Introduction to Steel Heat Treatment
Steel heat treatment involves heating, holding, and cooling the steel in solid-state to change its internal structure, thereby improving its mechanical and process properties. The main heat treatment methods include:
Annealing
Annealing involves heating the steel to a temperature above the critical temperature, then cooling it slowly in the furnace. Its purpose is to reduce hardness, increase plasticity, refine the crystalline structure, improve machining performance, and eliminate internal stresses caused by casting, forging, and welding. Specific types include:
- Full Annealing: Heating steel above Ac3 by 20-30°C, holding for a period, and then cooling slowly in the furnace to obtain a nearly equilibrium structure.
- Diffusion Annealing (Homogenization Annealing): Used to homogenize the internal chemical composition of the steel.
- Incomplete Annealing: Used for partial transformation of the crystal structure.
- Spheroidizing Annealing: Used for high carbon steel and tool steel to obtain a spheroidal carbide structure.
- Stress Relief Annealing: Heating the steel to a certain temperature below Ac1 (generally 500-650°C), holding, and then cooling slowly in the furnace to eliminate internal stresses.
- Recrystallization Annealing (Intermediate Annealing): Used to restore plasticity during processing.
Normalizing
Normalizing involves heating steel components to a temperature 30-50°C above Ac3, holding for a period, and then cooling in air. The cooling rate is faster than annealing but slower than quenching, which can refine grains, homogenize the internal structure, and eliminate internal stresses.
Quenching
Quenching involves heating steel to a temperature above the critical temperature, holding for a period, and then rapidly cooling in water or oil. Its purpose is to increase hardness and wear resistance. Quenched materials must undergo tempering. Specific types include:
- General Quenching: Common quenching method.
- Double Liquid Quenching: Quenching using two cooling media.
- Austempering: Holding at a constant temperature for a period after heating, then cooling.
- Solution Treatment: Mainly used for stainless steel and alloy steel to improve corrosion resistance.
Tempering
Tempering involves reheating quenched steel to a certain temperature (below the critical temperature), holding for a period, and then cooling in air. Depending on the tempering temperature, it can be divided into:
- Low-Temperature Tempering (150-250°C): Used to reduce brittleness and quenching stress while maintaining high hardness.
- Medium-Temperature Tempering (350-500°C): Improves elasticity and yield point, suitable for various springs and parts subjected to impact.
- High-Temperature Tempering (500-650°C): Combined with quenching, called quenching and tempering, suitable for important load-bearing parts.
Surface Hardening
Surface hardening is suitable for parts requiring high surface hardness and internal toughness. Methods include:
- Flame Heating Surface Hardening
- High-Frequency Heating Surface Hardening (20-10000kHz)
- Medium-Frequency Heating Surface Hardening (<10kHz)
- Laser Heating Surface Hardening
- Electron Beam Heating Surface Hardening
- Electrolytic Heating Surface Hardening
Chemical Heat Treatment
- Carburizing: Used for low carbon steel or low carbon alloy steel to increase surface hardness.
- Nitriding: Forms a high hardness nitrided layer on the surface, suitable for 38CrMoAl nitriding steel.
- Carbonitriding: Simultaneous infiltration of carbon and nitrogen into the surface.
- Others: Such as boronizing, aluminizing, and multi-element co-infiltration.
Functions of Heat Treatment
Heat treatment changes the microstructure or surface chemical composition of the workpiece to improve performance, enhance mechanical properties, relieve residual stress, and improve machinability. Heat treatment processes are classified into preparatory heat treatment and final heat treatment based on their purpose.
Preparatory Heat Treatment
Preparatory heat treatment aims to improve processing performance, relieve internal stress, and prepare for the final heat treatment. It mainly includes annealing, normalizing, aging treatment, and quenching and tempering.
Final Heat Treatment
The final heat treatment aims to improve hardness, wear resistance, and strength.
- Quenching: Includes surface quenching and overall quenching. Surface quenching causes less deformation, suitable for parts requiring high external strength and wear resistance.
- Carburizing and Quenching: Suitable for low carbon steel and low alloy steel to increase surface hardness.
- Nitriding Treatment: Increases surface hardness, wear resistance, fatigue strength, and corrosion resistance.
Defects in the Heat Treatment Process
Overheating and Overburning
- Overheating: Refers to significantly coarse austenite grains due to excessive heating temperature or prolonged holding time.
- Overburning: Refers to the formation of melting or oxidation at grain boundaries at temperatures close to the melting point, which is irreparable.
Oxidation and Decarburization
- Oxidation: Formation of oxide scales on the workpiece surface.
- Decarburization: Surface carbon loss, reducing surface carbon content.
Deformation and Cracking
Deformation and cracking of parts are caused by internal stress during quenching.
Important Mechanisms of Heat Treatment
Heat treatment involves two important mechanisms that can change the properties of alloys:
- Martensitic Transformation: Used to induce deformation, increasing strength and hardness.
- Metal Diffusion Mechanism: Used to alter homogeneity, making the material exhibit single-character tendencies.
Heat Treatment of Other Materials
Besides steel, aluminum, copper, magnesium, titanium, and their alloys can also be heat-treated to change their mechanical, physical, and chemical properties to achieve different performance requirements. For example, cryogenic treatment is a technique that cools steel to temperatures below zero after quenching, ranging from minus seventy to over a hundred degrees.
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