Introduction to the definition, characteristics and functions of tempering in tempering furnace

Tempering is one of the metal heat treatment processes, also known as matching fire. Tempering is generally used to reduce or eliminate internal stress in hardened steel, or to reduce its hardness and strength to improve its ductility or toughness. The tempering is to reheat the quenched workpiece to a suitable temperature lower than the lower critical temperature, and heat-treat the metal in a medium such as air or water or oil after being kept for a while. Or the quenched alloy workpiece is heated to a suitable temperature for a certain period of time and then slowly or rapidly cooled. Low temperature tempering, medium temperature tempering or high temperature tempering can be used according to different requirements. Generally, as the tempering temperature increases, the hardness and strength decrease, and the ductility or toughness gradually increases.

Steel workpieces have the following characteristics after quenching:
1. Unbalanced (ie, unstable) structures such as martensite, bainite, and retained austenite are obtained.
2. There is a large internal stress.
3. Mechanical properties can not meet the requirements. Therefore, steel workpieces are generally tempered after quenching.
The role of tempering:
1. Improve the stability of the structure, so that the workpiece does not undergo tissue transformation during use, so that the workpiece geometry and performance remain stable.
2. Eliminate internal stresses in order to improve the performance of the workpiece and stabilize the geometry of the workpiece.
3. Adjust the mechanical properties of steel to meet the requirements of use.

The reason why tempering has these effects is because the atomic activity is enhanced when the temperature is raised, and the atoms of iron, carbon and other alloying elements in steel can be diffused faster, realizing the rearrangement and combination of atoms, thereby making the instability The imbalanced organization has gradually transformed into a stable and balanced organization. The elimination of internal stress is also related to the decrease in metal strength at elevated temperatures. In general, when steel is tempered, the hardness and strength are lowered, and the plasticity is improved. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with higher alloying elements will precipitate some fine metal compounds in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.

Parts requiring different applications should be tempered at different temperatures to meet the requirements in use. 1 Tools, bearings, carburized and quenched parts, surface hardened parts are usually tempered at a low temperature below 250 ?°C. After low temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness is slightly improved. 2 The spring is tempered at 350-500 ?° C at medium temperature to obtain higher elasticity and necessary toughness. 3 Parts made of medium carbon structural steel are usually tempered at a high temperature of 500 to 600 ?° C to obtain a good fit of strength and toughness. The heat treatment process of quenching and high temperature tempering is collectively referred to as quenching and tempering.

When steel is tempered at around 300 ?°C, its brittleness is often increased. This phenomenon is called the first type of temper brittleness. Generally should not temper in this temperature range. Some medium carbon alloy structural steels tend to become brittle if they are slowly cooled to room temperature after tempering at high temperatures. This phenomenon is called the second type of temper brittleness. The addition of molybdenum to steel or cooling in oil or water during tempering prevents the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of tempered brittle steel to the original tempering temperature.

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