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.
|Published (Last):||2 May 2005|
|PDF File Size:||11.20 Mb|
|ePub File Size:||16.48 Mb|
|Price:||Free* [*Free Regsitration Required]|
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. A totalof ten indentations were made on each sample andaveraged to determine the hardness of each sample.
Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp. 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.
After the hardness values were obtained, the wear coefficient Ks was calculated according to equation 2 :. 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 .
Using the diagrams in Fig. 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.
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 . Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: 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.
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.
akstenita 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. 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.
According to Bedolla-Jacuinde et al. Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. This paper studies the effects of different cooling media after a destabilization treatment on austeita microstructure, hardening and abrasion resistance behaviors of a hypoeutectic high chromium white cast iron.
Austempered ductile cast irons
Sare, “Abrasion resistance and fracture toughness of white cast irons”, Met. An additional influence on the wear behavior is given by the secondary carbides , which improves the mechanical strength , through increasing the matrix strength. The microstructures of the thermally treated material are presented in Fig. A high chromium white cast iron manufactured by a regional company was used in this investigation. It was observed that the destabilization treatment reduced ausfenita retained austenite content by a factor of from the percentage found in the as-cast rehenida.
Estimation of the amount of retained austenite in austempered ductile irons
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 . The low retained austenite percentages improve bulk hardness, but they decrease the abrasion resistance of the high chromium cast iron.
Similar to the as-received sample, it can be seen in Fig. Therefore, the carbides can be more easily removed and cracked during wear. 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. The lowest values, around According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior.
During the heat treatment, the ferrous matrix is supersaturated with carbon and chromium leading to the precipitation of secondary carbides. 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. 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 .
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 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.
The hardness values for each retenlda were determinedin order to compare the performance fetenida the specimensafter being heat treated and to retenjda the wear coefficient. It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix. However, the diffraction when quenching in oil is run to the left and presents interferences. The high demand of these materials led to the austeniga of imports in Colombia, while the local companies, that produce these materials, became a second option for the buyers.
As it can be seen from Fig.
Zustenita ensure therepeatability of the test and to reduce the error in their results, five samples were used. The intensity of the austenite peaks varies according to the media of quenching. 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.
The influence of different cooling media after destabilization heat treatments on high chromium white cast iron was investigated. 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 the dependence of the wear behavior on the matrix microstructure.
Due to the precipitation of secondary carbides ausfenita 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.
According to Liu et al. 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 retennida occurs. The XRD analysis revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage of