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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The High Chromium White Cast Iron HCWCI is a material highly used in the mining 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. By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples. Also, the direction in which the carbides are oriented influences the abrasive wear resistance, since, if the carbides are perpendicular to the surface being subjected to friction, it will be more affected than in the case where the carbides are oriented parallel to the same area .
Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient. It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix.
Both the as-casting and the heat treated materials were structurally characterized in order to correlate the microstructural changes with the wear behavior. In order to identify the theoretical regenida of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior. A high chromium white cast iron manufactured by a regional company was used in this investigation.
This increase in imports is caused by the better performance of the tools, as the duration of the materials is about 4-four times higher, than the tools manufactured locally. After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 :. Although the cementite is practically removed due to the high proportion of chromium found in the used HCWCI, some traces of cementite may be present. It can be observed that the samples subjected to destabilization and cooled in air present a more homogeneous distribution of finer carbides in the structure, compared with the other samples.
While the as-cast presented a lower hardness and consequently a lower wear resistance, after the heat treatments the samples showed an improvement of these characteristics, due to the precipitation of secondary carbides within the martensite matrix and reduction of retained austenite. A particular feature of the analyzed high chromium white cast iron was the presence of small amounts of M 23 C 6 carbides, which represent the Fe, Cr 23 C 6 type carbides , besides the M7C3 carbides.
It was observed that the destabilization treatment reduced the retained austenite content by a factor of from the percentage found in the as-cast samples.
Gates, “A transformation toughening white cast iron”, Journal of Materials Science 32, pp.
The high chromium white cast irons implies a good wear resistance for an extended life service . A totalof ten indentations were made on each sample andaveraged to determine the hardness of each sample. Hardness tests of the analyzed samples were performed on a Brinell hardness tester. Therefore, a certain minimum percentage of retained austenite is required in order to provide the best wear performance. Therefore, the microstructure must present a tough matrix and high volume fraction of hard chromium carbides [9, 10], such as a high carbon hard martensite matrix hardened by secondary carbides, because retained austenite reduces the hardness which might lead to a decrease in the abrasion resistance.
The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples. The microstuctural behavior of the as-received cast iron is given in Figure 4a. V is the volume of the lost material mm 3 ,H represents the material hardness BrinellP is the load used in the tests kg and L is the sliding distance mm. Different investigations were made on the effect of destabilization treatment parameters on the nature and morphology of secondary carbide precipitation as the distribution of secondary carbides in the martensitic matrix after heat treatment is known to improve the wear resistance , due to their high hardness.
However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. After the heat treatments, the cast iron presented a transformation of the primary austenite to martensite, while the secondary chromium carbides M 7 C 3 and M 23 C 6 nucleated and grew within the dendritic matrix.
Estimation of the amount of retained austenite in austempered ductile irons
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.
These retenid are similar to those found by Hinckley et al.
It was determined that the matrix structure austeenita predominantly austenite austenite dendrites proeutecticwith an approximate 1. 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. Using the diagrams in Fig.
The microstructures of the thermally treated material are presented in Fig. The hardness values wustenita each sample were determinedin order to compare the performance of the specimensafter being heat treated and to calculate the wear coefficient. Additionally, the secondary carbides developed a typical laminar form because of the phase changes for both the matrix and the secondary carbides, due to the thermal change that occurs. The lowest values, around As it can be seen from Fig.
Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. The microstructure of the as-cast presented an austenitic matrix austenite retenidz proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries. According to Liu et al.
This paper studies the effects of different cooling media after a destabilization treatment on the microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron. The XRD analysis revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage of The best 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.
Austempered ductile cast irons
Along with the material composition and processing conditions the wear behavior is also influenced by heat treatment , which leads to a suitable rteenida , as the thermal activation provided by heat treatment allows precipitation of chromium carbides [12, 13]. According to Zhang et al. 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.
Following the investigation of Bedolla-Jacuinde et zustenita. To ensure therepeatability of the test and to reduce the error in their results, five samples were used.