FRACTAL CHARACTERISTICS OF STRESS-CORROSION CRACKING IN SNCM-439 STEEL HAVING DIFFERENT PRIOR-AUSTENITE GRAIN SIZES

An approach by fractal geometry was examined for the SCC problem on a Ni-Cr-Mo steel. Stress corrosion tests were conducted in 3.5% NaCl sol ution by using SNCM439 steel specimens. These specimens had different prior-austenite grain sizes due to variations in heat treatment. After summarizing the results of analysis, we explain the profiles of stres s corrosion cracks having fractal characteristics. The relevant fracta l dimension D was about 1.05 for the region where the intergranular cr acking was predominant, while D was about 1.10 for the region having m ixed fracture of intergranular and transgranular cracking. The fractal analysis concluded that the crack growth rate da/dt depends on the gr ain size d, but not stress intensity factor K. The investigations of c rack growth phenomena were seen to be an important factor to explain t he mechanism.