Effect of tempering temperature and hydrogen on deformation during tensile tests in a V-added high strength low alloy steel

Tensile behavior and hydrogen embrittlement have been investigated in a V-added high strength low alloy steel tempered at three different temperatures through slow strain-rate tensile tests and simultaneously hydrogen charging at different current densities. In the meantime, the in-situ strain field was quantitatively supervised using digital image correlation (DIC) technique. With increasing hydrogen content, the strain to fracture and work hardening exponent are greatly reduced regardless of the tempering temperature, which can indeed affect the susceptibility to hydrogen-induced embrittlement (HIE). However, in the absence of hydrogen, the steel per se displays decrement in strength, increment in ductility and slight increase in work hardening exponent with raising tempering temperature. These changes were elaborated by microstructure and a model of interaction between dislocation and hydrogen. With increasing tempering temperature, the tangled dislocation gets incompact. The hydrogen can pin the separated dislocation, but facilitate movement of the multiple dislocation. Hence the assistant force stemmed from hydrogen always increases with continuous deformation, reduces strain hardening ability, accelerates local deformation which can result in the very closely spaced dislocations being absorbed by grain boundary, then gives rise to the steel premature fracture.