Vacuum tempering process of steel

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Vacuum tempering is a process in which quenched steel is heated to a temperature below A1 to transform it into a stable tempered structure, and then cooled to room temperature in an appropriate manner. The main purpose of vacuum tempering is to reduce or eliminate quenching stress, ensure corresponding structural transformation, improve the toughness and plasticity of steel, and obtain an appropriate combination of hardness, strength, plasticity and toughness to meet the performance requirements of workpieces for various purposes. The most important factor that determines the structure and performance of the workpiece after vacuum tempering is the tempering temperature. According to the structure and performance requirements of the workpiece, vacuum tempering can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.

Types of vacuum tempering:

Low temperature tempering: heating at 150~250℃, holding time: 2h + conditional thickness × 1min per 1mm. Cooling method: air cooling or hot water cooling.

Purpose and application: (1) Reduce the internal stress and brittleness of the workpiece after quenching, but do not reduce the hardness or rarely reduce the hardness (1~3HRC)

(2) Tools that have been tempered at low temperature will not break during work.

(3) Mainly used in various cutting tools, tools, cold deformation molds, ball bearings, carburized parts, and high-frequency quenching parts to maintain high hardness after tempering.

High-temperature tempering: heating at 500~~650℃, holding time: 10 min + conditional thickness per 1 mm × 1 min, cooling method: (1) air cooling (2) alloy steel with tempering brittleness, use hot water or oil cooling

Purpose and application: (1) Obtain tempered sorbite structure

(2) Make the parts have a certain strength and hardness, as well as good plasticity and toughness combined with comprehensive mechanical properties

(3) Machine parts for structural steel, such as connecting rods, crankshafts, bolts, etc.

Tempering in quenching and tempering treatment: heating at 500~670℃, holding time: 10 min + condition thickness × 1 min per 1 mm, cooling method: same as high temperature tempering.

Purpose and application: (1) Quenching and tempering is high temperature tempering at a temperature slightly lower than Ac1 after quenching

(2) H is used to refine the structure of the steel, prepare the structure for the next step of heat treatment, and increase the toughness.

(3) Quenching and tempering can be used as the final heat treatment process to increase toughness and obtain the required mechanical properties. It can also be used as an intermediate heat treatment process to prepare the microstructure for the next heat treatment and ensure uniform properties obtained by the final heat treatment.

(4) Reduce hardness and improve plasticity to ensure the cutting performance of steel

Commonly used vacuum tempering methods

Ordinary tempering: At the selected tempering temperature, overall heating and tempering is adopted. This is the most widely used method.

Partial tempering: quickly heat and temper the parts that require low hardness, and then cool them in oil or directly send them to a low-temperature tempering furnace to continue tempering the parts that require higher hardness.

Self-tempering: The method of tempering using the residual heat after quenching and cooling of the workpiece is called self-tempering. After heating the workpiece to austenitize it, put the part that needs to be hardened into the coolant, stay there for a certain period of time to harden the workpiece, and then take it out and place it in the air to allow the heat from the unhardened end to be conducted to the hardened end. Perform tempering. After it reaches the tempering temperature, immediately put all the workpieces into the coolant to prevent the tempering temperature from continuing to rise and causing insufficient hardness.

Strength and plasticity of steel during vacuum tempering

When the vacuum tempering temperature is low, the strength of the steel increases due to the gradual elimination of quenching stress. However, when the tempering temperature is higher than 250°C, the strength of the steel gradually decreases due to the precipitation of cementite. During the entire vacuum tempering process, the plasticity of steel gradually increases as the tempering temperature increases.

Since high carbon steel is very brittle during low-temperature tempering, early brittle fracture occurs during tensile testing, so the strength value cannot be measured. The general rule is the same: the strength of steel gradually decreases as the tempering temperature increases, and the plasticity of steel gradually increases as the tempering temperature increases.

The elastic limit peaks between 300 and 400°C. The low elastic limit value after high-temperature tempering is because the strength of the steel after high-temperature tempering is too low; while the low elastic limit value after low-temperature tempering is because the internal stress has not been fully eliminated.

Therefore, spring steel is generally vacuum tempered at 300 to 400°C.

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