The Effect of Second Tempering on Hydrogen Embrittlement of Ultra-High-Strength Steel
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INTRODUCTION
THE demand for ultra-high strength construction machinery martensitic steels with excellent strength and toughness is increasing.[1,2] For the ultra-high strength steels, the hydrogen embrittlement (HE) is a potential problem. Hydrogen is unavoidable in many environments and manufacturing processes, and drastic reductions in ductility and unpredictable catastrophic failure may occur in the presence of hydrogen.[3] Meantime, the susceptibility of HE increases as the strength increases.[4–7] Consequently, HE is an essential issue that restrains the use of ultra-high-strength construction machinery martensitic steels in applications.[8,9] Consequently, it is important to make efforts to improve HE resistance for commercial ultra-high strength construction machinery using martensitic steel. In recent decades, numerous studies have widely assessed the effect of hydrogen traps on HE resistance in martensitic steels. Regarding hydrogen traps with high binding energy, Nagao et al.[10] demonstrated that the addition of (Ti/Mo) enhanced HE resistance due to the
ZHENG WANG, BO KAN, JUANPING XU and JINXU LI are with the Corrosion and Protection Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected]. Manuscript submitted November 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(Ti/Mo) C precipitates. The addition of V and Nb can also effectively enhance HE resistance because of the (V/ Nb) C precipitates.[7,11] In addition to hydrogen traps with high binding energy, numerous studies have assessed hydrogen traps with low binding energy through analysis of the grain boundaries, dislocations, vacancies and interfaces between the cementite and matrix.[12,13] Regarding grain boundaries, Fuchigami et al.[14] demonstrated that the refinement of the grain size can improve HE resistance because of the increased grain boundaries. In addition, a fine-grain structure revealed mainly quasi-cleavage that differed from the intergranular fracture of the coarse-grained structure.[15] Regarding dislocations, hydrogen diffusion is restrained because of hydrogen trapping by the dislocations.[16–18] However, the concept of hydrogen transportation by mobile dislocations is an old theory called hydrogen-enhanced localized plasticity (HELP).[19,20] Vacancies are generated during plastic deformation, which is called the hydrogen-enhanced strain-induced vacancy (HESIV) mechanism, and the diffusible hydrogen increases as the the vacancies increase.[21] Finally, regarding cementite, uniform distribution of fine cementite showed higher HE resistance under plastic deformation than large needle-shaped cementite.[2,13,22,23] The reason is attributed to the transformation from low to high binding energy by forming strained interfaces between the cementite and matrix. In contrast, Wei et al.[24,25] reported that cementite plays a negligible role in the
hydrogen trapping capability, and the hydrogen content decreases continuously as the temperin
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