Here we take a closer look at the different types of heat treating and how these affect the parts during precision machining.

What is heat treatment?

Heat treatment process is the controlled heating or cooling of plain carbon steel or alloy steel. Heat treatment of steel is done to change its molecular structure, in order to obtain certain mechanical properties. These mechanical properties could be hardness, strength or ductility. Various types of heat treatment processes are done to obtain various types of mechanical properties.

Why do precision parts need heat treatment? 

Heat treating a precision machined component is a controlled heating and cooling process that alters the physical properties of the metal to achieve a specific requirement. In many cases, heat treatment is used to increase strength, hardness, and wear resistance.

Heat Treating Hardening

In heat treating to harden a metal, the metal is heated to a temperature where the elements in the metal become a solution. Before doing this, defects in the crystal lattice structure of metal are the primary source of ‘give’ or plasticity. Heat treating addresses those deficiencies by bringing the metal into a reliable solution with fine particles to strengthen the metal. Once the metal is thoroughly heated to the right temperature to produce a solid solution, it is quickly quenched to trap the particles in solution.

In precipitation hardening, impurity particles are added to the metal alloy to increase strength further.

Heat Treating CASE HARDENING

In the process of case hardening, the external layer of metal is hardened while the interior metal remains soft. For metals with a low carbon content such as iron and steel, additional carbon has to be infused into the surface. Case hardening is a process often used as a final step after the piece has already been machined. High heat is used in combination with other elements and chemicals to produce a hardened outer layer. Because hardening can make metals more brittle, case hardening can be useful for applications that require a flexible metal with a durable wear layer.

precision machining, heating

ANNEALING

Annealing is a heat treatment method where a metal such as aluminum, copper, steel, silver, or brass is heated to a specific temperature, held at that temperature for some time to allow transformation to occur, and then air cooled. This process increases the metal’s ductility and decreases hardness to make the metal more workable. Copper, silver, and brass can be cooled quickly or slowly, whereas ferrous metals like steel must always be cooled gradually to allow annealing to occur. Annealing may be used before a metal is machined to improve its stability, making harder materials less likely to crack or fracture.

NORMALIZING

Normalizing is an annealing process for steel where it is heated 150-200°F higher than in annealing and held at the critical temperature long enough for the transformation to occur. Steel treated in this way must be air cooled. The heat treating in normalization causes smaller austenitic grains, while air cooling produces more refined ferritic grains. This process improves machinability, ductility, and strength of the steel. Standardization is also useful to remove columnar grains and dendritic segregation that can occur during the casting of a part.

TEMPERING

Tempering is a method of heat treating used to increase the resilience of iron-based alloys like steel. Iron-based metals are very hard, but they are often too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, and strength of metal, which usually makes it easier to machine. The metal will be heated to a temperature below the critical point as lower temperatures reduce brittleness while maintaining hardness.  For increased plasticity with less hardness and strength, higher temperatures are required.

Another option is to purchase material that has been hardened or to harden the material before machining. While this makes it more difficult to machine, it eliminates the risk for the part sizes changing, unlike a post-machining heat treatment process. It can also eliminate the need for a grinding shop to get the tight finishes or tolerances.

Defects of heat treatment processes

Decarburization

Loss of carbon from the metal surface due to prolonged heating is known as decarburization.

Over heating

Heating metal for a long period of time at high temperature causes loss of ductility and impact strength of material.

Oxidation

Oxidation of metal surface is also a defect which normally arise due to heat treatment processes. It can be prevented by using an inert atmosphere.

Soft spots

Localized decarburization and bubble formation in metal results in soft spots.

Quenching cracks

Cracks arising in metal surfaces due to higher cooling rates is known as quenching cracks.

Warping

Non-uniform heating or cooling of the metal causes warping.

What are the types of heat treatment for steel?

The 4 Types of Heat Treatment Steel Undergoes

Heat Treatment Steel: Annealing.

Heat Treatment Steel: Normalizing.

Heat Treatment Steel: Hardening.

Heat Treatment Steel: Tempering.

What is heat treatment explain different types and their objectives?

Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching.

What are types of heat treatment?

What are the 4 Types of Heat Treating Processes? Common types of heat treating methods include annealing, hardening, quenching, and stress relieving, each of which has its own unique process to produce different results.