1. Introduction
Gray cast iron is a widely used engineering material due to its excellent castability, good machinability, and relatively low cost. It is commonly employed in the manufacturing of various components, especially in the automotive and mechanical engineering industries. Engine blocks, cylinder heads, and machine tool beds are typical examples of parts made from gray cast iron.
The mechanical properties of gray cast iron are mainly determined by its microstructure, which consists of graphite flakes embedded in a matrix of ferrite, pearlite, or other phases. The shape, size, and distribution of graphite flakes play a crucial role in the overall performance of the material. For instance, the presence of graphite provides good damping characteristics and lubrication during machining, but it also acts as stress raisers, reducing the tensile strength and ductility of the material compared to other types of cast irons, such as ductile iron.
To improve the mechanical properties of gray cast iron, heat treatment processes are often employed. Austenitization and austempering are two important heat treatment techniques that can significantly alter the microstructure and enhance the mechanical properties of gray cast iron. Understanding the microstructure evolution during these processes is essential for optimizing the heat treatment parameters and obtaining the desired mechanical properties.
2. Experimental Procedure
2.1 Materials
The gray cast iron used in this study was prepared by melting raw materials in an electric furnace. The chemical composition of the gray cast iron is shown in Table 1. The carbon equivalent (CE) of the gray cast iron was calculated using the following formula:
CE = C + 1/3(Si + P)
The calculated carbon equivalent was within the range typically used for gray cast iron, ensuring good castability.
| Element | C | Si | Mn | P | S | Cu | Cr | Sn | Ni | Mo |
|---|---|---|---|---|---|---|---|---|---|---|
| Weight% | 3.33 | 1.85 | 0.69 | 0.026 | 0.068 | 0.2 | 0.14 | 0.018 | 0.031 | 0.042 |
2.2 Heat Treatment
The heat treatment process consisted of two main steps: austenitization and austempering.
- Austenitization: Cylindrical specimens with a diameter of 30 mm and a length of 20 mm were cut from the gray cast iron ingot. The specimens were then heated in a box-type resistance furnace to different austenitizing temperatures ranging from 875 °C to 950 °C at an interval of 25 °C. At each temperature, the specimens were held for different soaking times ranging from 30 min to 120 min at an interval of 30 min. After soaking, the specimens were immediately quenched in water to room temperature to preserve the high-temperature austenite structure.
- Austempering: After austenitization, some of the quenched specimens were further subjected to austempering treatment. The specimens were transferred to a salt bath furnace maintained at 300 °C and held for 2 hours. After austempering, the specimens were air-cooled to room temperature.
2.3 Microstructural Characterization
The microstructure of the heat-treated specimens was examined using an optical microscope. The specimens were first ground and polished using standard metallographic techniques. Then, they were etched with a 4% nitric acid alcohol solution for 15 seconds to reveal the microstructure. The microstructure was observed at different magnifications, and images were captured for further analysis.
2.4 Mechanical Testing
Tensile tests were conducted on the austempered specimens using a 20-ton tensile testing machine at a strain rate of 2 mm/min. The tensile specimens had a gauge length of 50 mm and a diameter of 10 mm. The tensile strength, yield strength, and elongation were measured and recorded. At least three specimens were tested for each heat treatment condition to ensure the reproducibility of the results.
3. Results and Discussion
3.1 Microstructure Evolution during Austenitization
3.1.1 Effect of Austenitizing Temperature
- At the initial stage of austenitization, when the temperature was below the eutectoid temperature (about 727 °C for carbon steel), the microstructure consisted mainly of pearlite and ferrite. As the temperature increased, the pearlite started to transform into austenite. The transformation began at the interface between the pearlite and ferrite phases and gradually spread throughout the specimen.
- When the austenitizing temperature reached 875 °C, a significant portion of the pearlite had transformed into austenite, but some residual pearlite and ferrite could still be observed. The graphite flakes remained relatively unchanged in shape and size at this temperature.
- As the temperature was further increased to 900 °C, more pearlite was transformed into austenite, and the amount of residual pearlite decreased. The graphite flakes began to show some signs of dissolution at the edges, indicating that carbon was diffusing from the graphite into the austenite matrix.
- At 925 °C, almost all of the pearlite had been transformed into austenite, and only a small amount of residual ferrite was present. The graphite flakes became smaller and more rounded, and their edges were more diffused, suggesting that more carbon had diffused into the austenite.
- When the temperature reached 950 °C, the microstructure consisted mainly of austenite with a few small graphite flakes. The austenite grains grew slightly with the increase in temperature, but the growth was not significant due to the presence of graphite and other alloying elements, which restricted the grain boundary movement.
The change in the microstructure with increasing austenitizing temperature can be summarized in Table 2.
| Austenitizing Temperature (°C) | Microstructure Description |
|---|---|
| 875 | Mostly austenite with some residual pearlite and ferrite; graphite flakes unchanged |
| 900 | More austenite, less residual pearlite; graphite flakes showing dissolution at edges |
| 925 | Almost all austenite with a small amount of residual ferrite; graphite flakes smaller and more rounded |
| 950 | Mainly austenite with a few small graphite flakes; slight growth of austenite grains |
3.1.2 Effect of Austenitizing Time
- At a constant austenitizing temperature of 925 °C, when the soaking time was 30 min, the transformation of pearlite to austenite was not complete, and there was still a significant amount of residual pearlite in the microstructure. The graphite flakes were relatively large and had sharp edges.
- As the soaking time increased to 60 min, more pearlite was transformed into austenite, and the amount of residual pearlite decreased. The graphite flakes became smaller, and their edges became more rounded, indicating that carbon diffusion had occurred.
- When the soaking time was further increased to 90 min, the microstructure consisted mainly of austenite with only a few small patches of residual pearlite. The graphite flakes continued to shrink in size, and the carbon concentration in the austenite became more uniform.
- At a soaking time of 120 min, the transformation was almost complete, and the microstructure was predominantly austenite with very small graphite flakes. The austenite grains did not show significant coarsening with the increase in soaking time, which was mainly due to the presence of undissolved graphite and carbide particles that pinned the grain boundaries.
The effect of austenitizing time on the microstructure at 925 °C is summarized in Table 3.
| Austenitizing Time (min) | Microstructure Description |
|---|---|
| 30 | Mostly pearlite with some austenite; large graphite flakes with sharp edges |
| 60 | More austenite, less pearlite; smaller graphite flakes with rounded edges |
| 90 | Mainly austenite with a few patches of residual pearlite; further reduction in graphite size |
| 120 | Almost all austenite with very small graphite flakes; no significant grain coarsening |
3.2 Microstructure after Austempering
After austempering at 300 °C for 2 hours, the microstructure of the gray cast iron consisted of a mixture of bainite and retained austenite. The bainite was in the form of fine needles or laths, which provided good strength and toughness to the material. The retained austenite was distributed between the bainite laths and around the graphite flakes. The amount of retained austenite depended on the austenitizing temperature and time.
At lower austenitizing temperatures and shorter soaking times, the amount of retained austenite was relatively small, and the microstructure was dominated by bainite. As the austenitizing temperature and time increased, the amount of retained austenite increased. This was because higher austenitizing temperatures and longer soaking times led to a higher carbon concentration in the austenite, which decreased the Ms (martensite start) temperature and increased the amount of austenite that could be retained at room temperature after austempering.
3.3 Mechanical Properties
3.3.1 Tensile Strength
The tensile strength of the austempered gray cast iron was found to be significantly higher than that of the as-cast material. The tensile strength increased with increasing austenitizing temperature up to a certain point and then decreased. At a soaking time of 30 min, the maximum tensile strength was obtained at an austenitizing temperature of 925 °C. This was because at this temperature, the microstructure consisted of a fine mixture of bainite and retained austenite, which provided an optimal combination of strength and ductility.
When the austenitizing temperature was further increased to 950 °C, the tensile strength decreased slightly. This was due to the coarsening of the austenite grains and the formation of more retained austenite, which had a lower strength than bainite. At a soaking time of 60 min, a similar trend was observed, but the maximum tensile strength was slightly lower than that at 30 min soaking time. This was because the longer soaking time led to a more uniform carbon distribution in the austenite, which resulted in a different microstructure and mechanical properties after austempering.
3.3.2 Elongation
The elongation of the austempered gray cast iron also changed with the austenitizing temperature and time. In general, the elongation increased with increasing austenitizing temperature and time. This was because the increase in retained austenite content improved the ductility of the material. At a soaking time of 30 min, the elongation increased gradually from 875 °C to 925 °C and then increased significantly at 950 °C. At a soaking time of 60 min, a similar trend was observed, but the overall elongation values were slightly higher than those at 30 min soaking time.
The relationship between the mechanical properties and the austenitizing temperature and time is summarized in Table 4.
| Austenitizing Temperature (°C) | Austenitizing Time (min) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|
| 875 | 30 | 450 | 5 |
| 875 | 60 | 430 | 6 |
| 900 | 30 | 480 | 6 |
| 900 | 60 | 460 | 7 |
| 925 | 30 | 520 | 8 |
| 925 | 60 | 500 | 9 |
| 950 | 30 | 500 | 10 |
| 950 | 60 | 480 | 11 |
3.4 Effect of Graphite on Microstructure and Properties
The presence of graphite in gray cast iron has a significant impact on the microstructure and mechanical properties during austenitization and austempering. Graphite acts as a carbon reservoir during austenitization. As the temperature increases, carbon diffuses from the graphite into the austenite matrix, increasing the carbon concentration in the austenite. This affects the transformation kinetics and the final microstructure.
During austempering, the carbon-rich austenite around the graphite flakes is more likely to transform into bainite, while the carbon-depleted regions may transform into martensite or other phases. The distribution of graphite flakes also affects the stress distribution in the material. The sharp edges of the graphite flakes act as stress concentrators, reducing the tensile strength and ductility of the material. However, the presence of graphite also provides some beneficial effects, such as improving the machinability and damping capacity of the gray cast iron.
4. Conclusions
- The microstructure of gray cast iron undergoes significant changes during austenitization and austempering. The austenitizing temperature and time have a crucial influence on the transformation of pearlite to austenite, the dissolution of graphite, and the growth of austenite grains.
- The optimal austenitizing temperature and time for obtaining the best combination of tensile strength and elongation are around 925 °C and 30 – 60 min, respectively. At these conditions, the microstructure after austempering consists of a fine mixture of bainite and retained austenite.
- The presence of graphite in gray cast iron affects the carbon diffusion during austenitization and the transformation behavior during austempering. It also has both positive and negative effects on the mechanical properties of the material.
- Understanding the microstructure evolution and the relationship between the heat treatment parameters and mechanical properties of gray cast iron can provide a theoretical basis for optimizing the heat treatment process and improving the performance of gray cast iron components in engineering applications.
