1. Tensile strength test

The experimental results of tensile strength of samples after different high temperature oxidation cycles are shown in Table 1. When the heat treatment cycle increases, the tensile strength of the sample at each temperature decreases. After 50 times of high temperature oxidation treatment, the tensile strength at 400 ℃ is about 64% lower than that after 10 times of treatment, while the corresponding values at 600 ℃ and 800 ℃ are 49% and 34% lower respectively. At the same time, for the samples with the same heat treatment cycle, the tensile strength decreases significantly with the increase of temperature. After 10 times of heat treatment, the tensile strength at 800 ℃ is about 24% of that at 400 ℃, while the corresponding value of 50 times of heat treatment is 43%. Under multiple high-temperature oxidation, the difference of tensile strength at each temperature decreases gradually, which is mainly the result of the deterioration of low-temperature tensile properties.

2. Graphite and matrix phase observation

Figure 1 shows the graphite morphology and matrix structure near the 400 ℃ fracture of the sample after different high-temperature oxidation cycles. It can be seen from the figure that the graphite in the sample is basically type A, and its thickness and distribution are relatively uniform. With the increase of heat treatment cycle, the carbon in graphite is fully dissolved in the matrix and diffused. At the same time, with the occurrence of oxidation and decarburization, the thickness of graphite sheet gradually decreases and the range of flocculent band around it gradually increases. After 30 times of high temperature oxidation treatment, the matrix has basically transformed into ferrite; After that, when the heat treatment is continued, the grain coarsens gradually and secondary cementite precipitates at the grain boundary. These two factors may be the main reason for the decrease of tensile strength. In addition, the color of flocculent band is close to that of graphite after erosion, and the phenomenon that its range increases with the increase of heat treatment times is more obvious.

3. Analysis of fracture morphology and phase composition

Fig. 2 shows the fracture morphology of the sample at 600 ℃ after different high-temperature oxidation cycles. When gray cast iron is stretched, the graphite flakes are cleaved into long stripes, hexagonal domains and deformation structures, and pearlite is cleaved into dimple and lamellar characteristics. It can be seen in Figure 2 that all fractures reflect the brittle fracture properties of the material, and the cleavage of graphite sheet and matrix can be observed. The three-dimensional morphology of flake graphite in a eutectic cluster is flower like, and the flower is the transformation product of eutectic austenite, that is, pearlite or ferrite.

Because the strength of graphite is much lower than that of the matrix, microcracks initiate at the graphite sheet and then propagate into the matrix. Microcracks propagate along the interface between graphite sheet or graphite sheet and matrix, and finally connect with each other to form fracture. From a large number of graphite flakes exposed on the fracture, it can be concluded that the fracture of gray cast iron belongs to multi-source fracture, and each graphite flake can become the origin of microcracks. In fact, the matrix can really hinder the crack propagation. It can be seen in Figure 2 that with the increase of heat treatment times, the exposure rate of graphite sheet at the fracture is increasing, while the matrix cleavage is less and less obvious. This shows that after the sample is oxidized at high temperature, the damage effect of graphite sheet on the continuity of matrix is more and more significant, while the hindrance effect of matrix on crack is less and less.

Fig. 3 shows the morphology of flocs around graphite under scanning electron microscope and the results of energy spectrum analysis. It can be seen from the figure that these flocs are not micropores formed by the oxidation of graphite itself, but oxides of silicon, manganese and iron. Under the condition of periodic high-temperature oxidation, oxygen atoms diffuse into the sample along the graphite sheet and penetrate into the matrix, which will inevitably lead to the oxidation of carbon, silicon, manganese, iron and other elements in graphite and its nearby matrix. The oxidation phenomenon of the matrix far away from the graphite sheet, as shown in the “1” position in Figure 3, is not obvious, which fully proves that the flake graphite is the channel for oxygen atoms to enter the sample.

4. Graphite and matrix at high temperature

Figure 4 shows the morphology of graphite and matrix at different times under ultra-high temperature laser confocal microscope. The figure shows that the thickness of graphite sheet in the sample at high temperature is significantly less than that at room temperature, which is because the carbon solubility in austenite is greater than that of ferrite. After 900 ℃ heat preservation treatment, the thickness of the original graphite sheet basically does not change when the sample is cooled. This shows that after a large amount of carbon is dissolved in the matrix at high temperature, it does not precipitate at the original graphite when cooling. At the same time, new graphite is formed in the area far from the original graphite, as shown in areas I and II in Fig. 4 (f). The newly generated graphite is relatively short, small and fragmented, and its distribution is scattered, which is basically not connected with the original graphite.

Because the carbon atoms are saturated and dissolved in austenite at high temperature, the carbon far away from the graphite sheet has no time to diffuse around the original graphite sheet during cooling, that is, nucleation at the grain boundary to form graphite particles; The supersaturated carbon in the surrounding area also diffuses and precipitates here, and then forms new graphite. On the contrary, the carbon in the area near the original graphite can diffuse to the original position and precipitate when cooling, but can not form new graphite, as shown in Area III in Fig. 4 (f). In addition, secondary cementite precipitates at the original austenite grain boundary when the sample after high-temperature treatment is cooled. This phenomenon can also be observed in the sample matrix after high-temperature oxidation treatment for 30 and 50 times in Figure 1, which will inevitably lead to the weakening of grain boundary and the decrease of matrix strength.

5. Carbon and silicon content

Table 2 contents of carbon and silicon in samples after different high temperature oxidation cycles. When the number of heat treatment increases, the carbon content in the sample decreases significantly, but the change of silicon content is not obvious. It should be noted that the carbon content decreased the most between 20 and 30 times of high-temperature oxidation treatment, which corresponds to the basic disappearance of pearlite in the sample matrix in Fig. 1.

It can be seen that there is a process of matrix accelerated decarburization before pearlite transformation is inhibited; After it is inhibited, the matrix decarburization rate decreases significantly. After 50 times of high temperature oxidation treatment, the carbon content of the sample is 1.44%, which is less than half of the initial carbon content. If pearlite is formed when austenite is cooled, cementite with carbon content of 6.69% must be formed by uphill diffusion. Compared with the initial concentration, the concentration value of about 1.5% in the later stage of heat treatment requires greater driving force and diffusion time. Pearlite transformation is difficult to complete under the same cooling conditions. With the increase of heat treatment cycle, the decarburization degree of the sample becomes more and more serious, and the proportion of pearlite decreases gradually until it disappears completely.

Park et al. Believe that the tensile strength of gray cast iron mainly depends on its carbon and silicon content. Since the silicon content in this experiment is basically unchanged, it can be considered that the tensile strength is mainly affected by the carbon content. Fig. 5 shows the variation curve of tensile strength with carbon content at different temperatures. The figure shows that the carbon content of gray cast iron has a significant effect on the tensile strength at all temperatures. When the carbon content is less than 1.55%, the tensile strength of the sample decreases significantly with the decrease of carbon content. After 30 times of high temperature oxidation treatment, the sample matrix has been basically ferrite; The effect of carbon content on the deterioration of tensile strength is much greater than that of pearlite matrix. The fitting relationship between tensile strength and carbon content of pearlite matrix and ferrite matrix samples at different temperatures is shown in Table 3. It can be seen from the table that whether the matrix is pearlite or ferrite, the influence of carbon content on tensile strength at low temperature is greater than that at high temperature.