Solidification and abrasion wear of white cast irons alloyed with 20% carbide forming elements

Abstract

Three different white cast irons with compositions of Fe–3%C–10%Cr–5%Mo–5%W (alloy no. 1), Fe–3%C–10%V–5%Mo–5%W (alloy no. 2) and Fe–3.5%C–17%Cr–3%V (alloy no. 3) were prepared in order to study their solidification and abrasion wear behaviors. Melts were super-heated to 1873 K in a high frequency induction furnace, and poured at 1823 K into Y-block pepset molds. The solidification sequence of these alloys was investigated. The solidification structures of the specimens were found to consist of austenite dendrite (γ); (γ+M 7 C 3 ) eutectic and (γ+M 6 C) eutectic in the alloy no. 1; proeutectic MC; austenite dendrite (γ); (γ+MC) eutectic and (γ+M 2 C) eutectic in the alloy no. 2, and proeutectic M 7 C 3 and (γ+M 7 C 3 ) eutectic in the alloy no. 3, respectively. A scratching type abrasion test was carried out in the states of as-cast (AS), homogenized (AH), air-hardened (AHF) and tempered (AHFT) using the abrasive paper with 120 mesh SiC and 10 N application load. In all the specimens, the abrasion wear loss was found to decrease in the order of AH, AS, AHFT and AHF states. Abrasion wear loss was lowest in the specimen no. 2 and highest in the specimen no. 1 except for the as-cast and homogenized states in which the specimen no. 3 showed the highest abrasion wear loss. The lowest abrasion wear loss of the specimen no. 2 could be attributed to the fact that it contained proeutectic MC carbide, eutectic MC and M 2 C carbides having extremely high hardness. The matrix of each specimen was fully pearlitic in the as-cast state but it was transformed by heat-treatments to martensite, tempered martensite and austenite. From these results, it becomes clear that MC carbide is a significant phase to improve the abrasion wear resistance of white cast iron. Keywords Solidification Abrasion wear White cast iron Heat-treatments 1 Introduction Alloyed white cast irons with many kinds of strong carbide-forming elements are recently developed wear resistant materials for the application to the hot strip and mineral pulverizing mills [1–11] . They contain carbide forming elements such as Cr, V, Mo and W, and their carbon content is relatively higher than that of high-speed tool steel with similar alloying elements. MC, M 2 C, M 6 C, M 7 C 3 and NbC carbides can be precipitated as proeutectic and/or eutectic carbides during solidification. In addition, the matrix can also be varied by the heat-treatments such as air-hardening and tempering, and particularly hard matrix can be obtained due to the precipitation of numerous fine secondary carbides and the transformation of matrix from austenite to martensite. Properties such as abrasion wear resistance, surface roughening resistance and seizing or sticking resistance are essentially important to these alloyed white cast irons used for the rolls and other wear resistant parts of steel rolling and mineral pulverizing mills. Among these properties, the abrasion wear resistance is reported to be dependent upon not only type, morphology, amount and distribution pattern of the carbides precipitated from the melt, but also the type of matrix structure. Nevertheless, the abrasion wear behavior of these irons was little researched systematically. In this work, alloyed white cast irons with three different chemical compositions were selected for the investigation of solidification and abrasion wear behaviors. In order to clarify the solidification process, the specimen was quenched into water from different temperatures during thermal analysis to interrupt freezing. On the other hand, heat-treatments such as air-hardening and tempering were employed to obtain the different type of matrix structures. Then, the effect of carbide type and matrix structure on the abrasion wear resistance was investigated using a scratching type abrasion wear testing machine. The worn parts of the specimens were also examined by the scanning electron microscope (SEM) to derive the mechanism of abrasion wear. 2 Experimental procedure 2.1 Specimen preparation To obtain the alloys with several combinations of the different carbides, the four alloying elements such as Cr, Mo, W and V were designed so as to make their sum approximately 20 mass%. The alloys were melted in a 15 kg-capacity high frequency induction furnace. Initial charge materials were clean pig iron and steel scrap. Ferro-alloys such as Fe–60%Cr, Fe–80%V, Fe–60%Mo and Fe–75%W were added to a slag-free molten iron so as to minimize the oxidation loss and the slag formation. The melt was subsequently super-heated to 1873 K and transferred into a pre-heated teapot ladle. After removal of any dross and slag, the melt was poured at 1823 K into the pepset molds to produce Y-block ingots. The chemical analysis, density and co-existent carbides of the alloys are shown in Table 1 . 2.2 Thermal analysis For studying the solidification sequence of each alloy, 50 g were remelted at 1723 K in an alumina crucible using a silicon carbide resistance furnace under argon atmosphere. Then, the molten iron was cooled at the rate of 10 K/min to reveal all the solidification reactions of the alloy through a cooling curve. Based upon the cooling curve, the molten iron was quenched into water from several temperatures on the way of thermal analysis. 2.3 Heat-treatment Before air-hardening and tempering, the as-cast specimens were homogenized at 1223 K for 5 h under vacuum atmosphere. Then, they were air-hardened in forced air after austenitizing at 1323 K for 2 h in vacuum atmosphere and followed by tempering at 573 K for 3 h. 2.4 Measurement of austenite The volume fraction of austenite was calculated from the ratio of peak areas of (2 0 0) and (2 2 0) for ferrite (α) and martensite (M), and those of (2 2 0) and (3 1 1) for austenite (γ). The diffraction patterns were obtained by employing a...