The effects of residual elements and deoxidation practice on the mechanical properties and stress relief cracking susceptibility of ½%CrMoV turbine castings

Abstract

The effects of residual element content on the mechanical properties and stress relief cracking susceptibility have been investigated for basic electric arc melted ½%CrMoV steel castings deoxidized by using either aluminium or titanium practices. The residual element contents ranged from the lowest readily available in commercial practice to levels substantially higher than those common at present. In normalized and tempered material, deoxidation by the use of aluminium resulted in lower creep rupture ductility than deoxidation by using titanium, regardless of residual element content. Only in one cast did low purity appear to correlate with low ductility. Similarly, in simulated heat-affected zone material, the susceptibility to stress relief cracking was less in casts deoxidized with titanium. Increasing the residual element content had a slightly deleterious effect on stress relief cracking susceptibility, but austenite grain size refinement gave significant improvement. It is concluded that adoption of the titanium deoxidation practice used in the present work would improve the creep rupture ductility and stress relief cracking resistance of ½%CrMoV steel, but that at present there is no need for very low residual element contents. Regardless of deoxidation practice, sound welds should be obtainable when adequate grain refinement can be produced by control of the welding process.