The Effects of Rolling and Testing Temperature on Hydrogen Embrittlement of 301 Stainless Steel and its Laser Welds
Date Issued
2011
Date
2011
Author(s)
Huang, Yen-Cheng
Abstract
The effects of rolling and testing temperature on hydrogen embrittlement (HE) of AISI 301 stainless steel and its laser welded specimens were evaluated in both the unrolled and rolled conditions. Slow displacement rate notched tensile tests were carried out in air and gaseous hydrogen at 25 to 100℃ to assess HE of various specimens. The rolled specimens were processed at either room temperature or 150℃ with 30% thickness reduction. Due to the temperature dependence of strain-induced martensitic transformation in 301-type stainless steel, the microstructure showed substantial differences between the specimens rolled at distinct temperatures. The HE was seen to be affected by the amount and type of martensites (α'' and ε) transformed in the specimens. Apart from reducing the notched tensile strength (NTS), the effect of HE also caused ductility loss of the specimens. For the base metal specimen (the B specimen), hydrogen atoms could be transported to the stress concentration region through mobile dislocations during plastic deformation in hydrogen. This would promote localized plasticity and lead to the formation of strain-induced α''. It was also found that the amount of strain-induced α'' and HE susceptibility decreased as the test temperature increased. For the base metal rolled at 25 and 150℃ specimens, i.e. the B-CR and B-HR specimens, their stress-displacement curves revealed no plastic deformation during notched tensile tests in hydrogen, implying that hydrogen atoms were diffused to the crack tip and caused brittle fracture. Because hydrogen had a much faster diffusivity in the α''-phase (BCC or BCT) than in the γ-phase (FCC), the HE resistance of the B-HR specimen (0.3% α'') was better than that of the B-CR specimen (23.8% α''). The results clearly demonstrated that the formation of considerable amounts of α'' increased the HE susceptibility of 301 stainless steel.
The HE mechanism of the weld metal specimen (the W specimen) was similar to that of the B specimen during notched tensile tests in hydrogen. However, the presence of δ-ferrite after weld metal solidification the led to a reduction of α'' transformation in the W specimen. Moreover, the γ/δ interfaces could also act as hydrogen traps, resulting in the HE susceptibility of the W specimen less than that of the B specimen. Unlike the B specimen, the W specimen does not contain annealing twins to serve as potential crack propagating paths along twin boundaries. As a result, the reduced HE susceptibility for the W specimen could be expected. For the notched tensile testing in hydrogen, hydrogen transport was controlled predominantly by diffusion for both the weld metal and base metal specimens after rolling. However, some slip bands in the welded specimens were impeded by solidification sub-grain boundaries so that both the content of strain-induced α'' and the HE susceptibility of the W-CR and W-HR specimens were lower than those of the corresponding B-CR and B-HR specimens. It is noteworthy that the NTS loss of the W-HR specimen was close to 0% at 100°C, but its ductility loss was approximately 10% in hydrogen. For testing in air, the fracture surface exhibited ductile dimples for all specimens. However, the region affected by HE on the fracture surface changed to brittle fracture in hydrogen. When the test temperature was raised, the hydrogen adsorption and the amount of strain-induced α'' adjacent to sharp notches of the specimens decreased. Accordingly, the HE susceptibility and the extent of embrittlement on the fracture surface of a given specimen reduced.
Subjects
AISI 301 stainless steel
laser welding
rolling
notched tensile strength
hydrogen embrittlement
strain-induced martensitic transformation.
Type
thesis
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