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Effect of Microstructure on Strain Aging and Hydrogen Embrittlement Behavior of Bake Hardening Steels

The strain aging and hydrogen embrittlement behavior of two bake hardening (BH) steels with different microstructures were investigated in this study. The single-phase BH steel was composed of fully ferrite, while the dual-phase BH steel consisted of ferrite and 3 % martensite. The BH index and aging index (AI) of the two BH steels were measured to compare the effect of microstructure on strain aging behavior. As a small amount of martensite is present in the dual-phase BH steel, the BH index value of the dual-phase BH steel was higher than that of single-phase BH steel, and the AI showed a relatively low value. After the BH steels were pre-strained up to 20% and electrochemically pre-charged with hydrogen, on the other hand, a slow strain rate test (SSRT) was carried out to examine the effect of pre-strain on hydrogen embrittlement behavior. The silver decoration technique and thermal desorption analysis (TDA) were performed to understand the hydrogen embrittlement mechanism from the standpoint of hydrogen trapping. Regardless of pre-strain the dual-phase BH steel had lower hydrogen embrittlement resistance than the single-phase BH steel because martensite formed in the dual-phase BH steel acts as a reversible hydrogen trap site. The hydrogen embrittlement resistance of the two BH steels decreased with increasing amount of pre-strain which increases dislocation density. Based on these results, it can be said that the introduction of martensite and dislocation in BH steels have a negative impact on hydrogen embrittlement resistance.