Response of the Cr-alloyed PM Steels on Vacuum Sintering and Heat Treatment

The present paper deals with the evaluation of different vacuum heat treatments on the fracture surfaces of a low alloyed sintered steel in correlation with the mechanical and plastic properties achieved. The heat treatment conditions consist of the sintering process in vacuum furnace at 1393 K for 1800 s with an integrated final tempering at 473 K for 3600 s. The average cooling rates were calculated in the range of 1393 K to 673 K and were 0.1 K/s, 0.235 K/s, 3 K/s and 6 K/s, respectively. Vacuum heat treatment performed is supporting bainite/martensitic microstructure with increasing cooling rates; it provides a marked increase in strength coupled to a decrease in ductility. The typical microstructure appearances on the fracture surfaces show that microstructure-fracture developments are closely associated with strength/plastic properties. The fracture surface mode at higher magnification revealing two main micromechanisms of fracture: brittle (transgranular cleavage fracture) and ductile (dimples and plastic deformation by slip). Finally, the cooling rate of the vacuum furnace in order of 3 K/s is sufficient to obtain suitable properties of strength and plasticity which can however be accepted for several applications.