Heat treatment of metals involves modifying the crystallographic structure of a metal part for a specific purpose. Certain techniques can be used toimprove the hardness of parts and increase their resistance to stress. There are many different heat treatment processes for metals, each of which affects different properties. How does metal heat treatment work, and how do we choose the most appropriate one?
Different types of metal heat treatment
The term "heat treatment of metals" actually refers to a set of industrial processes applied to metal parts for a clearly defined purpose. In practice, the material is subjected to a specific environment that modifies its physical and chemical properties. To achieve this, these treatments act on the time, atmosphere and temperature in which the metal part is placed. The metal then passes through three phases: heating, holding and cooling. Most of these processes take place in a gaseous, vacuum or liquid environment.
There are two main types of heat treatment for metals: those that soften the metal's resistance, thus improving its machinability or formability, and those that, conversely, contribute to its hardening. In this case, the main effect is on the part's hardness or mechanical strength, and the aim is to make it more resistant to stress and improve its mechanical performance.
Two types of hardening treatment
Hardening treatments can be applied either to the entire volume of the part, or only to its surface. The former is referred to as " heat treatment in the mass ", the latter as " surface treatment ".
Treatment in the mass
Among the best-known mass treatments are annealing, quenching and tempering.
Annealing technique
As the name suggests, annealing involves heating a part. First heated to a temperature ranging from 450 to 1,100 degrees Celsius, it is maintained at this temperature and then cooled. The timing of each phase remains crucial, and is the subject of precise analysis in order to influence the desired mechanical properties or metallographic structure. Annealing mainly serves to eliminate or relax certain stresses, such as those obtained during part processing (forging, casting, machining...) or during welding operations. Annealing can also be used to reduce the risk of deformation, or to modify the structure of the part for subsequent processing.
Tempering technique
Hardening follows the same three stages, but the difference lies in the cooling phase: this can take place in a gaseous or aqueous medium such as water or oil. This stage is faster than annealing, and the metal is instantly hardened by martensitic transformation. Similarly, the quenching phase is generally followed by a final step, such as tempering.
Tempering technique
Tempering generally follows quenching, and involves heating the part slowly one or more times. After quenching, the metal's structure is relatively brittle and unstable, so tempering makes the metal more ductile and tough.
Surface heat treatment
Surface treatments modify only the surface properties of the part, and are generally used to protect it against wear, friction, fatigue or corrosion. Here again, there are many different processes available, each conferring different properties on the final part.
Case-hardening process
One of the most famous surface heat treatment processes is case-hardening. The aim of this technique is to harden the surface of the metal part, while preserving a resilient interior, making the whole less brittle. The carburizing process involves the diffusion of carbon onto the surface of the part, usually in a gaseous medium.
Nitriding process
Nitriding is also one of the most effective surface heat treatments for improving a part's resistance to wear and corrosion. Here, the part is exposed to nitrogen. This reacts with the iron to form a combination layer and a diffusion layer of iron nitrides. This surface layer considerably improves the strength of the part's surface and extends its service life.
How do you choose the right process?
Choosing the right heat treatment is not an easy task. This decision obviously depends on the part's end use, but also on the stresses it will undergo during operation.
This is based on a calculation of forces and stresses, also known as resistance to materials (RTM). The treatment to be preferred varies according to the stresses to which the final equipment is subjected.
Examples of metal heat treatment applications
Heat treatment of metals is now widely used in many industrial sectors. Without these processes, an airplane can't fly, a car can't run, a rocket can't take off...
Also used in the design of medical equipment, heat treatment of metals seems to be everywhere, from the axles and crankshafts that make up the structure of a car, to the racks and machine tools used in heavy industry.
Thermi-Lyon has recognized expertise in heat treatment for all types of metal parts. Let's talk about your project.
