White cast iron is a versatile material known for its high abrasion resistance, making it ideal for applications requiring crushing, grinding, and other heavy-duty tasks. However, to further enhance its durability and performance, heat treatment plays a crucial role.
In this blog post, we will explore the process of heat treatment for white cast iron and its impact on the material’s properties.
Understanding White Cast Iron
White cast iron exhibits superior hardness and abrasion resistance due to the presence of eutectic carbides, which are significantly harder than iron carbide. The addition of elements like chromium, nickel, molybdenum, and copper helps prevent the formation of graphite and pearlite, while also increasing the hardenability of the material.
Chemical compositions of White Cast Iron
White cast iron is characterized by its high carbon content and the presence of alloying elements that contribute to its specific properties. The chemical compositions of different types of white cast iron may vary, but here are some common alloying elements and their typical ranges:
- Carbon (C): White cast iron usually contains a high carbon content ranging from 2.7% to 3.6%. The carbon content contributes to the hardness and abrasion resistance of the material.
- Chromium (Cr): Chromium is commonly added to white cast iron to enhance its wear resistance and corrosion resistance. The chromium content can range from 1.1% to 28%, depending on the specific type of white cast iron.
- Nickel (Ni): Nickel is often alloyed with white cast iron to improve its toughness and impact resistance. The nickel content can range from 1% to 11% or higher.
- Molybdenum (Mo): Molybdenum is another alloying element that can be added to white cast iron to enhance its wear resistance and toughness. The molybdenum content typically ranges from 0% to 3%.
- Copper (Cu): Copper is sometimes used as an alloying element in white cast iron to improve its hardenability and impact resistance. The copper content is generally kept below 3%.
- Other Elements: In addition to the above elements, white cast iron may contain small amounts of other elements such as silicon (Si), manganese (Mn), and others. These elements can influence the material’s properties, including its hardness, strength, and microstructure.
| Alloying Element | Typical Range (%) |
|---|---|
| Carbon (C) | 2.7 – 3.6 |
| Chromium (Cr) | 1.1 – 28 |
| Nickel (Ni) | 1 – 11 |
| Molybdenum (Mo) | 0 – 3 |
| Copper (Cu) | < 3 |
| Silicon (Si) | Varies |
| Manganese (Mn) | Varies |
| Other Elements | Varies |
Types of High-Alloy White Cast Irons
- Ni-Hard Type I to IV: Nickel-Chromium White Irons:
- These alloys contain nickel, chromium, and carbon, offering excellent abrasion resistance and impact toughness.
- Different compositions within this category provide varying hardness and strength levels, making them suitable for specific applications.
- Chromium-Molybdenum White Irons:
- These alloys consist of chromium, molybdenum, and other elements like nickel or copper.
- With their high hardness and eutectic carbides, they offer exceptional abrasion resistance and reasonable toughness.
- High Chromium White Irons:
- These alloys contain a high percentage of chromium, along with other elements such as molybdenum and nickel.
- They offer a unique combination of excellent abrasion resistance, toughness, and corrosion resistance.
Heat Treatment of Ni-Hard White Irons
Special precautions should be taken when heating or cooling white iron castings, as they are prone to thermal-shock cracking due to their brittle nature, especially when dealing with complex shapes.
Stress-Relieving Treatment (Tempering)
To alleviate the stresses induced by the transformation of austenite to martensite in the as-cast matrix structure, a stress-relieving treatment is conducted. This involves heating the castings to a temperature range of 205-260°C for at least 4 hours.
The treatment serves to temper the material, increasing its strength and impact toughness by 50 to 80% without compromising hardness or abrasion resistance. During the cooling process from the stress-relieving temperature, some retained austenite may transform into martensite.
High Temperature Treatment
When the as-cast hardness of the white iron is not sufficient, a high-temperature treatment is employed. The castings are heated to a temperature range of 750-790°C for 8 hours, followed by controlled air or furnace cooling at a rate not exceeding 30°C/h.
Tempering can be performed at temperatures of 205-260°C. Alternatively, sub-zero treatment can be utilized to achieve a hardness of 550 BHN when the matrix contains 60% martensite, or a hardness of 650 BHN when the martensite content is 80-90%.
To transform retained austenite into martensite and reduce its presence, the castings are subjected to refrigeration at temperatures ranging from -70 to -180°C for a period of half to one hour after the austenitization process. Martensite, retained austenite, and carbides are observed in Ni-hard IV alloy after undergoing deep-freezing. Subsequently, tempering is conducted at temperatures of 205-260°C.
Annealing
To improve the machinability of castings, subcritical annealing or full annealing can be performed. Subcritical annealing involves heating the castings within the range of 690-705°C and maintaining them at this temperature for a period of 4 to 12 hours.
This process leads to a hardness level of 400-450 BHN. On the other hand, full annealing involves heating the castings to a higher temperature range of 950-1010°C, followed by a slow cooling process down to 760°C.
The castings are then soaked at this temperature for a duration of 10 to 50 hours, depending on their composition. Full annealing results in even lower hardness levels. After the annealing process, the castings can undergo further steps such as hardening and tempering after machining.
Heat Treatment of Nickel-Chromium White Irons
Nickel-chromium white iron castings undergo a stress-relief heat treatment due to their martensitic matrix structure when cast properly. Tempering is performed within the temperature range of 205 to 260°C (400 to 450°F) for a minimum duration of 4 hours.
This tempering process not only enhances the toughness and strength of the martensite but also alleviates some of the stresses caused by the transformation. Upon cooling from the tempering temperature, additional martensite may form. Importantly, this heat treatment does not compromise the hardness or abrasion resistance of the castings.
In the heat treatment of any white cast iron, it is crucial to prevent cracking caused by thermal shock. It is advised to avoid placing the castings in a hot furnace or subjecting them to rapid temperature changes during heating or cooling. The risk of cracking is higher for castings with complex shapes and thicker sections.
For austenitizing heat treatment, the castings are typically heated to temperatures ranging from 750 to 790°C (1380 to 1450°F) and held at this temperature for 8 hours. Cooling is then conducted either in air or in a furnace, with a cooling rate not exceeding 30°C per hour.
Subsequently, a tempering or stress-relief heat treatment is performed. Refrigeration heat treatment has become a more commonly used method today to address low hardness issues.
Importance of Heat Treatment
Heat treatment of white cast iron involves subjecting the material to controlled heating and cooling processes to modify its microstructure and enhance its mechanical properties. The two primary heat treatment methods for white cast iron are stress-relieving treatment (tempering) and high-temperature treatment.
- Stress-Relieving Treatment (Tempering):
- This treatment involves heating the cast iron to a specific temperature for a designated period, followed by slow cooling.
- Tempering reduces stress-induced during solidification, increases strength and impact toughness without compromising hardness or abrasion resistance.
- High-Temperature Treatment:
- When the as-cast hardness is insufficient, high-temperature treatment is employed.
- This treatment involves heating the castings to a specific temperature and then air or furnace cooling at a controlled rate.
- Tempering may follow the high-temperature treatment to achieve the desired hardness.
Benefits of Heat Treatment
- Increased Hardness: Heat treatment transforms the matrix of white cast iron, resulting in a harder material that offers enhanced abrasion resistance.
- Improved Toughness: By relieving stresses and optimizing the microstructure, heat treatment improves the material’s toughness and ability to withstand repeated impact loading.
- Customization for Specific Applications: Different heat treatment techniques can be employed to tailor the properties of white cast iron to meet the requirements of specific applications.