Microstructural influence on sag resistance of Cr containing and Cr free spring steels

Abstract

The effects of Cr addition and tempering temperature on mechanical properties, especially sag resistance, with respect to the microstructural evolution during tempering in 1.5%Si spring steels, were investigated by performing hardness tests, tensile tests, torsional Bauschinger tests, and examination by transmission electron microscopy. The results obtained showed that the sag resistance depended on the distribution of tempered carbides. With increasing tempering temperature, sag resistance increased to a maximum value at 350 degrees C and then decreased. The CI addition did not affect the behaviour of tempered carbides at low tempering temperatures, while at high temperatures, above 400 degrees C, the Cr addition reduced the coarsening rate of the cementite, resulting in finer particle distribution of the cementite. The Cr addition improved the sag resistance as well as the hardness at tempering temperatures above 400 degrees C, but it decreased the ratio of loop area in the torsional Bauschinger test (the sag resistance) to hardness, regardless of tempering temperature.