Efeito de parâmetros microestruturais e da temperatura do meio na susceptibilidade à fragilização por hidrogênio de aços de alta resistência e baixa liga

Abstract

Hydrogen embrittlement (HE) is a phenomenon that affects many steels and other metal alloys, drastically reducing their fracture strength. Despite being studied for over a century. HE is a complex phenomenon that still needs a better understanding. Therefore, the present work aims to evaluate the influence of hardness, prior austenitic grain (PAG) size, dual-phase microstructure (DP) and environment temperature on the hydrogen embrittlement susceptibility (HES) of two high-strength low alloy (HSLA) steels: 4137-M and 4130-M. This evaluation was carried out using the Incremental Step- Loading (ISL) test, with the development of a methodology capable of identifying the fracture threshold load of low hardness steels was set as an additional objective. The results revealed that the developed methodology to identify the Pth of lower hardness steels than that recommended by the ASTM F1624 standard was valid. It was observed that the microstructure high angle boundaries (HAB) act a strong H traps, in such a way that the microstructure with high HAB density exhibited lower HES. This effect was evidenced by noticing a reduction in the HES and also in the diffusible hydrogen content in the microstructure with lower PAG size and the DP microstructure. The PAG boundary, which is a HAB, prevented part of the H from diffusing to the crack nucleation sites. For the DP microstructure, two main types of HAB were obtained, the PAG boundary and the martensite / ferrite interface, which resulted in a higher density of HAB, and, consequently, in the lower HES presented by the DP microstructure. Furthermore, the ferrite localized deformation in DP microstructure reduced the stress concentration and directed the H to ferrite, which is less susceptible than martensite. The hardness reduction decreased the steel HES, since the decrease in hardness is accompanied by lower density of H traps in the PAG boundary and greater plastic deformation capacity, factors that increase the HE resistance. Moreover, greater HES was noted at 24 °C and 4 °C, with no significant difference in HES between these two temperatures. However, at 54 °C a reduction in HES was observed. This behavior was attributed to the enhancement in H diffusivity caused by the increase in temperature, which reduced the capacity of dislocations to trap and transport hydrogen to the notch root. Finally, it was observed that H diffusion combined with the H transport by dislocations saturate the high-angle boundaries, inducing the crack to propagate through these boundaries. When the HAB were mainly the PAG boundaries, the fracture was intergranular. On the other hand, if the H-saturated HAB were the martensite packages and blocks or the martensite / ferrite interface, the fracture had a quasi-cleavage aspect.