Cracking Process in Delayed Fracture of High-Strength Steel after Long Atmospheric Exposure

Abstract

This paper is the first microscopic observation of the entire cracking process in delayed fracture of highstrength steel bolt after long-term atmospheric exposure. A sufficiently fresh fracture surface exhibits the initiation of the propagating crack in a thin zone beneath the screw groove, resulting from the merging of multiple cracks nucleated therein. The fracture morphology is initially intergranular, exhibiting the threedimensional shape of prior austenite grains, but the stress and strain states at the nucleation sites are not uniquely specified. The fracture morphology alters as the crack extends from intergranular to quasicleavage and fine dimples, associated with increasing stress intensity under a constant-displacement condition. The change from inter-to trans-granular fracture is continuous, implying affinity among different morphologies associated with the increased density and the distribution of potential crack nucleation sites in the crack front. The crack propagation in the quasi-cleavage and fine dimple regions is step-wise of about 50 µm per step. Recent studies about the function of hydrogen in embrittlement are referred to in respect of the accumulation of strain-induced damage. The enhanced generation of strain-induced vacancies is the presumable function of hydrogen compatible with the present findings.

Hydrogen Content Dependence of Crack Initiation and Propagation Behavior of Hydrogen Embrittlement in Tempered Martensitic Steel

“…In the present study, the two crack initiation sites with the low hydrogen content at the level absorbed by atmospheric corrosion are consistent with the previously reported result that the crack initiation sites of the bolts that fractured after 16 years of exposure to the atmosphere originated from multiple sites near the screw grooves 17) . Crack initiation in the case of delayed fracture at a low hydrogen content absorbed in atmospheric conditions is not necessarily consistent with the site of the principal maximum stress and the maximum local hydrogen concentration due to hydrostatic stress.…”

“…Multiple cracks probably propagated together in a step-like manner on the fracture surface. 17) It has been reported that the maximum diffusible hydrogen content absorbed in tempered martensitic steel through atmospheric corrosion under applied stress was approximately 0.2 mass ppm (referred to here as simply ppm). 18) Therefore, crack initiation and subsequent propagation under low hydrogen content levels 1), 18) absorbed by atmospheric corrosion may not be sufficiently explained by crack initiation from a single site, as reported for conventional stress-or strain-controlled types.…”

Modeling and Evaluation of Damages in Bismuth-containing Stainless Steel Rolling

“…Where ρ is the density of the specimen; c is the specific heat of the sample; η is heat transfer efficiency, determined by Eq. (2). By substituting the data obtained from the thermal simulation experiment into Eqs.…”

“…Sheng-De HU, 1,2) Lin ZHU, 1) * Mao ZHANG, 2) Xin-Yun WANG 2) and Sheng LIU 1) the understanding of material damage and fracture behavior.…”

“…Therefore, the prediction and prevention of damage and fractures during material forming processes have always been a focal point of international research. [1][2][3][4][5] Scholars from different countries have carried out a significant amount of research on the micro mechanisms, fracture criteria, and process optimization related to damage and fracture in metal materials. [6][7][8][9][10][11] These studies involve various materials, different forming methods, diverse damage patterns, and distinct fracture criteria.…”