Solidification Structure and Heat Treatment Behavior of Multi-alloyed White Cast Iron with Extensive Molybdenum Content for Applying to Hot Work Rolls

In this work, the effect of Mo content on solidification structure and heat treatment behavior of multi-alloyed white cast iron was investigated. Cast irons with varying Mo content from 0.12 to 7.66 wt% under the other alloying elements with basic composition were prepared. The solidification sequences were investigated by means of thermal analysis methods. After annealing, the specimens were hardened from 1050 °C by liquid nitrogen spray and subsequently tempered at 400–600 °C. It was found that the solidification sequence of all specimens started with primary austenite dendrite (γp) and was followed by eutectic reactions. The first eutectic reactions began with L1→(γ+MC) and are followed by L2→(γ+M7C3) for specimens with Mo less than 1.17 wt% and L2→(γ+M2C) for specimens with Mo more than 3.02 wt%, respectively. As Mo content rose, the solidification temperature range of (γ+MC) eutectic decreased but that of (γ+M2C) eutectic increased when the Mo content rose over 3.02 wt%. The area fraction of (γ+MC) eutectic decreased while (γ+M2C) eutectic increased with an increase in Mo content. With respect to heat treatment behavior, hardness in the as-hardened state increased proportionally but volume fraction of retained austenite (Vγ) dropped progressively as Mo content increased. All the tempered hardness curves showed more or less secondary hardening depending on the Mo content. The maximum tempered hardness (HTmax) was obtained at 525 °C tempering where the Vγ value was less than 3%. The highest values of HTmax, 913 HV30 for macro-hardness and 890 HV0.1 for micro-hardness, were obtained, respectively, in the specimen with 4.98 wt%Mo.