Abstract
In the work that is described in this thesis we studied a new cast iron based composite material that was produced by a double casting technique using sedimentation sand casting. The material is based on the high Cr white cast irons (WCI) 15, 20 % Cr-Mo- LC and 25 % Cr matrixes, according to ASTM A532-75a and was designed to exceed the wear life of wear resistant materials that are used in cement, mining, coal extraction and chemical and process industries currently. The working region of the material extends to a depth of 5-8 mm below the surface and is an in situ and ex situ particle reinforced composite. WC particles of different sizes (1-3 and 3-5 mm) were selected as the ex situ reinforcements. Thus, the composite consisted of a WCI matrix, which was reinforced with WC and other transition metal carbide particles. A sand casting method was developed during which the WC particles were directed to a specific location in the ingot and were distributed uniformly in the near surface area of the composite (the working region of the material) during casting/solidification of the ingot. This ensured chemical bonding between the high Cr WCI and the particle reinforced composite (the working region) at a well defined interface parallel to the working surface. Solidification of the melt started from the WC particles around which three reaction zones were formed. Owing to partial dissolution of the WC particles and the resulting interdiffusion of elements such as W, Co, Fe, C and Cr, carbides containing Fe, Cr, W and Co were formed in the reaction zones. It is shown that current models for the interaction between a reinforcing particle and an advancing S/L interface in liquid route MMCs cannot describe the present case successfully because the solidification of the melt starts around the WC particles after the latter have settled in the near surface region of the casting. Two new approaches have been studied; in the first the WC particles travel through the iron melt and settle at the bottom of the mould and in the second WC particles settled at the bottom of the mould experience the forces of the liquid flow. The solidification paths of the three WCI matrixes and MMCs have been simulated with the Scheil-Gulliver model using the Thermo-Calc software for different iron based alloy systems, by changing the W, Cr and C concentrations. In the WCI, carbides solidify after the austenite. The increase in Cr from 14 to 25 wt. %, for fixed C (at 2, 2.5 or 3 wt. %), causes the stabilization of phases according to cementite M7C3 FCC (stable). Only for Cr up to 30 wt. % and C up to 2.5 wt. % the cementite is replaced by the BCC phase. The increase in C content from 2 to 3 wt. % for fixed Cr (at 14, 20 or 25 wt. %) has the same effect. In the MMC, the increase in W from 15 to 25 wt. % causes the stabilization of M6C. As the C increases from 2.5 to 6 wt. %, first the M7C3 is stabilised and for C > 4 wt. % the MC phase is favoured. The increase in Cr leads in the formation of the M23C6 while simultaneous increase in Cr and C promotes the M7C3 phase and restrains the formation of the M6C that resulted from the increase of W. Finally the simultaneous increase of C, W and Cr stabilize the MC phase for > 4 wt. % C. The wear of the new materials was evaluated both in an industrial scale and in the laboratory (pin-on-disc). Segments of an industrial pulverising ash mill used in the cement industry were manufactured using the casting method and materials developed in this thesis. The results showed an improvement in the wear life of the component of several times compared to the standard high Cr WCI material used by the same industry to date.