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Highlight Lecture

Hydrogen embrittlement mechanism in crack growth of microalloyed advanced high strength steels

Friday (28.09.2018)
10:30 - 10:45 S1/01 - A2
Part of:

It is known that as the strength of steels exceeds about 800MPa they become increasingly susceptible to hydrogen embrittlement. This is important in advanced high strength steels (AHSS) for automotive applications with a continuous push to increase strength to well above 1000MPa. In this work, model high strength AHHS steels based on Ti-Mo, V, and V-Mo microalloyed chemistries were studied. The effects of hydrogen on tensile crack growth behaviour and ductility after slow strain-rate testing was investigated in detail. As expected, charging by hydrogen resulted in a loss in strength and ductility, although post yield work hardening rates were similar. By using double notched specimens, it was possible to undertake detailed investigation of the crack initiation and propagation around the notch site that did not fail. In the uncharged specimen then failure tended to be by conventional tensile necking with ductility. In the hydrogen charged specimens, multiple cracking was observed, with intergranular cracking in purely ferrite steel and transgranular cracking in dual phase steels. Cracks grew within the ferrite along the (100) plane, while cracks were preferentially along the in martensite/ferrite grains or (110) plane in martensite. In addition, the hydrogen charging increased the dislocation density in the bulk unstrained sample. Hydrogen charging did not appreciably change the work hardening rate during tensile testing. However, after tensile testing, the hydrogen charged specimens exhibited a much higher dislocation density, despite a lower strain compared to the uncharged sample. In charged tensile samples, a well developed subgrain structure formed, with cracking occurring along subgrain boundaries. For the equivalent condition in the uncharged tensile sample, dislocations were tangled, but did not form a subgrain structure. The implication for these observations on the mechanism of hydrogen embrittlement in these steels is discussed.

Prof. W. Mark Rainforth
The University of Sheffield
Additional Authors:
  • Dr. Peng Gong
    University of Sheffield
  • Prof. Brad Wynne
    University of Sheffield