Effect of Nitrogen on the Fatigue Crack Growth Behavior of 316L Austenitic Stainless Steels
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TRODUCTION
AUSTENITIC stainless steels (SSs) are known to undergo stress- or strain-induced phase transformation.[1–5] This transformation, normally associated with the shear deformation process, can impart a good combination of strength and toughness to many structural materials,[2–4] including ceramics.[6] For the austenitic SSs, it involves the retained austenite phase undergoing deformation-induced martensitic transformation (DIMT) ahead of the crack tip due to the crack tip stress or strain fields. The extent of the retained austenite present for transformation depends on the heat treatment. Several parameters, such as strain rate, temperature, state of stress and/or strain, etc., can influence the transformation due to plasticity ahead of the crack tip.[7–10] The present study is focused on
M. NANI BABU and G. SASIKALA are with the Materials Development & Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603102, India. Contact e-mail: [email protected] K. SADANANADA is with Technical Data Analysis Inc., 3190 Fairview Park, Falls Church, VA 22043. Manuscript submitted December 8, 2018. Article published online April 23, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
analyzing the role of nitrogen on the fatigue crack growth (FCG) resistance in type 316L austenitic SSs.
II.
BACKGROUND
It is known that austenitic SSs are prone to intergranular stress corrosion cracking due to sensitization. The carbon content is lowered to minimize the sensitization, and nitrogen (N) is added to compensate the consequent loss of strength.[11] Nitrogen, while stabilizing the austenite phase, also (1) improves the solid solution strengthening, (2) causes grain refinement thereby contributing to additional strengthening and (3) increases toughness via strain hardening. It also improves the resistance to corrosion such as pitting, crevice corrosion and intergranular stress corrosion by inhibiting the carbide formation near the grain boundaries. Hence, nitrogen is considered one of the essential elements for industrial application of austenitic SSs.[12] It is also known that austenitic SSs (300 series) are prone to deformation-induced martensite formation, which can influence the corrosion and mechanical behavior, including crack growth, especially at ambient and lower temperatures. The nitrogen additions can stabilize the retained austenite and hence inhibit the stress-induced transformation to martensite, which in turn can VOLUME 50A, JULY 2019—3091
influence its mechanical behavior, including crack growth resistance. This was demonstrated by the results of the classical experiments of Mei and Morris[13] shown in Figure 1. They tested 304L SS with and without nitrogen (304LN and 304L) at room temperature and at 77 K. The absence of N in 304L favors localized transformation-induced toughening at the crack tip, enhancing the FCG. Figure 1 shows that, at room temperature, FCG rates in both 304L and 304LN are the same, indicating that any beneficial effects of nitrogen on t
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