Hydrogen-assisted cracking and damage tolerance of high-strength lath-martensite steel bars for structural applications

The paper addresses the hydrogen-assisted cracking and damage tolerance of high-strength lath martensite steel bars assessed from slow strain rate tensile tests. These were carried out on fatigue precracked specimens simultaneously charged with hydrogen in a standardized 20 % aqueous solution of ammonium thiocyanate at 50 degrees C and in air, at room temperature, for comparison. The fractographic analysis of the tested specimens showed the steel sensitivity to hydrogen uptake, which gives rise to the sub-critical extension of the pre-existing crack in two phases, initiation and growth, with both hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) being synergically involved as damage mechanisms. Despite this, the damage tolerance behavior of the steel is highly satisfactory because the crack size causing the steel failure in tension slightly exceeds the theoretical limit of plastic collapse under combined tension and bending loading, regardless of the environment aggressiveness. Although H-assisted failure loads fit the theoretical limit loads, the fast propagation of the crack under constant load beyond the small-scale yielding regime could represent a real threat for the structural use of these steel bars in aggressive media unless the service loads are limited in accordance with such behavior.