Effect of W Content in a CrMoV Forging Steel on Creep Property Under Multiaxial Stress and Rupture Time Prediction Methods

Abstract

Recently, a modified CrMoV forging steel was developed with superior creep strength property by adding tungsten to conventional CrMoV forging steels which are widely used for high temperature components. Generally, creep damage proceeds at stress concentration portion under multiaxial stress gradient. It is important to clarify creep rupture property and damage evolution process under the multiaxial stress states for the modified CrMoV forging steels to achieve life extention. In this study, creep tests were conducted using plain and four kinds of circular notch specimens with tip radius in 0.1 mm(R0.1), 0.5 mm(R0.5), 2.0 mm(R2.0) and 4.0 mm(R4.0) of the modified and a conventional CrMoV forging steels as well as finite element creep analyses of the notch specimens. It was found that creep rupture times of the modified CrMoV forging steel are three times longer than those of the conventional one showing notch strengthen effect. Rupture times of the notch specimens of the modified one are longer as the tip radius decreases. Although the distribution tendency of creep void number density at the notch root sections is similar in both CrMoV forging steels and it corresponds to the distribution tendency of the maximum principal stress, number of the creep voids in the modified one is smaller than that in the conventional one. The difference of creep rupture property and void number density between the modified and the conventional ones may be caused by the difference of initial dislocation density and contents of Laves phase. It was demonstrated that creep rupture times of both the modified and conventional ones were precisely predicted based on the area average damage concept considering effective damage region with the principal creep strain as a criterion.