Effect of trace elements on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb steel

Abstract

The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C,B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.