Steel Heat Treatments: Types and Processes
Steel Heat Treatments
Steel heat treatments are operations of heating and cooling, subject to certain temperatures and conditions, to obtain the properties and characteristics best suited to your job or transformation.
Development of Heat Treatments
Heat treatment consists of three phases:
Heating to the Set Temperature
The temperature rise should be even. When a piece is heated, the temperature increases very slowly or will maintain a time at intermediate temperatures before passing through the critical points. This last step is the heating temperature set.
Sojourn Time
The goal of this phase is the complete transformation of the starting structural constituent. A sojourn time of about two minutes per inch of thickness can be considered sufficient to achieve complete austenitizing in the center and surface. Long maintenance, especially at high temperatures, is very dangerous because the austenitic grain of the steel grows rapidly, leaving the final structures coarse and brittle.
Cooling from the Set Temperature to Room Temperature
The cooling must be carefully controlled depending on the type of treatment carried out.
Classification of Different Types of Heat Treatment
Treatments in the Mix
- Annealed
- Normalized
- Quenching
- Tempering
Surface Treatments
- Surface Hardening
- Thermochemical Treatments
- Carburizing
- Carbonitriding
- Nitriding
- Boriding
Surface Treatments (Deposits)
Most Used Mass Treatments
A) Normalizing
It is done by heating the steel to a temperature about 50°C above the critical temperature. Once austenitized, it is allowed to cool in still air. The cooling rate is slower than in quenching and faster than annealing. This treatment refines and homogenizes the structure. This treatment is typical of carbon steel construction from 0.15% to 0.60% carbon. As the diameter of the bar increases, the cooling will be slower. Therefore, the yield strength decreases and elongation increases slightly. This variation is more pronounced the closer to the center the test is conducted.
B) Annealing
Several treatments are known by this name, whose main objective is to “soften” the steel to facilitate subsequent machining. It is also used to regenerate the grain or eliminate internal stresses. Annealing does not generally provide the most suitable characteristics for the use of steel. Almost always, the material undergoes further treatment to obtain the best characteristics. When this happens, annealing is also called a “preliminary heat treatment,” and the subsequent treatment is called a “quality heat treatment.”
Types of annealing are:
- Regeneration Annealing
- Thickening of Grain Annealing
- Globular or Spherical Annealing
- Globular Subcritical Annealing
- Regular Austenitizing Globular Annealing
- Incomplete or Oscillating Annealing
- Homogenization Annealing
- Subcritical Annealing (Softening or Stress Relief)
- White Isothermal Annealing
C) Quenching
Quenching is a process of heating followed by cooling, usually rapid, with a minimum speed called “critical.” The goal to be achieved with the hardening cycle is to increase the hardness and mechanical strength, transforming the entire mass into austenite with heating. Then, by rapid cooling, austenite becomes martensite, which is the typical constituent of tempered steels.