Fluctuations of properties of Cr-Mo-V hot work die steels by artificial increment of vanadium

4Cr5Mo2V steels with different contents of vanadium (V) (0.15V, 0.55V, and 1.25V wt%) were prepared by smelting, homogenizing, forging, quenching and tempering. The target Rockwell hardness of 46-49 HRC and Vnotch impact toughness of 16-22 J/cm2 were both achieved. The ultimate tensile strengths at 25 degrees C/620 degrees C of 0.15V, 0.55V, and 1.25V steels were 1369/698 MPa, 1347/714 MPa and 1381/662 MPa, respectively. The effect of V on precipitation, mechanical properties as well as thermal stability of 4Cr5Mo2V steels were investigated. TEM observations testified that the amount of MC-type stable precipitations increased with V content, inversely, both thermal stability and thermal conductivity declined according to finite element simulation on quenching & tempering. Higher content of V in experimental steels not only postpones secondary hardening but also exacerbates coarsening of carbides as well as reduction of hardness. Strengthening contribution stemmed from VC carbides for 0.15V, 0.55V and 1.25V steels were calculated to be 770, 730 and 765 MPa, respectively. Avrami exponent n of 0.15V, 0.55V and 1.25V steels, correlates well with the degree of precipitation and segregation during tempering, were calculated to be 0.272, 0.309 and 0.556, respectively.