1. 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

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

3. 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.)

4. 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%)

5. 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).

6. Microstructure characterization

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

8. X-ray analysis
Co K source

9. 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.

10. Fractography
without
H charging
after
H charging
24 hours

11. Microstructure – quantitative characteristics of the cracks

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

13. What is hidden behind?
P/P

15. Hydrogen diffusion coefficients

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

17. 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.

18. 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.

19. Thank you for your attention!