The Study of Fatigue Crack Characteristics of 316L and A508 Steels
Date Issued
2012
Date
2012
Author(s)
Tsai, Yueh-Hsuan
Abstract
This study focused on two commonly used materials, 316L stainless steel and A508 low-alloy steel, in the nuclear power industry. In the fatigue tests, several differently treated 316L specimens were used: 20% cold rolled (316L-CR), cold rolled and sensitized (316L-CRS), grain-coarsened (316L-CG), and solution-treated base metal (316L-BM). The A508 steel was tested in three conditions including as-received (A508-BM), solution-treated and water-quenched (A508-SQ), and quenched and tempered (A508-QT). A series of fatigue crack growth rate (FCGR) tests was conducted at 25°C and in air 300°C, and the results were compared. The FCGR tests for various 316L and A508 specimens were also conducted in gaseous hydrogen at 25°C to investigate the influence of hydrogen embrittlement (HE). To identify the specimens tested at various conditions, the numbers (temperature) together with A (air) or H (hydrogen) were attached to the specimen''s designation for simplicity. For instance, the 316L-CR-25H represented the 316L-CR specimen which was tested in hydrogen at 25°C. Experimental results revealed that the 316L-BM-25A, 316L-CR-25A and 316L-CRS-25A specimens had similar FCGRs, implying that cold-working and sensitization treatment had little influence on FCGRs in air at 25°C. Under the same condition, the 316L-CG-25A specimen exhibited higher FCGRs at high ΔK (stress intensity factor range), possibly due to fewer grain boundaries to retard the motion of dislocations. For specimens tested in air at 300°C, both the 316L-CR-300A and 316L-CRS-300A specimens possessed similar FCGRs. Due to the lack of strain-induced martensitic transformation at 300°C, the 316L-CR-300A specimens with pre-existing (alpha)'' had higher strength / hardness and lower FCGRs than the 316L-BM-300A specimen. In addition, the 316L-CG-300A specimen had FCGRs close to that of the 316L-BM-300A specimen, possibly owing to the effect of roughness-induced crack closure. The FCGRsof the A508 specimens did not differ from each other significantly under various heat treatments and test temperatures (25°C and 300°C) in air. Nevertheless, the results of the 316L and A508 specimens tested in H2 at 25°C were remarkably different from those tested in air at 25°C. The 316L-BM-25H specimen had the lowest FCGR among the four groups of 316L specimens. Furthermore, the A508-SQ-25H specimen, with a greater amount of untempered martensite, possessed higher FCGRs than the A508-BM-25H specimen due to the effect of HE. In general, the average FCGR of a given 316L or A508 specimen tested at 300°C was higher than that at 25°C, owing to improved ductility at elevated temperatures. Aditionally, the absence of strain-induced martensitic transformation, i.e., no phase transformation-induced crack closure, could also contribute to the higher FCGRs of 316L specimens at 300°C. Although the sensitization is not apparent for 316L steel, the FCGRs of specimens tested at low temperatures in hydrogen were accelerated. This phenomenon is associated with strain-induced martensite formation at the crack’s front and agrees with the Hydrogen Enhanced Localized Plasticity (HELP) theory.
Subjects
316L stainless steel
A508 steel
strain-induced martensitic transformation
fatigue crack growth rate
heat treatment
cold work
hydrogen embrittlement.
Type
thesis
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