Understanding Heat Treatments For CNC Machined Parts

Heat treatments are popular in industrial part manufacturing, often used to improve the material’s specific physical and chemical composition. The process involves heating metal alloys to extreme temperatures and the immediate cooling of the heated metals under monitored conditions. This rapid heating and cooling drive the changes in the chemical composition and atomic structures of the metal alloys.

Heat treatments can deliver physical properties like improved hardness, strength, malleability, and processability. Heat treatments are also widely suitable for most materials that are used with CNC machining. This article describes the factors necessary for heat treatment and the many benefits of the process for CNC machined parts.

What factors are essential in heat treatments?

The effectiveness of heat treatment operations grossly lies within process control. Three factors are essential if a heat treatment process is to achieve the desired results. They include:

i. Temperature

The temperature to which the material is heated must be sufficient to cause changes in its physical and chemical composition.

ii. Time 

How long the metal alloy is maintained at the elevated temperature 

iii. Cooling rate 

The rate of cooling the metal alloy back to normal temperature has a crucial effect on the metal’s final physical characteristics.

When can you heat-treat your parts?

Parts to be CNC machined may be heat-treated before or after machining operations. Here are some insights into the benefits of applying heat treatments before or after:

i. Before machining 

Applying heat treatments before machining means that you will be working with standardized materials. This is often faster, but the increased hardness may make machinability more challenging.

ii. After machining

Heat treatment may be considered as a type of finishing solution. This helps to deliver the hardness and strength to the part after it has been produced. While this extends the overall leadtimes, the machinability of the material is more straightforward.  

What are the types of heat treatment available for CNC machined parts? 

There are numerous heat treatments for CNC machined parts. These treatments serve varying purposes. In this article, we shall cover 6 techniques and the purposes they serve. Here are 5 types of heat treatments available:

i. Annealing 

Annealing is a metal heat treatment technique that involves heating the metal to a high temperature and cooling the material to achieve the desired atomic structure. Annealing is done to improve the malleability and machinability of the metal alloy. It softens the part for further processing (if any) and may be done before or after the part has been made.

ii. Quenching 

Quenching, like annealing, begins by heating the metal alloy to a very high temperature. The hot metal is then rapidly cooled by dipping the metal in water, oil, or super cool air. The rapid cooling process performs the function of making permanent changes to the structure of the material. The process usually follows after CNC machining. The primary aim of quenching is to achieve more hardness in the metal material.

iii. Aging 

Aging, also called precipitation hardening, is used to increase the strength and hardness of the material. The process begins with heating the material to a high temperature, then rapid cooling or quenching comes next. Lastly, the cooled material is reheated to a lower temperature for extended periods to complete the aging process.

iv. Case hardening 

Case hardening is used when CNC machinists want to increase the material’s hardness only at the surface level. The process keeps the underlining materials (core) in their brittle and soft state. It can be likened to how materials like aluminum can be anodized to increase the surface hardness and thickness of their oxide layer.

v. Stress relieving 

As the name suggests, stress relief is used to eliminate the residual stress from the CNC machining process. It is used for parts that have a significant need for excellent mechanical properties. Stress relieving will involve heating the part to high temperatures, but not as high as annealing temperatures.

vi. Tempering 

Tempering is a heat treatment operation that reduces the brittleness of the metal alloy following a quenching operation. It is widely applicable to mild steels and steel alloys and aids the material’s mechanical performance. Tempering, like stress relieving, takes place at a temperature lower than annealing.

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