Hydrogen diffusion characteristics in advanced high strength steels SOJKA Jaroslav, VODÁREK Vlastimil, VŠB – Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Ostrava, Czech Republic

Collaboration Silesian University of Technology, Faculty of Materials Engineering and Metallurgy, Katowice, Poland Ecole Centrale Paris, France

Introduction Automotive industry – advanced high strength steels (AHSS) – high toughness, high strength and good formability. AHSS – mostly multiphase steels TRIP steels very promising . Corrosion protection – zinc coating. Some operations – risk of hydrogen provoked degradation Research initiated by companies performing coatings (Electropoli France etc.)

TRIP steels C-Mn-Si 0.20% C; 1.5 % Mn; 1.5 % Si Si – problems during hot dip galvanizing; Modifications: C-Mn-Si-Al (part of Si is replaced by Al) – advantageous from the point of view of galvanizing but other problems appear. C-Mn-Si-Al-P (or C-Mn-Si-P) – phosphorus content between 0.05 and 0.10 %. Very important – microstructure – ferrite, bainite + retained austenite (10-15%)

Some parts of the research Sheets with thickness of 1.5 mm. Laboratory heats, industrial heats. 2 step annealing: – intercritical annealing (825°C/6 min./ rapid cooling + – annealing in the range of bainitic transformation (425°C/5 min./air cooling).

Microstructure characterization

Microstructure – TEM as-received 10 % deformation DF in 111

X-ray analysis Co K source

Hydrogen provoked degradation Various testing modes: hydrogen sulfide, without any external loading; hydrogen sulfide; loading in the region of elastic deformation; tensile test after previous electrolytival hydrogen charging; slow strain rate testing (tensile test with simultaneous hydrogen charging). Evaluation: Microscopy; fractography, mechanical properties.

Fractography without H charging after H charging 24 hours

Microstructure – quantitative characteristics of the cracks

Hydrogen diffusion characteristics – electrochemical permeation method Specimen – diam. 20 mm; thickness ~ 0.5 mm; Exit side – palladium coated; Argon bubbling in the output cell.

What is hidden behind? P/P

Hydrogen diffusion coefficients

Fitting experimental results / theoretical model – best for the second build-up transient first build-up transient second build-up transient

Sub-surface hydrogen concentration (ppm) As-received state 5 % tensile deformation 10 % tensile deformation 25.9 35.2 27.0 High sub-surface concentration of hydrogen: very important – equilibrium between the concentration of diffusible hydrogen and reversibly trapped hydrogen. High sub-surface concentration of hydrogen CH0 results in high hydrogen concentration around (in) reversible traps – critical concentration – crack initiation and growth.

Conclusions Methods of microstructure characterization, hydrogen provoked degradation evaluation and hydrogen diffusion characteristics analysis make possible a comprehensive study of hydrogen impact on many metallic materials.

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