Laser transformation hardening of iron-carbon and iron- carbon- chromium steels
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INTRODUCTION
L A S E R transformation hardening of steel surfaces is a process that can offer greater precision and less distortion than more conventional surface hardening techniques. The initial microstructure of most laser hardened steels consists primarily of pearlite colonies with some proeutectoid ferrite. When heated above the critical temperature, the pearlite colonies are first to transform to austenite by simultaneous dissolution of the ferrite and cementite constituents. The austenite continues to grow at the expense of any proeutectoid ferrite. Concentration gradients in the single phase austenitic structure are then removed by further carbon diffusion. Successful laser transformation hardening requires that the power, size, and shape of the laser beam, in addition to the speed at which it scans the workpiece, be carefully adjusted so that the absorbed energy is sufficient to austenitize the steel to a depth of 0.5 mm or more without melting the surface, tl-9] Rapid self-quenching by conduction of heat into the workpiece transforms the austenite layer to martensite. mrata, ll0'll'12lLi, [6]Ashby and Easterling, t71and others [9'13'141 have analyzed the effects of processing and materials parameters on the heat transfer and phase transformations aspects of laser hardening of plain carbon steel. Unfortunately, modeling the process is complicated by the fact that the steel surface must first be coated with an absorptive material to enhance coupling of the 10.6/xm CO 2 laser radiation with the underlying metal; otherwise most of the radiation would be reflected. I4'5'1~ Among the most significant features of laser surface hardening are heating and self-quenching rates typically on the order of 104 deg per second or greater, f~'3'6'~7~Furthermore, the period for which the steel is heated above the critical temperature is extremely brief, often - 0 . 1 second or less. These factors,
JOHN R. BRADLEY, Staff Research Scientist, and SOOHO KIM, Senior Research Scientist, are with General Motors Research Laboratories, Warren MI, 48090-9055. Manuscript submitted October 2, 1987. METALLURGICALTRANSACTIONS A
particularly in the presence of a relatively slowly diffusing solute such as Cr, would suggest the possibility that nonequilibrium transformation conditions may obtain during austenitization, and that an understanding of the effects of alloying elements upon the mechanism and kinetics of the pearlite to austenite transformation would be central to successful laser heat treatment. In contrast to the abundance of research reported on the kinetics and mechanisms of austenite decomposition, the study of austenite formation during heating has produced a much smaller body of literature. The subject has been reviewed by Paxton. t~81It was the consensus among most early workers that, at least in fairly pure Fe-C alloys, austenite nucleated primarily at pearlite colony intersections, and that austenite growth was mainly carbon diffusion controlled, the characteristic diffusion distance being the interlamellar spacin
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