PSI - Issue 17

Berta Igor et al. / Procedia Structural Integrity 17 (2019) 509–513 Berta, I., Pokusová, M./ Structural Integrity Procedia 00 (2019) 000 – 000

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continuous operation, there occurs a massive wear of the material and the knife need to be replaced.

Fig. 1. A knife for grinding cocoa beans after several weeks of operation compared to the original shape

Abrasion resistant chromium cast iron is well-known because of its extremely high abrasion resistance, but with very limited practical applicability due to its brittleness and very low fracture resistance. However, their further research in our laboratories shows that the plastic properties of chromium cast iron can be effectively improved by appropriate alloying, opening up a wide range of applications for this material category. 2. Theoretical background All basic metallurgical textbooks as a Walton (1981), Laird (2000), Murgaš (2003) mention that the resistance of chromium cast iron increases with increasing C and Cr content. The 0.3 % Cr content prevents graphitization in thin walls of the castings and the 2 % Cr content causes the cast iron to always solidify as white (without graphite). Increasing the amount of carbides by increasing the C and Cr content is beneficial only in the eutectic region. When it has exceeded the higher values of this elements, hypereutectic carbides are formed, which then coarsen and increases in number, thereby increasing the brittleness of the casting material. Thus, the resistance depends on the shape, type, size and amount of carbides, matrix properties and bond strength of the matrix-carbide. At a content of less than 10 % Cr only carbide (FeCr) 3 C is formed in the structure, the hardness is of 840 - 1100 HV30. An increase in abrasive resistance can be achieved by alloying 3 to 5% Ni and 1% Mo , at which the martensitic matrix is already formed. A significant increase in abrasion resistance is achieved when alloying more than 10% Cr , where more carbides, especially (FeCr) 7 C 3 , have a hardness of 1200 to 1800 HV30. In practice, cast iron containing 16 to 28% Cr and 2.5 to 3.2% C with M 7 C 3 carbides and small amounts of M 23 C 6 carbides have proven their worth. Massive eutectic carbides are known to cause high brittleness of the material and are splitting out from the matrix by abrasion. This phenomenon is possible to restrict by replacing the part of Cr with 2-4% of W , V and Ti . These elements create the special carbides ( WC , VC and TiC ), which are induced at higher temperatures independently of M 7 C 3 i.e. M 23 C 6 . These carbides are smaller and the cast iron is not as brittle. The appreciable increase in toughness and fracture resistance of cast iron can only ensure that some austenite is formed in the matrix. The mainly austenitic matrix can be achieved by alloying more than 2.5 % Mo or by combining 0.6 % Mo + 1 % Ni + 1.5 % Mn to provide a matrix with content when casting in sand molds even at walls larger than 50 mm about 60 % austenite. The dominantly austenitic structure of matrix gives cast iron higher abrasion resistance than pearlite and comparable to martensitic structure. This positive result is wear caused the transformation of austenite to martensite not only in deformed areas (on the surface of the grooves) but also in their surroundings. Reinforcing relatively large volumes of transformed austenite gives the cast iron high resistance to abrasive wear even at low surface pressures. The well-known low resistance of the austenitic matrix of Hadfield