Effects of alloy additions and tempering temperature on the sag resistance of Si-Cr spring steels

Abstract

The effects of Mo, W, and V additions and tempering temperatures on the sag resistance in relation to the microstructural evolution during tempering of Si-Cr spring steels were investigated by performing TEM examination, torsional Bauschinger tests, hardness tests and tensile tests. The hysteresis loop area measured in torsional Bauschinger tests, which is closely related with the sag resistance, is directly influenced by the distribution of precipitates. It increases and then decreases with tempering temperature, after reaching its maximum value at 350 degrees C. The additions of Mo and/or W result in a finer distribution of tempered carbide particles due to the decrease in the rate of the spheroidization and of the coarsening of carbide particles above 400 degrees C. However, they do not affect the behavior of tempered carbides at low tempering temperatures. Accordingly, the additions of Mo and/or W increase the hysteresis loop area at tempering temperature above 400 degrees C. The V addition increases the hysteresis loop area as well as hardness due to the precipitation of vanadium carbonitrides, regardless of tempering temperature. In considering industrial application of spring steels. a new parameter, the ratio of the hysteresis loop area in the torsional Bauschinger test to hardness, is suggested. (C) 2000 Elsevier Science S.A. All rights reserved.