The Effect of Cobalt on the Structure and Properties of No. 5 Die Steel

Abstract

The tempering characteristics of No. 5 die steel, with varying amounts of cobalt, have been investigated. Microstructural changes were determined by preparing extraction replicas and examining these with the electron microscope. The carbides were extracted electrolytically, identified by X-ray diffraction and analysed by classical chemical techniques. The transformation temperatures (Ms, Mf, Ac1, and Ac3) were determined for No. 5 die steel with graduated cobalt additions by modified dilatometric methods. Hardenability and T.T.T. diagrams were also determined in order to compare the effect of cobalt on these properties. An attempt was made, by using electron probe microanalyser, to find evidence of partitioning of cobalt at the ferrite-austenite and ferrite-carbide interfaces during transformation. Impact tests were performed on three steels and, lastly, a die wear test was carried out under hot forging conditions. The results indicate that the tempering resistance increases with increasing amount of cobalt. Cobalt refines the carbide particle size and reduces agglomeration. Cementite (Fe3C type structure) is the only carbide in tempered 3%Co steel. No partitioning of cobalt is observed in any stage of transformation and cobalt has very little effect on impact strength and impact transition temperature of No. 5 die steel. All the transformation temperatures rise with cobalt content. A drastic fall in hardenability occurs with the addition of cobalt. The die wear test shows a significant improvement in the die wear properties with increasing cobalt content. All these observations can be explained on the assumption that in the absence of large quantities of strong carbide formers, cobalt has the effect of reducing the carbon equivalent of the steel.