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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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 revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage of This increased hardness could be the result of the precipitation of secondary carbides, which destabilized the austenite leading to the formation of a martensite matrix, by increasing the matrix strength through a dispersion hardening effect; the fine secondary carbides can increase the mechanical support of the eutectic carbides .
The microstructure of the as-cast presented an austenitic matrix austenite dendrites proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries.
This behavior could be due to the increase of carbides without reyenida matrix support , leading to a reduced toughness, which resulted from brittle carbides. This paper studies the effects reteniida different cooling media after a destabilization treatment on the microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron.
The material composition is summarized in Table I.
Following the investigation of Bedolla-Jacuinde 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.
Additionally, the secondary carbides developed a typical laminar form because of the phase changes for both the matrix and the secondary carbides, austeinta to the thermal change that occurs.
According to Bedolla-Jacuinde et al.
Austempered ductile cast irons
Upon cooling, the austenite matrix becomes martensite because of the secondary carbide precipitation. Given the above problem, the aim of the present investigation is to establish the effect of different cooling media used after destabilization treatment on the wear resistance of a white cast iron.
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.
The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples. However, the diffraction when quenching in oil is run to the left and presents interferences.
Therefore, the as-cast microstructure is made of dendrites, which remain fully austenitic retenica 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. 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 was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1.
Estimation of the amount of retained austenite in austempered ductile irons
These results are similar to those found by Hinckley et al. 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.
In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. The microstuctural behavior of the as-received cast iron is given in Figure 4a.
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.
While it was considered that the presence of residual austenite in the microstructure causes volumetric expansion which may also lead to microcracks because of the developed stresses, some investigations determined that a certain percentage of retained austenite could improve the abrasion resistance, due to its work-hardening properties [3, 4], ductility and thermodynamic metastability at room temperature .
The chemical composition of the studied high chromium white cast iron was marked with 1 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.
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 microstructures of the thermally treated material are presented in Fig. The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. 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.
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. Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient.
Due to the precipitation of secondary carbides within the martensite matrix, after the destabilization heat treatment, the samples present an increase in the hardness which leads to a wear resistance higher than that of the as-received material.
The resulting carbide percentage was around Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: However, it was determined that even though a low percentage of retained austenite could improve the hardness values, it could negatively affect the wear resistance, as it can be seen for the samples subjected to destabilization followed by cooling in air and subsequently overcooled in CO rwtenida.