Influence of microstructure on the hydrogen diffusion behavior in dual-phase steels: an electrochemical permeation study

This work investigates the impact of various microstructural features obtained by heat treatment on the hydrogen diffusion and trapping behavior in dual-phase steel (DP-980). The heat treatment was carried out in the inter-critical (IC) region at two different temperatures followed by water quenching (WQ) (i.e., IC-650 °C-WQ, IC-800 °C-WQ). The microstructural characteristics were examined using a scanning electron microscope equipped with electron backscattered diffraction (SEM-EBSD). The electrochemical hydrogen permeation technique was used to characterize the hydrogen diffusion behavior. Heat treatment led to variation in martensite- and ferrite-phase volume fraction, distribution, morphology, and local misorientation, geometrically necessary dislocations (GNDs) in the microstructure. EBSD characterization revealed that the overall GNDs density values increase from 1.61 × 1014 to 2.99 × 1014 m−2 with an increase in martensite fraction. The permeation results revealed that the highest apparent diffusion coefficient (Dapp) value of 2.7 ± 0.3 × 10–7 cm2 s−1, the lowest apparent solubility (Capp) 4.4 ± 0.2 × 10–4 mol H cm−3, and trapping sites density of 2.2 ± 0.3 × 1021 cm−3 in IC-650 °C-WQ are compared to other studied microstructures. It could be attributed to the microstructure with lower GNDs and lower martensite volume fraction (38%) with the semi-continuous distribution in IC-650 °C-WQ, whereas in IC-800 °C-WQ, the microstructure consists of martensite with (66%) volume fraction having lath morphology. In the decay transient, the IC-650 °C-WQ specimen shows a slower rate of decay with a lower lattice diffusion coefficient (DL) value of 2.1 ± 0.01 × 10–6 cm2 s−1, higher diffusible hydrogen concentration (Cdiff) value of 8.8 ± 0.09 ppm, and higher reversible trapping sites among the studied microstructures.