Effect of Air Environment on Creep-Fatigue Properties of 21/4 Cr-1 Mo Steel

Abstract

The strain range partitioning (SRP) life relation was experimentally determined at 550°C both in air and in vacuum (<2× 10^-6mmHg) for the annealed and the normalized and tempered 21/4 Cr-1 Mo steels, and the effects of air environment and heat treatment on the SRP life relation of 21/4 Cr-1 Mo steel were examined.
All of Δε_ij-N_ij properties were found to be sensitive to the environment in the strain range lower than (Δε_ij)_cr while not in the strain range higher than (Δε_ij)_cr. The value of (Δε_ij)_cr was larger in the annealed material than in the normalized and tempered one.
The effects of heat treatment on Δε_pp-N_pp and ΔεN_cc-N_cc properties were not so remarkable for the both materials, whereas Δε_pc-N_pc and Δε_cp-N_cp properties of the annealed material were superior to those of the normalized and tempered one.
The tendency that in the annealed material N_pc was smaller than or equal to N_cp in air can be fully attributed to the environmental effect, because the present vacuum data of both materials show that N_cp tends to be smaller than N_pc independent of the value of the inelastic strain range.
The present vacuum condition (<2× 10^-6mmHg) was not a perfect one for obtaining the SRP life relation in vacuum for 21/4 Cr-1 Mo steel. This conclusion was derived from the result that all theΔε_cc-N_cc data in the present vacuum condition gave shorter lives than those predicted by Manson's equation Δε_cc=D_c^0.6N_cc^-0.6.
Based on these results and the facts reported in the literatures, a method to obtain the SRP life relation in perfect vacuum from both air data and imperfect vacuum data was proposed. The SRP life relation in perfect vacuum thus obtained was found to be useful to explain quantitatively the effects of air environment and heat treatment.