1, Performance changes after normalizing treatment
Refine grain size: Heat the steel pipe to a temperature above austenitization by normalizing, and cool it in air after insulation. This process can refine the grain size of steel and make the internal structure more uniform. For example, it can alleviate or eliminate the banded structure and mixed crystal phenomenon formed during the deformation process of steel pipes, making the microstructure of steel pipes more regular and improving their overall performance.
Improving cutting performance: For medium C steel and low-alloy steel, normalizing can replace annealing to improve the cutting performance of steel pipes. Meanwhile, normalizing can also change the residual stress state, such as eliminating some internal stresses, making the steel pipe more stable during subsequent processing or use.
2, Performance changes after annealing treatment
Improve plasticity and toughness:
Annealing is the process of heating steel pipes to a temperature above or below the critical point (Ac3 or Ac1), and then slowly cooling them after insulation. Generally, high C, low alloy, and alloy steel pipes can be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven structure, refine crystalline structure, and facilitate steel pipe machining. For example, making it easier for previously hard and brittle steel pipes to undergo plastic deformation processes such as bending and stretching.
It can also improve the comprehensive mechanical properties of steel pipes, reduce or eliminate residual internal stresses generated during quenching, stabilize the size of steel pipes, and prevent changes in the structure of steel pipes during use.
3, Performance changes after quenching treatment
Hardness improvement:
Quenching is the process of heating steel pipes to 30-50 ℃ above Ac3 (hypoeutectoid steel) or 30-50 ℃ above Ac1 (hypereutectoid steel), followed by rapid cooling to obtain martensitic structure (M) after insulation. The microstructure of carbon steel after quenching is composed of martensite and a certain amount of residual austenite. Martensitic microstructure generally has high hardness, so the hardness of steel pipes after quenching will be significantly increased. A hardness greater than HRC55 (up to HRC62) after quenching is considered qualified (such as seamless pipes after quenching).
But the microstructure after quenching is more brittle because martensite itself is relatively brittle and requires subsequent tempering treatment to improve toughness and other properties.
Strength change: After quenching, the strength of the steel pipe will also be improved due to the high strength characteristics of the martensitic structure. However, if the quenching process is not properly controlled, such as insufficient heating temperature (for hypoeutectoid steel), ferrite may appear in the quenched structure, resulting in a decrease in strength and hardness.
4, Performance changes after tempering treatment
Reduce brittleness: Tempering is a treatment applied to quenched steel pipes. The martensite structure after quenching has high brittleness, and tempering can reduce this brittleness and improve the toughness of steel pipes. For example, low-temperature tempering (150-250 ℃), medium temperature tempering (350-500 ℃), and high-temperature tempering (500-650 ℃) have different effects on the properties of steel pipes in different tempering temperature ranges, but overall they all help improve the brittleness problem after quenching.
Adjusting hardness and strength: Depending on the tempering temperature, the hardness and strength of the steel pipe can be adjusted to a certain extent. Generally speaking, the higher the tempering temperature, the lower the hardness and strength, but the toughness will be further improved.For example, after high-temperature tempering, the hardness and strength of the steel pipe will decrease compared to quenching, but the toughness will be better improved, and the comprehensive mechanical properties will be better.
