Heat Treatment of Gray Iron Castings
Gray iron is one of the most commonly used casting materials, primarily because it is an inexpensive and versatile material with a wide range of applications. In addition to its low cost, gray iron is also popular for its high strength, wear and corrosion resistance, good thermal conductivity, and relative ease of machining and finishing. Despite these useful characteristics, gray iron requires specific heat treatment to achieve its maximum potential and obtain the desired properties for a particular application.
The heat treatment of gray iron castings involves the use of high temperatures to increase the strength, castability, machinability and other characteristics of the material. Generally, the heat treatment of gray iron focuses on ensuring the presence and proper size of graphite flakes, which act as both a source of strength and a lubricant during machining operations. The heat treatment of gray iron is composed of two distinct processes, namely, preheating and hardening.
The preheating process of gray iron castings involves the heating of the material to a temperature of 538 – 752°C (1000 – 1400°F) for 20 – 30 minutes. The purpose of preheating is to reduce the risk of thermal shock and to allow for faster cooling during the hardening process. It also allows for stress relief and improved machinability. The preheating temperature should be closely monitored and controlled to ensure the integrity of the casting.
The hardening process of gray iron involves further heating of the material to a temperature ranging from 816 – 867°C (1500 – 1600°F) and then quickly cooling it in an oil bath. After cooling, the casting can be rapidly transferred to a tempering oven to further strengthen and soften it. During the tempering process, metal is heated at a lower temperature (generally less than 315°C/600°F) for a few minutes and then cooled. Depending on the application, the tempering oven can be set to different temperatures ranging from 250 – 800°C (480 – 1480°F).
After the heat treatment has been completed, the casting is then ready for machining or use in whatever application it was originally intended for. Heat treatment is an important part of ensuring the performance and quality of grey iron casting components. By following the correct heat treatment process, castings can achieve the desired strength and wear properties for their intended end use. Additionally, heat treatment increases the machinability and assists in achieving the desired surface finish of the final product.
