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Hydrogen Embrittlement of the Low Alloy Cr and CrMo Steel

Hydrogen embrittlement (HE) of the low alloy Cr and CrMo steels quenched and tempered at 700^oC was tested at constant load to 80% of their measured yield strength in the 0.5 M H_2SO_4 solution with 10 mg/L As_2O_3 s promoter. The charging current density was 5 mAcm^-2. Scanning electron microscopy (SEM) showed transgranular fracture morphology in the presence of hydrogen. Energy dispersive X-ray spectrometry (EDXS) suggested transgranular cracking with flat fracture zones in which the inclusion was centrally located in both materials. CrMo steel had higher resistance to HE than Cr steel under identical charging conditions. This was due primarily to the beneficial effect of molybdenum. The most significant difference between the two investigated alloys was finer substructure and carbide size in the CrMo alloy, the result consistent with the increased tempering resistance due to its higher Mo content . X-ray diffraction analysis of the extracted carbides evidenced that CrMo steel, in addition to Fe_3C, contained fine Mo_2C carbides. Thus, it appeared that the finer carbide structure was the key microstructural parameter for improved hydrogen embrittlement of CrMo steel.

low alloyed Cr and CrMo steel; hydrogen embrittlement; microstructure; voltammetry; SEM; EDXS

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