The functions of heat treatment and surface treatment:
Heat treatment:
Enhances mechanical and process properties, including hardness, strength, toughness, wear resistance, and fatigue resistance.
Surface treatment:
Reinforces surface performance, improving corrosion resistance, preventing rust, increasing surface hardness, enhancing wear resistance, and imparting special properties such as electrical conductivity, smoothness, aesthetics, coating, and film covering.
Common heat treatment methods：
Internal stress:
When an object undergoes deformation due to external factors such as temperature, external forces, or its own gravity, internal forces are generated within the object to resist the deformation, attempting to restore the object to its original state but cannot fully recover, resulting in residual stress.
Ways to eliminate stress include: Thermal aging: annealing, normalizing, tempering, and holding below tempering temperature after heating. Natural aging: Purely preventing in the natural environment for a period of time, with slow release due to temperature difference, exposure to sunlight, and rain.
Vibrational aging: Artificially striking or vibrating stress-relief devices.
Heat treatment:
The four-step process of heating, quenching, tempering, and cooling, where faster cooling rates result in higher internal stress and brittleness.
Annealing:
Heating followed by holding at a constant temperature and then cooling in the furnace, used to reduce hardness for easier machining, refine the microstructure, and relieve internal stress.
Normalizing:
Heating followed by holding at a constant temperature and then cooling in still air at room temperature, used to increase the hardness and strength of low carbon steel, refine the microstructure, improve machining precision, and relieve internal stress.
- Low carbon steel to increase hardness for easier machining, refine the microstructure to improve strength, and enhance machining precision.
- Relieve internal stress and prepare for subsequent heat treatment.
- Some less critical components undergo final heat treatment, such as welding frames.
Quenching:
Rapid cooling after heating, using water or oil as the cooling medium, to increase material hardness and strength, but may lead to increased internal stress, typically requiring tempering treatment.
- Water quenching: Faster cooling rate than oil, used for carbon steel.
- Oil quenching: Slightly slower cooling rate, used for alloy steel.
- Good hardenability: Results in less deformation and lower internal stress during heat treatment, suitable for precision components.
- Hardenability: Depends on carbon content and alloying elements.
Tempering:
Heating followed by holding at a constant temperature and then cooling in the furnace, used to relieve internal stress after quenching, improve toughness, strength, wear resistance, and fatigue strength. Tempering temperature: low, medium, high. The higher the tempering temperature and the longer the time, the more internal stress is relieved, resulting in greater reduction in hardness and improved toughness.
Annealing:
Quenching + high-temperature tempering. Excellent comprehensive mechanical properties.
Processing without heat treatment:
Cutting – Rough machining – Semi-finishing – Finishing – Surface treatment
Heat treatment:
Cutting – Rough machining – Annealing, Normalizing – Semi-finishing – Quenching and tempering – Finishing – Carburizing, Nitriding – Grinding.
After quenching and tempering:
Aging treatment – Further elimination of internal stress – Finishing.
Common surface treatment methods:
Surface modification: Altering surface stress and microstructure without changing chemical composition.
Surface conversion coating: Forming protective coatings through chemical reactions.
Surface coating: Applying coatings or platings where substrate elements do not participate in film formation.
Surface alloying: Enhancing surface wear resistance and hardness through processes like carburizing and nitriding.
Common surface treatment methods for common components:
Surface quenching:
such as high-frequency induction quenching, flame quenching, to enhance surface wear resistance, hardness, and fatigue strength.
Laser surface treatment:
used for rust removal, cleaning, and quenching. Shot blasting / sandblasting: cleaning, improving surface quality, removing burrs, matte finish, removing oxide scale, and increasing adhesion for coatings.
Rolling:
improving surface smoothness and quality. Knurling: increasing surface friction coefficient.
Drawing / polishing:
improving aesthetics and reducing surface roughness.
Carburizing / nitriding:
increasing surface wear resistance and fatigue strength.
Carburizing:
improving surface wear resistance, hardness, and fatigue strength by about 2~3mm.
Nitriding:
for alloy steels, significantly increasing surface wear resistance, hardness, and fatigue strength by about 0.4mm.
Special nitrogen steel:
about 0.7mm. Gas nitriding and ion nitriding are better.
Carbonitriding:
carbon can assist nitrogen penetration.
Anodizing:
suitable for aluminum parts, providing protection and aesthetics, with options like natural anodizing, bright silver, black, matte black, etc. Hard anodizing: insulating and wear-resistant.
