Prior-Austenite Grain Size Effect on Crack Growth and Fractal Character of Stress Corrosion Cracking in SNCM439 Steel

Abstract

Stress corrosion cracking tests were conducted in 3.5%NaCl solution by using SNCM439 steel specimens which have three different prior-austenite grain sizes, d=12, 78 and 172μm. The fracture surfaces obtained were analyzed from a viewpoint of fractal geometry.
The results obtained are summarized as follows:
(1) The crack growth rate, da/dt, increased with increasing the stress intensity factor, K, and with decreasing prior-austenite grain size, d.
(2) The grain boundary fracture area, φ, decreased with increasing K. At a given K level, φ increased with increasing d.
(3) The profiles of stress corrosion cracks had the character of fractal. The relevant fractal dimension, D, was about 1.05 in the region c where the intergranular cracking was predominant, while it was about 1.10 in the region b where the mixed feature of intergranular and transgranular cracking appeared.
(4) The fractal analysis explained the dependence of da/dt on d.