Abstract
The microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel were investigated after austenitising over a range of temperature between 1133 and 1483 K. The experimental results show that the isotropy of impact toughness and mechanical properties were greatly improved due to the disappearance of undissolved aligned second phase when the austenitising temperature was over 1233 K. When the austenitising temperature was over 1383 K, martensite lath and packet abnormally grew up due to dissolution of spheroidal Nb rich carbonitrides; both the platelet size and morphology of martensite were changed, which has an effect on the mechanical properties of the samples. It was noticed that the finer self-tempered carbides, which strengthened martensitic matrix, appeared after austenitising temperature over 1283 K. The strength profiles show a marked plateau for the samples austenitised from 1283 to 1433 K; however, the strength was deteriorated due to coarsening of these self-tempered carbides at 1483 K. It was confirmed that calcium treatment can help improve the isotropy of mechanical properties by modifying sulphide inclusion morphology.
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