Laser transformation hardening of tempered 4340 steel
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INTRODUCTION
L A S E R transformation hardening of steels is one of many successful applications in laser material processing. It is characterized by a very short interaction time, with the intense energy density which accelerates the cooling rate of the laser-impinged area. When the laser beam is removed, rapid self-quenching by conduction of heat into the bulk material causes the martensitic transformation in the surface layer. The process is suitable for selective surface treatments and is primarily used in steels with sufficient hardenability for improved wear resistance and fatigue strength, tl,2J The mechanism for laser transformation hardening involves the formation of austenite on the heating cycle and then the transformation to martensite upon cooling. It is known that the microstructure of the material exerts a great influence on the formation of austenite during laser processingJ 31 The case depth is deeper for AISI 4150 specimens with bainitic and tempered martensitic structures than for those with pearlitic and spheroidized structures.141 The extent of the laser-hardened zone for specimens with various microstructural features can be related to the size and distribution of carbides in the steel. Finer and more evenly distributed carbides would produce a better hardening effect.J3.41 Molinder tSj conducted investigations on a hypereutectoid steel with widely spaced carbides and observed that the growing austenite occasionally enveloped the carbides. Continued dissolution of carbides then took place by carbon diffusion through the austenite envelope I6,71 in the austenite temperature range. Finally, the growing austenite regions impinged and the complete transfor-
R.K. SHIUE, formerly Graduate Research Assistant, Institute of Materials Science and Engineering, National Taiwan University, is Engineer, Taiwan Power Company, Taichung, Taiwan, Republic of China. C. CHEN, Professor, is with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10764, Republic of China. Manuscript submitted January 30, 1991. METALLURGICAL TRANSACTIONS A
mation to austenite was achieved. Speich et al. c8] studied austenite formation during intercritical annealing of 1.5 pct Mn dual-phase (ferrite and pearlite) steels, indicating that the time for complete dissolution of pearlite to form austenite was very short (0.2 to 200 ms) at temperatures between 780 ~ and 900 ~ After the dissolution of pearlite, further growth of austenite into ferrite was controlled by the carbon diffusion in the austenite at high temperatures of 850 ~ to 900 ~ At low temperatures (740 ~ to 780 ~ the growth of austenite was controlled by Mn diffusion in ferrite, which required a much longer time (4 to 24 hours) to proceed. Apparently, the thermal cycles (at various depths) of laser-treated specimens included all of the possible reactions at corresponding temperature ranges, as mentioned above. Ashby and Easterling 19] have analyzed the temperature as a function of time at various depths for laser surface tre
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