Surface hardening of a gray cast iron used for a diesel engine cylinder block using high-energy electron beam irradiatio
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I. INTRODUCTION
SINCE gray cast iron contains a high percentage of flake graphite, it has special properties such as excellent machinability, the ability to resist galling with restricted lubricant, and excellent vibration damping.[1–4] Cylinder blocks and heads are some of the ways in which the damping capacity of gray cast iron is utilized. As the demand for high performance and durability in automobiles has recently soared, strenuous efforts have been made to improve the wear resistance of engine parts by hardening their surfaces. Notably, the surface modification of cast irons by highenergy beams such as the laser beam and electron beam has been studied extensively by many investigators.[5–11] Since an electron beam has an energy range from 50 to 200 keV, it usually requires a vacuum chamber, which is one of the many limitations of an electron beam as compared to the laser beam.[11,12,13] Recently, a high-energy electron-beam (energy range: 0.5 to 1.5 MeV) irradiation technique, which can be extracted into air, has been developed.[14,15,16] A high-energy electron beam can penetrate up to several tens of centimeters of air. Because the depth of the hardened surface layer is proportional to the electron penetration depth, a layer depth of about 1 mm can be hardened by a high-energy electron beam without surface melting, which is almost impossible by the laser technique. Furthermore, many overlappings are not needed to harden a broad area, because the scanning width of an electron beam can be easily controlled up to several tens of centimeters by changing the magnetic field. Blacking is not necessary in electron-beam surface hardening. In spite SEONG-HUN CHOO, Research Assistant, and SUNGHAK LEE, Professor, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. SOON-JU KWON, Associate Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, 790784 Korea. Manuscript submitted September 30, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
of these advantages, little attempt has been made to apply it to industrial practice. The present study aims to improve the durability of automotive parts by applying high-energy electron-beam irradiation to the gray cast iron used for an automotive diesel engine cylinder block and, thereby, modifying its surface structure. To attain the maximum hardened depth and the peak hardness without surface melting, it was essential to establish the optimum irradiation conditions. Surface hardness and wear properties can be enhanced by the phase transformation from pearlite to martensite, while flake graphites remain and play their own distinctive roles.[2,3] The surface hardening mechanism was clarified by investigation of the microstructural modification and the phase transformation both before and after irradiation. Correlations between process parameters, microstructures, and surface properties were also investigated. Furthermore, thermal analysis was c
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