Hydrogen-induced Delayed Fracture Behavior Of Several High Strength Fastener Steels

Delayed fracture is one of the major challenges for the development of higher strength level steels for fasteners, mainly for the purposes of higher performance, weight reduction and cost saving. In this paper, a series of slow strain rate tensile (SSRT) tests, constant loading tensile(CLT) tests and thermal desorption spectrometry(TDS) analysis and SEM were conducted to investigate their delayed fracture behaviors of steels 30CrMnSiA and 30CrMnSiNi2A for fasteners, as well as 42CrMoVNb steel with different nickel content. Emphasis was given to the effects of microstructure transformation, alloying elements and hydrogen traps on delayed fracture behavior of high stress steels.The result of 30CrMnSiA and 30CrMnSiNi2A steels show that the delayed fracture resistance of the tested steel increases with increasing tempering temperature and comparatively better delayed fracture properties could be obtained when tempered at 510℃.The delayed fracture properties of 30CrMnSiNi2A steel was better than 30CrMnSiA steel at the same strength especially for high strength level. The fracture characteristics of specimens without hydrogen charging was quasi-cleavage failure with a small quantity of dimple rupture, while after hydrogen charging it changed to intergranular failure when tempered at 250℃,350℃ and 400℃, the specimens tempered at 510℃ remains unchangedThe result of 42CrMoVNb steel show that the delayed fracture resistance of the tested steel tempered at 400～600℃also increase with increasing tempering temperature. The fracture characteristics of specimens without hydrogen charging changed from intergranular failure to quasi-cleavage failure as the tempering temperature increaseing. Hydrogen evolution curves of hydrogen charged specimens show that the absorbed hydrogen content increases slowly with increasing tempering temperature up to 500℃; when the tempering temperature exceeds 500 ℃, the absorbed hydrogen content increases sharply and reaches its peak at 600℃; thereafter the absorbed hydrogen content declined sharply as the tempering temperature increased. Further phase analysis show that fine dispersed (V,X)C carbide precipitate at 600℃, which could be served as strong hydrogen trap with its hydrogen trap activation energy Ea of about 28.74 kJ/molThe results of 42CrMoVNb steels with different nickel content (0～1.0%) show that the addition of nickel has little effects on both strength and toughness, whereas it has noticeable effect on hydrogen induced delayed fracture resistance. That is to say, the delayed fracture resistance can be increased approximately 8% for CLT test, or 15 % for SSRT test, with the addition of 0.5% Ni, and the delayed fracture resistance remains little increase with further nickel addition.