The effect of niobium contents on mechanical properties of the large forged Cr-Mo-V steel

The effect of Niobium(Nb) contents on the mechanical properties of the CrMoV forgings was investigated as a preliminary study to manufacture large forgings having a low Fracture Appearance Transition Temperature(FATT) and high tensile strength. The experimental forgings were varied in the Nb contents as 0%(A), 003%(B) and 0.05%(C). The tensile strength at room temperature and 600 .deg. C were almost equal regardless of Nb contents, while the FATT lowered in order of alloy C(32 .deg. C), B(35 .deg. C), and A(62 .deg. C). These were resulted from the grain refining. The grain size of alloy A was ASTM no. 5.0 and those of B and C were ASTM no. 6.5 respectively. Based on the preliminary experimental results, large forgings containing 0.03%Nb(D) and 0.05%Nb(E) were manufactured from 30 ton ingot and then those mechanical properties were tested. The higher tensile strength was obtained in both D and E forgings, whereas, the FATT of D and E forgings were 33 deg. C and 67 .deg. C, respectively. The ductile fractured surfaces were developed in the forgings D, while serrated fractographes were alternately occurred with the eutectic niobium carbides and ductile dimples in the forgings E. The eutectic NbC and normal matrix were alternately developed in a severely Nb segregated region of the forgings E. The alternated structure might develop a serrate fracture surfaces, which caused forgings E to increase its FATT. Therefore, in order to manufacture large CrMoV forgings by adding Nb, Nb content to be added should be decided by considering segregation of Nb during solidification. The coarse eutectic niobium carbides were formed in the severely Nb segregated region. The serrated fracture might be caused to increase the FATT of large CrMoV forgings containing Nb