Microstructure Characteristics and Their Effects on the Mechanical Properties of As-Served HR3C Heat-Resistant Steel

Abstract

The precipitation of secondary phases, the austenite grain growth and their effects on the mechanical properties of the HR3C heat-resistant steel in long-term service at 605 °C in an ultra-supercritical power generation unit were investigated. The results show that during the service period of the HR3C steel, the precipitation of M23C6 (M=Cr, Fe, Ni and Mo) follows the Kurdjumov–Sachs orientation relationship. Polygonal M23C6 particles preferentially precipitate and continuously distribute at the austenite grain boundaries. Meanwhile, the nanoscale NbCrN particles precipitate on the dislocations in the austenite grains. The austenite grain growth occurs in three stages with a growth exponent of 5.92 in the third stage, indicating that it is strongly restrained by the precipitates. In the as-served HR3C steel, the precipitation of the secondary phases, rather than the austenite grain growth, results in the strong improvement of the strength and hardness, albeit at the expense of a clear degradation of the toughness and plasticity.