Susceptibility To Hydrogen Embrittlement Of The Steel For High-strength Drive Shaft

Delayed fracture has been a problem which is always puzzling the research and application of high strength steels.It is known that high strength martensitic steels are susceptible to hydrogen embrittlement, and the susceptibility is usually increasing with the increase of the strength level.Hydrogen transport plays a very important roal in the fracture process caused by hydrogen embrittlement.In this study, hydrogen transport in two quenched and tempered martensitic steels was investigated by means of TDS(thermal desorption spectrometry), and hydrogen embrittlement susceptibility of the steels was investigated by means of slow strain rate tensile tests, respectively.Results show that:(1) It resulted in some diffusible hydrogen transport into the investigate steel, and an additional hydrogen desorption peak at 170℃, after the electrochemical method hydrogen incharged. Hydrogen in the steel with a high Cr low and A1 content transports obviously more slowly than in the steel with a low Cr and minor Al content. The diffusion coefficient of hydrogen in the D1 steel was determined to be 1.52×10-7cm2/s. while that in the D2 steel was 5.3×10-8cm2/s, from the variation of the hydrogen content in the hydrogen charged specimen with exposing time at ambient temperature. There are a large number of fine carbides in the D2 steel, which can prevent hydrogen from diffusing and reduce the hydrogen diffusion coefficient, and they may also act as hydrogen trap sites and increase the hydrogen content charged. Thus, the hydrogen embrittlement susceptibility of the D2 steel is lower than that of the D1 steel.(2) Both 25CrNi2MoVNb and 18Cr2Ni4W steel have a martensitic micro structure after quenching and tempering, their tensile-strength above 1500 MPa and 1300MPa, respectively. Compared with the 18Cr2Ni4W steel, the 25CrNi2MoVNb steel has a higher strength level of over 1500 MPa due to its higher carbon content and finer grain size, and it also has higher susceptibility to hydrogen embrittlement. It resulted in some diffusible hydrogen transport into the investigate steels, and an additional hydrogen desorption peak at 180℃,after the electrochemical method hydrogen incharged.. From the variation of the hydrogen content in the hydrogen-charged specimens with exposing time at ambient temperature, it has been found that hydrogen diffusion coefficients in the 25CrNi2MoVNb and 18Cr2Ni4W steels are 7.87×10-7cm2/s and 3.99x10-7cm2/s, respectively. Hydrogen can transport more easily in the 25CrNi2MoVNb steel, indicating that the mechanical properties of the 25CrNi2MoVNb steel may decrease more evidently when charged with a similar diffusible hydrogen content.(3)The diffusible hrdrogen content in the 25CrNi2MoVNb steel decreases with increasing austenitizing temperature, it's lowest at 1050℃, then increases with increasing austenitizing temperature. It resulted in some diffusible hydrogen transport into the investigate steel, and an additional hydrogen desorption peak at 170℃, after the electrochemical method hydrogen incharged. The diffusion coefficient of hydrogen in the steel with different austenitizing temperature of 880,1000,1100 and 1200℃was determined to be 6.25×10-7 cm2/s,6.12×10-7 cm2/s,3.69×10-7 cm2/s and 4.88×10-7cm2/s. respectively.