Effect of Carbon, Nitrogen and Nickel on Long-term Creep Rupture Strength of 10 Cr Steels Containing Boron

Microstructure and precipitates of creep ruptured specimens were investigated in order to understand the effects of carbon (C), nitrogen (N) and nickel (Ni) on long-term creep rupture strength of 10Cr heat-resistant steels containing boron (B). The low-N steels showed higher creep rupture strength than the high-N steels. In long-term creep rupture region such as over 10,000 h at 650°C, the deterioration of creep rupture strength was not observed in the low-N, high-C steel. On the other hand, the creep rupture strength of the low-N, low-C steel dropped to the strength level of the high-N steels. The addition of N to the B containing steels promoted the recovery of microstructure. The formation of coarse BN in the high-N steels led to the decrease of the amount of effective B that dissolved in M23(C,B)6 and suppressed its coarsening. From EDS analysis of precipitates, the fraction of M23C6 in the low-C steels was less than that in the high-C steels, while the fraction of coarse Laves phase in the low-C steels was more than that in the high-C steels. In the low-N, low-C steel, the coarsening of precipitates caused the deterioration of the creep rupture strength after 10,000 h exposure. Ni lowered Ac1 and Ac3 transition temperature, but it did not affect the fraction of precipitates according to the calculation of thermodynamic equilibrium using Thermo-Calc. It is concluded that C and N are more effective to the stability of microstructure than Ni.