Transformation hardening of En3 steel with a Nd:YAG 10 millisecond pulsed laser

Abstract

The power densities available with a laser enable a thin surface of the steel to be raised to temperatures higher than an appropriate critical value characteristic of the austenitic transition in a very short time. Very low values of energy are absorbed per unit depth which implies a reduction in heat distortion. This together with high rates of quenching produced by heat conduction processes alone enable the process of transformation hardening of steel to be effected satisfactorily. The low power density process of transformation hardening on the surface involves heating without melting. The principal operation consists of the irradiation of the surface for short times which in the present case is given by a pulse duration of 10 ms. The heat energy from the laser can be as high as 25 J in a single pulse and this energy passes into the metal by conduction processes. The thin surface layer is maintained above the critical temperature for a sufficient time to ensure that the austenitic transition takes place in the form of a localised surface phenomenon. The increase in hardness is the result of the formation of martensite due to the quenching process. The quenching rate achieved by heat conduction effects is of the order of 105 °C s−1. The hardness achieved is a factor of the order of 3.2 times that of the base metal. The depth of hardening achieved is 0.63 mm in these experiments. The use of pulsed lasers achieves a better quenching rate compared to that realised by mechanically scanning the specimen with a continuous laser beam. Such a comparison of the depth of hardening that can be achieved is carried out in this paper. Comparison of the experimental measurements were made with empirical formulae and with the mathematical model developed by Davis, Kapadia, Dowden, Steen and Courtenay (1986).The power densities available with a laser enable a thin surface of the steel to be raised to temperatures higher than an appropriate critical value characteristic of the austenitic transition in a very short time. Very low values of energy are absorbed per unit depth which implies a reduction in heat distortion. This together with high rates of quenching produced by heat conduction processes alone enable the process of transformation hardening of steel to be effected satisfactorily. The low power density process of transformation hardening on the surface involves heating without melting. The principal operation consists of the irradiation of the surface for short times which in the present case is given by a pulse duration of 10 ms. The heat energy from the laser can be as high as 25 J in a single pulse and this energy passes into the metal by conduction processes. The thin surface layer is maintained above the critical temperature for a sufficient time to ensure that the austenitic transition takes plac...