14th of March ,2022

The "quenching" process of heat treatment

1. Quenching process

The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypo-eutectoid steel) or Ac1 (hyper-eutectoid steel), keep it for a period of time to make it all or part of austenite, and then rapidly cool it at a rate greater than the critical cooling rate. A heat treatment process for martensite (or bainite) transformation to below Ms (or isothermal near Ms).

Process: heating, heat preservation, cooling.

The essence of quenching is to transform martensite or bainite into martensite or bainite by supercooling austenite.

The purpose of quenching: (1) To greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel, so as to meet the different requirements of various mechanical parts and tools. (2) Through quenching, the special physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels can be satisfied.

Scope of application: The quenching process is the most widely used, such as tools, measuring tools, molds, bearings, springs and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft, etc. The parts are all using the quenching process.

2. Quenching medium

The medium used for quenching and cooling the workpiece is called quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress.

Commonly used quenching media are water, aqueous solution, mineral oil, molten salt, molten alkali, etc.

● water

Water is a quenching medium with strong cooling capacity.

Advantages: Wide source, low price, stable composition and not easy to deteriorate.

Disadvantages: The cooling capacity is unstable, and it is easy to deform or crack the workpiece. In the "nose" region of the C curve (about 500-600°C), the water is in the vapor film stage, and the cooling is not fast enough, and a "soft spot" will be formed; while in the martensitic transformation temperature region (300-100°C), the water is in the In the boiling stage, if the cooling is too fast, it is easy to make the martensite transformation speed too fast and generate great internal stress, resulting in deformation or even cracking of the workpiece. When the water temperature rises, the water contains more gas or insoluble impurities (such as oil, soap, mud, etc.) are mixed in the water, which will significantly reduce its cooling capacity.

Application: It is suitable for quenching and cooling of carbon steel workpieces with small section size and simple shape.

● Brine and alkaline water

Add an appropriate amount of salt and alkali to the water, after the high-temperature workpiece is immersed in the cooling medium, the crystals of salt and alkali are precipitated in the steam film stage and burst immediately, the steam film is destroyed, and the oxide skin on the surface of the workpiece is also blasted. The cooling capacity of the medium in the high temperature area has the disadvantage that the medium is highly corrosive.

Application: In general, the concentration of brine is 10%, and the concentration of caustic soda aqueous solution is 10% to 15%. It can be used as a quenching medium for carbon steel and low-alloy structural steel workpieces. The operating temperature should not exceed 60 °C. After quenching, it should be cleaned and rust-proofed in time.

● oil

The cooling medium generally uses mineral oil (mineral oil). Such as engine oil, transformer oil and diesel oil. No. 10, No. 20, and No. 30 oil are generally used for engine oil. The larger the oil number, the higher the viscosity, the higher the flash point, the lower the cooling capacity, and the correspondingly higher operating temperature.

quenching process of heat treatment

3. Quenching method

● Single liquid quenching

It is a quenching operation method in which the austenitized workpiece is immersed in a certain quenching medium and cooled to room temperature. The single-liquid quenching medium includes water, brine, alkaline water, oil and specially formulated quenching agent.

Advantages: Simple operation, conducive to mechanization and automation.

Disadvantages: The cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality.

Application: For carbon steel, single-fluid quenching is only suitable for workpieces with simpler shapes.

● Double liquid quenching

It is to immerse the austenitized workpiece in a medium with strong cooling ability, take it out before the steel reaches the temperature of the quenching medium, and immediately immerse it in another medium with weak cooling ability to cool, such as water first and then oil , water first, then air, etc. Double-liquid quenching reduces the tendency of deformation and cracking, the operation is not easy to master, and there are certain limitations in application.

● Martensitic graded quenching

It is to first immerse the austenitized workpiece in a liquid medium (salt bath or alkali bath) with a temperature slightly higher or lower than the martensitic point of the steel, and keep it for an appropriate time until the inner and outer layers of the steel workpiece reach the medium. After the temperature is taken out, it is taken out for air cooling to obtain a quenching process of martensitic structure, also known as graded quenching.

Advantages: Staged quenching can effectively reduce phase transformation stress and thermal stress, and reduce quenching deformation and cracking tendency due to air cooling after the staged temperature stays at the same temperature inside and outside the workpiece.

Application: It is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes.

● Bainite isothermal quenching

It is a quenching process that austenitizes the steel, makes it quickly cool to the bainite transformation temperature range (260 ~ 400 ° C) and maintains it isothermally, and transforms austenite into bainite. The time is 30 to 60 minutes.

● Compound quenching

The workpiece is quenched to below Ms to obtain 10% to 20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method can obtain M+B structure for workpieces with larger cross-sections. The martensite formed during prequenching can promote the bainite transformation, and then temper the martensite at the isothermal time. Compound quenching is used for alloy tool steel workpieces, which can avoid the first type of temper brittleness and reduce the amount of retained austenite, that is, the tendency of deformation and cracking.

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