Abstract

An equation for dislocation density during creep was introduced for martensitic heat-resistant steel using Orowan’s equation and a thermal activation equation of Kauzmann type based on several assumptions. The dislocation density discussed here corresponds to the number of dislocations swept out of an imaginary deformable domain in an activated state and is not the so-called mobile dislocation density. The one adjustable parameter, the actual deformable domain size, was adopted in the equation as a size intermediate of the lath martensite and packet. The activation energy and volume,  and , were calculated using creep curves for 9Cr-1W tempered martensitic steel as a function of creep strain. The changes in the dislocation density during creep were estimated nondestructively to be roughly  to  using the values of  and . The obtained densities were approximately comparable to the values of total dislocation density reported for directly observed ferritic/martensitic steels. It was found that the dislocation density of 9Cr-1W steel initially increased and then decreased in a transition creep range, which relate to hardening and recovery, respectively. This point of inversion from hardening to recovery can be confirmed easily and nondestructively far before the time to minimum creep rate and as a matter of course the time to rupture. It is suggested that monitoring of the dislocation density can predict the unexpected decrease in long-term rupture strength in advance.

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