Effect of heat treatment on microstructure and mechanical properties of a Ti-bearing hypereutectic high chromium white cast iron
Introduction
High chromium cast irons (HCCIs) have long been considered candidate materials for wear-resistant parts in the mining and minerals industry due to their excellent abrasion resistance imparted by the hard (HV 1300–1800) M7C3 carbides present in the microstructure [1], [2], [3], [4], [5], [6], [7], [8]. HCCIs may be hypoeutectic, eutectic or hypereutectic compositions [9]. In most cases, HCCIs are used in hypoeutectic compositions containing 10–30 wt.% Cr and 2.0–3.5 wt.% C in engineering [9], [10]. Compared with hypoeutectic HCCIs, hypereutectic HCCIs have more volume fractions of hard and wear-resistant M7C3 carbides and are often the preferred alloys for many hardfacing applications [9], [11]. However, the hypereutectic HCCIs generally are not favored for casting, due to high scrap and high rejection rate which are mainly caused by the coarser and larger primary carbides [11].
Unfortunately, there is little research in the literature about the hypereutectic HCCIs, which is mainly focused on the refinement of primary carbides [10], [12], [13], [14], [15]. Peng and Zhang [16] reported the hardness, impact toughness and wear resistance of a hypereutectic HCCI (containing 3.7–4.0 wt.% C, 27 wt.% Cr and up to 2.5 wt.% Mo) air-quenched at the temperature of 950 °C, but they did not research the effect of heat treatment on the mechanical properties of the hypereutectic HCCI systemically. In an earlier paper [14], we added titanium into a 4 wt.% C–20 wt.% Cr hypereutectic HCCI and refined the primary M7C3 carbides apparently. In this study, the previous study was continued and the effect of heat treatment on the microstructure and mechanical properties of a hypereutectic HCCI containing about 1.0 wt.% Ti was discussed.
Section snippets
Experimental procedures
The studied alloy was prepared in a 20 kg-capacity medium frequency induction furnace. Initial charge materials were low silicon pig iron and steel scrap. Ferro-alloys such as Fe–63% Cr and Fe–60% Mo were added to a slag-free molten alloy so as to minimize the oxidation loss and slag formation. Then about 0.10% pure aluminum was added into the molten alloy to deoxidize at the temperature of 1580–1600 °C. At last, ferrotitanium (Fe–30% Ti) was pressed into the molten alloy to minimize oxidation.
As-cast microstructure and properties of the hypereutectic HCCI
Fig. 1 shows the as-cast microstructure of the hypereutectic HCCI. Fig. 2 is the XRD analysis results of the as-cast specimen. The as-cast microstructure of the specimen mainly consists of refined hexagonal-shaped primary M7C3 carbides [14] and eutectic carbides (Fig. 1a) in a matrix of austenite and pearlite (Fig. 1b), there are also a small amount of martensite around carbides, and TiC (Fig. 1c) particles in the alloy.
The compositions of the primary carbides, eutectic carbides, austenite in
Microstructural development through heat treatment
The concentration of carbon, silicon and titanium elements in the hypereutectic HCCI is relatively high (Table 1) and those elements can promote the pearlite formation and decrease the hardenability [20], [24]. So there is certain amount of pearlite in the as-cast matrix (Fig. 1b). The chromium concentration in the primary carbides is about 4 wt.% higher than that in the eutectic carbides (Table 4), which is consistent with the research of Peng and Zhang [16]. When molybdenum is added into
Conclusions
- (1)
The as-cast microstructure of the alloy mainly consists of refined hexagonal-shaped primary M7C3 carbides, eutectic M7C3 carbides, austenite, pearlite, a small amount of martensite and TiC particles.
- (2)
With the increase of the heat treatment temperature, the precipitated carbides changes from M3C to M7C3 and the retained austenite content increases.
- (3)
The bulk hardness and matrix microhardness both increase with the increase of the heat treatment temperature and reach peak values of 64.6 HRC and 850
Acknowledgement
This work is supported by the Natural Science Foundation of China, under contract no. 50571078.
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