Analysis of hydrogen trapping behaviour in plastically deformed quenching and partitioning steel in relation to microstructure evolution by phase transformation

Abstract

The relationship between plastic deformation and hydrogen transport behaviour in the quenching and partitioning (Q&P) steel is studied. The evolution of microstructure after different levels of plastic deformation is characterized using the electron backscatter diffraction (EBSD) and X-ray diffraction technique. Also, the hydrogen diffusion and trapping of the plastically deformed Q&P steel are investigated using the hydrogen permeation electrochemical method and thermal desorption spectroscopy (TDS) on pre-strained specimens up to 14% engineering strain. The major findings through the experimental approach can be summarized as follows: (1) retained austenite fraction in Q&P steel decreased after plastic deformation; (2) the geometrically necessary dislocation (GND) density measured by KAM analysis increased; (3) hydrogen diffusivity decreased, while solubility increased as the plastic deformation increased. The increase in hydrogen trapping in the Q&P steel with plastic deformation can be attributed to the enhanced hydrogen trapping environment by increased dislocation density in ferrite and martensite transformed from metastable austenitic phase. The TDS analysis could support the hydrogen-trapping characteristics of Q&P steel, in which two major peaks were measured in different temperature ranges. Moreover, the EBSD observation and scanning transmission electron microscope (STEM) using transmission Kikuchi diffraction (TKD) analysis provided good correlation with the result of TDS analysis.