Creep Deformation and Creep-rupture Behavior of Cr-Mo-V Steel Forgings

Abstract

In order to investigate creep deformation and the fracture mechanism of Cr-Mo-V steel, creep tests up to 104 h were carried out in a temperature range of 550-675°C. Metallographic observations were also conducted on the fractured specimens. The results obtained are summarized as follows: (1) Stress dependence of steady-state strain rate changes at a certain stress level, at all the temperatures studied. The stress levels of the transition in creep rate are close to those in fracture mode from transgranular to intergranular. (2) The stress level at which instantaneous plastic deformation starts (regarded as the Orowan stress), is estimated to be σ/E = 1.5 - 2.0 × 10-3. This Orowan stress corresponds with the transition stress described in (1). (3) Above the Orowan stress, creep occurs mainly by the motion of dislocations within grains, which is controlled by the Orowan-bowing mechanism. In this stress range, creep rupture ductility is high. (4) Below the Orowan stress, dislocations slowly climb over particles. Because of the slow deformation within grains, the contribution of intergranular sliding increases. This results in a greater chance of intergranular cavitation, and consequently, results in a low creep rupture ductility. © 1990, The Iron and Steel Institute of Japan. All rights reserved.