Effect of Thermal Aging on Deformation and Fracture Properties of SS 304HCu

Austenitic stainless steel 304HCu is one of the candidate materials for superheater and re-heater tubing of advanced ultra-supercritical power plant. Though the material is expected to have Cu-rich nanosized precipitates in the matrix in the initial stages of aging, the other elements like Cr, Nb, C and N may lead to precipitations which significantly influence the deformation and fracture properties after long-term aging. This study carries out a detailed microstructural investigation for aging duration up to 20,000 h at 700 degrees C. The variation of hardness and room temperature tensile deformation parameters has been studied in the light of the microstructural variation during aging. It is understood that though the nanosized Cu-rich precipitates give rise to the initial hardening, the tensile and deformation properties are ultimately governed by the M23C6-type precipitates at the grain boundary and MX-type precipitates at the matrix. The as-received SS 304HCu shows a fracture toughness of 300 kJ m(-2), which dropped to 100 kJ m(-2) after 10,000 h of aging. This is attributed to grain boundary embrittlement arising from continuous network of M23C6-type precipitates.