Effect of heat input on the stress corrosion cracking behavior of weld metal of nitrogen-added AISI type 316 stainless steel

Round tensile specimens of AISI Type 316N (UNS S31651) stainless steel weld metal, made by manual metal arc welding (MMAM process using heat inputs ranging from 3.07 kJ/cm to 7.41 kJ/cm, were subjected to stress corrosion cracking (SCC) tests in boiling acidified sodium chloride (NaCl) solution (initial stress level = 250 MPa) and in boiling 45% magnesium chloride (MgCl2) solution (initial stress level = 120 MPa), using the constant load technique. In boiling acidified NaCl tests, the open-circuit potential (vs saturated calomel electrode [SCE]) was monitored with respect to time to determine the critical cracking potential (CCP) at the time-of-failure. In boiling acidified NaCl solution, the SCC time-to-failure (t(f)) increased while the CCP decreased with increasing heat input. In boiling 45% MgCl2 solution, no significant change in tf was observed. The t(f) in acidified NaCl solution was far greater than that in 45% MgCl2 solution. Failure occurred by a combination of transgranular stress corrosion cracking (TGSCC) of austenite and dissolution of delta-ferrite in boiling NaCl solution, and by a combination of TGSCC of austenite and cracking of the ferrite/ austenite interface in MgCl2 solution. Optical micrographic studies indicated a higher degree of discontinuity, coarser delta-ferrite particles, and increased interdendritic spacing with increase in heat input. Microhardness measurements indicated softening of the austenite matrix with increasing heat input due to thermal recovery caused by reheating of the previous passes. The improved SCC resistance on increasing heat input was attributed to thermal recovery in the austenite and increased discontinuity of the delta-ferrite network.