The Effect of Frequency and Strain Range on Environmental Fatigue Cracking in 20%Cr/25%Ni/Nb Steel at 1025 K (750°C)

Abstract

High strain fatigue endurances of a 20%Cr/25%Ni/Nb stabilized stainless steel at 1025 K (750° C) in CO₂, air, and vacuum at strain rates 10⁻³–10⁻⁵s⁻¹ are presented. In vacuum tests, bulk density decreases, cavitation and internal cracking, and general structural changes were observed only at high plastic strain ranges (∼0·08). At low strain ranges (<0·01) the variation of endurance with atmosphere was complex, depending on strain rate and strain range. Because endurance testing does not reveal the proportion of life spent in crack initiation and propagation, limited interpretation only of the data is possible. Examination of oxide in cracks and of the adjacent metal, however, enabled some estimate of crack-propagation rates to be made.