Effect of microstructural evolution in thermal exposure on mechanical properties of HP40Nb alloy

Abstract The phase transformation of HP40Nb alloy at various temperatures and times was investigated and the microstructural evolution in aging process was characterised. Additionally, the mechanical tests were implemented for aged specimens. Experimental results indicate the microstructure of as-cast alloy consists of the austenite matrix with skeletal-shaped primary carbides. NbC carbide changes into G phase from 700 °C to 990 °C, and the nose temperature of transformation ranges from 900 °C to 950 °C. M7C3 carbide tends to transform into M23C6 carbide at higher temperature. Meanwhile, numerous secondary carbides are precipitated in the initial aging stage and dissolves gradually in further process. The interdendritic precipitates coalesce and coarsen continuously in aging treatment. The tensile strengths and the micro-hardness increase slightly followed by the continuous reduction in exposure, while the elongation and impact toughness decrease continuously. The microstructural evolution during aging process is consistent with the variation of mechanical properties.