The presented paper deals with hydrogen diffusion and hydrogen provoked degradation in the TRIP 800 Mn-Si-Al steel with increased aluminum content. The steel was studied after standard heat treatment including two step annealing – an intercritical annealing and an annealing in the range of bainitic transformation. A part of specimens was submitted to a plastic deformation in tension (5 and 10 %) in order to modify both structure and mechanical properties. Microstructure of the steel consisted predominantly of ferrite, bainite and retained austenite, the amount of which decreased with increasing extent of the plastic deformation. Hydrogen diffusion was studied by electrochemical permeation method. Hydrogen diffusion coefficients were calculated using the “time lag” method. The lowest values of hydrogen diffusion coefficients were always observed for the first build-up transient, while the highest values were recorded for the second build-up transient due to the less intense hydrogen trapping. As to the impact of deformation, the highest hydrogen diffusion coefficients were observed after 5 % tensile deformation. Subsurface hydrogen concentration reached maximal value approx. 14 mass ppm. Hydrogen embrittlement of the steel was studied using tensile tests after previous electrolytic charging in diluted sulfuric acid solution. Hydrogen embrittlement manifested itself mainly by a drop of elongation at fracture. As to the fracture morphology, hydrogen embrittlement resulted in a change of fracture mode from transgranular ductile fracture to transgranular cleavage fracture with an appearance of “fish eyes” initiated on aluminum rich non-metallic inclusions.Keywords: TRIP steel, electrochemical permeation method, hydrogen diffusion, hydrogen embrittlement
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