What is tempering?

Posted by chase Liu on December 27th, 2022

Tempering is a heat treatment process in which the quenched seamless pipe or welded pipe is heated to a certain temperature, held for a certain period of time, and then cooled in a certain way. Tempering is an operation performed immediately after quenching, and is usually the last process of heat treatment of the workpiece, so the combined process of quenching and tempering is called final treatment. The main purpose of quenching and tempering is to:

1) Reduce internal stress and reduce brittleness. Quenched parts have great stress and brittleness. If they are not tempered in time, they will often deform or even crack.

2) Adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness.

3) Stable workpiece size. The metallographic structure can be stabilized by tempering to ensure that no deformation will occur during future use.

4) Improve the cutting performance of some alloy steels

The role of tempering is to:

① Improve the stability of the structure, so that the steel pipe will no longer undergo structural transformation during use, so that the geometric size and performance of the steel pipe will remain stable.

② Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometric dimensions of the steel pipe.

③ Adjust the mechanical properties of steel to meet the requirements of use.

The reason why tempering has these effects is that when the temperature rises, the activity of atoms increases, and the atoms of iron, carbon and other alloying elements in steel can diffuse quickly to realize the rearrangement of atoms. So that the unstable unbalanced organization is gradually transformed into a stable and balanced organization. The relief of internal stress is also related to the decrease in metal strength as the temperature increases.

Generally, when steel is tempered, the hardness and strength decrease and the plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with high content of alloying elements will precipitate some fine-grained metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening

Tempering requirements: workpieces with different uses should be tempered at different temperatures to meet the requirements in use.

① Cutting tools, bearings, carburized and quenched parts, and surface quenched parts are usually tempered at a temperature below 250°C. After low-temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness improves slightly.

② The spring is tempered at a medium temperature at 350-500°C to obtain high elasticity and necessary toughness.

③ Parts made of medium carbon structural steel are usually tempered at a high temperature of 500-600 ° C to obtain a good combination of strength and toughness.

When steel is tempered at around 300°C, its brittleness often increases. This phenomenon is called the first type of temper brittleness. Generally, it should not be tempered in this temperature range. Some medium carbon alloy structural steels are also prone to become brittle if they are slowly cooled to room temperature after high temperature tempering.

This phenomenon is called the second type of temper brittleness. The addition of molybdenum to the steel, or cooling in oil or water during tempering, can prevent the second type of temper brittleness. This brittleness can be eliminated by reheating the second type of temper brittle steel to the original tempering temperature.

In production, it is often based on the requirements for the performance of the workpiece. According to different heating temperatures, tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering. The heat treatment process combining quenching and subsequent high-temperature tempering is called quenching and tempering, that is, it has good plasticity and toughness while having high strength.

1. Low temperature tempering: 150-250℃, M times, reduce internal stress and brittleness, improve plastic toughness, have higher hardness and wear resistance. Used to make measuring tools, knives and rolling bearings, etc.

2. Tempering at medium temperature: 350-500°C, T time, with high elasticity, certain plasticity and hardness. Used to make springs, forging dies, etc.

3. High temperature tempering: 500-650℃, S time, with good comprehensive mechanical properties. Used to make gears, crankshafts, etc.