Effect of Long-Term Aging on the Microstructure Evolution and Fracture Mechanism of 9% Cr Steels

Abstract

The fracture toughness of pressure pipe steels is vital for their design and safe operation. Understanding the deterioration of the fracture toughness and the corresponding fracture mechanisms is instructive and meaningful for material selection and development. The elastic–plastic fracture toughness of 9% Cr steels aged at 873 K and 923 K from the initial state to 20000 h was investigated at ambient temperature and 873 K. The deterioration of the fracture toughness and the corresponding fracture mechanisms are discussed in detail. Voids were mainly initiated from primary inclusions such as silicides, sulfides, alumina and manganides in the as-received 9% Cr steels, while voids initiated from both primary inclusions and secondary particles such as the Laves phase in aged steels. These voids grew and coalesced with neighboring voids and crack tips subsequently until ultimately fracture. After aging at 873 K, the fracture toughness values tested at 873 K decreased from the initial state to 3000 h and then increased to 10000 h. The peculiar deterioration of the fracture properties after 3000 h of aging at 873 K can be attributed to the high number density of Laves phase particles in the steels. It was found that the densely distributed hard Laves phase particles induced stress concentration at the interface between the Laves phase and the matrix in plane-strain state and initiated a large number of voids. It was suggested that dispersed voids with short interspaces were more susceptible to coalesce into crack tip, resulting in lower fracture resistance. After aging at 923 K from 10000 h to 20000 h, the number density, size and volume fraction of precipitates, block size, misorientation angle and dislocation density remained almost constant. The slight increase in the fracture toughness from 10000 h to 20000 h can be mainly ascribed to the coarsening of the martensite lath in this stage.

Keywords: 9% Cr heat-resistant steel, Fracture toughness, Aging, Laves phase, void initiation