Have you ever wondered how steel can be made both incredibly strong and flexible? This blog post will uncover the fascinating world of quenching, a crucial heat treatment process in mechanical engineering. You’ll learn about various quenching methods and their unique applications, providing you with a deeper understanding of how everyday tools and machinery are crafted for optimal performance.
Quenching is a critical heat treatment process widely used in metallurgy and materials science to enhance the mechanical properties of metals and alloys. In the context of steel, quenching involves a precisely controlled thermal cycle:
The primary goal of quenching in steels is to form martensite, a supersaturated solid solution of carbon in iron with a body-centered tetragonal (BCT) crystal structure. This results in significantly increased hardness and strength. In some cases, quenching may be designed to produce bainite through isothermal treatments near the martensite start temperature (Ms).
It’s important to note that quenching is not limited to ferrous alloys. The term also encompasses heat treatment processes for other materials:
The specific quenching parameters, including heating temperature, holding time, cooling rate, and quenchant selection, are carefully tailored to the material composition and desired final properties. Modern quenching processes often utilize computer-controlled systems and advanced quenchants to optimize performance and minimize distortion.
Quenching is a heat treatment method that involves heating steel above its critical temperature, holding it for a certain period, then cooling it at a rate greater than the critical cooling speed to obtain a predominantly martensitic unbalanced structure (although bainite or a single-phase austenite may also be obtained as needed).
Quenching is the most widely applied method in steel heat treatment processes.
There are roughly four basic processes in steel heat treatment: annealing, normalizing, quenching, and tempering.
Annealing
This involves heating the workpiece to an appropriate temperature, holding it for a duration dependent on the material and workpiece size, and then slowly cooling it (slowest cooling rate). The goal is to bring the internal structure of the metal to or near equilibrium, achieving good process performance and use performance, or preparing the structure for further quenching.
Normalizing
After heating the workpiece to a suitable temperature, it is cooled in the air. The effect of normalizing is similar to annealing, but it produces a finer structure. It is commonly used to improve the cutting performance of materials, and sometimes used as the final heat treatment for parts with less demanding requirements.
Tempering
To reduce the brittleness of steel pieces, those that have been quenched are maintained at a temperature higher than room temperature but below 710℃ for an extended period before cooling. This process is known as tempering.
Quenching
This is a heat treatment process that involves heating the workpiece to austenitize it, then cooling it in a suitable manner to obtain a martensite or bainite structure. Common methods include water quenching, oil quenching, and air quenching.
Annealing, normalizing, quenching, and tempering are the “four fires” in integral heat treatment. Quenching and tempering are closely related, often used in conjunction, and both are indispensable.
There are ten methods for quenching in the heat treatment process, which are:
In this process, the workpiece is heated to the quenching temperature and is then rapidly cooled by immersing it into a quenching medium. This is the simplest quenching method and is commonly used for simple shaped carbon steel and alloy steel workpieces. The choice of quenching medium is based on factors such as the heat transfer coefficient, hardenability, size, and shape of the parts.
Fig. 1 Single medium (water, oil, air) quenching
In the heat treatment process, the workpiece that has been heated to the quenching temperature is cooled rapidly to the point close to the martensite start (MS) in a strong cooling medium. The workpiece is then slowly cooled to room temperature in a slower cooling medium, which creates a range of different quenching temperatures and ideal cooling rates.
This method is used for workpieces with complex shapes or large workpieces made of high-carbon steel, alloy steel, and carbon tool steel. The common cooling media include water-oil, water-nitrate, water-air, and oil-air. Water is typically used as a quick cooling medium, while oil or air is used as a slower cooling medium. Air is used less frequently.
The steel is austenitized, and then it is immersed in the liquid medium (salt bath or alkali bath) with a temperature slightly higher or lower than the upper martensite point of the steel for a specific time. The steel is then taken out for air cooling, and the undercooled austenite transforms slowly into martensite.
This method is generally used for small workpieces with complex shapes and strict deformation requirements. High-speed steel and high-alloy steel tools and dies are also commonly quenched using this method.
The workpiece is cooled quickly in the bath when the bath temperature is lower than the MS (martensite start) point and higher than the MF (martensite finish) point. This results in the same outcome as using a larger bath size.
This method is commonly used for low hardenability steel workpieces of large size.
The workpiece is quenched into a bath with a lower bainite temperature for isothermal treatment, causing the formation of lower bainite. This process is typically performed by keeping the workpiece in the bath for 30 to 60 minutes.
The isothermal quenching of bainite process consists of three steps:
This method is commonly used for small-sized parts made of alloy steel and high-carbon steel, as well as ductile iron castings.
Martensite with a volume fraction of 10% to 30% is obtained by quenching the workpiece below the MS point, followed by an isothermal treatment in the lower bainite region.
This method is commonly used for alloy tool steel workpieces.
This quenching method is also referred to as step-up austempering. The process involves first cooling the parts in a bath with a lower temperature (above MS) and then transferring them to a bath with a higher temperature to undergo isothermal transformation of austenite.
This method is appropriate for steel parts with low hardenability or large size, as well as workpieces that must be austempered.
In the precooled isothermal quenching process, the parts are pre-cooled to a temperature slightly above Ar3 or Ar1 using air, hot water, or a salt bath. Then, single medium quenching is performed.
This method is often used for parts with complex shapes, significant differences in thickness, and minimal deformation requirements.
The quenching and self-tempering process involves heating all the workpieces, but only immersing the parts to be hardened (usually the working parts) in a quenching liquid for cooling during quenching.
Once the glow of the un-immersed parts disappears, the quenching process is immediately removed for air cooling.
This method allows heat to transfer from the center to the surface to temper it, and is commonly used for tools that must withstand impacts such as chisels, punches, hammers, etc.
The quenching method of spraying water onto the workpiece can be adjusted in terms of water flow, depending on the desired quenching depth. Jet quenching avoids the formation of a steam film on the surface of the workpiece, which results in a deeper hardened layer compared to normal water quenching.
This method is mainly used for localized surface quenching.
The primary objective of quenching is to induce a phase transformation in steel, converting supercooled austenite into martensite or bainite. This transformation results in a microstructure that significantly enhances the mechanical properties of the material. Quenching, followed by controlled tempering at specific temperatures, allows for precise tailoring of steel properties, including increased hardness, wear resistance, fatigue strength, and toughness. This versatility enables manufacturers to meet the diverse requirements of various mechanical components and tools across industries.
Quenching is a critical heat treatment process that involves heating a metal workpiece to a specific austenitizing temperature, holding it for a predetermined time to ensure complete phase transformation, and then rapidly cooling it in a quenching medium. The choice of quenching medium—such as brine, water, polymer solutions, mineral oils, or even forced air—depends on the desired cooling rate and the specific alloy composition. Each medium offers different cooling characteristics, allowing metallurgists to control the microstructural evolution and resultant properties.
The rapid cooling during quenching creates a supersaturated solid solution, trapping carbon atoms within the iron lattice and forming the metastable martensite phase. This martensitic structure is characterized by extremely high hardness and wear resistance but can be brittle. Subsequent tempering processes are often employed to optimize the balance between strength, toughness, and ductility, tailoring the material properties to specific application requirements.
Beyond mechanical property enhancement, quenching plays a crucial role in developing specific physical and chemical properties in specialty steels. For instance, it can significantly improve the ferromagnetic properties of permanent magnet steels, enhance the corrosion resistance of stainless steels, and modify the electrical properties of silicon steels used in transformer cores.
The quenching process is particularly critical for steels due to their allotropic nature and the ability to form various microstructures based on cooling rates. When steel is heated above its critical temperature (typically in the range of 723-912°C, depending on composition), its room temperature structure transforms into austenite. The subsequent rapid cooling prevents the diffusion-dependent formation of ferrite and pearlite, instead forcing the face-centered cubic (FCC) austenite to transform into body-centered tetragonal (BCT) martensite through a diffusionless shear mechanism.
However, the rapid cooling inherent to quenching introduces significant thermal stresses within the workpiece. These stresses, if not properly managed, can lead to distortion, warping, or even cracking of the component. To mitigate these risks, metallurgists employ various techniques such as interrupted quenching, selective quenching, or the use of specialized quenchants with controlled cooling characteristics.
Quenching processes can be broadly categorized based on the cooling method employed:
The selection of the appropriate quenching process and parameters is crucial for achieving the desired microstructure and properties while minimizing the risk of quench-related defects. Advanced quenching techniques, such as intensive quenching or cryogenic treatments, continue to evolve, offering new possibilities for enhancing material performance in demanding applications.
The quenching process includes three stages: heating, holding, and cooling. Here, the principles for selecting process parameters for these three stages are introduced using the quenching of steel as an example.
Quenching Heating Temperature
Based on the critical point of phase transformation in steel, the heating during quenching aims to form fine and uniform austenitic grains, obtaining a fine martensitic structure after quenching.
The quenching heating temperature range for carbon steel is shown in the figure “Quenching Heating Temperature”. The principle for selecting the quenching temperature shown in this figure also applies to most alloy steels, especially low-alloy steels. The heating temperature for hypoeutectoid steel is 30-50℃ above the Ac3 temperature.
| Chinese Grade | Critical point /℃ | Quenching temperature /℃ | |
| Ael | Aes(Acm) | ||
| 20 | 735 | 855 | 890~910 |
| 45 | 724 | 780 | 830~860 |
| 60 | 727 | 760 | 780~830 |
| T8 | 730 | 750 | 760~800 |
| T12 | 730 | 820 | 770~810 |
| 40Cr | 743 | 782 | 830~860 |
| 60Si2Mn | 755 | 810 | 860~880 |
| 9CrSi | 770 | 870 | 850~870 |
| 5CrNiMo | 710 | 760 | 830~860 |
| 3Cr2W8V | 810 | 1100 | 1070~1130 |
| GCr15 | 745 | 900 | 820~850 |
| Cr12MoV | 810 | / | 980~1150 |
| W6Mo5Cr4V2 | 830 | / | 1225~1235 |
From the “Quenching Heating Temperature” figure, we can see that the state of steel at high temperature is in the single-phase austenite (A) region, hence it is called complete quenching. If the heating temperature of hypoeutectoid steel is higher than Ac1 and lower than Ac3 temperature, then the previously existing proeutectoid ferrite is not completely transformed into austenite at high temperature, which is incomplete (or subcritical) quenching. The quenching temperature of hypereutectoid steel is 30-50℃ above the Ac1 temperature, this temperature range is in the austenite and cementite (A+C) dual-phase region.
Therefore, the normal quenching of hypereutectoid steel still belongs to incomplete quenching, and the structure obtained after quenching is martensite distributed on the cementite matrix. This structure has high hardness and high wear resistance. For hypereutectoid steel, if the heating temperature is too high, too much of the proeutectoid cementite will dissolve, even completely dissolve, then the austenite grains will grow, and the carbon content of austenite also increases.
After quenching, the large martensite structure increases the internal stress in the micro-regions of the quenched steel, increases the number of microcracks, and increases the tendency of the part to deform and crack. Because the carbon concentration in austenite is high, the martensite point drops, the amount of retained austenite increases, and the hardness and wear resistance of the workpiece decrease. The quenching temperature of commonly used steels is shown in the figure “Quenching Heating Temperature”, and the table shows the heating temperature for quenching of commonly used steels.
In actual production, the choice of heating temperature needs to be adjusted according to specific conditions. For example, when the carbon content in hypoeutectoid steel is at the lower limit, when the furnace charge is large, and when the depth of the quench hardening layer of the part is desired to be increased, the upper limit temperature can be chosen; if the workpiece shape is complicated, and the deformation requirements are strict, the lower limit temperature should be adopted.
Quenching Holding
The quenching holding time is determined by various factors such as equipment heating mode, part size, steel composition, furnace charge amount, and equipment power. For through-hardening, the purpose of holding is to make the internal temperature of the workpiece uniformly converge.
For all kinds of quenching, the holding time ultimately depends on obtaining a good quenching heating structure in the required quenching area. Heating and holding are important steps that affect the quality of quenching. The structure state obtained by austenitization directly affects the performance after quenching. The austenite grain size of general steel parts is controlled at 5-8 levels.
| Steel grade | Isothermal temperature /℃ | Isothermal time /min | Grade | Isothermal temperature /℃ | Isothermal time /min |
| 65 | 280-350 | 10-20 | GCr9 | 210~230 | 25-45 |
| 65Mn | 270-350 | 10-20 | 9SiCr | 260-280 | 30-45 |
| 55Si2 | 300-360 | 10-20 | Cr12MoV | 260-280 | 30-60 |
| 60Si2 | 270-340 | 20-30 | 3Cr2W8 | 280-300 | 30-40 |
| T12 | 210~220 | 25-45 |
Quenching Cooling
To make the high-temperature phase in the steel – austenite, transform into the low-temperature metastable phase – martensite during the cooling process, the cooling speed must be greater than the critical cooling speed of the steel. During the cooling process of the workpiece, there is a certain difference between the cooling speed of the surface and the core. If this difference is large enough, it may cause the part with a cooling rate greater than the critical cooling rate to transform into martensite, while the core that is less than the critical cooling rate cannot transform into martensite.
To ensure that the entire cross-section transforms into martensite, a quenching medium with sufficient cooling capacity needs to be selected to ensure that the core of the workpiece has a high enough cooling speed. But if the cooling speed is large, the internal stress caused by uneven thermal expansion and contraction inside the workpiece may cause the workpiece to deform or crack. Therefore, considering the above two conflicting factors, it is important to choose the quenching medium and cooling method reasonably.
The cooling stage is not only about obtaining a reasonable structure for the parts, achieving the required performance, but also maintaining the size and shape accuracy of the parts. It is a key link in the quenching process.
Workpiece Hardness
The hardness of the quenched workpiece affects the effect of quenching. The hardness of the quenched workpiece is generally determined by its HRC value measured by a Rockwell hardness tester. The HRA value can be measured for thin hard steel plates and surface quenched workpieces, while for quenched steel plates with a thickness less than 0.8mm, surface quenched workpieces with a shallow layer, and quenched steel bars with a diameter less than 5mm, a superficial Rockwell hardness tester can be used to measure their HRC values.
When welding carbon steel and certain alloy steels, quenching may occur in the heat-affected zone and become hard, which is prone to cold cracking. This is something to prevent during the welding process.
Due to the hardness and brittleness of the metal after quenching, the residual surface stress generated can cause cold cracks. Tempering can be used as one of the methods to eliminate cold cracks without affecting the hardness.
Quenching is more suitable for use with parts of small thickness and diameter. For larger parts, the quenching depth is not enough, and carburizing has the same problem. At this time, consider adding alloys such as chromium to the steel to increase strength.
Quenching is one of the basic means of strengthening steel materials. Martensite in steel is the hardest phase in iron-based solid solution structures, so steel parts can obtain high hardness and high strength by quenching. However, martensite is very brittle, and there is a large quenching internal stress inside the steel after quenching, so it is not suitable for direct application and must be tempered.
Single-Medium Quenching: The workpiece is cooled in one medium, such as water or oil. The advantages are simple operation, easy mechanization, and wide application. The disadvantage is that quenching in water causes large stress, making the workpiece prone to deformation and cracking; quenching in oil has a slow cooling rate, small quenching diameter, and it is difficult to quench large workpieces.
Double-Medium Quenching: The workpiece is first cooled to about 300℃ in a medium with strong cooling capacity, and then cooled in a medium with weaker cooling capacity. This method can effectively reduce internal stress due to martensitic transformation and reduce the tendency of workpiece deformation and cracking.
Staged Quenching: The workpiece is quenched in a low-temperature salt bath or alkali bath, with the temperature near the Ms point. The workpiece stays at this temperature for 2-5 minutes and then is air cooled.
Isothermal Quenching: The workpiece is quenched in an isothermal salt bath, the salt bath temperature is at the lower part of the bainite zone (slightly higher than Ms). The workpiece stays at the same temperature for a long time until the bainite transformation is complete, and then is air cooled.
Surface Quenching: Surface quenching is a method of partially quenching the surface layer of a steel piece to a certain depth, while the core remains unquenched.
Induction Hardening: Induction heating uses electromagnetic induction to generate eddy currents in the workpiece for heating.
Cryogenic Quenching: This involves immersing in a strong cooling ability of ice water solution as the quenching medium.
Partial Quenching: This involves quenching only the parts of the workpiece that need to be hardened.
Gas-Cooling Quenching: Specifically refers to heating in a vacuum and quenching in a high-speed circulating negative pressure, normal pressure, or high-pressure neutral and inert gas.
Air-Cooling Quenching: This involves using forced flowing air or compressed air as the cooling medium for quenching.
Brine Quenching: This involves using a salt water solution as the cooling medium for quenching.
Organic Solution Quenching: This involves using a water solution of organic polymer as the cooling medium for quenching.
Spray Quenching: This involves using a jet liquid flow as the cooling medium for quenching.
Hot Bath Cooling: This involves quenching the workpiece in a hot bath such as molten salt, molten alkali, molten metal, or high-temperature oil.
Double-Liquid Quenching: After heating the workpiece to form austenite, it is first immersed in a medium with strong cooling capacity, and when the organization is about to undergo martensitic transformation, it is immediately transferred to a medium with weak cooling capacity for cooling.
Pressurized Quenching: After heating the workpiece to form austenite, it is quenched under specific fixture clamping, with the aim of reducing quenching cooling distortion.
Through-Hardening: This involves quenching the workpiece from the surface to the heart entirely.
Isothermal Quenching: The workpiece is quickly cooled to the bainite transformation temperature interval to maintain isothermality after heating to form austenite, allowing the austenite to become bainite.
Staged Quenching: After heating the workpiece to form austenite, it is immersed in an alkali bath or salt bath with a temperature slightly higher or lower than the M1 point for a certain time, and after the whole workpiece reaches the medium temperature, it is taken out for air cooling to obtain martensite.
Sub-Temperature Quenching: Hypoeutectoid steel workpieces are quenched after being austenitized in the Ac1-Ac3 temperature range to obtain martensite and ferrite structures.
Direct Quenching: This involves directly quenching the workpiece after carburizing.
Double Quenching: After carburizing the workpiece, it is first austenitized at a temperature higher than Ac3 and then quenched to refine the core structure. It is then austenitized at a slightly higher than Ac3 temperature to refine the carburized layer structure.
Self-Cooling Quenching: After the workpiece is quickly heated to austenitize locally or on the surface, the heat from the heating area spreads to the unheated area on its own, causing the austenitized area to cool quickly.
Quenching is a critical heat treatment process extensively employed in modern mechanical manufacturing. Virtually all crucial components in machinery, particularly steel parts used in automobiles, aircraft, and aerospace applications, undergo quenching to enhance their mechanical properties. To meet the diverse technical requirements of various components, numerous specialized quenching processes have been developed.
Quenching methods can be categorized based on several factors:
1. Treatment area:
2. Phase transformation during heating:
3. Phase transformation during cooling:
Each quenching method has specific characteristics and limitations, making them suitable for particular applications. Among these, induction heating surface quenching and flame quenching are the most widely utilized. Emerging high-energy density heating quenching methods, such as laser beam and electron beam heating, are rapidly gaining attention due to their unique capabilities and precision control.
Surface quenching finds extensive application in machine components fabricated from medium carbon tempered steel or ductile iron. This process is particularly effective for medium carbon tempered steel, as it allows for the maintenance of high overall mechanical properties in the core while achieving superior surface hardness (>HRC 50) and wear resistance. Common applications include machine tool spindles, gears, diesel engine crankshafts, and camshafts.
The principle of surface quenching can also be applied to various iron-based materials with compositions similar to medium carbon steel, such as:
Among these, ductile iron exhibits the best process performance and high overall mechanical properties, making it the most widely used material for surface quenching applications.
For high carbon steels, surface quenching significantly improves surface hardness and wear resistance. However, the core’s plasticity and toughness remain relatively low. Consequently, surface quenching of high carbon steel is primarily employed for tools, measuring instruments, and high cold-hardened rolls that experience minimal impact and alternating loads.
Low carbon steels, on the other hand, show minimal strengthening effects after surface quenching and are thus rarely subjected to this treatment.
The selection of an appropriate quenching method and material depends on the specific component requirements, including mechanical properties, wear resistance, and operating conditions. Advancements in quenching technologies continue to expand the possibilities for enhancing material properties in various industrial applications.
As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.
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