Malleable cast irons are a type of cast iron that undergoes a prolonged annealing process, resulting in a microstructure of tempered carbon within a matrix of ferritic, pearlite, or both. While the manufacturing process of malleable cast iron does not typically involve heat treatment, there are instances where heat treatment can be applied.
In this article, we will explore the various heat treatment methods for malleable cast iron, including stress relieving, hardening and tempering, austempering, martempering and tempering, and surface hardening.
Heat Treatment of Malleable Cast Iron
Heat Treatment of Malleable Cast Iron can be carried out using:
- Stress Relieving
- Hardening and Tempering
- Austempering
- Martempering and Tempering
- Surface Hardening
Stress Relieving
In most cases, malleable castings are used without any additional heat treatment, unless specifically required. However, stress relieving treatment is commonly performed before machining the castings.
This treatment is similar to other types of cast irons and involves heating the castings to a temperature range of 500-650°C at a rate of 25-150°C per hour. The castings are then held at this temperature for 1 hour plus an additional hour for every 25 mm of section thickness.
Afterward, the castings are cooled to a temperature of 200-300°C at a rate of 25-100% per hour and finally cooled in open air or with the assistance of an air blast. Stress relieving can also be included as part of the malleablizing annealing process.
Hardening and Tempering
For a shortcut method of hardening, the casting can be air-cooled from the first stage of malleablizing annealing until it reaches a temperature below the critical point. The casting is then reheated to a temperature range of 820-950°C and held at this austenizing temperature for 30 minutes to 1 hour.
Afterward, it is quenched in warm agitated quenching oil at a temperature range of 50-100°C. In the case of fully annealed malleable castings, the material is heated at a rate of 25-150°C per hour to a temperature range of 820-950°C and held at that temperature for 30 minutes to 1 hour. It is then quenched in warm oil.
The latter process results in a lower quenched hardness compared to the former process, so the tempering temperature needs to be adjusted accordingly to achieve the desired hardness level. The tempering process should follow the guidelines applicable to ductile iron, as discussed in the appropriate clauses of the standard.
Austempering
Austempering is a heat treatment method that produces a less brittle and softer structure called bainite, as opposed to the martensite structure obtained through normal quench hardening.
To achieve austempering, the casting is quenched from a temperature above the austenite transformation range (850-950°C) in a hot salt, oil, or lead bath maintained at a temperature range of 250-400°C. It is held at this temperature for 20-30 minutes to allow complete bainitic transformation and then air cooled.
Martempering and Tempering
Martempering is a heat treatment method that minimizes distortion and cracking of the casting by transforming the martensite structure. The casting is quenched from a temperature (850-950°C) above the austenite transformation range in a hot salt, oil, or lead bath maintained at a temperature slightly above the martensite start temperature (MS + 50°C).
The casting is held in the bath until it reaches the bath temperature and then air cooled. The tempering treatment for martempering is the same as the normal hardening and tempering process discussed in the appropriate clause of the standard.
Surface Hardening
Surface hardening of malleable cast iron can be achieved through flame hardening, a process where the surface of the casting is heated using a flame and then rapidly cooled. This method increases the hardness of the surface layer, providing improved wear resistance.
Conclusion
In conclusion, heat treatment can be applied to malleable cast iron to enhance its properties and meet specific requirements. The choice of heat treatment method depends on the desired outcome and the characteristics of the castings.
Whether it is stress relieving, hardening and tempering, austempering, martempering and tempering, or surface hardening, each method offers unique benefits and considerations for achieving the desired results.