High Chromium Cast Iron: Part One
High chromium cast irons (HCCI’s) exhibit very good mechanical properties and offer benefits for a range of manufacturing applications.
With carefully controlled heat treatment the cast iron properties can be further influenced to yield higher strength and ductility depending on the desired application.
High-chromium white iron is an erosion resistant ferrous alloy widely used in manufacturing, it has for long been applied to components in mining and minerals industry due to its excellent abrasion resistance, imparted by the hard alloy eutectic carbides present in the microstructure. Furthermore heat treatment can improve properties, all of interest, depending upon the particular applications.
High-chromium irons have good mechanical properties, high hardness in a cast state, low fluctuation of hardness when properties change. High-chromium irons almost don’t concede in fluidity to regular gray cast iron that allows using parts sufficiently difficult form directly after casting without the following machining.
Table 1: Chemical composition of high-chromium wear resistant irons
The mechanical properties of High chromium cast iron HCCI mainly depend on type, size, number, morphology of hard carbides and the matrix structure (γ or α). The hypereutectic HCCI with large volume fractions of hard carbides is preferred to apply in wear applications. However, the coarser and larger primary M7C3 carbides will be precipitated during the solidification of the hypereutectic alloy and these will have a negative influence on the wear resistance.
As mentioned above, high chromium cast irons (HCCIs) are mainly used in mining, milling, earth-handling and manufacturing industries which require materials with exceptional wear and corrosion resistance. The exceptional wear resistance of high chromium cast irons is due to the high volume fraction of hard chromium carbides, although the toughness of the matrix also contributes to the wear resistance. The high controlled percentage of chromium helps to retard the formation of graphite and stabilize the carbides.
The article of D. Kopyciński et al. presents results of heat treatment on the high chromium cast iron. The study was carrying out on samples cut from the casting made from chromium cast iron. Those were hardened at different temperatures, then tempered and soft annealed.
Hardness was measured at three points sample, and two at the edges. Figures 1 and 2 show results of hardness after hardening, tempering Vickers and Rockwell method respectively hardness of as-cast sample.
Test results indicate that with proper selection of heat treatment parameters the hardness and composition of phase microstructure of chromium cast iron can be controlled. Hardening affects positively the hardness of castings, whereas tempering and soft annealing improves ductile properties of chromium cast iron. By adjusting the heat treatment parameters, the material properties can be customized for a particular application. The studies let to determinate the optimum heat treatment for this type of cast iron. The best properties were obtained for hardening at 950°C, for other temperature of heat treatments the hardness increased and exceeded 60 HRC units.
Table 2: Heat treatment parameters for individual samples
Figure 1: The Vickers hardness of the samples tempering and soft annealing, and as-cast
Figure 2: The Rockwell hardness of the samples after hardening, tempering and soft annealing, and as-cast