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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Gates, “A transformation toughening white cast iron”, Journal of Materials Science 32, pp.

This behavior was encountered in other investigations [21] and could be explained by the slow solidification of the alloy. During the heat treatment, the ferrous matrix is supersaturated with carbon and chromium leading to the precipitation of secondary carbides.

Sare, “Abrasion resistance and fracture toughness of white cast irons”, Met. 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.

After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 [18]:. The chemical composition of the studied high chromium white cast iron was marked with 1 in Fig.

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. The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated.

A correlation between hardness and wear behavior volumetric loss and wear coefficient is given in Fig. Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp. It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1. The microstructure of the as-cast presented an austenitic matrix austenite dendrites proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries.

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The XRD analysis revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage of 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. Following the investigation of Bedolla-Jacuinde et al. Similar to the as-received sample, it can be seen in Fig.

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 the austenitic, minimizes cracking and removal during wear.

Austempered ductile cast irons

This increased hardness could be the result of the precipitation of secondary carbides, which destabilized the austenite leading to the asutenita 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 [24].

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 retenidaa cooling in air and subsequently overcooled in CO 2.

The microstructures of the thermally treated material are presented in Fig. 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 retwnida lead to a decrease in the abrasion resistance.

Hardness tests of the analyzed samples were performed on a Brinell hardness tester. According to Bedolla-Jacuinde et al.

It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix. 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.

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These results are similar to those found by Hinckley et al. According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior.

The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples. This transformation process is critical for the wear behavior of high chromium cast irons because it is believed that the austenite generates the spalling process, the main cause of damage to this material under working conditions [27]. 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 [2], besides the M7C3 carbides.

Upon cooling, the austenite matrix becomes martensite because of the secondary carbide precipitation. However, because of the austenitic matrix found in reteniida as-cast state, an adequate heat treatment cycle is necessary.

Estimation of the amount of retained austenite in austempered ductile irons

Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. A totalof ten indentations were made on each sample andaveraged to determine the hardness of each sample. 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.

Although the cementite is practically removed auatenita to the high proportion of chromium found in the used HCWCI, some traces of cementite may be present.

In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed.