Studying the Machinability of Tin-Bearing Free-Cutting Steel

Resumo

The machinability of 0.2% Sn free-cutting steel produced by vacuum melting and hot shaping under pressure using the method of hammer forging has been studied. The distribution of elements (Sn, S, Mn) in the steel has been investigated by secondary ion mass spectrometry (SIMS). It has been established that the doping with tin appreciably increases the machinability index of the free-cutting steel to k_v = 1.72. The effect of S and Mn segregation with the formation of manganese sulfide MnS precipitates has been revealed. Tin in the form of fine particles is uniformly distributed in crystalline grains and along their boundaries and segregates on MnS particles. In the process of cutting, low-melting tin particles are melted to initiate the Rehbinder effect (liquid-metal embrittlement), thus embrittling the steel. Moreover, tin has a lubricating effect at the point of contact between the cutter and the machined surface. All these effects improve the machinability of tin-bearing free-cutting steel.