Los resultados muestran que a pesar de que el enfriamiento al aire, seguido por inmersión en CO2, puede reducir eficazmente la austenita retenida, esto no es. microestructura del material está formada por dendritas finas de austenita men de austenita retenida depende de manera crítica de los parámetros del. microestructuras son extraordinariamente duras ( HV) y resistentes (2,5 GPa) . Palabras clave. Bainita. Austenita retenida. Aceros. Transformaciones de fase.
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Also, the secondary carbides are distributed more homogeneously in the treated microstructures than in the as-cast one, this behavior was also found by Wang et al.
In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. As the martensitic structure is recognized to provide a higher wear resistance, it was assumed that reducing the retained austenite to low percentages would lead to a better wear behavior.
Therefore, the as-cast microstructure is made of dendrites, which remain fully austenitic at room temperature, while the eutectic micro-constituent is a continuous network of chromium-rich carbides and eutectic austenite, similar to the investigation realized by Hann et al.
The austeenita combination of hardness and wear resistance was found in the samples cooled in air, due to the percentage of retained austenite and a moderate precipitation of chromium carbide. The high demand of these materials led to the increment of imports in Colombia, while the local companies, that produce these materials, became a second option for the buyers.
This hardness value is lower than the one obtained by Marathray et al. According to Zhang et al.
The results show that although air cooling followed by immersion in CO 2 can effectively reduce the retained austenite, this is not enough to transform completely the retained austenite into martensite.
The high chromium white cast irons implies a good wear resistance for an extended life service . According austenjta Liu et al.
During the heat treatment, the ferrous matrix is supersaturated with carbon and chromium leading to the precipitation of secondary carbides. The chemical austenitq of the studied high chromium white cast iron was marked with 1 in Fig.
The study is performed in order to determine the most suitable microstructure along with improved mechanical properties of HCWCIs produced in Colombia, through an appropriate heat treatment that could increase the wear resistance and hardness, and thus improving the production approach to international standards, and helping the local industries to strengthen their position in the international market. The resulting carbide percentage was around It should be noted that in the analyzed materials, the a phase is mainly associated to the ferrite phase.
Hardness tests of the analyzed samples were performed on a Brinell hardness tester.
Thus, the high degree of strain hardening that occurs in the austenitic matrix, as a result of the plastic deformation caused by the normal and the tangential forces of the moving abrasive particles, leads to a lower wear resistance in the as-cast material . It is presumed that the behavior of this kind of Colombian materials, is caused by the large percentage of retained austenite, due to a austebita treatment performed improperly . Thus, when the undercooling is smaller because of the heat released by the formation of the M 7 C 3 carbidesthis type of carbide shape is favored .
The hardness values for each sample were determinedin order to compare the performance of the specimensafter being heat treated and to calculate the wear coefficient.
The High Chromium White Cast Iron HCWCI is a material highly used in the retenifa and oil industry, to manufacture crushing hammers and drilling rigs, due to the presence of a significant proportion quantity of chromium rich carbide phase in their microstructures.
Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles.
Estimation of the amount of retained austenite in austempered ductile irons
High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance. Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: Gates, “The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron”, Mater.
To ensure therepeatability of the test and to reduce the error in their results, five samples were used. The lowest values, around austeniha The microstructures of the thermally treated material are presented in Fig.
It can be seen that the as-received cast iron presents a lower hardness and higher values of volumetric loss and wear coefficient than the heat treated samples, showing the dependence of the wear behavior on the matrix microstructure.
Austempered ductile cast irons
By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples. An additional influence on the wear behavior is given retenlda the secondary carbides , which improves the mechanical strength , through increasing the matrix strength.
In order to obtain a better wear performance, the high chromium white cast irons should present a martensitic structure, because the martensitic formation, compared to aaustenita austenitic, minimizes cracking and removal during wear.
As it can be seen from Fig. A correlation between hardness and wear behavior volumetric loss and wear coefficient is given in Fig.