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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Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. High-Chromium White Cast Iron is a material highly used in mining and drilling shafts for oil extraction, due to its high wear resistance.
However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. This paper studies the effects retenids different cooling media after a destabilization treatment on the microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron. 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 low retained austenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron.
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. 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. The high chromium white cast irons implies a good wear resistance for an extended life service .
The high demand of these materials led to the increment of imports in Colombia, while the local companies, that produce these materials, became a second option for the buyers.
Austempered ductile cast irons
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.
It is presumed that the behavior of this kind of Colombian materials, is caused by the large percentage of retained austenite, due to a heat treatment performed improperly . According to the literature, retwnida microstructure of the high-chromium white cast irons, influences the wear behavior.
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.
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 .
According to Liu et al. An additional influence on the wear behavior is given by the secondary carbides , which improves retehida mechanical strength , through increasing the matrix strength. 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.
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].
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.
Following the investigation of Bedolla-Jacuinde et al. 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 study is performed in order to determine the most suitable microstructure along with 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.
A high chromium white cast iron manufactured by a regional company was used in this investigation. The material composition is summarized in Table I. To ensure therepeatability of the test and to reduce the error in their results, five samples were used. 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. Hardness tests of the analyzed samples were performed on a Brinell hardness tester.
A totalof ten indentations were made on each sample andaveraged to determine the hardness of each sample. After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 :. 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.
In order to identify the theoretical structure of the investigated alloy, the binary diagrams for Fe-C and Fe-Cr were analyzed. The chemical composition of the studied high chromium white cast iron was marked with 1 in Fig. 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. Both the as-casting and the heat treated materials were structurally characterized in order to correlate the microstructural changes with the wear behavior.
The microstuctural behavior of the as-received cast iron is given in Figure 4a. It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1. According to Bedolla-Jacuinde et al.
Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient. 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.
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 , besides the M7C3 carbides. This behavior was encountered in other investigations  and could be explained by the slow solidification of the alloy.
It should be noted that in the analyzed materials, the a phase is mainly associated to the ferrite phase. The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples.
The resulting carbide percentage was around Using the diagrams in Fig. According to Zhang retenica al. The microstructures of the thermally treated material are presented in Fig. During the heat treatment, the retenoda matrix is supersaturated with carbon and chromium leading to the precipitation of secondary carbides. Upon cooling, the austenite matrix becomes martensite because of the secondary carbide precipitation. It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix.
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.
Thus, when the undercooling is smaller because retenixa the heat released by the formation of the M 7 C 3 carbidesthis type of carbide shape is favored .
These results are similar to those found by Hinckley 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. 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.
Similar to the as-received sample, it can be seen in Fig. 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 .
The intensity of the austenite peaks varies according to the media of quenching. However, the diffraction when quenching in oil is run to the left and presents interferences. By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples. 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 lowest values, around 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.
A correlation between hardness and wear behavior volumetric loss and wear coefficient is given in Fig.
Estimation of the amount of retained austenite in austempered ductile irons
It can be seen that the as-received cast iron presents a lower hardness and higher values of volumetric loss and wear coefficient auztenita the heat treated samples, showing the dependence of the wear behavior on the matrix microstructure. 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 retejida forces of the moving abrasive particles, leads to a lower wear resistance in the as-cast material .
This hardness value is lower than the one obtained sustenita Marathray et al. Due to the precipitation of secondary carbides within the martensite matrix, after the destabilization heat treatment, the samples present an increase in the austeniha which leads to a wear resistance higher than that of the as-received material.
This increased hardness could be the result of the precipitation austeinta 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 . As the martensitic structure is recognized to provide a higher wear resistance, it was assumed that reducing the retained austenite to low percentages would lead to a better wear behavior.
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 2.
This behavior could be due to the increase of carbides without enough matrix support , leading to a reduced toughness, which resulted from brittle carbides. Therefore, the carbides can be more easily removed and cracked during wear. 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 .
Therefore, a certain minimum percentage of retained austenite is required in order to provide the best wear performance. As it can be seen from Fig. The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. The microstructure of the as-cast presented an austenitic matrix austenite dendrites proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries.
The XRD analysis revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage of 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. 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.
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. Sare, “Abrasion resistance and fracture toughness of white cast irons”, Met. Gates, “A transformation toughening white cast iron”, Journal of Materials Science 32, pp. Gates, “The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron”, Mater.
Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp.