FATIGUE BEHAVIOUR OF 316 (N)/316(N) WELD JOINTS AND 316(N) WELDS

Abstract

In fast reactors, components operating at high temperatures are often subjected to cyclic thermal stresses as a result of temperature gradients that occur on heating and cooling during start-ups and shutdowns or temperature. Further, on load periods introduce creep loads and combination of these two causes creep-fatigue interaction. Low cycle fatigue and creep fatigue are therefore important considerations in the design of high temperature systems subjected to thermal transients’ variations during steady state operations.

In this paper, a comparative study of the Low Cycle Fatigue (LCF) and creep-fatigue interaction behaviors of indigenously developed 316(N) base metal, 316(N) weld metal and weld joints is presented. Total axial strain controlled fatigue tests were carried out in the temperature range of 773 – 873 K at strain amplitudes in the range of ±0.25 to 1.0%. Creep-fatigue tests were performed at 873 K introducing hold times at peak strain in the tension portions of the cycle. Further, the effects of strain rate on LCF were studied in the strain rate of 3 x 10^-5 s^-1 to 3 x 10^-2 s^-1 at 773 K.

Cyclic stress response behavior and fatigue life variations of the weld metal and weld joints under different testing conditions are compared with the base metal. Base metal showed an initial hardening followed by a saturated response while the weld metal and weld joints showed a softening regime in between. The initial hardening observed is attributed to dynamic strain ageing and cyclic softening observed in weld metal and weld joints to annihilation of dislocations. At high temperatures δ ferrite got transformed into brittle phases like σ phase and carbides leading to crack deflection in the weld metal. In general, the LCF and creep-fatigue lives exhibited by the base metal were higher than the of weld metal. Weld joints exhibited the lowest life. The differences in fatigue and creep-fatigue lives of the weld metals and base metal under various testing conditions have been found to be associated with the basic differences in chemical composition and transformation of δ ferrite to brittle phases. The differences in  LCF and Creep-Fatigue behaviors of base and weld metal and weld joints were rationalized on the basis of deformation and fracture behavior. Further, the data generated is compared with that available from the literature and RCC-MR design curves. Life prediction has been carried out using Ostergren Frequency Modified Damage Function (FMDF) and predicted lives are found to within a factor of 2.