Quenching

Quenching is a heat treatment process that heats steel above the critical temperature, holds it for a certain period of time, and then cools it at a rate greater than the critical cooling rate to obtain an unbalanced structure mainly composed of martensite (or to obtain bainite or maintain single-phase austenite as needed). Quenching is the most widely used process method in steel heat treatment. [1]

There are four basic processes for steel heat treatment: annealing, normalizing, quenching, and tempering.

annealing

Heat the workpiece to an appropriate temperature, use different insulation times according to the material and workpiece size, and then slowly cool (with the slowest cooling rate) to achieve or approach equilibrium state of the internal structure of the metal, obtain good process and service performance, or prepare the structure for further quenching.

normalizing

After heating the workpiece to a suitable temperature and cooling it in air, the effect of normalizing is similar to annealing, except that the obtained microstructure is finer. It is commonly used to improve the cutting performance of materials, and sometimes used as the final heat treatment for some parts with low requirements.

tempering

In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature above room temperature but below 710 ℃ for a long time, and then cooled. This process is called tempering.

quenching

A heat treatment process in which the workpiece is heated and austenitized, and then cooled in an appropriate manner to obtain martensite or bainite structure. The most common ones include water-cooled quenching, oil-cooled quenching, air-cooled quenching, etc.

Annealing, normalizing, quenching, and tempering are the "four fires" in overall heat treatment, among which quenching and tempering are closely related and often used together, and none of them are indispensable.

The purpose of quenching is to transform undercooled austenite into martensite or bainite, obtain martensite or bainite structure, and then cooperate with tempering at different temperatures to significantly improve the rigidity, hardness, wear resistance, fatigue strength, and toughness of steel, thereby meeting the different usage requirements of various mechanical parts and tools. Quenching can also meet the special physical and chemical properties of certain special steels, such as ferromagnetism and corrosion resistance.

The metal heat treatment process involves heating the metal workpiece to an appropriate temperature and maintaining it for a period of time, followed by rapid cooling by immersing it in a quenching medium. Common quenching media include salt water, water, mineral oil, air, etc. Quenching can improve the hardness and wear resistance of metal workpieces, and is widely used in various tools, molds, measuring tools, and parts that require surface wear resistance (such as gears, rollers, carburized parts, etc.). By combining quenching with tempering at different temperatures, the strength, toughness reduction, and fatigue strength of metals can be significantly improved, and a combination of these properties (comprehensive mechanical properties) can be obtained to meet different usage requirements. In addition, quenching can also enhance the physical and chemical properties of some special performance steels, such as enhancing the ferromagnetism of permanent magnet steels and improving the corrosion resistance of stainless steels. Quenching process is mainly used for steel parts. When commonly used steel is heated above the critical temperature, the original structure at room temperature will be completely or largely transformed into austenite. Subsequently, the steel is immersed in water or oil for rapid cooling, and austenite is transformed into martensite. Compared with other structures in steel, martensite has the highest hardness. Rapid cooling during quenching can cause internal stress in the workpiece, and when it reaches a certain level, the workpiece may undergo distortion, deformation, or even cracking. Therefore, it is necessary to choose an appropriate cooling method. According to the cooling method, quenching processes are divided into four categories: single liquid quenching, dual medium quenching, martensitic graded quenching, and bainitic isothermal quenching.

This includes three stages: heating, insulation, and cooling. Taking the quenching of steel as an example, the principles for selecting process parameters in the above three stages will be introduced.

Quenching heating temperature

Based on the critical point of steel phase transformation, small and uniform austenite grains should be formed during heating and quenching, and fine martensitic structure should be obtained after quenching. The quenching heating temperature range of carbon steel is shown in the figure "Quenching Heating Temperature", and the quenching temperature selection principle shown in this figure is also applicable to most alloy steels, especially low alloy steels. The heating temperature of hypoeutectoid steel is 30-50 ℃ above the Ac3 temperature. From the graph of "Quenching Heating Temperature", it can be seen that the steel is in the single-phase austenite (A) zone at high temperatures, hence it is called complete quenching. If the heating temperature of hypoeutectoid steel is higher than Ac1 and lower than Ac3, the partially eutectoid ferrite at high temperature has not completely transformed into austenite, which is called incomplete (or subcritical) quenching. The quenching temperature of hypereutectoid steel is 30-50 ℃ above the Ac1 temperature, which is in the dual phase zone of austenite and cementite (A+C). Therefore, the normal quenching of eutectoid steel still belongs to incomplete quenching, and after quenching, the structure of carbides distributed on the martensitic matrix is obtained. This organizational state has high hardness and wear resistance. For hypereutectoid steel, if the heating temperature is too high and the first eutectoid carbides dissolve too much or even completely, the austenite grains will grow and the austenite carbon content will also increase. After quenching, the coarse martensite structure increases the stress and microcracks in the quenched microstructure of the steel parts, leading to an increase in deformation and cracking tendency of the parts; Due to the high concentration of austenite carbon, the martensite point decreases and the amount of residual austenite increases, resulting in a decrease in the hardness and wear resistance of the workpiece. The temperature for quenching common steel grades is shown in the figure "Quenching Heating Temperature", and the table shows the heating temperature for quenching common steel grades.

In actual production, the selection of heating temperature should be adjusted according to specific situations. If the carbon content in hypoeutectoid steel is the lower limit, the upper temperature limit can be selected when the furnace loading is large and the depth of the quenching layer of the parts needs to be increased; If the shape of the workpiece is complex and the deformation requirements are strict, a lower temperature limit should be used.

Quenching insulation

The quenching insulation time is determined by various factors such as equipment heating method, part size, steel composition, furnace charge, and equipment power. For overall quenching, the purpose of insulation is to make the internal temperature of the workpiece uniform and consistent. For various types of quenching, the holding time ultimately depends on obtaining a good quenching heating structure in the area that requires quenching. Heating and insulation are important factors affecting the quality of quenching, and the microstructure obtained from austenitization directly affects the properties after quenching. Generally, the austenite grain size of steel parts is controlled at grades 5-8.

quench cooling

To transform the high-temperature phase austenite in steel into a low-temperature metastable phase martensite during the cooling process, the cooling rate must be greater than the critical cooling rate of the steel. During the cooling process of the workpiece, there is a certain difference in the cooling speed between the surface and the center. If this difference is large enough, it may cause the part above the critical cooling speed to transform into martensite, while the part below the critical cooling speed cannot transform into martensite. To ensure that the entire cross-section transforms into martensite, it is necessary to choose a quenching medium with sufficient cooling capacity to ensure a sufficiently high cooling rate at the center of the workpiece. However, due to the high cooling rate, uneven thermal expansion and contraction inside the workpiece can cause internal stress, which may cause deformation or cracking of the workpiece. Therefore, it is necessary to consider the above two contradictory factors and choose a reasonable quenching medium and cooling method.

The cooling stage is not only to obtain a reasonable structure of the parts and achieve the required performance, but also to maintain the dimensional and shape accuracy of the parts, which is a key link in the quenching process.

Workpiece hardness

The hardness of the quenched workpiece affects the quenching effect. Quenched workpieces are generally measured for their HRC value using a Rockwell hardness tester. Quenched thin and hard steel plates and surface quenched workpieces can be measured for HRA values, while quenched steel plates with a thickness less than 0.8mm, shallow surface quenched workpieces, and quenched steel bars with a diameter less than 5mm can be measured for their HRC values using a surface Rockwell hardness tester.

When welding medium carbon steel and certain alloy steels, quenching may occur in the heat affected zone, leading to hardening and the formation of cold cracks, which should be prevented during the welding process.

Due to the hardness and brittleness of the metal after quenching, residual surface stress can cause cold cracks. Tempering can be one of the means to eliminate cold cracks without affecting hardness.

Quenching is more suitable for parts with smaller thickness and diameter. For parts that are too large, insufficient quenching depth and carburization also have the same problem. In this case, it should be considered to add chromium and other alloys to the steel to increase strength.

Quenching is one of the basic methods for strengthening steel materials. Martensite in steel is the hardest phase in iron-based solid solution structure, so quenching steel parts can achieve high hardness and strength. However, martensite has a high brittleness, and there is a significant quenching internal stress in the steel parts after quenching, so it is not suitable for direct application and must be tempered.

Single medium quenching

The workpiece is cooled in a medium, such as water quenching or oil quenching. The advantages are simple operation, easy mechanization, and wide application. The disadvantage is that the quenching stress in water is high, and the workpiece is prone to deformation and cracking; Quenching in oil results in a low cooling rate, small quenching diameter, and difficulty in quenching large workpieces.

Dual medium quenching

The workpiece is first cooled to around 300 ℃ in a medium with strong cooling capacity, and then cooled in a medium with weak cooling capacity, such as water quenching followed by oil quenching, which can effectively reduce the internal stress of martensitic transformation and reduce the tendency of workpiece deformation and cracking. It can be used for quenching workpieces with complex shapes and uneven cross-sections. The disadvantage of dual liquid quenching is that it is difficult to grasp the timing of dual liquid conversion. If the conversion is too early, it is easy to harden, and if the conversion is too late, it is easy to crack. In order to overcome this drawback, the graded quenching method has been developed.

Graded quenching

The workpiece is quenched in a low-temperature salt or alkali bath furnace. The temperature of the salt or alkali bath is near the Ms point, and the workpiece stays at this temperature for 2-5 minutes before being taken out for air cooling. This cooling method is called graded quenching. The purpose of graded cooling is to achieve a more uniform temperature inside and outside the workpiece, while undergoing martensitic transformation, which can greatly reduce quenching stress and prevent deformation and cracking. The grading temperature was previously set slightly above the Ms point, and after the temperature inside and outside the workpiece became uniform, it entered the martensitic zone. Improved to a temperature classification slightly below the Ms point. Practice has shown that grading below the Ms point is more effective. For example, high carbon steel molds undergo graded quenching in an alkali bath at 160 ℃, which not only hardens but also reduces deformation, making them widely used.

Isothermal quenching

The workpiece is quenched in an isothermal salt bath, with the salt bath temperature at the lower part of the bainite zone (slightly higher than Ms). The workpiece remains isothermal for a long time until the bainite transformation is completed, and is taken out for air cooling. Isothermal quenching is used for steels above medium carbon, with the aim of obtaining lower bainite to improve strength, hardness, toughness, and wear resistance. Low carbon steel is generally not subjected to isothermal quenching.

Surface quenching

Surface quenching is a local quenching method that hardens the surface layer of a steel piece to a certain depth, while keeping the core part in an unhardened state. During surface quenching, rapid heating is used to quickly reach the quenching temperature on the surface of the workpiece. Before the heat can penetrate the center of the workpiece, it is immediately cooled to achieve local quenching.

Induction hardening

Induction heating is the use of electromagnetic induction to generate eddy currents within a workpiece and heat it up.

Cold extract

Quenching and cooling by immersing in a cold water solution with strong cooling ability as the cooling medium.

Localized quenching

Quenching is only performed on the parts of the workpiece that require hardening.

Air-cooled quenching

It specifically refers to heating in vacuum and quenching and cooling in neutral and inert gases under high-speed circulation of negative pressure, atmospheric pressure, or high pressure.

Surface quenching

Quenching only performed on the surface of the workpiece, including induction quenching, contact resistance heating quenching, flame quenching, laser quenching, electron beam quenching, etc.

Air-cooled quenching

Quenching cooling using forced flowing air or compressed air as the cooling medium.

Saltwater quenching

Quenching cooling using aqueous solutions of salts as cooling media.

Organic solution quenching

Quenching cooling using an aqueous solution of organic polymer as a cooling medium.

Spray quenching

Quenching cooling using jet liquid flow as cooling medium.

Spray cooling

The workpiece undergoes quenching and cooling in a mist sprayed with a mixture of water and air.

Hot bath cooling

The quenching and cooling of workpieces in hot baths such as molten salt, molten alkali, molten metal, or high-temperature oil, such as salt bath quenching, lead bath quenching, alkali bath quenching, etc.

Double liquid quenching

After heating and austenitizing the workpiece, it is first immersed in a medium with strong cooling ability. When the microstructure is about to undergo martensitic transformation, it is immediately transferred to a medium with weak cooling ability for cooling.

Pressure quenching

The quenching and cooling of workpieces after austenitization and clamping with specific fixtures is aimed at reducing quenching and cooling distortion.

Through quenching

Hardening of the workpiece from the surface to the center.

Isothermal quenching

After heating and austenitizing the workpiece, it is rapidly cooled to the bainite transformation temperature range and kept isothermal, resulting in the quenching of austenite into bainite.

Graded quenching

After the workpiece is heated and austenitized, it is immersed in an alkaline or salt bath with a temperature slightly higher or lower than the M1 point for an appropriate period of time. After the workpiece reaches the medium temperature as a whole, it is taken out for air cooling to obtain martensite quenching.

Subcritical quenching

After austenitizing in the Ac1-Ac3 temperature range, hypoeutectoid steel workpieces are quenched and cooled to obtain martensite and ferrite structures.

Direct quenching

The process of directly quenching and cooling the workpiece after carbon infiltration.

double quenching

After carburizing and cooling the workpiece, it is first austenitized and quenched at a temperature higher than Ac3 to refine the core structure, and then austenitized at a temperature slightly higher than Ac3 to refine the microstructure of the carburized layer.

Self cooling quenching

After rapid heating and austenitization of the workpiece locally or on the surface, the heat in the heating zone transfers to the unheated zone on its own, resulting in rapid cooling and quenching of the austenitized zone.

Quenching technology is widely used in modern mechanical manufacturing industry. Almost all important parts in machinery, especially steel parts used in automobiles, airplanes, and rockets, have undergone quenching treatment. To meet the diverse technical requirements of various parts, various quenching processes have been developed. For example, according to the treated parts, there are overall, local quenching, and surface quenching; According to whether the phase transformation is complete during heating, there are complete quenching and incomplete quenching (for hypoeutectoid steel, this method is also known as subcritical quenching); According to the content of phase transformation during cooling, there are graded quenching, isothermal quenching, and under speed quenching.

In addition, due to the characteristics and limitations of each quenching method, they are all applied under certain conditions, among which induction heating surface quenching and flame quenching are the most commonly used. Laser beam heating and electron beam heating are rapidly developing high-energy density heating and quenching methods, which are attracting people's attention due to their unique characteristics that other heating methods do not have.

Surface quenching is widely used in machine parts made of medium carbon quenched and tempered steel or ductile iron. Because medium carbon quenched and tempered steel, after pre-treatment (quenching or normalizing), can be surface quenched to maintain high comprehensive mechanical properties in the center, as well as to make the surface have high hardness (greater than HRC 50) and wear resistance. For example, machine tool spindles, gears, diesel engine crankshafts, camshafts, etc. In principle, pearlite ferrite based gray cast iron, ductile iron, malleable cast iron, alloy cast iron, etc. with a matrix equivalent to medium carbon steel can all undergo surface quenching, and ductile iron has the best process performance and high comprehensive mechanical properties, making it the most widely used.

After surface quenching of high carbon steel, although the surface hardness and wear resistance are improved, the plasticity and toughness of the core are lower. Therefore, surface quenching of high carbon steel is mainly used for tools, measuring tools, and high cold hard rolling rolls that work under small impacts and alternating loads.

Due to the insignificant strengthening effect after surface quenching of low-carbon steel, it is rarely used.