Analysis of the Interaction between Dislocation and Interstitial Carbon and Hydrogen in Press Hardening Steel

Abstract

Internal friction measurement were carried out on a press hardened steel (PHS) grade after thermal cycles simulating the continuous annealing, press hardening, and bake hardening processes. The peaks in the internal friction spectrum of the press hardened steel with a lath martensite microstructure were analyzed by comparison with previously published data. This analysis was supplemented by comparison of the spectrum with the internal friction spectra of the same steel with a ferrite-pearlite microstructure after deformation at room temperature, and after recrystallization annealing and quenching to obtain solute carbon in supersaturation. The relation between the peaks in the internal friction spectrum of press hardened steel with a lath martensite microstructure, and the -peak, Snoek peak and Snoek-Kê-Köster peak observed for ferritic steels, is discussed. An unstable low temperature shoulder peak was observed on the Snoek-Kê-Köster peak. It is argued that this peak is related to the interaction between dislocations and C atoms located out of the slip plane, rather than twins, as recently suggested. Press hardened steel is known for its high strength and this made it susceptible to hydrogen embrittlement especially in aluminized press hardened steel. Hydrogen embrittlement phenomena in PHS were observed by tensile test and bending test. The room temperature aging also conducted to observe diffusible hydrogen behavior. The hydrogen analysis were carried out by thermal desorption analysis (TDA) to obtain hydrogen desorption rate peak. This hydrogen desorption rate was measured to determine the hydrogen trap site in press hardened steel. Dislocation were observed as trap site in PHS especially long screw dislocation segment. It is argued that the interaction between dislocation, carbon atoms and hydrogen atoms were the source of hydrogen embrittlement of PHS grade.