order to investigate the phenomena on this heat treatment. Additionally, the relationship be /01/$ see front matter 2001 Elsevier Science Inc. All rights reserved. PII: S (01)

2 T. Hirogaki et al. / Precision Engineering 25 (2001) Table 1 Properties of S45C C % Cr % Thermal conductivity W/cm C Thermal diffusivity cm 2 /s Prediction of temperature during laser irradiation tween the residual austenite on the surface and the hardness was researched after the heat treatment by means of this system. 2. HEAT TREATMENT SYSTEM 2.1 Construction of system Fig. 1. Heat treatment system. The heat treatment system is shown in Fig. 1. The laser used in this system is a 300W YAG laser, emitting continuous wave at 1.06 m wavelength. This emitting source is equipped at a place away form the machining center. The laser beam is delivered via an optical fiber more than 50 m in length. The fiber output housing that holds the lenses is attached to the spindle of the machining center with a tool holder. The focal length from the fiber output housing is kept at 120 mm to prevent the interference. The laser beam is focused at the workpiece surface giving a beam spot size of 1.4 mm diameter. Figure 2 shows the quenching part where the laser irradiation is done in this report. Fig. 2. Quenching part by laser irradiation. WithaCO 2 laser beam, the absorption ratio on the steel surface is considered to range from 0.6 to 0.7, which varies with the coated thickness of the absorption material [4]. The temperature of the steel during the laser heat treatment processes was clarified by many studies. However, there have been few studies dealing with this temperature during the processes without coating absorption material using a YAG laser. In this section, we attempt to analyze of the temperature of the steel during the YAG laser irradiation to predict the range of the hardened area. The temperature Tv is indicated by equation (1) when a heat source of the Gauss mode moves along the x axis on the surface. Here, the y axis shows the width direction, and the z axis the depth direction. 2r 0 is the diameter of the heat source. v the feed speed. the absorption ratio, and and a are the thermal conductivity and the thermal diffusivity, respectively. T v 2a0.5 x P a r 2 0 exp 2 x vt 2 2y 2 2 z2 (1) 8a r 0 4a d The material properties of S45C steel, which is widely used, is shown in Table 1. The absorption ratio is considered to be 0.35 [8]. The range of the austenite transformation is considered the most important factor, because the cooling speed of the self-quenching is sufficient in the case of the laser heat treatment. From the above conditions, the depth from the surface and the width on the surface were determined where the temperature was higher than Ac1. Figure 3 gives the quenched depth and width calculated from Ac1 temperature when varying the feed speed in irradiation. Here, the value of the width is defined as a distance from the spot center (y 0) to the end of the width on the surface. The appearance of the quenched parts is confirmed from these results. It is indicated that the heat treatment by this system can be done without coating the absorption material on the surface. Figure 4 shows the micrograph of the cross section after the laser irradiation under feed speed 700 mm/min. The white part due to the heat treatment can be observed in the above figure. The depth and the width of this part are 470 m and 820 m, respectively. These results are in good agreement with the calculated ones shown in Fig. 3.

3 214 T. Hirogaki et al. / Precision Engineering 25 (2001) Table 2 Properties of materials Material C% Cr% Mn% V% Ni% Cu% Mo% W% Si% SUJ420J SKD SNCM HMD SKH SUJ HMD SKS Cowry-Y SKD EXPERIMENTAL RESULTS AND DISCUSSION There are many kinds of die steels. Therefore, we attempt to apply this heat treatment to them in the experiments. The steels used are shown in Table Hardness after heat treatment for various steels Fig. 3. Calculated hardened depth and width. From the above results, the heat treatment is found to be sufficient by means of the small power YAG laser (300 W) in this system. Figure 5 shows the relationship between micro Vickers hardness (load 980N) and the distance from the surface after heat treatment of SKD61, SKS93, S45C and SKD11. The maximum hardness indicates 820 HV at SKS93 containing high carbon (1.05%) and 700 HV at S45C containing low carbon (0.45 %). On the other hand, a unique phenomenon occurs at SKD11 in that sufficient hardness can not be obtained near the surface. This type material is considered unsuitable for this heat treatment. Thus, the relationship between carbon content and the maximum hardness after this heat treatment was researched without SKD11. We would investigate the characteristics of SKD11 in section 3.3 and 3.4. Figure 6 shows the maximum hardness of the other steels including different carbon content. The broken line indicates the hardness after conventional heat treatment. The tendency is also found to increase the hardness up to a Fig. 4. Micrograph of cross section (Feed speed 700 mm/min, S45C). Fig. 5. Relationship between hardness and distance from surface (Feed speed 700 mm/min).

4 T. Hirogaki et al. / Precision Engineering 25 (2001) Fig. 6. Relationship between carbon content and maximum hardness. carbon content of 0.6% and to maintain constant value of more than 0.6% carbon content after the laser heat treatment. Especially, the hardness after this heat treatment indicates higher values (50 HV) than conventional one in the case of less than 0.6% carbon content. 3.2 Surface roughness Figure 7 shows the section profile of the surface after the laser heat treatment. A roughness of 20 m can be seen on the surface. However, it is considered that there is no problem because this roughness can be modified by grinding with a machining center after the heat treatment. 3.3 Residual austenite It is considered that the residual austenite increases after quenching as the carbon content of the steel increases due to decreasing the martensite start temperature (Ms point). Therefore, the residual austenite on the surface was measured by means of X-ray after the laser heat treatment. Figure 8 shows the relationship between the residual austenite and the carbon content. The materials are SKD61, HMD1, HMD5 and SKD11. The residual austenite tends to increase as the carbon content increases. An Especially large value can be seen at SKD 11. As a result, the residual austenite on the surface causes the insufficient hardness at SKD11 after this heat treatment. In general, the quantity of the residual austenite is related to the martensite start temperature (Ms point). Fig. 8. Relationship between carbon content and residual austenite. (Feed speed 700 mm/min). Therefore, the hardness after conventional heat treatment is compared with that after this heat treatment for the steels with various Ms points. Figure 9 shows the hardness ratio of the conventional method to this one. Here, Ms points were calculated by Eq. (2). Ms C C% 20 Cr% 40 Mn% 35 V% 17 Ni% 10 Mo% 5 W% (2) The figure indicates the tendency for the hardness to decrease for steels having Ms points of less than 100 C. The insufficient hardness after this heat treatment is obtained for the steels with low Ms points. High Ms point steels are found to be suitable for this heat treatment. 3.4 Hardness and heat treatment temperature It seems that quenching temperature is especially important to obtain sufficient hardness after quenching when Fig. 7. Surface roughness after quenching (Feed speed 700 mm/min). Fig. 9. Relationship between hardness ratio and Ms point. (Feed speed 700 mm/min).

5 216 T. Hirogaki et al. / Precision Engineering 25 (2001) Fig. 10. Calculated temperature (SKD11). Fig. 12. Hardened depth and width after heat treatment. quenching an alloy steel including much carbon and chromium, such as SKD 11. The appropriate temperature of SKD 11 is from 1050 C to 1100 C in quenching. In this section, we therefore investigate the relationship between the temperature during the laser irradiation and the distance from the surface. The thermal conductivity and the thermal diffusivity a are 0.253W/cm C and cm 2 /s, respectively. Figure 10 shows the calculated results for SKD 11. Figure 11 shows the hardness and the distance from the surface after heat treatment for SKD 11 using this system. In the case of feed speed 700 mm/min, the temperature reaches more than 1100 C to more than 200 m depth in Fig. 10. On the other hand, it can be seen in Fig. 11 that sufficient hardness is not obtained to this depth. In general, it is known that the decrease in hardness occurs due to the increase in the residual austenite when quenching the SKD 11 at a temperature higher than the appropriate temperature. Thus, sufficient hardness near the surface cannot be obtained due to the high temperature in quenching. It is only at a suitable depth that the hardness increases where the appropriate quenching temperature is maintained. On the other hand, in the case of feed speed 2000 mm/min, the high hardness value can be seen near the surface, because the appropriate temperature appears near the surface in Fig. 10. Thus, provided the feed speed is higher, the high hardness can also be obtained near the surface for SKD 11, but the hardened depth is shallow in this case. 3.5 Hardened depth and width Fig. 11. Hardened depth (SKD11). As stated above, it is clear that the steels with a high Ms point are suitable materials for application of this heat treatment. The maximum hardness is also different due to the carbon content. In this section, we investigate the hardened depth and width when quenching steels with different carbon content among the suitable materials. In general, in the case of high carbon steels, the effective hardness is more than 500 HV, and, in the case of low carbon steels, more than 400HV. Then, the hardened

6 T. Hirogaki et al. / Precision Engineering 25 (2001) depth and width for the SKS93 (high carbon steel) and the SUS420J2 (low carbon steel) are measured. These are more than 500 HV and 400 HV, respectively. Figure 12 gives the measured results. The horizontal axis indicates the energy density( (laser energy)/(spot diameter) (feed speed)). The difference can hardly been seen between these materials in Fig. 12. It can be confirmed that there are few differences in the effective hardened width and depth among the suitable materials. 4. Conclusions 1. We proposed an in-situ heat treatment system using a YAG laser with a machining center. It is confirmed that sufficient heat treatment can be achieved without coating the absorption materials on the steel surface by means of this system. 2. The martensite start temperature (Ms point) of materials is an important factor in estimating the application of this heat treatment. Steels with a high Ms point are suitable materials for application of this heat treatment. 3. The tendency is to increase the hardness up to the carbon content 0.6% and to maintain a constant carbon content of 0.6% after laser heat treatment the same as after conventional heat treatment. Therefore, there are few differences in the effective hardened width and depth among suitable materials. Acnowledgement This work was supported by the research grant from the Ministry of Education, Japan (2000, Commendatory Research (A) No ). References [1] Kakino Y, Matubara A, Kita Y, Nakagawa H, Otubo H and Tozawa S. Development of Grinding Support System GSX for Grinding Center, Advancement of Intelligent Production, 1994; 455. [2] Nakagawa H, Hirogaki T and Fukuda Y. Study on Honing with a Machining Center (Development of a Honing Unit), Int Journal of JSPE 1999;33(3):197. [3] Bubrujiaud B, Fontes A, Forget P, Papaphilippou C, Sainte-Catherine C, Vardavoulias M and Jeandin M. Surface Modification using High Power Laser Influence of Surface Characteristics on Properties of Laser Processed Materials, Surgical Engineering 1997;13(6):461. [4] Woo H G and Cho H S. Three-dimensional temperature distribution in laser surface hardening processes, Journal of Engineering Manufacture, part B, 1999;213:695. [5] Zhang XM, Man HC and Li HD. Wear and Friction Properties of Laser Surface Hardened En31 Steel, Journal of Materials Processing Technology, 1997;69:162. [6] Guan YH, Chen TL, Wang HG and Zhang JT. The Prediction of the Mechanical Properties of Metal During Laser Quenching, Journal of Materials Processing Technology 1997;63:614. [7] Chen TL, Guan YH, Wang HG and Zhang JT. A Study on Austenite Transformation during Laser heating, Journal of Materials Processing Technology 1997;63:546. [8] Matuyama S, Kunieda M and Satuta T. Influence of Physical Properties of Workpiece Material on Laser Piercing difficulty, Journal of Society for Precision Engineering 1972;66(5):699 (in Japanese).