Hydrogen Embrittlement of Cold-worked 304L Stainless Steel and its Welds
Date Issued
2011
Date
2011
Author(s)
Lai, Chien-Lin
Abstract
The susceptibility to hydrogen embrittlement (HE) of AISI 304L austenitic stainless steel and its welds was evaluated by slow displacement rate notched tensile tests in gaseous hydrogen. The notched tensile tests were carried out in different combinations of temperature (25 to 135℃) and environment (air or hydrogen) to assess the HE of various specimens. The base metal and weld metal specimens were conducted in both the unrolled and cold rolled (20% and 40% reduction in thickness) conditions. During testing in air at 25℃, all specimens underwent strain-induced α''-martensite transformation at the beginning of plastic deformation. For the specimens tested at 80℃, such a transformation started to increase only after reaching the notch tensile stress (NTS) and was considerably less than at 25℃. When the test temperature was further increased to 135℃, the α''-martensite transformation was generally not observed. In hydrogen-containing environments, hydrogen atoms can be transported through dislocation or by diffusion to the stress concentration region, resulting in HE of the specimens. In the case of the base metal specimen (the B specimen), plastic deformation occured at low stress levels. Hydrogen atoms were transported through mobile dislocations and facilitated localized plastic deformation. This caused the formation of strain-induced α'' in front of the notch tip and resulted in cracks along a narrow α'' region, leading to a significant NTS loss in the B specimen at 25℃. For the B specimen after 20% thickness reduction, i.e. the B20R specimen, the density of hydrogen traps increased and the number of mobile dislocations decreased, hence the HE susceptibility was lowered. In the 40% cold-rolled base metal specimen (the B40R specimen), hydrogen was transported mainly by diffusion, and the presence of 70% γ and ε in the specimen greatly decreased the diffusion of hydrogen. Therefore, the B40R specimen had lower HE susceptibility than the B20R specimen if the NTS loss was adapted to index the relative HE susceptibility. The much slower movement of hydrogen by diffusion compared with dislocation transport also explains why the B40R specimen has better resistance to HE.
For the welded specimens, the γ / δ interfaces are trapping sites for hydrogen atoms and δ-ferrite inhibits strain-induced α'' transformation in the deformation process. Consequently, the weld metal specimen (the W specimen) and its 20% cold-rolled specimen (the W20R specimen) exhibited lower HE susceptibility than the corresponding base metal specimens (the B and B20R specimens). As for the 40% cold-rolled specimen (the W40R specimen), the hydrogen atoms could diffuse along a tunnel or continuous path which consisted of α'' and δ, resulting in a slightly higher susceptibility to HE than the W20R specimen. This tendency is different from the cold-rolled base metal specimens, where the B40R specimen had slightly lower susceptibility to HE than the B20R specimen. Regardless of the experimental group, the fracture appearance displayed ductile dimples for the specimens tested in air, but changed to quasi-cleavage with secondary cracks in gaseous hydrogen. Additionally, the effect of HE decreases as the test temperature increases for all specimens. This is due to the fact that both the hydrogen adsorption on metal surface and the α'' content in front of the notch tip are reduced. During the notched tensile test of metastable austenitic stainless steels, hydrogen atoms enhanced localized plasticity and caused further strain-induced α'' transformation. This is the main cause of HE of the experimental material and generally agrees with the hydrogen enhanced localized plasticity (HELP) theory. In 310S austenitic stainless steel with no strain-induced α'' transformation, HE was not observed between 25 and 135℃, which clearly indicates that the formation of α'' is an important factor to cause HE in metastable austenitic stainless steels.
Subjects
304L stainless steel
welding
cold rolling
hydrogen embrittlement
notched tensile strength
martensitic transformation
Type
thesis
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