Stress Corrosion Cracking Of 316LN Austenitic Stainless Steel Weld Joints

Austenitic stainless steel type 316LN is selected as the primary piping material of AP1000. Its mechanical, corrosion properties have been studied intensively. However, quantitative evaluation of stress corrosion cracking (SCC) of the 316LN base materials and weld joints are relatively less, due to the difficulty in obtaining precise results in experiment. The purpose of this research work focuses on the quantitative study to stress corrosion cracking and intergranular corrosion behaviors of 316LN base material and weld joints.As received material was cold rolled to different rolling reduction and followed by solution annealing heat treatment, and weld joints were prepared using gas tungsten arc welding (TIG) and gas metal arc welding (MIG). Compact tension specimens were prepared from the base metal and weld joints for constant load tensile test in boiling saturated MgCl2 solution. Crack growth rate (CGR) was measured by direct current potential drop method. Susceptibility to intergranular corrosion was evaluated by electrochemical potential reactivation (EPR) method. Experimental results of base metal corrosion tests show that solution heat treatment is effective in lowering CGR for cold worked and heat treated 316LN base material specimens, and under the same solution heat treatment process, CGR increases with the increase of cold rolling reduction. EPR test results show that cold rolled and heat treated specimens are more susceptible to intergranular corrosion, and the higher cold reduction the more sensitive to inergranular corrosion. It is doubt that the solution heat treatment cooling rate was low.Study on TIG and MIG weld joints shows that the SCC CGR of MIG specimen is lower than TIG specimens. SCC tests of MIG specimens show that 80%Ar+20%CO2 cover gas (1107 process) was effective in lowering SCC CGR than 100%CO2 gas (1108 process). In respect to intergranular corrosion, TIG weld specimens are more sensitive than MIG weld specimen, while 1108 MIG welding process is more sensitive to intergranular corrosion than 1107 process. For the same weld joint, weld metal has the highest CGR, while CGR of fusion zone specimen lies in the middle and that of heat affected zone (HAZ) is the lowest. However, with respect to intergranular corrosion, weld joint specimen shows the lowest susceptibility, fusion zone still lies in the middle and HAZ is most sensitive to intergranular corrosion. The experimental results provide references for manufacturing and field assembling of primary loop piping of Chinese third-generation nuclear power plant.