Austenitic cast iron is a versatile material widely used in various industries due to its excellent corrosion resistance, high strength, and good machinability. However, to further enhance its mechanical properties, heat treatment processes are often employed.
In this article, we will explore the heat treatment techniques used for austenitic cast iron and their impact on its strength and durability.
Heat Treatment of Austenitic Cast Iron
Austenitic cast irons have a diverse range of uses because of their exceptional qualities, such as resistance to oxidation, heat, wear, strength, and corrosion. Additionally, their non-magnetic and high electrical resistivity properties make them crucial in electric industries. Both types of austenitic cast irons typically undergo the following types of heat treatment:
- Stress relieving,
- Spherodizing annealing, and
- High temperature stabilizing.
Stress relieving of austenitic cast iron
Residual stress builds up in austenitic cast irons during both the cooling of the casting and the machining process. Therefore, it is preferable to undergo stress relieving treatment after rough machining rather than after fettling. This approach helps to preserve dimensional accuracy by counteracting distortion caused by residual stress.
The stress relieving treatment involves the following steps:
- a) Placing the castings into the furnace at a temperature below 100°C.
- b) Heating the casting gradually to a range of 600-675°C at a rate of 25-100°C per hour.
- c) Maintaining the casting at that temperature for 1 hour per 25 mm section size.
- d) Cooling the castings at a rate of 25-100% per hour until they reach a temperature of 200-300°C.
- e) Finally, allowing the castings to cool naturally in the air.
Spheroidizing Annealing
Spheroidizing Annealing is a process used to improve the structure of austenitic cast iron that contains a higher amount of chromium. In certain cases, such as in thin sections or chilled castings, carbides can form in the microstructure, resulting in a stressed and hard structure.
To address this, the treatment involves heating the casting to a temperature range of 970-1025°C and maintaining it at that temperature for a duration of 30 minutes to 5 hours, depending on the thickness of the section.
Subsequently, the casting is cooled down gradually to a range of 200-300°C at a rate of 25-100% per hour, followed by air cooling. This process helps dissolve some of the carbides and transform the remaining ones into a spherical shape, resulting in a more uniform structure.
High Temperature Stabilizing Treatment
To prevent significant changes in the microstructure, such as grain growth and carbon rejection, as well as distortion during service conditions, it is recommended to apply a stabilization treatment to austenitic cast iron under cyclic loading at high temperatures.
It is advisable to perform this treatment before the final machining stage, as it may cause some distortion in the casting. The process involves gradually heating the casting to approximately 870°C and holding it at that temperature for a specific duration based on the section thickness (minimum 1 hour per 25 mm). Afterward, the casting is cooled in the furnace to a range of 200-300°C at a rate of 25-100°C per hour, followed by further cooling in open air.
Dimensional stabilizing treatment
A different method known as dimensional stabilizing treatment is administered to castings that necessitate precise dimensions for components made with austenitic iron casting in specific applications.
This treatment is an expansion of the high temperature stabilizing treatment. In this process, the machined component is first soaked according to the high temperature stabilizing treatment, then cooled in a furnace at a rate of 25-100°C per hour to a temperature range of 200-300°C.
Afterward, it is air cooled and subjected to finish machining and stress relieving at a temperature of 250-300°C for 1 hour per 25 mm section thickness. Finally, it is air cooled again.
Summary of Heat Treatment for Austenitic Cast Iron
- Annealing: Annealing is a common heat treatment process used to refine the microstructure of austenitic cast iron. The material is heated to a specific temperature and then slowly cooled to room temperature. This process relieves internal stresses, improves ductility, and reduces hardness. Annealing also helps to eliminate any casting defects and refine the grain structure, resulting in improved mechanical properties.
- Solution Treatment: Solution treatment involves heating the austenitic cast iron to a high temperature, typically above its critical temperature, followed by rapid cooling. This process dissolves any carbides present in the material, resulting in a homogeneous solid solution. Solution treatment improves the material’s toughness, corrosion resistance, and high-temperature strength.
- Precipitation Hardening: Precipitation hardening is a heat treatment technique used to further enhance the strength and hardness of austenitic cast iron. After solution treatment, the material is aged at a lower temperature to allow the formation of fine precipitates. These precipitates act as obstacles to dislocation movement, resulting in increased strength and hardness. Precipitation hardening also improves the material’s resistance to wear and fatigue.
- Nitriding: Nitriding is a surface hardening process that involves the diffusion of nitrogen into the austenitic cast iron. The material is heated in a nitrogen-rich atmosphere, forming a hard nitride layer on the surface. Nitriding improves the material’s wear resistance, hardness, and fatigue strength. It also enhances the corrosion resistance of austenitic cast iron, making it suitable for applications in aggressive environments.
- Stress Relieving: Stress relieving is a heat treatment process used to reduce residual stresses in austenitic cast iron. The material is heated to a temperature below its critical range and then slowly cooled. This process helps to minimize distortion and cracking that may occur during machining or welding. Stress relieving also improves the material’s dimensional stability and reduces the risk of premature failure.
Conclusion:
Heat treatment plays a crucial role in enhancing the mechanical properties of austenitic cast iron. Annealing, solution treatment, precipitation hardening, nitriding, and stress relieving are some of the commonly employed techniques. These processes refine the microstructure, improve strength, hardness, and corrosion resistance, making austenitic cast iron suitable for a wide range of applications.