Microstructure and tensile properties of gray cast iron under low frequency thermal fatigue load

Schematic illustration of cracking mechanism of grey cast iron under thermal cyclic loads

Gray cast iron is a common material for the production of ingot mold, chassis, sintering grate and industrial valves. Its service temperature is often higher than 800 ℃, and most of them bear low-frequency thermal fatigue load, accompanied by oxidation and decarburization. Due to the poor high temperature thermal fatigue performance of gray cast iron, the service life of its parts is short, which may cause safety problems in production.

In recent years, the research on deeply understanding the high temperature thermal fatigue behavior of gray cast iron and further improving the service life of its products has gradually attracted extensive attention. Drapkin et al. Pointed out that the thermal fatigue cracking behavior of gray cast iron is related to the change of internal friction of matrix after multiple thermal cycle treatment; By analyzing the changes of chemical composition and microstructure of thermal cycling samples at different temperatures, Shea found that the samples with more graphite content and thicker lamella have stronger crack resistance; The research of roehrig et al. Shows that the thermal fatigue damage of gray cast iron is mainly caused by the cyclic stress caused by thermal expansion and thermal shrinkage, and the austenite transformation at high temperature is an important reason for the decline of thermal fatigue performance; Fine summarized and analyzed the thermal fatigue mechanism of gray cast iron and steel on the basis of summarizing the previous research on fatigue behavior of metal materials; Rukaddikar measured the cracks of 23 kinds of high carbon gray cast iron at different thermal cycle temperatures by using an automatic thermal fatigue tester. They found that adding a certain amount of Mo, Cr, Cu and other alloy elements had better thermal fatigue resistance; Buni et al. Observed the fatigue behavior of four kinds of gray cast iron under low frequency thermal load, and considered that the thermal fatigue performance of gray cast iron directly depends on its mechanical properties; Collini measured the tensile strength and thermal fatigue resistance of three kinds of gray iron through experiments. It was found that the tensile strength and fatigue resistance of gray iron were improved when the graphite content was reduced; Sharma reported the changes of composition and structure of scrap ingot mold after analyzing different service times. It was found that the ferrite content in the matrix was not directly related to the service life of gray iron mold, and it was considered that pearlite spheroidization, graphite and matrix oxidation had a direct impact on the inner wall cracking of ingot mold. Lee et al. Deeply studied the thermal fatigue resistance of austenitic cast iron on the basis of analyzing the crack behavior and microstructure changes of gray cast iron with different materials, and evaluated the thermal fatigue properties of gray cast iron, nodular cast iron and vermicular cast iron according to the thermal crack index. In addition, some other scholars have carried out research on the alloying of cast iron composition, solidification rate and microstructure control, in order to improve the mechanical properties of products. Domestic scholars began to pay attention to the material and properties of gray cast iron from 1980s to 1990s. After decades of research, some achievements have been made.

1) With the increase of high temperature oxidation times, the tensile strength of gray cast iron decreases at all temperatures, and the decreasing trend of tensile strength at low temperature is greater than that at high temperature. After 30 times of high temperature oxidation treatment, the matrix has basically transformed into ferrite; When the heat treatment is continued thereafter, the grains are coarsened gradually. In addition, secondary cementite precipitates at the grain boundary of the original austenite when the sample after high temperature treatment is cooled, and new graphite is formed in the region far away from the original graphite;

2) When the number of high temperature oxidation treatment increases, the exposure rate of graphite sheet at the fracture of gray cast iron increases, and the cleavage of matrix is less and less obvious. Flocs around graphite are not micropores formed by the oxidation of graphite itself, but oxides of silicon, manganese and iron. Flake graphite is the channel for oxygen atoms to enter the sample;

3) Before the pearlite transformation is inhibited, there is a process of matrix accelerated decarburization; After it is inhibited, the matrix decarburization rate decreases significantly. After 30 times of high temperature oxidation treatment, the carbon content of the sample is about 1.5%, and the pearlite transformation is difficult to complete under the same cooling conditions;

4) The relationship between tensile strength and carbon content of gray cast iron with different matrix structure was fitted. After matrix ferrite, the tensile strength of the sample decreases significantly with the decrease of carbon content. Whether the matrix is pearlite or ferrite, the effect of carbon content on tensile strength at low temperature is greater than that at high temperature.

Although the fatigue behavior of gray cast iron and its influence on service state have been analyzed, there are still deficiencies in the understanding of the thermal fatigue failure mechanism of gray cast iron under the condition of low-frequency and high-temperature oxidation, and the research on the correlation and variation rules of its chemical composition, matrix structure and high-temperature mechanical behavior still needs to be further carried out. By analyzing the relationship between tensile strength, fracture morphology, phase composition and carbon and silicon content of samples after periodic high temperature oxidation treatment, the variation characteristics of thermal fatigue resistance of samples under high temperature oxidation are revealed, and the micro macro mechanism of thermal fatigue failure of gray cast iron is discussed.

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