Effects of hydrogen concentration, specimen thickness, loading frequency and temperature on the hydrogen enhanced crack propagation of low alloy steel

Crack propagation of SCM440H low alloy steel under varying load is enhanced by absorbed hydrogen. Substantial acceleration of crack propagation rate up to 1 000 times was observed compared with that of uncharged material. The role of factors affecting enhanced acceleration was investigated by changing hydrogen concentration absorbed in metal, specimen thickness, loading frequency and temperature. Results are as follows, (1) 0.2 mass ppm diffusible hydrogen in metal was enough to cause enhanced acceleration. The predominant fracture mode showing acceleration was quasi cleavage. (2) In the case of specimen as thin as 0.8 mm, the constraint of the crack was weak, and the enhanced crack propagation did not appear. However, the introduction of side-groove to 0.8 mm thick specimen resulted in enhanced acceleration. (3) The crack propagation rate in time domain was almost constant irrespective of loading frequency. Lower loading frequency resulted in higher crack propagation rate in cycle domain. (4) The crack propagation at different temperature was controlled by thermal activation process. The crack propagation rate in time domain is controlled by the diffusion of hydrogen. Enough concentration of hydrogen, enough constraint and low loading frequency resulted in enhanced acceleration of crack propagation.