Abstract
The effects of niobium and heat treatments on the creep and creep-rupture properties of 15Cr-14Ni heat resisting steels and the electron-microscopic observation have been studied. The relationships between creep properties and several structural factors are discussed. The results are as follows;
In vacuum-melted 15Cr-14Ni steels, the addition of niobium improves the creep and creep-repture strengths but reduces the rupture ductility. In 0.15C-15Cr-14Ni steels containing niobium, the creep and creep-rupture strengths have maxima at some niobium contents, which increase with increasing the solution temperature. At higher niobium contents, the creep and creep-rupture strengths increase with increasing the niobium content. A decrease of the niobium content or an increase of the solution temperature gives a loss of the rupture ducility in these steels. We find that the increment of the creep rupture strength is proportional to the product of {(%C) sol}0.4×(%Nb)
sol0.4×(n1)0.2×(Dγ)0.4, where (%C) sot and (%Nb) sol are the contents of carbon and niobium in solution, respectively, (n1) is the distribution density of undissolved carbides and (Dγ) is the mean austenite grain diameter after solution treatment. This is explained by the combination of the following factors; the solid-solution hardening effects by carbon and niobium, the dispersion strengthening effects by carbides, M23C6 and NbC, and the effect of the grain size.
