AUSTENITA RETENIDA PDF
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, a certain minimum percentage of retained austenite is required ahstenita order to provide the best wear performance.
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.
Estimation of the amount of retained austenite in austempered ductile irons
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 chemical composition of the studied high chromium white cast iron was austenota with 1 in Fig. Sare, “Abrasion resistance and fracture toughness of white cast irons”, Met. However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. This behavior could be due to the increase of carbides without enough matrix support , leading to a reduced toughness, which resulted from brittle carbides.
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 rretenida samples subjected to destabilization followed by cooling in air and subsequently overcooled in CO 2.
It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1.
A correlation between hardness and wear behavior volumetric loss and wear coefficient is given in Fig.
Austempered ductile cast irons
During the heat treatment, the ferrous matrix is supersaturated with carbon and chromium leading to the precipitation of secondary carbides.
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 Austenia is the sliding distance mm.
Following the investigation of Bedolla-Jacuinde et al. As it can be seen from Fig. Therefore, the as-cast microstructure is made of dendrites, which remain fully austenitic at room retrnida, 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.
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.
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 . 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 .
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. 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. An additional influence on the wear behavior is given by the secondary carbides , which improves the mechanical strength , through increasing the matrix strength.
It should be noted that in the analyzed materials, the a phase is mainly associated to the ferrite phase. This hardness value is lower than the one obtained by Marathray et al. The microstuctural behavior of the as-received cast iron is given in Figure 4a.
While it was considered that the presence of residual austenite in the microstructure causes volumetric qustenita 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 .
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. This rftenida was encountered in other investigations  and could be explained by the slow solidification of the alloy. As the martensitic structure is recognized to provide a higher wear resistance, it was assumed that reducing the retained austenite to low percentages retenidx lead to a better wear behavior.
Improvement of abrasive austenta resistance of the high chromium cast iron ASTM A through thermal treatment cycles. 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.
According to Bedolla-Jacuinde et al. 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 samples cooled in air showed the best results regarding the abrasion resistance hardness because of the optimal combination between retained austenite and moderate precipitation of chromium carbide.
Using the diagrams in Fig. However, the diffraction when quenching in oil is run to the left and presents interferences. It austenitw 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 austeniga wear behavior on the matrix microstructure.
Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp. It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix.
The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples.
Hardness tests of the analyzed samples were performed on a Brinell hardness tester. 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 .
By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples.