This article comprehensively studies the microstructure evolution of gray cast iron during austenitizing and its impact on mechanical properties. Through a series of experiments and analyses, it is found that within the range of austenitizing temperature 875 – 950 °C and austenitizing time 30 – 120 min, the austenite grains do not coarsen significantly, with the maximum grain size being about 80 μm. The presence of graphite affects the carbon concentration in austenite and the mechanical properties after austempering. By adjusting the austenitizing process, the properties of gray cast iron can be effectively controlled.
1. Introduction
Gray cast iron is widely used in manufacturing engine components such as cylinder blocks and cylinder heads due to its excellent comprehensive properties. However, with the continuous development of engines towards lightweight and high-efficiency, the performance requirements for gray cast iron components are gradually increasing. Isothermal quenching has been shown to improve the mechanical properties of gray cast iron, but the research on the austenitizing process of gray cast iron is relatively limited. Therefore, this study aims to investigate the microstructure evolution law of gray cast iron during austenitizing and its influence on mechanical properties, providing a theoretical reference for the development of reasonable production processes.
2. Experimental Materials and Methods
2.1 Experimental Materials
The chemical composition of the experimental gray cast iron is shown in Table 1. The metallographic structure mainly consists of flake pearlite, flake graphite, and a small amount of blocky ferrite.
| Chemical Composition (mass fraction, %) | C | Si | Mn | P | S | Cu | Cr | Sn | Ni | Mo |
|---|---|---|---|---|---|---|---|---|---|---|
| Content | 3.33 | 1.85 | 0.69 | 0.026 | 0.068 | 0.2 | 0.14 | 0.018 | 0.031 | 0.042 |
2.2 Experimental Methods
Cylindrical samples and tensile samples were cut from the gray cast iron test bars. The samples were austenitized at different temperatures (875 °C, 900 °C, 925 °C, and 950 °C) and held for different times (30 min, 60 min, 90 min, and 120 min), followed by water quenching to room temperature. The microstructure was observed using an optical microscope after polishing and etching. Tensile tests were carried out on the samples after isothermal quenching.
3. Experimental Results and Discussion
3.1 Equilibrium Phase Diagram Calculation
The equilibrium phase diagram of the experimental gray cast iron was calculated using Jamtpro software. As shown in Figure 3, when the heating temperature reaches 764 °C, pearlite and ferrite start to transform into austenite. At 785 °C, all the ferrite in pearlite and the free ferrite in the structure are completely transformed into austenite, while most of the cementite remains in the structure. With the further increase of temperature, the cementite gradually dissolves, and the volume fraction of austenite increases. When the temperature reaches 1080 °C, all the cementite is dissolved, and the volume fraction of austenite reaches the maximum. In addition, it can be seen from Figure 3(b) that the volume fraction of graphite decreases suddenly at 940 °C.
3.2 Influence of Heating Temperature on Microstructure
3.2.1 Transformation of Pearlite to Austenite
The transformation of pearlite to austenite in gray cast iron occurs in four stages: nucleation, growth of nuclei towards ferrite and cementite, dissolution of remaining carbides, and homogenization of austenite composition. In this experiment, the heating temperature was set to 875 °C, 900 °C, 925 °C, and 950 °C. The metallographic structures after holding at different temperatures for 120 min are shown in Figure 4. It can be seen that the matrix structure is mainly martensite, flake graphite, and undissolved dot-like cementite, indicating that the high-temperature structure has completely transformed into austenite at 875 – 950 °C.
3.2.2 Growth of Austenite Grains
With the increase of austenitizing temperature, the growth trend of austenite grains is relatively weak. When the austenitizing process is 950 °C for 120 min, the grain size is about 80 μm. The presence of undissolved cementite in the structure hinders the migration of austenite grain boundaries, which is evidenced by the fine dot-like cementite at the austenite grain boundaries in Figure 4(d).
3.2.3 Change of Graphite Morphology
The length and width of flake graphite decrease with the increase of temperature. This is because during the austenitizing process, austenite nuclei first nucleate at the interface between two phases, such as cementite/ferrite and graphite/ferrite, and grow into one of the phases by diffusion. The growth of austenite nuclei is a process of interface migration, which leads to the dissolution of cementite, graphite, and ferrite into austenite, resulting in the decrease of graphite size.
3.2.4 Appearance of Massive Martensite
At 900 °C and 925 °C, massive martensite appears in the structure, mainly distributed at the grain boundaries. This is because with the increase of temperature, the carbon concentration in the matrix phase increases, especially at the grain boundaries, which leads to a decrease in the Ms point temperature and an increase in the critical transformation rate, resulting in the occurrence of massive transformation. At 875 °C and 950 °C, no massive martensite is found in the gray cast iron structure.
3.3 Influence of Heating Time on Microstructure
The metallographic structures of gray cast iron after holding at 925 °C for different times are shown in Figure 6. With the extension of holding time, the grain size does not coarsen significantly, the blocky martensite at the grain boundaries first increases and then decreases, and the length and width of flake graphite gradually decrease, indicating that the carbon concentration in the matrix phase gradually increases and the homogenization degree of austenitization gradually increases.
3.4 Mechanical Properties after Isothermal Quenching
The tensile mechanical properties of gray cast iron after isothermal quenching at 925 °C for different holding times are shown in Figure 7. It can be seen that with the extension of holding time, the tensile strength decreases, and the plastic elongation before fracture increases. This is mainly due to the different contents of high-carbon austenite in the structure after isothermal quenching.
4. Conclusion
In this study, the microstructure evolution of gray cast iron during austenitizing and its influence on mechanical properties were investigated. The main conclusions are as follows:
- Within the range of austenitizing temperature 875 – 950 °C and austenitizing time 30 – 120 min, the austenite grains do not coarsen significantly, and the maximum grain size is about 80 μm.
- The presence of graphite in gray cast iron affects the carbon concentration in austenite and the mechanical properties after austempering. With the increase of austenitizing temperature and time, the tensile strength after austempering decreases, and the elongation before fracture increases.
- By adjusting the austenitizing process, the properties of gray cast iron can be effectively controlled, providing a theoretical basis for the optimization of production processes.
In future research, further studies can be carried out on the influence of other factors on the austenitizing process and mechanical properties of gray cast iron, such as alloying elements and cooling rates, to further improve the performance of gray cast iron components.