Characteristics of decomposition of residual austenite under the action of shock waves excited by laser pulses

Abstract

After laser thermal treatment of high-alloy steels, in the quenched surface layer a significant amount of austenite remains, depending on the original structural state, the grade of steel, and the thermal cycle of the laser quenching. Generally the amount of residual austenite increases with an increase in the carbon content in the steel and with an increase (within a certain interval) in the cooling rate. There are two ways to decrease the amount of residual austenite after traditional quenching: the thermal method and the deformation method. The thermal method is most widely used to regulate the content of residual austenite in machined pieces and tool. In order to decrease the amount of residual austenite in high-alloy steels, high-temperature tempering at 400-500°C or cold treatment is used. The deformation mechanism for decomposition of the austenite is used more rarely. In the case of local laser hardening, the amount of residual austenite can be regulated by changing the laser treatment conditions. In [1], decomposition of residual austenite was observed under the action of short, intense pressure pulses excited by giant laser pulses. In this paper we have studied the quantitative characteristics of decomposition of residual austenite in quenched steel under the action of shock waves initiated by laser radiation. In particular, we have studied the change in the amount of residual austenite as a function of the amplitude of the shock wave and the number of laser pulses. The studies were done on samples of 45, U8, and R6M5 steel. Surface laser quenching was accomplished by the emission from a YAG laser operating in the free oscillation regime. The irradiation parameters were: energy, 25 J; pulse length, 5 msec; inversion ratio, 2. The mechanical characteristics and structural features of layers hardened upon laser treatment have been rather well studied (see, for example, [2]). Accordingly, in tiffs paper we limited ourselves to x-ray phase analysis, which was done on the DRON-3 apparatus in filtered cobalt Kcz radiation.