Crystallographic study of hydrogen-induced fatigue crack growth in type 304 austenitic stainless steelThursday (29.09.2016) 09:30 - 09:45 Part of:
Stable austenitic stainless steels such as 316L have been used in the hydrogen industry. Although reducing the content of rare metal elements such as Ni is necessary from the economic viewpoint, the reduction of the Ni content decreases the stability of austenitic phase, leading to severe hydrogen embrittlement. In metastable austenitic stainless steels such as 304, deformation-induced martensitic transformation complicates the understanding of the HE mechanism, particularly, under cyclic loading conditions. In this study, the relationship between fatigue cracks and martensite was analysed from crystallographic perspective using multi-scale mechanical testing. The materials used in this study were commercial types of 304 austenitic stainless steel. Small compact-tension (CT) specimens with 50μm thickness and 1mm width were fabricated for through-thickness grain samples using laser and focused iron beams. Solute hydrogen was introduced by cathodically charging in a sulfuric solution with pH = 3.5 maintained at a temperature of 353 K. The saturated hydrogen content was determined to be approximately 100 mass ppm using thermal desorption spectrometry. Fatigue crack growth tests were performed at a frequency of 1 Hz under a stress ratio of 0.1. It was found that the hydrogen charge facilitated cracking along twin boundaries in bulk polycrystalline specimens. In the uncharged small CT specimen, cracks grew by alternating slip shearing at the low stress intensity factor range (ΔK). With increasing ΔK, more martensite was formed ahead of the crack tip, resulting in the crack growth in the formed martensite region. In contrast, crack shielding effect due to martensitic transformation ahead of the crack tip was reduced by hydrogen charging. Despite the fact that the formation of martensite was suppressed by hydrogen charging, martensite variants with the habit plane parallel to twin boundaries were occasionally formed, and they may become a crack path.