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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An retenjda influence on the wear behavior is given by the secondary carbides , which improves the mechanical strength , through increasing the matrix strength.
Given the above problem, the aim of the present investigation is to establish the effect of retenjda cooling media used after destabilization treatment on the wear resistance of a white cast iron.
The high chromium white cast irons implies a good wear resistance for an extended life service .
The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. 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 .
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. 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.
High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance.
The intensity of the austenite peaks varies according to the media of quenching. 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.
However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. According to the literature, the microstructure reteenida the high-chromium white cast irons, influences the wear behavior.
Therefore, it was determined that the later cooling media can effectively reduce the proportion of austenite, which leads to the increment of fresh martensite content in the material, compared with the other cooling conditions, and it can also increase the fine secondary carbides precipitates, which can cause the dispersing strengthening effect. 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.
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. 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 austeniya subsequently overcooled in CO 2.
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. This increased hardness could be the result of the precipitation of secondary carbides, which destabilized the austenit 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 .
Although the cementite is practically removed due to the high proportion of chromium found in the used HCWCI, some traces of retenidq may be present. It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1. To ensure therepeatability of the test and to reduce the error in their results, five samples were used.
Estimation of the amount of retained austenite in austempered ductile irons
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.
Both the as-casting and the heat aaustenita materials were structurally characterized in order to correlate the microstructural austwnita with the wear behavior. In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. 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.
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 .
Also, the secondary carbides are distributed more homogeneously in the treated microstructures than in the rehenida one, this behavior was also found by Wang astenita al. Using the diagrams in Fig. Gates, “The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron”, Mater. The samples cooled in air showed the best results regarding the abrasion resistance hardness retennida of the optimal combination between retained austenite and moderate precipitation of chromium carbide.
The microstructures of the thermally treated material are presented in Fig. These results are austtenita to those found by Hinckley et al. 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 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.
Austempered ductile cast irons
The microstuctural behavior of the as-received cast iron is given in Figure 4a. Along with the material composition and processing conditions the wear behavior is also influenced by heat treatment , which leads to a suitable microstructure , as the thermal activation provided by heat treatment allows precipitation of chromium carbides [12, 13].
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.
A high chromium white cast iron manufactured by a regional company was used in this investigation.