Hydrogen Embrittlement Susceptibility Evaluation of Tempered Martensitic Steels Showing Different Fracture Surface Morphologies

Abstract

The effects of the crosshead speed, hydrogen content and temperature on fracture strength and fracture surface morphology were investigated using a tempered martensitic steel containing 1.67 mass % of Si (H-Si) and one containing 0.21 mass% of Si (L-Si). When L-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from quasi-cleavage (QC) to intergranular-like (IG-like) to intergranular (IG) at room temperature. In contrast, when H-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from QC to IG-like at room temperature. This transition in the fracture surface morphology can be explained by the magnitude relationship between intergranular and transgranular strengths under hydrogen charging. At a temperature of-196 °C, hydrogen did not lower the fracture strength nor did it change the fracture surface morphology. Hence, hydrogen embrittlement at room temperature was presumably caused by hydrogen accumulation and lattice defect formation during stress application as well as by hydrogen trapped before stress was applied. Fracture strength decreased and converged to a constant value (lower critical stress) with decreasing crosshead speed. The crosshead speed for obtaining lower critical stress decreased as the fracture surface changed from IG to IG-like to QC. Therefore, the crosshead speed for obtaining lower critical stress should not be treated as a constant but should be determined experimentally for each type of fracture surface.

Examples of Fractured Steel Parts during Actual Usage due to Hydrogen Embrittlement and Consideration Regarding the Behavior of Hydrogen at the Prior Austenite Grain Boundary

“…The diffusion of hydrogen from less dangerous sites to dangerous sites, such as the prior austenite grain boundary, especially with stress during holding, can be supported by other reports. Figure 8 illustrates the change in fracture strength and fracture mode as a function of cross-head speed in tensile tests, as reported by Ogawa et al 14) . With a reduction in cross-head speed, the duration of testing increases, indicating that diffusion of hydrogen from less dangerous sites to more dangerous sites, such as the prior austenite grain boundary, is enhanced.…”

“…Fig. 8 A schematic illustration showing the effect of crosshead speed in tensile test on fracture strength and fracture mode, after Ogawa et al 14) Fig. 9 The effect of holding time before baking after Zn plating in a 10% HCl solution on hydrogen embrittlement ratio, modified graphitization after data by Doi et al 15) Fig.…”

“…Fig.8A schematic illustration showing the effect of crosshead speed in tensile test on fracture strength and fracture mode, after Ogawaet al14) …”