Effects of nitrogen in 9Cr-3W-3Co ferritic heat resistant steels containing boron

Many researchers revealed that the addition of small amount of boron (B) significantly improves the creep strength of ferritic heat resistant steels. However the mechanism for such an effect caused by B has not been clarified yet. In the present study, the effect of nitrogen (N) on the behavior of B in 9Cr-3W-3Co base steels was examined using Alpha-particle Track Etching (ATE), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and creep test. It was confirmed that creep lives have significantly increased by lowering the N content in boron containing steels. The prediction of creep life using Larson-Miller parameter suggests that high N-high B steel shows lower creep strength compared with high N-low B, low N-low B and low N-high B steels. SEM showed that B increases the precipitates stability against coarsening. TEM revealed that the stability of the matrix against recovery reaches maximum in low N-high B steels, while this stability is very weak in high N-high B steel. ATE showed that B segregating along packet and block boundaries and free B homogeneously distributing in the matrix are more effective on strengthening than B contained in M23C6 carbides. In addition, B in steels of a suitable content of N is contributable to the enhancement of creep strength through increasing the amount and the stability of fine precipitates such as VN. High content of N is expected to reduce such effects of B probably through the precipitation of BN, although no experimental evidence has been obtained.