Abstract
The testing method for delayed fracture has not been standardized yet. The quantitative evaluation method is necessary to develop low alloy high strength steels with higher delayed fracture resistance. The sustained load tests under cathodic hydrogen charging condition have been investigated considering the severity of the actual environment. The delayed fracture and hydrogen absorption behavior have also been investigated especially in 0.5C-0.3Mn-lCr-0.7Mo0.03Nb-0.3V-low P-low S steels.
The maximum hydrogen permeation coefficient, 0.1μA/cm, has been determined at the lowest pH=3.5 realized in the local environment such as in a crevice. The sustained load tests using notched round bar specimens have been carried out under cathodic hydrogen charging condition corresponding to 0.1μA/cm. The results show that this steel has the enough resistance to delayed fracture as 1.3GPa grade-high strength bolts. Although the apparent diffusible hydrogen content is much higher than 1.1GPa grade-JIS SCM440 steel, this steel absorbs the diffusible hydrogen evolved at more than 200°C in thermal analysis. It has been clarified that this diffusible hydrogen evolved at elevated temperatures has no relation with hydrogen embrittlement. Therefore, the susceptibility for embrittlement would be lower in this steel because of the uniform carbides dispersion and the decrease in the internal strain due to the high temperature tempering.
