Formation of stresses in surface layers of R6M5 steel in the process of electrospark alloying and laser treament

Abstract

Simultaneous electrospark alloying and laser treatment allow one to improve the characteristics of surface layers and decrease the cost of alloyed materials and energy [ l 1. Laser treatment of electrospark coatings increases the depth of the strengthened layer from 30-50 gm to 150-180 gin, which is significant for the improvement of service properties of cutting tools [ t ]. The aim of the present work is to investigate the influence of the power density of the laser pulse on the value and distribution in surface layers of residual stresses of the fist kind.Specimens made of R6M5 steel 10 x 10 x 10mm in size after thermal treatment (hardness is 62-65 HRC) were cut on a 4531 electrospark discharge machine into specimens of width 2.2 mm which were polished in soft modes to a final size of 70 x 10 x 2.Electrospark coatings made of hard alloys based on tungsten carbide were applied with an "Elitron-22" instrument with a current of 1.5 A and an alloying time of 1.5 min/cm 2. Laser treatment was performed with a "Kvant-15" instrument. The duration of the pulse is 4 msec, the density of the energy pulse is 1-2 J/mm 2, the diameter of the spot is 1 -1.4 mm, and the overlap coefficient of spots K u in a row and between rows is, respectively, 0.5 and 0.3, where K u = S/D, S is the displacement step and D is the diameter of the zone of radiation.To find the residual stresses, we use the Davidenkov method by continuously etching a strengthened layer and automatically recording sags [2]. The sign of stresses is defined by the sign of the slope of a tangent line to the bending curve at a certain point. We measured the microhardness of the coating with a PMT-3 instrument and the roughness with an MIS-I 1 microscope. The cutting instrument strengthened with electroslag alloying and laser treatment was tested according to the known method [1]. 30KhGSA steel served as a processing material. Residual stresses were calculated by the Davidenkov formulas [2].As the power density of the laser pulse increases, the maximum of tensile stresses in the strengthened layer (400-800 MPa) rises and reaches the limiting level at a power density of 380 MW/m 2 (Fig. 1). Further increase in the power density is accompanied by a decrease in the maximum, which is caused by the smaller rate of cooling of the bath of melt.Metallographic analysis shows that the zone of laser influence consists of the melting zone and thermal influence zone (Figs. 2 and 3) [1]. The maximum tensile stresses are in the surface layer of the melting zone (Fig. 4). After laser treatment, the residual stresses decrease upon approaching the surface (curves 2-5), which can be caused by their relaxation due to nucleation and growth of cracks (curves 2 and 3). For power densities of 250-310 MW/m 2, cracks grow to the depth of the melting zone of 40-60 gm, which results in their relaxation (curves 2 and 3). As the power density increases to values of 380-440 MW/m 2, the size of the melting zone becomes greater and the size of the thermal influence zone decreases (Fig. 3). Extensio...