Heat treatment refers to a metal heat processing process that facilitates processing and improves the strength or hardness of finished products by heating, holding and cooling materials in solid state. This process is usually used to treat steel workpieces before and after CNC machining.
The purpose of heat treatment of CNC machining materials is mainly to improve the mechanical properties of metal materials, give full play to the potential of materials, save materials and extend the service life of parts. Secondly, heat treatment can eliminate the residual stress of materials, improve the performance, and facilitate the cutting of materials
Heat treatment mode of CNC machining
Heat treatment process generally includes three processes of heating, heat preservation and cooling, sometimes only two processes of heating and cooling. These processes are interconnected and uninterrupted. Metal heat treatment process can be generally divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment.
Overall heat treatment
Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. The overall heat treatment of iron and steel generally includes four basic processes: annealing, normalizing, quenching and tempering
Annealing treatment:
Annealing is a metal heat treatment process that heats metal parts to a certain high temperature, maintains them for a period of time, and then allows them to cool naturally.
Its main functions:
1. Reduce the hardness of parts and improve the cutting performance;
2. Eliminate residual stress of parts, stabilize dimensions, reduce deformation and crack probability;
3. Refine the grain, adjust the structure, and eliminate the material structure defects;
4. Unify the material structure and composition, improve the material performance or prepare for the subsequent heat treatment process.
Normalizing treatment
Normalizing treatment is to heat the metal parts to a certain high temperature for a period of time, and then let them cool in the air by means of water spray, spray, blowing, etc. Different from annealing treatment, its cooling speed is faster, so the obtained material structure is finer, and the mechanical properties are also improved.
Its main functions:
1. Remove the internal stress of materials;
2. Reduce the hardness and improve the plasticity of materials;
Quenching treatment:
Quenching is a heat treatment process that heats metal parts to a temperature above the critical temperature Ac3 or Ac1, maintains them for a period of time, makes them fully or partially austenitized, and then rapidly cools them to less than Ms at a cooling rate greater than the critical cooling rate for martensitic transformation.
Its main functions:
A. Greatly improve the rigidity, hardness, wear resistance and fatigue strength of parts;
B. Meet the physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels
tempering:
Tempering treatment is a kind of heat treatment in which the steel after quenching and hardening or normalizing treatment is immersed at a temperature lower than the critical temperature for a period of time, and then cooled down at a certain rate to increase the toughness of the material.
Its main functions:
1. Eliminate the residual stress generated during quenching of workpieces to prevent deformation and cracking;
B. Adjust the hardness, strength, plasticity and toughness of the workpiece to achieve better performance requirements;
3. Stabilize the structure and size, and ensure the accuracy;
4. Improve and improve processing performance
Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment, in which quenching and tempering are closely related and often used together
Surface heat treatment
In order to only heat the surface of the workpiece without transferring too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the workpiece per unit area, so that the surface or part of the workpiece can reach a high temperature in a short time or instantaneously.
The main methods of surface heat treatment are flame quenching and induction heating heat treatment. The common heat sources are oxyacetylene or oxypropane flame, induced current, laser and electron beam, etc
Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and properties of the surface layer of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the workpiece surface.
Chemical heat treatment is to heat the workpiece in the medium (gas, liquid, solid) containing carbon, nitrogen or other alloy elements for a long time, so that the surface of the workpiece is infiltrated with carbon, nitrogen, boron, chromium and other elements.
After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metallizing.