Influence of δ-ferrite content on transformation to intermetallic phases in heat-treated, type 316 austenitic stainless steel weld metal

New insights into thermomechanical fatigue behavior of AISI Type 316 LN SS weld joint

“…The difference in the amount of transformation at Δε mech /2 of ±0.25% and ±0.6% is found to be minimum (Figure 12). The presence of the δ‐ferrite influences the accumulation of localized damage and crack propagation in a complex manner under cyclic loading, 6,10,14–17,31,51 depending on the temperature, the time of exposure, and stress range 6–8,10–12,31,51–53 . At the low (±0.25%) strain amplitude, the transformation is largely controlled by thermal aging effects as the deformation largely restricted to the weaker BM/HAZ parts of the joint.…”

“…32,33,[35][36][37] The transformation of meta-stable δ-ferrite into carbides and intermetallic σ or χ or laves phases in the austenitic SS weld metal is an important degradation mechanism at high temperature, depending on the time of exposure and chemical composition. [7][8][9][10][11] The development of damage gets further enhanced in the presence of cyclic loading due to increase in the dislocation density which provides an easy path for the diffusion of solute atoms and increases the localized deformation along the interfaces of austenite/δ-ferrite/σ-phase. 10,12,57 In the present study, failure is observed in the weld region of the joint at the highest Δε mech /2 under both IF and TMF cycling (Figure 7C,D,G,H).…”

“…Safe‐life design approach must consider the complex interactions between the above loadings, taking into account the presence of metallurgically heterogeneous and structurally weaker weld joints (WJ). Key factors pertaining to the structural integrity of welded components are (a) metallurgical notch effect which arises due to a difference in the strength levels in its constituent regions 3–6 and (b) inherent microstructural instability when subjected to long periods of operation at elevated temperatures 7–12 . Either of the above can result in early failures under service loadings.…”

“…[7][8][9][10][11][12] Either of the above can result in early failures under service loadings. Extensive studies are available on the nature and kinetics of microstructural transformations in austenitic SS base metal (BM) 13 and weld metal [7][8][9][10][11] under different combinations of temperature and aging periods. However, the influence of cyclic loading on the microstructural transformation and fracture behavior of SS welds have received limited attention, compared with the base metal.…”

Susceptibility of As-Welded and Thermally Aged Type 316LN Weldments Toward Pitting and Intergranular Corrosion

“…Since delta-ferrite is ferrite region, thus leaving carbides isolated in the austenite matrix. 24 The mechanism of the formation of intermetallics is favored in delta-ferrite containing weld metals, compared to the wrought 316 SS (UNS S31600). Over the temperature range of 973 to 1173 K, delta-ferrite transformation to secondary austenite remains an important mechanism.…”