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Abstract
The mechanisms responsible for lower creep strength of fine-grained heat-affected-zone (HAZ) than base metal, the microstructural evolution in the HAZ during heating and the suppression of Type IV fracture by boron have been investigated using simulated-HAZ specimens and welded joint specimens of Gr.92, Gr.122 and newly developed 9Cr steel with boron. Creep tests were carried out at 650°C for up to about 30,000 h. In Gr.122, the time to rupture has its minimum after heating the specimens to a peak temperature near AC3, while the hardness has its minimum after heating to a peak temperature near AC1. In Gr.92, the reduction of boundary and sub-boundary hardening in fine-grained HAZ is more important than the grain refinement for a significant decrease in time to rupture after the AC3 thermal cycle. The addition of boron without the formation of any boron nitrides suppresses the formation of fine-grained HAZ and produces substantially the same microstructure as the base metal, even after the AC3 thermal cycle. A newly designed alloy 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits not only much higher creep rupture strength of base metal than P92 but also no Type IV fracture in welded joints.
