Study of Notched Bar Creep Rupture Strength of a 1% Cr-1% Mo-0.25%V Steel

Abstract

To establish the influence of heat treatments and notch sharpness of specimens upon the notchedbar creep rupture strength of a Cr-Mo-V steel for steam turbine rotor forgings, creep rupture tests were conducted with variously heat treated and notched bar specimens. Three ways of austenitization were employed, i.e., heating at 950°C for 20hr (Sample A), heating at 1000°C for 20hr (Sample B), and heating at 1000°C for 20hr, followed by cooling to 950°C and holding at the temperature for 1hr (Sample C). All the specimens were tempered at 675°C for 24hr. Two kinds of notch sharpness were used, one had a notch with elastic stress concentration factor kt of 6.5 and another of 2.5. The creep rupture tests were conducted with these specimens for about 4000hr at 550°C.The results obtained were as follows.(1) The sample B showed the highest plain bar creep rupture strength of theses three samples, the sample C was second to the sample B, and the sample A was the poorest.(2) Both the sample A and the sample C were notch strengthened within the range of this investigation. However the sample B austenitized at 1000°C was notch weakened at about 1300 hr for kt of 2.5 and about 500hr for kt of 6.5.(3) The plain bar creep rupture properties of the sample C were similar to those obtained with the sample B, while for the notched bar specimen the sample C showed a similar behavior to the sample A. From these results it was found that austenitizing at 1000°C was effective for the plain bar creep rupture strength, and cooling from 950°C was effective for the notched bar creep rupture strength.(4) The rupture time ratios of notched bar specimens for kt of 2.5 were higher than those for kt of 6.5 in all the three samples.(5) From the results of observation of cracks at the notch root, it was found that in specimens for kt of 6.5 cracks were formed in comparatively earlier stage of test, i.e., within 300 of rupture life, while in the specimens for kt of 2.5 no crack could be observed within 900 of rupture life.Consequently, it was supposed that in the specimens for kt of 6.5 almost all of rupture lives were consumed by the propagation of cracks, while rupture lives of the specimens for kt of 2.5 were decided by initiation of cracks.