Microstructural Evolution along the NiCrMoV Steel Welded Joints Induced by Low-Cycle Fatigue Damage

Abstract

The degradation of mechanical properties of materials is essentially related to microstructural changes under service loadings, while the inhomogeneous degradation behaviors along welded joints are not well understood. In the present work, microstructural evolution under low-cycle fatigue in base metal (BM) and weld metal (WM) of NiCrMoV steel welded joints were investigated by miniature tensile tests and microstructural observations. Results showed that both the yield strength and ultimate tensile strength of the BM and WM decreased after low-cycle fatigue tests, which were attributed to the reduction of dislocation density and formation of low-energy structures. However, the microstructural evolution mechanisms in BM and WM under the same cyclic loadings were different, i.e., the decrease of dislocation density in BM was attributed to the dislocation pile-ups along the grain boundaries, dislocation tangles around the carbides at the lower strain amplitudes (±0.3% or ±0.5%). Additionally, when the strain amplitude was ±8%, the dislocation density was further decreased by the formation of subgrains in BM. For WM, the dislocation density decreased with the increase of strain amplitude, which was mainly caused by the dislocation pile-ups along the grain boundaries and the formation of subgrains.

Effects of V-N Microalloying on Low-Cycle Fatigue Property in the Welded Joints of Constructional Steel

“…Wei Song et al [12] carried out LCF testing for the BM and undermatched welds of 10CrNi3MoV steel, and they found that larger acicular ferrite and granular carbides in the base material led to transgranular cracking easily; according to the evaluation of plastic strain energy density and total strain energy density, the fatigue resistance of the undermatched weld was better than that of the BM. Shuo Weng et al [13] investigated the mechanical property and microstructural transformation of the BM and WJ of NiCrMoV steel under the LCF condition, and they found that cyclic softening took place in the BM and WJ under various strain amplitudes. The decrease in dislocation density in the WJ was mainly attributed to the dislocation pile-ups along the grain boundaries and the formation of subgrains, which led to the decrease in the WJ strength.…”