Effect of Environment on Fatigue Crack Growth Behavior of Type 316 LN Stainless Steel and its Weldments

Corrosion fatigue crack growth (CFCG) behavior of type 316LN stainless steel (SS) and its weldments are studied in acidified 5M NaCl + 0.15M Na2SO4 + 2.5 ml/l HCl medium under open circuit condition at a stress ratio (R) of 0.5 (R = ΔKmin/ΔKmax) and at a frequency (η) of 0.1 Hz for different stress intensity factor range (ΔK). At a value of ΔK corresponding to stress intensity factor for stress corrosion cracking (KISCC = 27.5 MPa.m0.5), the Paris region deviates from linearity as the crack growth/cycle increases dramatically for the base and weld metal indicating Type B behavior of an environmentally assisted cracking process. CFCG rate is compared with fatigue crack growth (FCG) rate at room temperature (RT) and at 450°C (HT). Type 316N weld metal (WM) showed lower threshold (ΔKth) values and higher crack growth rates (CGRs) than type 316LN SS. Cracking initiated in the transgranular (TG) mode for both the metallurgical conditions. Besides TG cracking of the austenite, dissolution of delta-ferrite (δ-Fe) was observed in the weld metal.