2018 Volume 67 Issue 3 Pages 389-395
In this paper, delayed fracture strength of SUJ2 bearing steel in biodiesel fuel with water is investigated. In order to evaluate the delayed fracture strength in fuel environment, a newly constant load test procedure was established. When specimens were immersed in biodiesel fuel with water applying a constant load, fracture stress was measured at about 1400 MPa at the test time of 200 hours, which highlights a clear decrease compared to the tensile strength of 1893 MPa found in air. After the test, intergranular fracture was observed at the fracture origin, and corrosion traces were observed on the surface near the origin. Intergranular fracture and corrosion traces tended to increase with test time. From these observations, the cause of strength reduction can be considered as follows: (i) intergranular embrittlement by hydrogen absorbing, or (ii) stress concentration due to surface roughness. Influence of intergranular embrittlement on strength reduction was investigated by slow strain rate test (SSRT). On the other hand, influence of surface roughness on strength reduction was undertaken by tensile test using corroded specimen. According to experimental results, strength reduction in this condition was mainly induced by intergranular embrittlement due to hydrogen absorbing. Finally, the relationship between the square root of the intergranular area (√areag) and the diffusible hydrogen content as well as between the fracture stress and √areag were investigated. As a result, √areag increased with increasing the diffusible hydrogen content. Furthermore, the fracture stress decreased with increasing √areag. Therefore, the strength reduction due to intergranular embrittlement in biodiesel fuel with water should be the consequence of the hydrogen diffusion in grain boundaries, which makes grain boundaries brittle.
