What is heat treatment?

Although most people don’t know what heat treatment is, it’s actually an essential part of the manufacturing process. That’s because heat treating alter the microstructure of metal in order for the material to better withstand wear and tear. Heat treatment involves heating a metal or alloy to a specific temperature and then cooling it to harden the material. 

Some notable industries in which heat treatment plays an important role include aircraft, automobiles, hardware–such as saws and axes, computers, spacecraft, military, and the oil and gas industry.

Stage of heat treatment

There are three stages of heat treatment: 

• Heat the metal slowly to ensure that the metal maintains a uniform temperature 

• Soak, or hold, the metal at a specific temperature for an allotted period of time

• Cool the metal to room temperature

1.The heating stage

During the heating stage, the foremost aim is to make sure that the metal heats uniformly. You get even heating by heating slowly. If you heat the metal unevenly, one section may expand faster than another, resulting in a distorted or cracked section of the metal. 

2.The soaking stage

The purpose of the soaking stage is to keep the metal at the appropriate temperature until the desired internal structure takes shape.  The “soaking period” is how long you keep the metal at the appropriate temperature. To determine the correct length of time, you will need the chemical analysis and mass of the metal. For uneven cross-sections, you can determine the soaking period using the largest section. 

Generally, you shouldn’t bring the temperature of the metal from room temperature to the soaking temperature in one step. Rather, you’ll need to heat the metal slowly to just below the temperature where the structure will change, and then hold it until the temperature is consistent throughout the metal. After this step of “preheating”, you more quickly heat the temperature to the final temperature that you’ll need. Parts with more complex designs may require layers of preheating to prevent warping. 

3.The cooling stage

In the cooling stage, you’ll want to cool metal back to room temperature, but there are different ways to do this depending on the type of metal. It may need a cooling medium, a gas, liquid, solid, or combination thereof. The rate of cooling depends on the metal itself and the medium for cooling. It follows that the choices you make in cooling are important factors in the desired properties of the metal. 

What Types Of Heat Treatment Are There?

Four fires in heat treatment

1、Normalizing Heat Treatment:

Normalizing is heating the metal casting to an elevated temperature above the transformation temperature and then air cooling it back to room temperature.  This process alters the microstructure to reduce the variation in hardness and ductility within the casting.  The temperatures and times are typically lower and shorter than for homogenization.  Normalizing is often followed by tempering. 

2、Tempering Heat Treatment:

Tempering is heating the metal casting to a temperature below the transformation temperature to lower the hardness and improve the ductility of the metal.  

3、Hardening Heat Treatment And Quenching Heat Treatment:

The use of this treatment results in an improvement in metal casting mechanical properties, related especially to increasing hardness or durability.  It is often used for steel parts.  Metal is heated to an elevated temperature (above the transformation temperature) and then rapidly cooled (quenched).  This causes the softer initial material to transform its structure into a stronger one.

Quenching cools the metal casting after the initial heating process. The quench is typically performed in oil or water, depending on the material and specification requirements.  Tempering is the last step in the hardening and tempering process and is required after the quench.  It involves reheating the metal at a low temperature (below the transformation temperature) to achieve final specifications.  Tempering lowers the as-quenched hardness, restores ductility, and reduces stresses of fully hardened steel. 

4、Anneal Heat Treatment:

Annealing is heating and holding the metal at an elevated temperature followed by furnace cooling to obtain the desired hardness.  This is typically performed on “hardenable” ferrous alloys to reduce hardness, increase ductility, and improve the machinability of the casting.  This process is often used on tool steels, alloy steels, and martensitic stainless steels to allow for quick rough machining prior to any hardening and tempering operation.   

In addition to the above four fires, there are some heat treatment processes that are more commonly used at Protosoon.

• Homogenizing Heat Treatment:

This process involves heating the metal casting in a furnace to an extremely hot temperature and holding for an extended period to improve chemical uniformity by diffusion.  Homogenization is sometimes employed as a pre-treatment to make subsequent heat treatments more effective.