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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specimens (2 x 2 x 50 mm) for bending strength tests and fracture toughness test. When verifying the impact of the additional alloying elements, it was investigated that if they were introduced separately into the Fe-C-Cr base material, the predominantly austenitic matrix was only obtained by alloying more than 2.5 % Mo . Even at high levels, Ni did not produce the desired increase in austenite content, and the Mn self alloying gave a completely negative result because the casting brittleness increased. Further tests have shown that the role of deficient Mo is irreplaceable, but the lower limit of Mo can be reduced to 0.7% if the cast iron is additionally alloyed with either 1% Ni or 1.5% Mn . A combination of 0.7% Mo + 1% Ni + 1.5% Mn , which at a C content of about 3% and a Cr content of 17-24%, yields a matrix of approximately 60% austenite in the cast state, even at walls thicker than 50 mm and cast into sand molds (Murgaš 2002) . The results of the tests showed that castings with a predominantly austenitic matrix had a lower hardness, but their abrasion resistance, which ranged from 2.8 to 4.6%, was comparable to or higher than that of martensitic matrices with the same C and Cr content. This finding was consistent with Walton (1981) and Laird et all (2000), where the high abrasion resistance of this type of austenitic matrix is explained by the fact that plastic deformation at the groove site causes martensite to occur not only in the deformed areas but also in their immediate environs, thereby strengthening relatively large volumes material. The pattern of the microstructure of the cast iron without heat-treatment in the wall thickness of about 4 mm of the plaster die nozzle is shown in Fig. 3. It consists of an austenitic matrix in which there are a large number of massive primary carbides predominantly of the M 7 C 3 type with small islets of eutectoid - a mechanical mixture of ferrite and discrete fine carbides in which fine eutectoid carbides occur. The sample was electrolytic polished in VUZ solution no.16 and etching agents for anticorrosive steels for better structure visibility. For preparing the sample surface in Fig.1 was used etchant HCl + HNO 3 + glycerin (Vilella).

Fig. 3. Microstructure of abrasive resistant cast iron in 4mm thick wall

The obtained complex results allowed a comprehensive assessment of the impact of the individual components of the cast iron. Abrasion resistance increases almost linearly with C and Cr rising. When the 3% C threshold is exceeded, the brittleness is intolerable increased and the increase in Cr content above 22% has no significant effect on abrasion resistance. Alloying by Mo has a significantly positive effect for abrasion and fracture resistance.