Abstract
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.
