Microstructure and properties of high chromium cast irons: effect of heat treatments and alloying additions E. Karantzalis, A. Lekatou* and H. Mavros Different combinations of critical and subcritical heat treatments variously modify the initial as cast microstructure of high chromium white cast irons leading to secondary carbide precipitation of different extent and nature. Destabilisation (critical heat treatment) of austenite at 970uC for 2?5h followed by annealing (subcritical heat treatment) at 600uC for 13 h results in massive precipitation of M 23 C 6 carbide particles along with spheroidised M 7 C 3 . The reversed order of heat treatments leads to extensive precipitation of M 7 C 3 secondary carbide particles. Mo has a favouring effect on the hardness of the microstructures containing pearlite by limiting pearlite formation. The gradual increase in the alloying additions, C and Cr, increases the hardness of the materials at the different treatment states by inducing carbide precipitation. The increase in the Si content leads to the opposite effect by favouring pearlite formation. Keywords: High chromium white irons, Microstructure, Carbide precipitation, Subcritical heat treatment, Austenite destabilisation Introduction High chromium white cast irons are extensively used in applications where high friction and wear resistance are demanded, as in ore and mining industry, cement manufacturing and heavy slurry pumping applications. The attractive behaviour they show, especially as far as their wear resistance is concerned, is mainly attributed to their microstructure and the involved phases both in the as cast and after treatment conditions. Tabrett et al., 1 in their extensive review, address all the different para- meters that can affect the final microstructure of this family of materials. Their initial cast structure mainly consists of austeni- tic dendritic matrix along with a eutectic mixture of austenite and carbide M 7 C 3 . This initial morphology can be notably modified by the utilisation of various critical and subcritical heat treatments, the purpose of which is the precipitation of secondary carbides and the trans- formation of the destabilised austenite into more desirable morphologies, such as martensite. The critical heat treatments are usually conducted at 920–1060uC for 1–6 h. 1–10 The subcritical heat treatments usually follow the critical heat treatment and are carried out at 200– 600uC for 2–6 h. 1–10 Various research efforts have been focused on the formation, morphology and characteristics of the secondary carbides formed during the heat treatments. They have shown that the crystallography, stoichiome- try, orientation and extent of formation of these carbides are issues associated with compositional characteristics of the initial material and process parameters during heat treatment. 11–21 Despite the nominal high Cr content of these materials, the majority of this is engaged in the carbide structure leaving a Cr deficient matrix with low hard- enability. 22 Therefore, it is commonly accepted that, in order to increase their hardenability, further alloying elements should be added, especially in cases of large tool dimensions. 23 The most common alloying additions are Mo, Ni, Mn and Cu. 1–5,23,24 Generally, these additions prevent or enhance the pearlite formation, prevent or enhance the martensite transformation, prevent or enhance the secondary carbide precipitation and, hence, can significantly control the final micro- structure and properties. 25–39 The present research work is an initial effort to clarify the effect of different heat treatments and alloying additions on the microstuctural characteristics of high chromium white irons and their properties. Experimental procedure The alloys were prepared by induction melting at 1440– 1460uC and casting in bentonite sand moulds. Hardness was measured in HRC scale with a portable Equotip hardness tester. All castings were subjected to annealing at 600uC for 13 h. A destabilisation treatment at 970uC for 2?5 h followed. Table 1 summarises the composi- tions of the different alloys. Samples with composition of 2?35%C, 18?23%Cr and 0?58%Mo were subjected to various heat treatments, as shown in Table 2. Metallographic inspection was conducted at a Hund 600 optical microscope. A 10 wt-% ammonium Department of Materials Science and Engineering, University of Ioannina, Ioannina 45100, Greece *Corresponding author, email alekatou@cc.uoi.gr 448 ß 2009 W. S. Maney & Son Ltd. Received 2 January 2008; accepted 24 March 2009 DOI 10.1179/174313309X436637 International Journal of Cast Metals Research 2009 VOL 22 NO 6