Tensile specimens of 10B22 (22MnB5) sheet steels were austenitized, quenched to martensite, and tempered at temperatures between 150 and 520°C for various times. The heat treated specimens were charged with 1.7 ppm hydrogen and immediately tested. Fracture surfaces were examined by field emission scanning electron microscopy. As-quenched martensitic specimens exhibited the most severe embrittlement and failed by stress-controlled cleavage fracture at low stresses. The initiation of hydrogen-induced fracture in specimens tempered between 150 and 350°C was consistent with glide plane decohesion, and coarse inclusion particles served as sources of hydrogen for circular areas of hydrogen-induced cleavage. Specimens tempered at 460 and 520°C showed little sensitivity to hydrogen embrittlement. The progression of decreased sensitivity to hydrogen-induced fracture with increasing tempering temperature correlates with the reduction in dislocations, the principal hydrogen traps, and the formation of cementite particles, considered to be ineffectual traps, with increased tempering. Very small amounts of intergranular fracture were observed, only in as-quenched specimens, confirming that boron has little effect on hydrogen embrittlement of hardened low-carbon steels.

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Hydrogen Embrittlement of Hardened Low-carbon Sheet Steel

This work addresses the influence of microstructure on the hydrogen embrittlement of advanced high-strength steels. With sufficient hydrogen, fracture initiates at the ultimate tensile stress, but initiation is at the specimen surface. Fracture initiation is typically intergranular, transgranular or quasi-cleavage. These micro-fracture modes suggest that planar defects are important, such as prior austenite grain boundaries and lath and block boundaries. Fracture propagation is by shear fracture indicating a major influence of hydrogen-dislocation interactions. These hydrogen assisted fractures occur at a fast rate, probably approaching the speed of sound, that is at fracture velocities greater than 1000 m/s.

https://espace.library.uq.edu.au/view/UQ:572b73f/UQ572b73f_OA.pdf

The influence of microstructure on the hydrogen embrittlement susceptibility of martensitic advanced high strength steels

Role of copper and aluminum additions on the hydrogen embrittlement susceptibility of austenitic Fe–Mn–C TWIP steels

This study aims at the experimental analysis of the transformation induced plasticity (TRIP) phenomenon. Experiments are conducted in which martensite is allowed to grow under the influence of a series of externally applied stresses. The magnitude of the applied stresses is less than 67 % of the yield strength of austenite σγ(Ts). Since there is no obvious difference between the transformation plasticity under tension and the compression for the lower applied stresses, only compressive stresses are applied. The results confirm that the transformation plasticity is proportional to the applied stress if the latter does not exceed 67% of σγ(Ts). The TRIP-strain, the kinetics, and their dependence on the applied stresses are studied. The comparison between calculated results and experimental results shows that the model accurately describes the phenomenon.

Effects of Applied Stresses on Martensite Transformation in AISI4340 Steel

The delayed fracture behavior of medium-carbon high strength spring steel containing different amounts of boron (0.000 5%, 0.001 6%) was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron content from 0.000 5 % to 0.001 6 %. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350 °C, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initiation area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3 (C, B) phase.

Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel

Hydrogen-charged miniaturized Charpy-type specimens were subjected to three-point bending tests to investigate an influence of phosphorus and boron on the hydrogen embrittlement (HE) susceptibility of high strength low alloy (HSLA) steels for bolts. The tests were carried out under wide variety of deformation rate to examine an effect of deformation rate on the susceptibility also. The experimental results revealed that the HE susceptibility increased with decreasing deformation rate. This dependence of susceptibility on deformation rate seemed to be associated with velocity relations between the deformation rate and the diffusion rate of hydrogen. The susceptibility was more pronounced by the addition of phosphorus, even though the phosphorus segregation was not sufficient by itself to cause temper embrittlement. On the other hand, boron had almost no influence by itself on the susceptibility. A cyclic voltammetry and thermal desorption spectroscopic analysis were conducted for understanding the change in HE susceptibility with phosphorus and/or boron from the point of view of hydrogen adsorption/absorption characteristics, respectively. However, it was hardly observed that those elements influenced the hydrogen content and the trapping site of hydrogen in the steel under the present hydrogen charging condition. Consequently, it was considered that the increase in HE susceptibility with phosphorus mainly resulted from the reduction in grain boundary strength due to phosphorous, rather than the variation in hydrogen adsorption/absorption characteristics.

Shin-ichi Komazazki

Papers

Influence of phosphorus and boron on hydrogen embrittlement susceptibility of high strength low alloy steel