Permeação de hidrogênio em aço ultrabaixo carbono: influências da fração recristalizada e do acabamento superficial

Abstract

Hydrogen is a small size atom that can rapidly spread through the lattice of metal structures. This permeation may affect the mechanical properties of components from several industrial areas, especially when these parts are in contact with sources of hydrogen such as H2S. The wear effect is referred to as hydrogen embrittlement, which makes the material more susceptible to the formation of fragile fractures. This work was developed with the aim of understanding the mechanisms for the permeation of the specie in ultra-low carbon steel Interstitial Free, in which the microstructural simplicity reduces the parameters involved in the phenomenon and facilitates the comprehension about more complex alloys. The focus was to investigate the influences of recrystaliised fraction and surface roughness on the permeation curves behavior by using electrochemical polarization and Devanathan-Stachurski cell. Diffusion coefficients for the four determined steel conditions (full hard, partially annealed, IF, surface finished IF) were compared to each other, considering the theoretical values from Fick's Law and experimental values. Among those different conditions, it was slightly more difficult for full hard steel to demonstrate hydrogen permeation due to dislocations, on the other hand, IF steel presented an easy permeation after releasing internal stresses by means of heat treatment. The decrease in surface roughness impaired the specie input mechanism, resulting in a more restricted permeation. The behavior of the permeation curves were related to the characteristics of each sample and revealed an association between defects and metal hardness with the hydrogen traps and failures, as cracks and blisters. Effects on the steel were analyzed by metallographic tests for identification of microstructural modifications.