In order to investigate creep behavior and the fracture mechanism of a Cr-Mo-V steel, creep tests were carried out over long-term periods in a temperature range of 500675°C. Metallographic observations were also conducted on the fractured specimens.
The results obtained are summarized as follows:
(1)A transition in the plot of steady-state strain rate versus applied stress occurs at a certain stress level, yielding two values of stress exponents at any temperature within a range of 500650°C. These transition levels are much the same as those in transition behavior from transgranular to intergranular fracture.
(2)In the transgranular fracture region, dislocations are nonuniformly distributed and clusterings of dislocations are seen. In the intergranular fracture region, dislocations are uniformly distributed and the formation of subgrain structures are seen.
(3)The stress level at which instantaneous plastic deformation starts (regarded as the Orowan stress), was estimated to be σ/E=1.5 2.0×10^-3. This Orowan stress level corresponds to the transition level which was determined by the change in stress exponent and fracture mechanism.
From the above results, it is considered that creep occurs in a form of crystal deformation above the Orowan stress level. This is due probably to the dislocations by-passing particles. Below the Orowan stress level, dislocations move by climbing around particles; eventually, subgrain formation occurs.