Tempering of steel during laser treatment
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I.
INTRODUCTION
T E M P E R I N G of steel is a common practice and already has been studied for more than a century. Usually steel is heat-treated after quenching to improve its mechanical properties. Laser treatment is a comparable process. During the passage of a laser beam after possible melting and resolidification, the heat is conducted away to the substrate, and high cooling rates are attained. Several authors have investigated laser treatments on different types of steel. [1-13] The hardness and wear resistance is improved with respect to conventionally quenched steel. [4.7.9-12] When a laser is used in practical applications, it is usually necessary to scan the complete surface, i.e., the laser beam passes a previous track a small distance away. This area is exposed to a thermal cycle and is tempered during a short time (Figure 1). Some authors have reported on laser-treated material tempered in a rather conventional way. [1,8[ A fine distribution of carbides was found. Austenite was discovered to be rather stable. In a laser-tempered pass, a lower hardness has been reported together with a tensile stress compared with the higher hardness and the compressive stress in the not-tempered pass. [13[ An AISI 1045 steel showed a decreasing hardness and higher sensitivity to etching in the laser-tempered area. [5[ Nonetheless, the picture of laser tempering and its effect on the mechanical properties of a laser-treated surface is rather incomplete, and an appropriate explanation is not available. In this study, attention is paid to these subjects dealing with three different types of steel. After laser treatment, one type shows an austenitic, another a martensitic, and a third a mixed structure of 8-ferrite, austenite, and martensite. The layers are analyzed and mechanically tested. A simple analytical model is presented to reveal the reaH.J. HEGGE and H. De BEURS, formerly Graduate Students, Department of Applied Physics, Materials Science Centre, University of Groningen, are with Shell Corporation, The Hague, and Philips Corporation, Drachten, respectively. J. NOORDHUIS, Graduate Student, and J.Th.M. De HOSSON, Professor of Applied Physics, are with the Department of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands. Manuscript submitted March 7, 1989. METALLURGICALTRANSACTIONSA
son why the tempering effect is sometimes quite large despite the short duration time of the laser passage. II.
EXPERIMENT
The chemical compositions of the steels used are listed in Table I. The specimens were ground and grit-blasted to get a rough surface which absorbed well the laser light used (size: 38-mm diameter, 5-mm thickness). After grit blasting, the samples were ultrasonically cleaned and annealed during 90 minutes at 920 K. The hemispherical absorption for the wavelength used (10.6 pm) was more than 30 pct at room temperature. The specimens were mounted on a numerically controlled X - Y table and irradiated by a 1.5 kW Spectra Physics 820 CO2 laser. The
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