Effect of nitrogen content on creep rupture strength of 9Cr-1Mo-V-Nb steel

The effect of nitrogen content on creep rupture strength (CRS) was examined in 9Cr-1Mo-V-Nb steel and the metallurgical reasons have been discussed. Nitrogen content as low as 100 mass ppm causes the dense distribution of finer than 30 nm precipitates in matrix. This increases the CRS in the short rupture time region at 600℃. As the precipitates are evaluated to be (V, Nb)N_0.5, these precipitates can be estimated not to be so thermodynamically stable that they coagulates during creep test. For this reasons, the CRS of low nitrogen content steel falls down abruptly at about 8,000 h. Higher nitrogen content than 300 mass ppm changes the fine precipitates to the thermodynamically stable ones that have the approximate composition of (V, Nb)N. As the size of (V, Nb)N is as large as 100 nm, the inter-precipitate distance of the precipitates is longer than that of 100 mass ppm nitrogen steel. These precipitates being not so effective in increasing the CRS in shorter rupture time region, they maintain the effect up to prolonged rupture time. (author)