Corrosion Fatigue of AISI Type 316LN Stainless Steel and its Weld Metal

This paper describes the results obtained on the corrosion fatigue behavior of Type 316LN (UNS S31653) stainless steel base metal in boiling acidified sodium chloride (NaCl) solution, and of Type 316LN stainless steel base metal and Type 316N stainless steel weld metal obtained in boiling 45% magnesium chloride (MgCl₂) solution. Type 316LN stainless steel base metal was tested using the fracture mechanics approach under different loading and environmental conditions, while smooth specimens were used to compare the base and weld metals. In the studies on Type 316LN stainless steel base metal carried out at a frequency of 0.1 Hz, with different stress ratios (R-ratios) adjusted by varying both the maximum and minimum stress intensity factors, K_max and K_min, respectively, crack growth rate per cycle (da/dN) increased slightly with increasing R-ratio. At both R-ratios, a significant rise in da/dN was observed at values of ∆K (= K_max – K_min) greater than K_ISCC (= 27.5 MPa.m^0.5) of the material. Contrary to those findings, tests performed at a frequency of 0.1 Hz, with different R-ratios performed by keeping K_max constant and varying K_min, da/dN decreased with increasing R-ratio. Only a marginal increase in da/dN was observed with increasing temperature. In some of the crack growth tests at a frequency of 0.1 Hz, open-circuit potential (OCP) and acoustic emission counts and energy were monitored with respect to time. The results indicated a sudden increase in these parameters at about 7,000 s, and this was related to crack initiation. In the studies on base and weld metals at a frequency of 0.1 Hz and R-ratio of 0.25, Type 316N stainless steel weld metal possessed better resistance to corrosion fatigue than Type 316LN stainless steel base metal in boiling 45% MgCl₂ solution. No fatigue limit was observed for either the weld or base metals.