Heat Treatments Applied to Cast Iron

In this article in our materials science section, we’ll cover heat treatments applied to cast iron.

Since cast irons are brittle by nature, in some cases they’re subjected to processes such as hardening, increasing wear resistance (austempering), relieving internal stresses (stress-relief annealing), and preparing them for machining (softening annealing). Let’s now examine the heat treatments applied to cast iron in detail.

We come across cast iron in many areas of our lives. For example, radiators (above)

Which Heat Treatments Are Applied to Cast Iron

If metals are cooled in a controlled way after being annealed, heat treatment has been applied. As a result of heat treatment, significant changes occur in the properties of metals. This change in properties can manifest as hardness, but it’s also possible to achieve the opposite result, such as softening.

As with other metallic materials, it’s possible to change the properties of cast iron through certain heat treatments. The heat treatment to be applied varies depending on the type of cast iron.
Some of these are as follows;

Hardening

If alloyed cast irons intended to be hardened are annealed between 820°C-875°C and then rapidly cooled, they harden. Cooling is generally done in oil, though water and air are also used. Following this process, the alloyed cast iron is tempered. This way, the hardness and wear resistance of the alloyed cast iron are increased.

Austempering

This is a heat treatment also known as austempering. It’s a heat treatment applied to ductile cast iron when increased wear and strength are desired. For the austempering process, ductile cast iron is immersed in a salt bath at a temperature of 260°C-290°C and held there for between 15 minutes and 1 hour.

The properties austempering gives to cast iron are as follows:

Low distortion, high ductility. Homogeneous and consistent hardness. Toughness and greater wear resistance. High impact and fatigue strength. Resistant to hydrogen embrittlement, high toughness. High wear resistance. Increased fatigue strength, increased hardness, increased tensile and yield strength

Stress-Relief Annealing

The heat treatment applied to cast iron when internal stresses need to be relieved is known as stress-relief annealing. It’s held at temperatures between 425°C-600°C for half an hour to five hours. It’s then slowly cooled. On the other hand, a drop in hardness and strength values is observed as a result of this process.

Softening Annealing

One of the types of heat treatment for cast iron is softening annealing. To make machining easier, cast iron is desired to have a soft structure. This way, time is saved, which lowers production costs. The process carried out to achieve this goal is softening. If hardness is desired afterward, the cast iron can then be hardened.

Cast iron annealed at 760°C-820°C for softening is held for a period of time (longer than for steel). For soft alloyed cast iron, the application temperature is 975°C. The strength of cast iron subjected to softening treatment decreases. In addition, the amount of free carbon increases.

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In this article in our materials science category, we covered the heat treatments applied to cast iron. We hope it’s been useful.

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