重炭酸塩環境中における炭素鋼の応力腐食割れに及ぼす熱処理の影響

抄録

Effects of annealing conditions on stress corrosion cracking (SCC) of SM 400B carbon steel has been investigated in 1M sodium bicarbonate solution at 343K. A potentiostatic slow strain rate test equipped with a charge coupled device camera system was employed to evaluate SCC susceptibility from the viewpoint of the crack behavior. Fixing an annealing time of 3.6 ks, no SCC appeared on the specimens annealed below 973 K, but intergranular SCC occurred beyond 1073 K. In the case that the SCC occurred, an increase in annealing temperature led to a decrease in crack initiation time and an increase in crack propagation rate. Fixing an annealing temperature of 1173 K, it is found that the SCC occurred in the annealing time range from 0.6 to 86.4 ks and that the fracture strain showed maximum at 3.6 ks. Analyzing the crack behavior, crack initiation time and crack propagation rate were found to be maximum and minimum at 3.6 ks, respectively. Thus, it was concluded that the maximum fracture strain at 3.6 ks resulted from minimum SCC susceptibility evaluated by initiation as well as propagation of the cracks. On the other hand, the observation of microstructure of the specimen revealed that the specimen surface suffered from decarburization during the annealing treatment beyond 1073 K, and the SCC always occurred on the specimen showing the decarburized structure.