Effects of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at different temperatures

Abstract

The aim of this paper is to study the effect of hydrogen on the fracture toughness of 42CrMo4 steel quenched and tempered at 500, 550, 600, 650 and 700ºC. The influence of hydrogen was assessed by means of fracture toughness tests carried out on compact CT specimens, pre-charged with gaseous hydrogen in a pressurized reactor at 19.5MPa and 450ºC for 21h. Thermal desorption analysis (TDA) and finite element simulations (FEM) were employed to study the hydrogen diffusivity and solubility of the different steel grades. Additionally, X-ray diffraction (XRD) was used to determine the dislocation densities present in these steel microstructures and scanning electron microscopy (SEM) to analyse them and to identify the fracture micromechanisms that took place during the fracture toughness tests.According to the obtained results, dislocation density, hydrogen solubility and residual hydrogen strongly trapped in the steel microstructure were seen to decrease with increasing tempering temperature, following the apparent hydrogen diffusion coefficient the opposite trend. It was also observed that hydrogen embrittlement was much greater in the steel grades tempered at the lowest temperatures (with higher yield strength), and in tests performed at the lowest displacement rates. Moreover, a change in the fracture micromechanism was detected, from ductile (microvoids coalescence, MVC) in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) in the case of the hydrogen pre-charged steels with relatively low yield strengths under low displacement rates and, finally, to a fully brittle behaviour (mostly intergranular fracture, IG) in the case of steels with the highest strengths, tested at low displacement rates.It was finally demonstrated that only diffusible hydrogen, hydrogen atoms able to move at room temperature, is responsible for hydrogen embrittlement.

Hydrogen embrittlement analysis in a CrMoV steel by means of sent specimens

“…These situations are very common in pipes, vessels and off-shore structures [2][3][4]. As these structures are exposed to high loads, they may be constructed with materials with high mechanical properties, but it is also well known that hydrogen embrittlement increases with material strength [5][6][7].…”

Evolution of magnetic properties during tempering

“…The material selected for this study is SS2244 (42CrMoS4), which is commonly used in industrial applications and has been well studied by researchers. As such, there are plenty of articles about the influence of different types of heat treatment on the microstructure and phase transformation, e.g., [15][16][17][18][19]. This allows for greater focus on the changes in magnetic properties during heat treatment and their correlation with the microstructure and phase change in the material in the present study.…”

The role of hydrogen diffusion, trapping and desorption in dual phase steels

“…According to the ASTM F1624 standard, the so-called threshold stress is ''the combined residual and applied stress above which time-delayed fracture will occur (finite life) or below which fracture will never occur (infinite life)''. In addition to the hydrogen content, the threshold is sensitive to the applied loading rate [24]. The slower the loading rate, the longer is the total test duration.…”