Specific Effects of Hydrogen Concentration on Resistance to Fracture of Ferrite-pearlitic Pipeline Steels

The presented work is dedicated to evaluation of strain and fatigue behaviour of the ferrite-pearlite low-alloyed pipeline steels under known hydrogen concentration in a bulk of metal. Tensile test results have shown on the existence of some characteristic value of the hydrogen concentration CH at which the mechanism of hydrogen influence changes, namely: below this value the enhanced plasticity (decreasing of the yield stress value) takes place and above – the hydrogen embrittlement occurs. The ambiguous relationship between fatigue crack growth rate and hydrogen concentration CH in the bulk of steels under their cyclic loading in hydrogen-contained environments has been found. There is a certain CH value at which the crack growth resistance of steel increases and the diagram of fatigue crack growth rate shifts to higher values of stress intensity factor. The generalised diagram of hydrogen concentration effect on strength behaviour of low-alloyed ferrite-pearlite pipeline steels is presented and discussed with the aim of evaluation of different mechanisms of hydrogen effect, conditions of their realization and possible co-existence.