The Effect of Cobalt on the Structure and Properties of a Ni-MO Hot Work Die Steel

Abstract

The possibility of improving a commercial ah nickel 3% molybdenum hot work die steel by progressive additions of up to 10% cobalt has been explored. Several of the major property requirements for hot work steels have been studied and the effect of cobalt determined. Further development of the steel has been based on economics whereby cobalt was used to replace its cost equivalent of molybdenum. A series of ausformed alloys have also been investigated and a die wear test used as the criterion of performance of all these steels. The results indicate that cobalt is essentially neutral in its effect on the retention of austenite after quenching and air cooling. Tempering resistance increases with increasing amounts of cobalt and the rates of nucleation and agglomeration of the alloy carbides are increased in the presence of cobalt. Analysis of the extracted carbides from the tempered samples has shown that cobalt does not alter the general form of the precipitated carbides and no evidence can be cited for its partition to the carbide phase. Fracture toughness is reduced after tempering above 200° and the tensile properties greatly increased. Cobalt increases the A1, A3 and Ms, temperatures. Both pearlitic and bainitic incubation during the isothermal decomposition are reduced and the transformation rates in each region are increased by cobalt. Hardenability is reduced due to the promotion of the bainite transformation. The die wear test shows a significant improvement in die wear properties with the addition of up to 3% cobalt, thereafter the effect being only marginally advantageous. Reducing the molybdenum content of the basic steel leads to an increase in die wear. For the economically balanced steels, however, a per unit forging is reduced by up to 17% and several steels are suggested for further investigation and industrial trials. The action of cobalt in steels has been reviewed and several explanations proposed to account for the observed effects. Most of the results can be successfully explained on the basis of cobalt reducing the surface energy of the carbideferrite interface but the elevation of the transformation temperatures remains unanswered on this basis. All the results can be rationalised if it is accepted that cobalt lowers the fault energy of the austenite and these high faulted regions act as sinks for the carbon atoms or nucleation sites for the martensite transformation.