Fine carbide-strengthened 3Cr-3WVTa bainitic steel

Both the 3Cr-3WV and the 3Cr-3WVTa steels exhibit an acicular bainite microstructure under the normalized and the normalized-and-tempered condition. The addition of Ta to the 3Cr-3WV steel substantially decreases the prior austenite grain size, but it has little effect on the bainite packet size. Fine TaC precipitates are formed in the normalized 3Cr-3WVTa specimen. After further tempering of 3Cr-3WVTa steel, fine TaC particles are further precipitated and dispersed within grains. The carbides at the prior austenite grain boundaries in the Ta-containing steel are much smaller than those in the steel without Ta. Tensile tests and fracture toughness (K-IC) tests have been performed on both the 3Cr-3WV and 3Cr-3WVTa steels at room temperature. The 0.2 pet yield strength of the Ta-containing steel is higher than that of the steel without Ta, especially under the normalized-and-tempered condition. The 3Cr-3WVTa steel is primarily strengthened by a secondary-phase precipitation mechanism represented by the formation of fine carbides after tempering. The 3Cr-3WVTa steel exhibits higher fracture toughness than the 3Cr-3WV steel. The toughening mechanism is also discussed based on the dependence of the calculated fracture stress upon the carbide size and the bainite packet size.