Salt-bath heat treatment of sintered hard alloys

Abstract

This paper examines the process of isothermal treatment of hard alloy VK8 in electrode salt baths heated by current going through molten salt. 5×5×35 mm polished inserts (up to Ra = 0.160–0.320) and disposable four- and five-sided inserts of VK8 grade were used as material for this study. The authors looked at the dissolution of hard alloy VK8 heated in tempering nitre baths. Thus, the alloy was first heated up in nitre baths using different soaking times and then in barium chloride salt baths at 500 °C. The heat treatment process followed a certain sequence as per the experiment design matrix. The following parameters were selected for optimization: bending strength σи; cutting wear on the tail end (hзп) and the head end (hпп); hardness HV. The factors that influenced the parameters varied in the following ranges: quenching temperature Т = (1150±150) °C (Х1), isothermal quenching temperature Т = (350±150) °C (X2), cooling medium (X3). The microstructure of the hard alloys was analyzed with the help of a metallographic microscope μVizo-MET-221 and a JEOL JCM-6000 microscope. The VK8 disposable inserts were tested for wear resistance. For this, a number of steel 45 workpieces were cross-turned in a thread turning lathe 16K62. The facing was done from the centre out with no lubricoolant applied. After the isothermal treatment, the specimens were analyzed for hardness, bending strength, microstructure and wear resistance. The isothermal treatment of hard alloy VK8 in salt baths resulted in increased strength, hardness, and wear resistance.