EFFECT OF NITROGEN-CONTENT ON CREEP-RUPTU RE STRENGTH OF 9CR-1MO-V-NBSTEEL

The effect of nitrogen content on creep rupture strength (CRS) was exa mined in 9Cr-1Mo-V-Nb steel and the metallurgical reasons have been di scussed. Nitrogen content as low as 100 mass ppm causes the dense dist ribution of finer than 30 nm precipitates in matrix. This increases th e CRS in the short rupture time region at 600 degrees C. As the precip itates are evaluated to be (V, Nb)N-0.5, these precipitates can be est imated not to be so thermodynamically stable that they coagulates duri ng creep test. For this reason, 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 stabl e 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 t he precipitates is longer than that of 100 mass ppm nitrogen steel. Th ese precipitates being not so effective in increasing the CRS in short er rupture time region, they maintain the effect up to prolonged ruptu re time.