Modeling of Laser-Tempering Process for Hyper-Eutectoid Steels
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INTRODUCTION
THERMAL sources such as laser, plasma arc, etc. have been used as external heat sources to soften or harden the workpiece material depending on the application. In situ thermal softening techniques have been used for machining hard ceramics, metals, and metal matrix composites.[1,2] On the other hand, the phase transformation characteristic of steels has been employed in case-hardening to improve the surface hardness, fatigue strength, and wear resistance of the material. Several works are available in the literature for laser transformation hardening of low and medium carbon steels.[3–5] While laser surface hardening has been studied extensively, laser surface tempering has not received wide attention. Recently, Raghavan et al.[6] used laser surface tempering to achieve controlled reduction in the surface hardness of a through-hardened high carbon steel. In this work, the work material was case-tempered without affecting the bulk hardness and this material behavior was exploited to enhance the machinability of the hardened steel. The laser-tempering process described by Raghavan et al.[6] is not isothermal but has one or more thermal cycles with short interaction times. The tempering behavior under these conditions is difficult to explain SATYANARAYANAN RAGHAVAN, Graduate Research Assistant, and SHREYES N. MELKOTE, Professor, are with the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA. Contact e-mail: [email protected] tech.edu JUNG-IL HONG, Associate Professor, is with Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology, Daegu, Korea. Manuscript submitted November 4, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
using conventional transformation diagrams because the cooling rates and temperature fields are not only timevariant, but also vary spatially. Previous works have studied tempering in temperature-controlled furnace heat treatment,[7,8] but they are not suitable for understanding material property changes in laser tempering. A computational model is therefore necessary to predict the evolution of surface and subsurface hardness in laser tempering. In laser surface hardening, the metal surface is exposed to a high power laser beam until it reaches the phase transformation temperature and is then rapidly quenched by the cooler bulk material when the laser source is removed. The rapid cooling rate leads to the formation of a hard martensitic phase. Overlapping laser scans are typically employed to produce a uniformly hardened surface. However, overlapping laser scans are known to temper the material in the region of overlap,[9] which is undesirable in laser hardening. While several models have been reported for the determination of laser hardening parameters to minimize back tempering,[10,11] a model for uniform laser tempering due to overlapping thermal cycles has not been reported. Furthermore, prior works on back tempering have focused only on hypo-eutectoid steels. The objective of this paper is to
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