The interaction of fatigue and creep in wledments of AISI 316L austenitic steel after helium implantation

Abstract

Reverse bending fatigue tests with hold times have been performed on helium implanted (500 appm He) and unimplanted electron beam weldments in type 316L steel at 723 and 873 K. Parameters for the fatigue tests were: fatigue frequency 0.01 Hz ≤ vF ≤3 Hz; total strain range 0.6% ≤ δϵ≤ 2.4%; strain rate 4.8 × 10−4 s−1 ≤ ϵ ≤ 4.8 × 10−2 s−1; hold time at maximum strain 0 s ≤ tH ≤ 100 s. In general in all tests the number of cycles to failure, Nf, is reduced with increasing hold time and decreasing frequency. At 723 K the influence of helium upon the fatigue life is negligible whereas at 873 K Nf is reduced by a factor of 3–5. At this temperature Nf decreases with decreasing frequency for zero or short hold times whereas for long hold times the influence of frequency vanishes. Furthermore, the fracture mode changes from transgranular at high frequencies and small hold times to nearly complete intergranular at low frequencies and long hold times. Optical and scanning electron microscopy indicate that the “parting” in the middle of the weldment is the weakest region.