The heat treatment of steel tubes involves processes such as heating, holding, and cooling to alter their internal structure and properties, thereby meeting various application requirements. Below is a detailed introduction to the heat treatment of steel tubes.
I. Main Purposes of Heat Treatment
- Increasing Strength and Hardness: Through heat treatment, the internal structure of steel tubes changes, such as the formation of austenite, martensite, and bainite, significantly enhancing their strength and hardness.
- Improving Wear Resistance and Corrosion Resistance: Heat treatment alters the surface structure of steel tubes, forming a high-hardness and high-wear-resistant surface layer, thus improving their wear resistance and corrosion resistance.
- Eliminating Internal Stress: Steel tubes may develop internal stresses during manufacturing and processing, which can lead to deformation, cracking, or failure during use. Heat treatment effectively eliminates or reduces these internal stresses, maintaining the dimensional stability and shape accuracy of steel tubes.
- Improving Plasticity and Toughness: Heat treatment improves the plasticity, toughness, and impact resistance of steel tubes, making them less prone to fracture under high loads and impacts, thereby extending their service life.
- Enhancing Machinability: Heat-treated steel tubes have better machinability, making them easier to cut, weld, and form, reducing processing difficulty and cost.
II. Common Heat Treatment Processes
- Annealing
- Operation Method: Heat the steel tube to a certain temperature (usually below Ac1 or above it), hold for a certain period, and then slowly cool it in the furnace.
- Purpose: To reduce the hardness of the steel tube and increase its plasticity; to refine the grain structure and improve mechanical properties; to eliminate internal stresses generated during cold or hot processing.
- Normalizing
- Operation Method: Heat the steel tube to a critical temperature (such as above Ac3 or Accm by 30~50°C), hold for a period, and then cool it at a slightly faster rate than annealing.
- Purpose: Similar to annealing, normalizing can also reduce hardness, increase plasticity, refine grain structure, and eliminate internal stresses. Additionally, normalizing improves the machinability of steel tubes.
- Hardening (Quenching)
- Operation Method: Heat the steel tube to a phase transformation temperature (such as above Ac3 or Ac1), hold for a period, and then rapidly cool it in water, nitrate salt, oil, or air.
- Purpose: To transform the internal structure of the steel tube into hard and brittle martensite, thereby increasing its strength and hardness. However, quenching increases the brittleness of the steel tube, so it is usually followed by tempering.
- Tempering
- Operation Method: Reheat the quenched steel tube to a temperature below Ac1, hold for a period, and then cool it in air, oil, hot water, or water.
- Purpose: To reduce or eliminate internal stresses caused by quenching, reducing deformation and cracking of the steel tube; to adjust hardness, improve plasticity and toughness; to stabilize the dimensions of the workpiece.
- Quenching and Tempering (Quenching followed by High-Temperature Tempering)
- Operation Method: Heat the steel tube to the quenching temperature, transforming its internal structure into austenite, and then rapidly cool it at a speed greater than the critical quenching speed to transform the internal structure into martensite. Follow this with high-temperature tempering to ultimately transform the steel tube’s structure into a uniform tempered sorbite structure.
- Purpose: To increase the strength and hardness of the steel tube while combining its strength, plasticity, and toughness.
- Surface Hardening
- Operation Method: Through induction heating or flame heating, rapidly heat the surface of the steel tube and quench it, forming a high-hardness surface layer.
- Purpose: To increase the hardness and wear resistance of the steel tube’s surface.
- Carburizing and Nitriding
- Operation Method: At high temperatures, infiltrate carbon or nitrogen into the surface layer of the steel tube, forming a hard carburized or nitrided layer.
- Purpose: To enhance the wear resistance and corrosion resistance of the steel tube’s surface.
III. Application Examples of Heat Treatment
- Q345 Steel: Used for buildings, bridges, etc., with a yield strength of approximately 345 MPa. It is generally normalized to improve its comprehensive mechanical properties.
- GCr15 Steel: A bearing steel that, after quenching and tempering, exhibits high and uniform hardness, good wear resistance, and high contact fatigue performance.
- 45# Steel and 42CrMo Steel: Can directly undergo quenching and tempering to meet a hardness requirement of 45 HRC.
- 20CrMo Steel: Can also meet a hardness requirement of 45 HRC after carburizing, quenching, and tempering.
In summary, heat treatment is an important means of improving the performance and service life of steel tubes. By selecting appropriate heat treatment processes and parameters, various application requirements can be met.
