Delayed Failure Test of Synthetic Heat-Affected Zone of High-Strength Steel due to Hydrogen

Abstract

A delayed failure test was done with notched specimens of synthetic heat-affected zone of highstrength steel. The test method was a so-called "charge and stress method, in which the specimen was subjected, at first, to a weld thermal cycle, charged with hydrogen electrolytically and then loaded in sustained manner. The main conclusions obtained in the study are as follows:
(1) Root crack susceptibility of the heat-affected zone in the first layer weld of a low-alloy high-strength steel can be examined in the delayed failure test of the synthetic heat-affected zone of steel.
(2) A new criterion was proposed for comparison of susceptibilities to hydrogen embrittlement of the synthetic heat-affected zones of steels. The ratio of a critical stress in the delayed failure test (σt) to a normal notch strength (σt) of synthetic specimens of a steel was plotted against a diffusible hydrogen content of the specimen in a graph with logarithmic graduation. The value of the ratio at a specific hydrogen content (e.g. 1.0 cc/100 gr) was adopted as the criterion.
(3) A critical stress increased with an increase of cooling time in a weld thermal cycle except for a 9% Ni steel. The stress also increased with decreases in diffusible hydrogen content and notch sharpness.
(4) Susceptibility to hydrogen embrittlement of a low-carbon martensite was affected by other alloying elements even if the carbon content and the hardness were not changed.
(5) Susceptibility to hydrogen embrittlement of a low-carbon martensite was increased by rapid quenching and also by repeated quenching.
(6) A bainitic structure which was isothermally transformed just above the M, temperature had a high susceptibility to hydrogen embrittlement. The susceptibility was nearly equal to that of an autotempered martensite.
(7) There was a relation between the new criterion in the delayed failure test and the critical restraining stress for root cracking in the heat-affected zone in the weld of the same steel in the TRC (Tensile Restraint Cracking) test.