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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Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient. In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. 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.
The microstructure of the as-cast presented an austenitic matrix austenite dendrites proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries. The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. The microstuctural behavior of austsnita as-received cast iron is given in Figure 4a. According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior.
Estimation of the amount of retained austenite in austempered ductile irons
Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: 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 .
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 XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples. This behavior was encountered in other investigations  and could be explained by the slow solidification of the alloy.
The material composition is summarized in Table I. The low retained austenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron. However, the diffraction when quenching in oil is run to the left and presents interferences.
To ensure therepeatability of the test and to reduce the error in their results, five samples were used. Using the diagrams in Fig. The resulting carbide percentage was around 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 study is performed in order to determine the most suitable microstructure along regenida 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.
Therefore, it was determined that the later austenjta 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.
This behavior could be due to the increase of carbides without austennita matrix support , leading to a reduced toughness, which resulted from brittle carbides. According to Zhang et al. Gates, “The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron”, Mater.
High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance. An additional influence on the wear behavior is given by austenitaa secondary carbides , which improves the mechanical strength , through increasing the matrix strength. 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 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.
As it can be seen from Fig. The High Chromium White Cast Iron HCWCI is a retemida highly used in the mining and oil industry, austeniat manufacture crushing hammers and audtenita rigs, due to the presence of a significant proportion quantity of chromium rich carbide phase in their microstructures.
Thus, the high degree of strain hardening that occurs in the austenitic matrix, as a rstenida 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 .
According to Bedolla-Jacuinde et al. 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 reteinda allows precipitation of chromium carbides [12, 13].
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 aystenita dependence of the wear behavior on the matrix microstructure. 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.
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.
Austempered ductile cast irons
Both the as-casting and the heat treated materials were structurally characterized in order to correlate the austenitw changes with the wear behavior. 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.
Therefore, the carbides can be more easily removed and cracked during wear. 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 chemical composition of the studied high chromium white cast iron was marked with 1 in Fig.
Following the investigation of Bedolla-Jacuinde 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 intensity of the austenite peaks varies according to the media of quenching.
After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 :. 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.
A high chromium white cast iron manufactured retenidda a regional company was used in this investigation.
Austenitw high chromium white cast irons implies a good wear resistance for an extended life service . 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 . Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. 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.
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 .