Effect of carbon concentration on precipitation behavior of M23C6 carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M23C6 carbides precipitate along boundaries and fine NIX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 per C steel, there are no M23C6 carbide precipitates, and instead, fine NIX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount Of M23C6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are NIX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.