Effect of air environment on creep-fatigue properties of 2 1/4 Cr-1 Mo steel

The strain range partitioning (SRP) life relation was experimentally determined at 550 deg C both in air and in vacuum (< 2 x 10-6 mmHg) for the annealed and the normalized and tempered 2 1/4 Cr-1 Mo steels, and the effects of air environment and heat treatment on the SRP life relation of 2 1/4 Cr-1 Mo steel were examined. All of Δεij - Nij properties were found to be sensitive to the environment in the strain range lower than (Δεij)cr while not in the strain range higher than (Δεij)cr. The value of (Δεij)cr was larger in the annealed material than in the normalized and tempered one. The effects of heat treatment on Δεpp - Npp and Δεcc - Ncc properties were not so remarkable for the both materials, whereas Δεpc - Npc and Δεcp - Ncp properties of the annealed material were superior to those of the normalized and tempered one. The tendency that in the annealed material Npc was smaller than or equal to Ncp in air can be fully attributed to the environmental effect, because the present vacuum data of both materials show that Ncp tends to be smaller than Npc independent of the value of the inelastic strain range. The present vacuum condition (< 2 x 10-6 mmHg) was not a perfect one for obtaining the SRP life relation in vacuum for 2 1/4 Cr-1 Mo steel. This conclusion was derived from the result that all the Δεcc - Ncc data in the present vacuum condition gave shorter lives than those predicted by Manson's equation Δεcc = Dc0.6Ncc-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. (author)