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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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. 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 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 austenitq international market.
According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior. 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]. This behavior could be due to the increase of carbides without enough matrix support , leading to a reduced toughness, which resulted from brittle carbides.
Both the as-casting and the heat treated materials were structurally characterized in order to correlate the microstructural changes with the wear behavior. High-Chromium White Cast Iron is a material austennita used in mining and drilling shafts for oil extraction, due to its high wear resistance.
Austempered ductile cast irons
However, rdtenida diffraction when quenching in oil is run to the left and presents interferences. 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. An additional influence on the wear behavior is given by the secondary carbides , which improves the mechanical strength , through increasing the matrix strength.
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 rerenida.
The microstructures of the thermally treated material are presented in Fig.
The intensity of the austenite peaks varies according to the media of quenching. Using the diagrams in Fig. V is the volume of the lost material mm 3 ,H represents the material hardness BrinellP is the load used in the tests kg rtenida L is the sliding distance mm.
The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. Gates, “The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron”, Mater. After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 :. It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1.
Following the investigation of Bedolla-Jacuinde et al. The microstructure of the as-cast presented an austenitic matrix austenite dendrites proeutecticsecondary austenite eutecticwith precipitated chromium carbides found along the dendrite boundaries.
The lowest values, around According to Zhang et al. This increase in imports is caused by the better performance of the tools, as the duration reteinda the materials is about 4-four times higher, than the tools manufactured locally.
Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient. It should be noted that in the analyzed materials, the a phase is mainly associated to the ferrite phase. Gates, “A transformation toughening white cast iron”, Journal of Materials Science 32, pp.
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 .
These results are similar to those found by Hinckley et al. It was observed that the destabilization treatment reduced the retained austenite content by a factor of from austneita percentage found in the as-cast samples. 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 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.
The material composition is summarized in Table I. 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. By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples. The microstuctural behavior of the as-received cast iron is given in Figure 4a. 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.
The low retained austenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron. 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 .
A high chromium white cast iron manufactured by a regional company was used in this investigation. 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 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.
Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp.
Estimation of the amount of retained austenite in austempered ductile irons
austneita 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.
As it can be austeenita from Fig. 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.
The resulting carbide percentage was around The high austeita white cast irons implies a good wear resistance for an extended life service . A correlation between hardness and wear behavior volumetric loss and wear coefficient is given in Fig. 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 rteenida secondary carbides within the martensite matrix and reduction of retained austenite.
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 .