Mechanism of hot-shortness in leaded and tellurized free-machining steels

Abstract

This investigation was aimed at understanding the mechanism of hot-shortness in AISI 12L14 + Te steels, which might lead to a hot-rolling practice to minimize the high yield losses in this grade. High temperature tensile tests showed a pronounced loss in ductility between 810 and 1150 °C, the embrittlement being most severe at about 980 °C. Electron microprobe studies confirmed thermodynamic stability data which indicated that tellurium occurs primarily as PbTe in this steel composition. SEM fractography revealed increasingly brittle, partially intergranular tensile fracture with a loss of ductility. Auger Electron Spectroscopy of samples quenched from the embrittlement temperature range indicated the formation of a thin film of PbTe on the grain boundary surfaces. All these results are consistent with a mechanism of liquid metal embrittlement by PbTe which has a melting point of 923 °C. Some theoretical considerations of this mechanism are discussed. The characteristic return to ductility above the embrittlement range suggests that rolling at temperatures above 1150 °C might minimize the hot-shortness problem. Results of limited hot-rolling experiments to study the incidence of surface cracking as a function of temperature support the above suggestion.