Studies on the Low-Alloy Hot-Work Forging-Die Steel

In order to develope the low-alloy hot-work forging-die steels with a superior thermal-crack risistivity, the effec of alloy components: C, Si, Mn, Cr, Mo, and V, and the effect of additional alloyingelements: Ni, Cu, Co, Ti, W, Zr and Nb, on the properties of.the Si-Cr-Mo-V steel have been investigated.Critical transformation temperatures, length change due to transformation, a thermal expansioncoefficient, hardenability, elevated temperature mechanical properties as well as room-temperaturetensile and impact properties of variously heat-treated structures (quenched and tempered, normalizedand tempered) were determined. Si, which was considered to be most important alloying element, wasvery effective for the elevation in transformation temperatures and reduction in length change due totransformation. On the other hand, the variation in hardenability and transformation characteristicswith increasing Si content was relatively small. The effect of increasing Si content on the mechanicalproperties of tempered martensitic structure was small, up to 2%Si. As for normalized and temperedstructures, the variation of tensile properties with Si was small, but the impact value at the lowerhardness level was decreased as Si content was increased. By the thermal-crack resistivity measurements, it was shown that these Si-Cr-Mo-V steels have very excellent resistivity in comparison with theconventional forging-die steel.Characteristics of Si-Cr-Mo-V steels in the present investigation (basic composition: 0.35%C-1.5%Si-1%Cr-0.4%Mo-0.2%V) are considered as follows: high resistivity for softening during tempering, high yield and tensile strength at elevated temperature, relatively good ductility and toughness, andsuperior thermal-crack resistivity.