The mechanism of stress-corrosion(SC) crack propagation of AISI 4340 steel in aqueous 3.5 wt.% NaCl solution at 30℃ has been investigated as a function of applied potential, stress state, the amount of dissolved oxygen, and tempering temperature over a range of applied potential from -140 to -1000 $m^V$_SHE. The continuous transition of the stress state from plane strain to plane stress was possible using 2 mm thick single edge-notched (SEN) specimens under constant load. The SC crack lengths were estimated by electrical potential method.
As the amount of cathodic polarization was increased, the SC crack propagation rate was increased, whereas anodic polarization yielded the opposite results. These polarization effects can be attributed to the lowered reduction rate of hydrogen ions during anodic polarization. Because of the continuous change in the stress state, the usual rapid increase of SC crack propagation rate in V-K curve at high K, next to the region II, did not result in most cases, SC crack propagated with the intergranular fracture mode in the most region and dimpled rupture occurred near the surface in form of shear lip, irrespective of applied potential and tempering temperature. It was also found that the vigorous bubbling of $N_2$ gas for 3h accelerated the SC crack propagation rate.
It was suggested from above experimental results that SC crack propagation is caused by hydrogen embrittlement. The effect of tempering temperature exhibited the decreasing order of resistance to SC cracking: 400℃, 200℃, and 300℃, and was discussed in terms of hydrogen embrittlement.