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This is offered as a contribution to the study of the strengthening mechanism of the centrifugally cast HK40 steel. The approach was to clarify the correlation between the structural factors and the creep rupture strength of HK40 steel. A suitable as cast structure for the higher creep rupture strength was obtained by controlling the cooling rate during the centrifugal casting, which promoted the larger dendrite cell structures. Both the secondary structure and the grain boundary morphology were controlled by the addition of Ti'which induces the uniform precipitation of secondary carbides and the discontinuous grain boundary morphology, and by the addition of Nb which induces the fine precipitation of the lamellar carbides. The discontinuous and irregular boundary morphology and the lamellar carbides on the cell boundaries were found to be effective in retarding crack propagation during the tertiary creep period, causing an increase in creep rupture time. A combined addition of Ti and Nb to HK40 steel in the order (Ti + Nb)/C ^) 0.3 and Ti/(Ti + Nb) 2 0.3, in atomic ratios, was found to be very effective in giving the suitable structure for the optimum creep rupture strength. The result was considered to be an extraordinarily high creep rupture strength for HK40 steel, superior to that of IN519 alloy. The improved Ti and Nb added HK40 steel, when compared at the same temperature, has a strength about 1.7 times higher than the plain HK40 steel, and, at an equal stress, it can be used at temperatures about 100°C higherthan the plain HK40 steel.

On the Strengthening of the Centrifugally Cast HK 40 Tube