Microstructure Degradation in Stainless Steel Weld Metals Due to Thermal and Mechanical Histories

Abstract

In order to develop a long term creep-fatigue evaluation method of stainless steel weldment, that incorpo

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

“…The KAM values were see to decrease to lower levels as one moved away from the crack (Figure 11D). The crack propagation along the interface regions is in agreement with the earlier studies on austenitic SS welds under IF, 6,10,14,17 creep, 12 and TMF cycling. 31 A higher amount of cyclic softening during OP compared with IP TMF tests (Figures 4C and 5D) is also consistent with the deformation observed in the weld region in the former case, while the HAZ carried more significant amount of strain in the latter.…”

“…The effect of inhomogeneity in deformation and a strong interaction between the creep, oxidation, and DSA during IP TMF cycling led to a drastic reduction in fatigue life for 316 LN SS base metal 32,33,35–37 . 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 7–11 . 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 .…”

“…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.…”

“…Besides, it is to be noted that these studies are mostly restricted to isothermal low cycle fatigue (abbreviated as IF) without taking the effects of thermal cycling into consideration. 6,10,[14][15][16][17] The extent of microstructural transformation and the failure location of the joint, which have a strong bearing on the cyclic life, have been found to depend on the mechanical strain amplitude (Δε mech /2) and test temperature (T) 6,14,16,17 in austenitic SS under IF deformation. In addition, high temperature cyclic loadings (generally T ≥ 820 K) is known to activate many temperature and time-dependent damage mechanisms such as oxidation, dynamic strain aging (DSA), creep, and change in cyclic slip character which are expected to accelerate the degradation process.…”