TiC-FeCr local composite reinforcements obtained in situ in steel casting
Graphical abstract
Introduction
Metal-matrix composites (MMCs) based on Fe alloys reinforced with ceramic particles are characterized by good mechanical properties and high wear resistance. The majority of the research being realized is focused on the application of titanium carbide (TiC) as a reinforcing phase, which results from its favorable properties. Fraś et al. (2000) mentioned the following regarding Fe-based alloys: their high hardness, high melting point, chemical and thermodynamic stability, and (above all) excellent wettability. The manufacture of locally TiC-reinforced materials is possible through the application of technologies such as laser cladding, thermal spraying, surfacing, TIG welding, or powder metallurgy.
At present, considering the available technologies of MMC manufacturing, the best results are obtained from the application of the combustion reaction in the SHS method (Self-propagating High-temperature Synthesis). In their survey work on manufacturing MMCs using the SHS method, Tjong and Ma (2000) indicated that it allows us to obtain high-purity products with low energy efficiency in a short amount of time. The important characteristic feature of the SHS method is the spontaneously proceeding reaction of product formation that runs until the substrates become exhausted. This is confirmed by Wang et al. (2005a,b), who evaluated the temperature of the SHS reaction in an Ni-Ti-B4C configuration. They showed that the temperature of a synthesis of TiC- and TiB4-type ceramic phases is significantly lower than the Ni and Ti melting points. This result indicates that SHS reactions are initiated by releasing the heat generated during the reaction of substrate synthesis in the solid state. However, it should be noted that the SHS method also has its disadvantages. One of these is the risk of obtaining products with high porosity. This problem was described by Moore and Feng (1995) in their survey articles in which they estimated the levels of porosity sometimes reaching 50% of the theoretical density of compressed powders. Within the space of dozens of years, this problem was investigated in many research works. Usually, research teams were focused on the elimination of porosity by applying mechanical loading while the temperature of the final product was sufficiently high. As an example, LaSalvia et al. (1992),1996) proposed the method of pressing at elevated temperatures while applying adequately high dynamic loading. Olevsky et al. (2002) and Strutt et al. (2008) described a method allowing for the elimination of porosity in sintered final products by the application of isostatic pressing. This technique consists of placing PM compacts in a special chamber filled with a medium transmitting equal pressure in all directions. This may be applied at both low and high temperatures. However, the application of such techniques in series production is too complicated and costly. One of the methods that allows us to obtain ceramic phases in situ in composite materials of high quality is the application of casting technologies. In connection with this, the SHS method found its application in the manufacture of MMCs based on the reaction of TiC in-situ synthesis in Fe and its alloys.
This technique has many advantages when compared with conventional methods. Wang et al. (2001) described the method of in-situ manufacturing of TiC and VC composite reinforcements with the application of casting technologies. The results of their investigations indicated the possibility of manufacturing ceramic composite reinforcements in the form of fine-structured TiC while eliminating the formation of conglomerates at the same time. Wang and Wang (2007) and Zhang et al. (1999) proved that the methods of the in-situ manufacturing of reinforcing phases in Fe-based alloys allow us to obtain better integration with the matrix (interfacial properties) at the boundary between the ceramic phase and metallic matrix (which additionally remains free from impurities). Guang et al. (2002) indicated that the manufacturing of MMCs with the application of casting technology provides great freedom in producing elements of assumed shapes, and the technological process itself is simple and relatively cheap. Attempts of the in-situ manufacturing of TiC as a reinforcing phase in Fe or Fe-based alloys with the application of casting were made by many research teams. The majority of the realized experiments consisted in placing profiles containing pure substrates of TiC formation as well as the addition of boron carbides in sand casting molds. The factor initiating the reaction of the synthesis of ceramic phases was the high temperature of the casting alloy. This phenomenon is described as reactive infiltration. Wang et al. (2005a; 2005b) manufactured cylindrical preforms containing Ni, Ti, and B4C, placing them in a sand mold poured with a liquid casting alloy that initiated the synthesis. As a result, they obtained the reinforcement of a base alloy in the form of TiB2 – TiC. Due to their density differences, the synthesized suspended preforms moved towards the gates of a casting, from where the samples for further research were taken. Feng et al. (2005) combined the SHS synthesis reaction with conventional casting technologies. Similarly, they placed a mixture of reacting substances Ti and C in a sand mold. As a result of metallostatic pressure and its impact, the packets became disintegrated during the filling of the mold cavity, being then uniformly distributed within the volume of the casting. Similar solutions were also presented by Hu et al. (2013) and Jiang et al. (2006), who prepared packets containing compressed powders of ferrotitanium or pure titanium and graphite in order to manufacture TiC composite reinforcements in steel castings. The packets were placed successively in a sand mold, and the TiC synthesis reaction was initiated by the high temperature of the alloy steel. After crystallization and cooling, the composite castings were knocked out of the casting molds and cut in order to take samples for further research.
The majority of the research works mentioned above were focused on the manufacture of a composite material followed by its characterization from the point of view of structure, microstructure, mechanical properties, or even heat-effect measurement using thermal analysis methods during the TiC synthesis reaction. However, it is extremely difficult to find research in which the authors reveal the macrostructures of the composite reinforcements manufactured in situ. An evaluation of the macrostructure should be the starting point for an analysis of the local composite reinforcements obtained in situ in castings. This seems to be very important, considering the use of MMCs in industrial applications where the properties of a composite material should be uniform on a working surface.
In his work, Merzhanov (1996) determined the enthalpy of TiC formation (amounting to −187 kJ/mol) resulting in intensive energy production in the form of heat within the area of the composite zone manufactured in situ. This phenomenon is unfavorable due to local temperature increases, resulting in composite zone fragmentation. The composite zone is separated into parts that, due to their lower density and convective motion, move within a casting mold [Olejnik and Jesiołowska, 2017 International Patenet]. One of the latest methods of confining composite zone fragmentation is the application of a moderator addition, significantly decreasing the heat effect within the area of TiC synthesis reaction. This method allows us to manufacture local composite reinforcements (LCR) in castings characterized by homogeneous structures, high mechanical properties, and high functional qualities.
The moderator is to be defined as a mixture of metallic powders with non-metals that, as a result of crystallization processes, can influence the structure of the composite zones. The moderator additive ensures control of the SHS process during the metallurgical process through the effective abortion of excess heat energy; thanks to this, it is possible to obtain composite zones with predictable dimensions. In addition, the use of a moderator in the form of a powder beneficially affects the nucleation kinetics and crystal growth during the synthesis reaction. The composite zones generated in situ are characterized by a large number of TiC crystals, which are characterized by even distribution; this directly affects the abrasion resistance of the composite material. The selection of a moderator with a specific chemical composition affects the microstructure of the composite zones and their mechanical properties. The matrix of the generated composite zone in situ is characterized by different properties than the cast base alloy. For example, the introduction of a moderator additive that is a mixture of powders with a chromium cast iron composition allows us to obtain a ceramic phase in the form of TiC as well as precipitations of chromium carbides, which additionally increase the hardness and abrasion resistance of the composite zone. On the other hand, the addition of a moderator with the chemical composition of Hadfield cast steel will allow us to increase the impact strength of the composite zone as a result of the mechanism of self-decontamination and the resulting defects of the structure – the twinning mechanism. The method described above is a good candidate for the production of functional composite parts included in machines that operate under conditions of strong abrasive wear. This allows for the process of consciously designing the composite by choosing the right base alloy and moderator compositions that facilitates the fabrication of a material with comprehensive properties that take the environment and abrasive wear mechanism into account.
The objective of this research was the manufacturing of a steel casting reinforced locally with composite zones containing a TiC reinforcing phase. Due to the exothermic character of a TiC synthesis reaction, the addition of a moderator showing the composition of high-chromium cast iron was introduced into the initial powder mixtures. This was done in order to stabilize the process and obtain homogeneous composite reinforcements. The manufactured composite zones were subjected to examinations of the macrostructure and microstructure with scanning electron microscopy (SEM) and structural analysis using the X-ray diffraction method (XRD). Subsequently, the mechanical properties and functional quality were analyzed by means of Vickers hardness testing as well as abrasion testing using the ball-on-disc method.
Section snippets
Experiment
The mixture of substrates necessary for TiC in situ formation was prepared with an atomic ratio of 1 : 1. The commercial titanium powder by Stanchem, Poland (99.95% purity, 44 μm average diameter) and flake graphite by Sinograf, Poland (purity above 96%, 3 μm average diameter) were used. In order to control the process of reactive infiltration, the addition of 30, 50, 70, and 90 wt.% of moderator showing the composition of high-chromium cast iron was introduced into the mixture of titanium and
Macrostructure
The macrostructure of the LCR specimens is presented in Fig. 2. The observations carried out did not reveal the occurrence of gas-related casting defects within the regions in neither the LCR nor the base alloy. Table 4 collects the results of measurements of the widths of the manufactured composite zones. A quality analysis of the LCR dimensional stability showed that the A and B zones underwent partial fragmentation. This is manifested by the separation of the composite zone region with a
Conclusions
The local composite reinforcements were manufactured in a cast steel casting as a result of the TiC in situ synthesis, following the reactions proceeding directly in a casting mold cavity. The introduction of the moderator showing the composition of high-chromium cast iron allowed us to confine the process of reactive infiltration, resulting in dimensionally stable composite zones being obtained. The macroscopic observations revealed that, with increasing percentages of the moderator in the
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2022, Materials Chemistry and PhysicsCitation Excerpt :This is not desired that the local temperature increase leads to the fragmentation of the composite zone. E. Olejnik effectively reduces the reaction heat effect of TiC synthesis in the composite zone of Fe–Cr type and Fe–Mn type steel-based materials by adding a moderator to the preforms [36,37]. Unfortunately, most of the published papers add multi-phase moderators to the preform, which increases the complexity of the in-situ reaction process and makes it difficult to accurately explore the phase transition laws.