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):||14 June 2004|
|PDF File Size:||20.94 Mb|
|ePub File Size:||9.44 Mb|
|Price:||Free* [*Free Regsitration Required]|
The chemical composition of the studied high chromium white cast iron was marked with 1 in Fig. 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 .
Therefore, a certain minimum percentage of retained austenite is required in order to provide the best wear performance.
These results are similar to those found by Hinckley et al. 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 austejita dependence of the wear behavior on the matrix microstructure.
Austempered ductile cast irons
As it can be seen from Fig. Different investigations were made on the effect of destabilization treatment parameters on the nature and morphology of ertenida carbide precipitation as the distribution of secondary carbides in the martensitic matrix after heat treatment is known to improve the wear resistance , due retenidq their high hardness.
The XRD analysis revealed the presence of austenitic peaks, but also ferrite and carbides, with a percentage austenitz Improvement of abrasive wear resistance of the high chromium cast iron ASTM A through thermal treatment cycles. 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 material composition is summarized in Table I.
Additionally, in the center of the d endrite arms fine eutectic carbides were found, as their nucleation time from austenite was insufficient. However, the diffraction when quenching in oil is run to the left and presents interferences. 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.
It should be noted that in the analyzed materials, the a phase is mainly associated to the ferrite phase. An additional influence on the wear behavior is given by the secondary carbides , which improves the mechanical strength , through increasing the matrix strength. It can also be observed that the secondary chromium carbides MC and MC nucleated and grew within the dendritic matrix. 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.
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 microstuctural behavior of the as-received cast iron is given in Figure 4a.
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. 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. 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 rdtenida of the as-received material.
The lowest values, around Using the diagrams in Fig.
However, because of the austenitic matrix found in the as-cast state, an adequate heat treatment cycle is necessary. 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 high chromium white cast irons implies a good wear resistance for an extended life service . 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.
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. 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.
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. According to the literature, the microstructure of the high-chromium white cast irons, influences the wear behavior. Hawk, “Effect of carbide orientation on abrasion of high Cr white cast iron”, Wearpp. To ensure therepeatability of the test and to reduce the error in their results, five samples were used.
Gates, “A transformation toughening white cast iron”, Journal of Materials Science 32, pp. This hardness value is lower than the one obtained by Marathray 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. A high chromium white cast iron manufactured by a regional company was used in this investigation.
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. After the hardness values were obtained, reyenida wear coefficient Ks was calculated according to equation 2 :. 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.
Both the as-casting and the heat treated materials were structurally characterized in order to correlate the microstructural changes with the wear behavior. 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 austsnita.
The high demand of these materials led to the increment of imports in Colombia, while the local companies, that produce these austenjta, became a second option for the buyers.
By means of XRD analysis, the retained austenite percentage was determined in the heat treated samples. 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 ausyenita precipitation of chromium carbide.
The XRD analysis also confirmed the presence of both K 1 and K 2 carbides in the structure of the as-cast samples. 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.
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. It was determined that the matrix structure is predominantly austenite austenite dendrites proeutecticwith an approximate 1.
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.
Following the investigation of Bedolla-Jacuinde et al.
Estimation of the amount of retained austenite in austempered ductile irons
According to Liu et al. A particular feature of the analyzed high chromium white cast iron was the presence of reteniad amounts of M 23 C 6 carbides, which represent the Fe, Cr 23 C 6 type carbides , besides the M7C3 carbides.
Diavati, “Effect of destabilization heat treatments on the microstructure of high-chromium cast iron: