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MATERIALS RESEARCH

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Eutectoid temperature of carbon steel during laser surface hardening Chin-Cheng Chen,a) Chun-Ju Tao, and Lih-Tyan Shyu Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China (Received 20 March 1995; accepted 2 October 1995)

A new method was developed to determine the eutectoid temperature, Ac1 , of carbon steel during laser surface hardening. In the method a three-dimensional heat flow model with temperature-dependent physical properties was set up and solved for the temperature distribution employing a finite element method (FEM). Workpieces were heat-treated to produce a melted and hardened zone by a single pass of a continuous-wave TEM00 CO2 laser beam. The depth profile of the melted zone was used as a calibrator to solve the uncertainty imposed by the unknown surface absorptivity. Obtained was an Ac1 of, on average, 770 ±C, a superheat of 47 ±C compared to the equilibrium Ac1 of 723 ±C. Furthermore, the numerical model was also employed to predict the hardened depth, and the results show that, for a depth of more than 100 mm, the eutectoid temperature 770 ±C leads to a depth about 10% smaller than that predicted at 723 ±C. The use of the temperature-dependent physical properties is critical; an error up to 80% could result if constant physical properties are used.

I. INTRODUCTION

Laser surface transformation hardening of carbon steels is an efficient process, offering greater precision and less distortion than conventional surface-hardening methods.1 It has attracted a great deal of attention and research interest, and has been adopted in industrial production lines.2,3 The initial microstructure of carbon steel consists primarily of pearlite colonies embedded in ferrite. When heated above a critical temperature, the pearlite colonies are first transformed to austenite by simultaneous dissolution of the ferrite and cementite constituents. The austenite continues to grow at the expense of ferrite. Carbon concentration gradients in the single phase austenitic structure are then smoothed by further carbon diffusion. Successful laser transformation hardening requires that the power, size, and shape of the laser beam as well as the velocity of the workpiece be carefully chosen so that the absorbed energy is sufficient to austenitize the steel to a proper depth without surface melting. Melting results in an undesired layer of rough dendrite structure. Rapid self-quenching by the conduction of heat into the workpiece transforms the austenite layer to martensite. Numerical prediction of hardened depth in laser surface hardening offers a more effective and costsaving alternative to determining processing parameters other than direct experimental measurement. So far, there are many studies that report on the numerical a)

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J. Mater. Res., Vol. 11, No. 2, Feb 1996

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prediction of hardened depth,4 where it is normally a