Hydrogen embrittlement resistance in advanced high strength: vanadium bearing and TRIP-assisted steels

Abstract

Advanced high strength steels undergo a series of microstructural transitions associated with damage, these lead to failure especially when hydrogen enters the lattice and gets trapped in defects. It is desired to design advanced steels highly resistant to hydrogen embrittlement (HE), while preserving a balance between their superior strength and ductility. For this purpose, a new 100Cr6 bearing steel is proposed with 0.3 wt.% vanadium, compared to a previous grade containing 0.5 wt.% vanadium, for which a new heat treatment is proposed. Mechanical testing was carried out on hydrogen-free and hydrogen-charged specimens to evaluate the potency of V4C3 traps to reduce the HE susceptibility in such steels. The nature of crack propagation and HE mechanisms in 100Cr6, 100Cr6+0.3V and 100Cr6+0.5V were assessed using hydro-hardness technique, which provides direct observation of hydrogen diffusion to cracks. The results showed that pre-charged 100Cr6+0.5V are anomalously weaker compared to 100Cr6+0.3V. Slow strain rate tests (SSRT) on notched round bar specimens have been carried out at a constant tensile crosshead speed of 0.005 mm min^-1. Failure was more pronounced in vanadium-free specimens as V4C3 limited the transition from ductile to brittle fracture mode in 100Cr6+0.5V steel. Rolling contact fatigue (RCF) test was also carried out to study the propagation of cracks under rolling contact fatigue conditions. It was concluded that 100Cr6+0.5V steel has longer bearing life and higher resistance to hydrogen embrittlement than 100Cr6. However, microstructural analysis confirmed the presence of non-metallic inclusions (NMIs) in 100Cr6+0.5V steel and the disadvantageous effects on its mechanical properties were recorded. A crucial consequence of plastic deformation during RCF is the transformation-induced hydrogen desorption which occurs when retained austenite transforms into martensite. This mechanism can be investigated through studying the HE susceptibility of transformation-induced plasticity (TRIP) steels. An intensive study was carried out to optimise the TRIP steel heat treatment aiming to achieve an ultimate tensile strength (UTC) of 1303 MPa. The microstructure showed lower bainite structure consisting of ferrite plates, film-like and blocky retained austenite. The most promising microstructure with respect to the expected UTC was obtained after bainitic isothermal tempering at 302 Degree Celsius for 24 to 30 minutes. Thermal desorption analysis was used to study desorption of hydrogen from TRIP steel. The high activation energy of diffusion indicates that the hydrogen is trapped in retained austenite which is considered as irreversible trap. However, there was no evidence demonstrating the advantage of retained austenite on preventing embrittlement after saturation with hydrogen. As a result, it can be inferred that a finer and lower fraction of retained austenite can reduce the mass of trapped hydrogen minimising the susceptibility to HE in TRIP-assisted steels.