Looking to enhance the strength and durability of your metal parts? Heat treatment is the way to go. But what happens after you heat up your workpiece? That’s where quenching comes in.
Quenching is a crucial process in heat treatment that involves rapid cooling of the material to achieve the desired properties. But did you know that there are ten different methods for quenching?
From single-medium quenching to jet quenching, each technique has its own unique advantages and is suitable for specific types of workpieces.
In this comprehensive guide, we’ll take you through each quenching method, explain how they work, and highlight the ideal applications for each.
Whether you’re a seasoned metallurgist or just starting out in the field, this article is a must-read for anyone looking to improve the performance of their metal parts.
So, let’s dive in and explore the world of quenching together!
What Is Quenching?
Quenching is a heat treatment process. In the case of steel, quenching involves heating it to a temperature above its critical temperature Ac3 (for hypo-eutectoid steel) or Ac1 (for hyper-eutectoid steel), holding it for a certain period to enable complete or partial austenitization, and then rapidly cooling it below Ms (or near Ms for isothermal) at a rate faster than the critical cooling rate, enabling the transformation to martensite (or bainite).
Quenching is also used to refer to the heat treatment processes involving solution treatment or rapid cooling for materials like aluminum alloys, copper alloys, titanium alloys, and tempered glass.
Quenching Methods in the Heat Treatment Process
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.
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.
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.
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.
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:
- Single-medium quenching (using water, oil, or air);
- Interrupted quenching;
- Martempering below the Ms point;
- Bainite isothermal quenching;
- Compound quenching;
- Precooled isothermal quenching;
- Delayed cooling quenching;
- Quenching self-tempering;
- Jet quenching.
1. Single-medium (water, oil, air) quenching
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
2. Interrupted 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.
4. Graded martensitic quenching method below Ms point
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.
5. Isothermal quenching of bainite
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:
- Austenitizing treatment
- Cooling treatment after austenitizing
- Bainite austempering
This method is commonly used for small-sized parts made of alloy steel and high-carbon steel, as well as ductile iron castings.
6. Compound quenching
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.
7. Precooled isothermal quenching
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.
8. Pelayed cooling quenching
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.
9. Quenching self tempering
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.
10. Jet quenching
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.
Purpose of Quenching
The purpose of quenching is to induce the transformation of supercooled austenite into martensite or bainite, resulting in a martensitic or bainitic structure. Subsequent tempering at different temperatures can significantly increase the rigidity, hardness, wear resistance, fatigue strength, and toughness of steel, meeting the diverse requirements of various mechanical parts and tools. Quenching can also satisfy special physical and chemical properties such as the ferromagnetism and corrosion resistance of certain special steels.
Quenching is a metal heat treatment process that involves heating a metal workpiece to an appropriate temperature, holding it for a period, and then quickly cooling it by immersion in a quenching medium. Commonly used quenching media include brine, water, mineral oil, and air. Quenching can improve the hardness and wear resistance of metal workpieces and is widely used in various tools, molds, measuring tools, and wear-resistant parts (such as gears, rolls, carburized parts, etc.).
Through quenching and subsequent tempering at different temperatures, the strength of the metal can be greatly improved, and its toughness and fatigue strength reduced. This process can achieve a balance of these properties (comprehensive mechanical performance) to meet different usage requirements.
Moreover, quenching can also give certain physical and chemical properties to steels with special performance, such as enhancing the ferromagnetism of permanent magnetic steel, improving the corrosion resistance of stainless steel, etc. The quenching process is mainly used for steel pieces.
When common steel is heated above its critical temperature, the structure that existed at room temperature will entirely or largely transform into austenite. Subsequently, the steel is quickly cooled by immersing it in water or oil, causing the austenite to transform into martensite. Martensite has the highest hardness compared to other structures in steel. Rapid cooling during quenching causes internal stress in the workpiece, which, when sufficiently large, can cause the workpiece to distort, twist, or even crack. Therefore, a suitable cooling method must be chosen.
Based on the cooling method, quenching processes can be divided into four categories: single liquid quenching, dual medium quenching, martensite graded quenching, and bainite isothermal quenching.
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|
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.
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|
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.
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.
Various Types of Quenching Methods
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 widely used in modern mechanical manufacturing industry. Important parts in machinery, especially steel parts used in automobiles, aircraft, and rockets, have almost all undergone quenching. In order to meet the diverse technical requirements of various parts, various quenching processes have been developed. For example, according to the parts being treated, there are overall, partial and surface quenching; according to whether phase transformation is complete during heating, there are complete quenching and incomplete quenching (for hypo-eutectoid steel, this method is also called subcritical quenching); according to the content of phase transformation during cooling, there are staged quenching, isothermal quenching, and underspeed quenching.
In addition, due to the characteristics and limitations of each quenching method, they are all used 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 quenching methods. Because they have some characteristics that other heating methods do not have, they are attracting attention.
Surface quenching is widely used in machine parts made of medium carbon tempered steel or ductile iron. Because the medium carbon tempered steel can maintain high comprehensive mechanical properties in the core and high hardness (>HRC 50) and wear resistance on the surface after pre-treatment (tempering or normalizing) and then surface quenching. For example, machine tool spindles, gears, diesel engine crankshafts, camshafts, etc. In principle, surface quenching can be performed on pearlitic ferritic iron based grey cast iron, ductile iron, malleable cast iron, alloy cast iron, etc., which are equivalent to the composition of medium carbon steel. The process performance of ductile iron is the best, and it also has high comprehensive mechanical properties, so it is the most widely used.
After high carbon steel is surface quenched, although the surface hardness and wear resistance are improved, the plasticity and toughness of the core are relatively low, so the surface quenching of high carbon steel is mainly used for tools, measuring tools, and high cold-hardened rolls that bear small impacts and alternating loads.
Since the strengthening effect is not significant after low carbon steel surface quenching, it is rarely used.