Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect ScienceDirect Structuralonline Integrity 00 (2019) 000–000 Available atProcedia www.sciencedirect.com Structural Integrity Procedia 00 (2019) 000–000 ScienceDirect www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia Procedia Structural Integrity 17 (2019) 509–513 ICSI 2019 The 3rd International Conference on Structural Integrity ICSI 2019 The 3rd International Conference on Structural Integrity Austenitic cast iron resistant against the abrasive material Austenitic cast iron resistant against the abrasive material degradation degradation Berta Igor*aa, Pokusová Marcelaaa Berta Igor* , Pokusová Marcela Slovak University of Technology, Faculty of Mechanical Engineering, Institute of technologies and materials, Námestie slobody 17, 812 31 Slovak University of Technology, Faculty of Mechanical Engineering, Institute of technologies and materials, Námestie slobody 17, 812 31 Bratislava, Slovak republic, [email protected] Bratislava, Slovak republic, [email protected] a a Abstract Abstract In producing processes there are components which are exposed to a high degree of abrasion. This paper presents the criteria for In there are components are exposed to athat high degree of abrasion. paper presents the criteria for theproducing selecting processes chemical composition for whitewhich chromium cast iron has the high abrasionThis resistance and increased fracture the selecting chemical for are white chromium cast iron thatofhas high abrasion andthat increased fracturea resistance. The results composition of experiments focused on the selection the the alloying element resistance combination can produce resistance. Theaustenitic results ofmatrix experiments onthe thecarbide selection of the alloying element combination thatabout can produce predominantly with a are big focused portion of components. The paper brings information developeda predominantly matrix big portion of the carbide components. Theabrasion paper brings information about compositions ofaustenitic chromium cast with irons ahaving high resistance against fracture and in as-cast state and alsodeveloped the basic compositions ironsand having high resistance against and abrasion in in as-cast state and also the basic information onofthechromium producingcast method service properties of these castfracture irons successfully used practice. information on the producing method and service properties of these cast irons successfully used in practice. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published by Elsevier B.V. B.V. © 2019The TheAuthors. Authors. Published by Peer-review under responsibility of Elsevier the ICSI organizers. Peer-review under responsibility of the ICSI 2019 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers. Keywords: abrasion; cast iron; austenitic Keywords: abrasion; cast iron; austenitic 1. Introduction 1. Introduction The operation of machines and structures can lead to various causes and conditions of degradation material The operation machines and structures canaging, lead to various induced causes and conditions of degradation material mechanisms, like of corrosion, brittleness and steel vibrations failures, and others. Some of them are mechanisms, like corrosion, brittleness and steel aging, vibrations induced failures, and others. Some of them are difficult to predict at the design process (Garan 2019, Chmelko 2016). difficult to predict at the designresistance process (Garan 2016). Demands on high abrasion are be 2019, often Chmelko required e. g. in building industries (earthmoving machinery), Demands on high abrasion resistance are be often required e. g. in building industries (earthmoving in mixing granulates and bulk mixtures or in compacting of materials. Some material used for raw foodmachinery), processing in mixing granulates and bulk mixtures or in compacting of materials. Some material used for rawAfter food 3processing can also have high abrasiveness - Fig. 1 shows a knife image used for grinding of cocoa beans. weeks of can also have high abrasiveness - Fig. 1 shows a knife image used for grinding of cocoa beans. After 3 weeks of * Corresponding author. Tel.: +421-57296851 *E-mail Corresponding author. Tel.: +421-57296851 address: [email protected] E-mail address: [email protected] 2452-3216 © 2019 The Authors. Published by Elsevier B.V. 2452-3216 2019responsibility The Authors. of Published Elsevier B.V. Peer-review©under the ICSIby 2019 organizers. Peer-review under responsibility of the ICSI 2019 organizers. 2452-3216 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ICSI 2019 organizers. 10.1016/j.prostr.2019.08.067 Berta Igor et al. / Procedia Structural Integrity 17 (2019) 509–513 Berta, I., Pokusová, M./ Structural Integrity Procedia 00 (2019) 000–000 510 2 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)3C 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) 7C3, 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 M7C3 carbides and small amounts of M23C6 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 M7C3 i.e. M23C6. 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 Berta Igor et al. / Procedia Structural Integrity 17 (2019) 509–513 Berta, I., Pokusová, M./ Structural Integrity Procedia 00 (2019) 000–000 511 3 steel (13% Mn, 1.3% C) to abrasion applying at low pressures is due to the fact that the martensite is formed only in deformed regions. The generally known shortcoming is the brittleness and low resistance to fracture under abrasive loading with impacts. 3. Realization Due to the chosen orientation of the Fe-C-Cr system and regarding to the required good foundry properties (given by the minimum wall thickness of 3 mm), the selection was narrowed to sub-eutectic up to eutectic chromium cast iron with a content wt. % of C from 2.2 to 3.3 and Cr from 17 to 24. Such cast iron containing only basic components C and Cr have, at real cooling rates of castings a structure composed of coarse carbides C (FeCr)3C, C1 (FeCr)23C6 and C2 (FeCr)7C3 in a martensitic-pearlite matrix practically free of austenite, i.e. consistent with the stable phase diagrams of the Fe-C-Cr system according to Giršovič (1978) and Laird et all (2000) which are shown in Fig. 2. The casting alloyed only Cr in a C content range from 2.5 to 3.3 wt. % is extremely fragile and therefore the work was aimed at increasing its toughness by forming a sufficiently high proportion of austenite in the matrix and refining the carbide phases by micro-alloying. Fig. 2. Section in ternary stable diagram Fe-C-Cr: a) 17 wt% of Cr; b) 24 wt % of Cr. Toughness depends crucially on the amount of austenite. Its representation in the matrix can be supported either by alloying with -region expanding elements, or by those that shift the temperature of the martensitic transformation to low temperatures, so that metastable non-transformed austenite remains in the matrix. According to Giršovič (1978) and Walton (1981), elements Ni, Mn and Mo have come into consideration in terms of the technological properties and economics. The experimental casts were performed in a folding laboratory arc furnace with a crucible of 5 kg. The chemical composition of the melt of the realized casts was chosen to provide a review of the properties of the basic Fe-C-Cr system and together with the influence of selected types of alloying elements. From the specifically prepared melt by the furnace tilting was cast directly into the body the groups of specimens for abrasion resistance tests, metallographic tests, wedge specimens for leak in tests and small prismatic bar 512 4 Berta Igor et al. / Procedia Structural Integrity 17 (2019) 509–513 Berta, I., Pokusová, M./ Structural Integrity Procedia 00 (2019) 000–000 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 selfalloying 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 M7C3 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 + HNO3 + 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. Berta Igor et al. / Procedia Structural Integrity 17 (2019) 509–513 Berta, I., Pokusová, M./ Structural Integrity Procedia 00 (2019) 000–000 513 5 Alloying by Ni was slightly positive, but only when it was applied simultaneously with Mo. Additions B in an amount of about 0.01% or a combination of 0.05% Ti + 0.05% V, which are introduced into the melt just before casting, also have a beneficial effect. The effect of Mn on abrasion has not been demonstrated, but it was presented in the final composition, since it significantly improves the fluidity with contents above 1%. A further increase in fluidity was achieved after melt processing just before the end of melting with CaF2-based synthetic slag, CaO and Al2O3 in an amount of about 0.1%. Surprisingly high melt penetration with the Mn content of about 2% allowed casting bars of additional material to be welded 5-6 mm long 400-500 mm. 4. Conclusion For higher abrasion resistance, the chrome cast iron having a continuous closed carbide chain in microstructure with a predominantly austenitic matrix was designed. The hardness of this material reaches 65-68HRC, the carbides reach a hardness above 2000 HV30. The recommended composition is as follows: C 2,8-3,1 Cr 20-24 Mn 0,8-2 Si 0,6-1,0 Ni 0,8-1,3 Mo 0,7-1,0 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 significant positive effect for abrasion and fracture resistance. Alloying by Ni was slightly positive, but only when it was applied simultaneously with Mo. Additions B in an amount of about 0.01% or a combination of 0.05% Ti + 0.05% V, which are introduced into the melt just before casting, also have a beneficial effect. The effect of Mn on abrasion has not been demonstrated, but it was presented in the final composition, since it significantly improves the fluidity with contents above 1%. Acknowledgements This work was supported by the Slovak Research and Development Agency under the contract No. APVV-170666. 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