Microstructural Characteristics and Mechanical Properties of Welded Joints and Base Material of Ultra-Supercritical Pipelines Before and After Service

Haifeng Jiang

doi:10.54097/50pxpk75

Authors

Haifeng Jiang

DOI:

https://doi.org/10.54097/50pxpk75

Keywords:

HR3C, long-term service, precipitates, property degradation, microstructural evolution

Abstract

This study investigated the microstructural evolution and mechanical property degradation of ultra-supercritical welded joints and base material (HR3C steel) before and after service. Mechanical property tests, including tensile and impact property tests, were conducted. The results show that the tensile strength, yield strength, elongation, and impact properties of the welded joints and base material after service were lower than those of unserviced specimens. Furthermore, optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to observe the welded joint and base material specimens before and after service. In the initial state, the welded joints and base material had an austenitic microstructure and a diffusely distributed strengthening phase with the composition NbCrN. Moreover, high-temperature exposure led to precipitation processes. After service, Cr-rich M23C6 carbides with a chain-like structure precipitated near the grain boundaries of the welded joints and base material, leading to the aggregation of voids at the grain boundaries. This indicates that the coarsened M23C6 carbides at the grain boundaries negatively affect grain boundary strength, which is the main factor causing the reduction in steel plasticity and toughness. In addition, NbCrN mainly precipitated inside the grains, accompanied by dislocation pinning. The inhibitory effect of the dispersed particles on the dislocations allows the steel to maintain strength properties comparable to the initial state after service. However, the gradual deposition of NbCrN eventually degraded the strength properties.

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