Heat treatment is a common method of altering the mechanical properties of certain metals. Being able to change the hardness, toughness, and strength of a metal while keeping its chemical composition intact and virtually unchanged is a great way to tailor a metal to the environment and the needs of the job.
There are many different ways to heat treat metals, the most popular of which is through a method called quenching.
What is quenching?
Quenching is a metal heat treatment process. Quenching refers to the rapid cooling of metals to adjust the mechanical properties of their original state. To perform the quenching process, the metal is heated to a temperature above normal conditions, usually a temperature above its recrystallization temperature but below its melting temperature. The metal is kept at this temperature for a while for the heat to “soak” the material. Once the metal is held at the desired temperature, it is quenched in the medium until it returns to room temperature. The metal can also be quenched for an extended time so that the cooling from the quenching process is distributed throughout the thickness of the material.
Quenching process
In the quenching of steel, rapid cooling is achieved by bringing the hot surface of an object into contact with some cooler material, which may be gaseous, liquid, or solid. This operation is called quenching and includes cooling by air, water, or other liquid jets – immersion in liquids such as brine, water, polymer quenching agents, salt baths, and interplate cooling.
Quenching process
However, the rate of cooling (the rate of heat transfer from the body of the hot metal to the quenching medium) depends on the cross-sectional size of the object, its temperature, its thermal properties, the condition of its surface as to the nature and degree of roughness of the oxide film, and the initial temperature of the cooling agent, its boiling point, the specific heat of coolant, latent heat of vaporization, the specific heat of vapor, its thermal conductivity, its viscosity and its speed through immersed objects.
Before starting to consider the cooling properties of commonly used coolants, it is best to investigate what happens when a heated steel object (say 840°C) is dropped into a still bath of cold water.
The cooling curve shows not a constant cooling rate but three stages throughout the quenching process:
Phase A – Steam Coverage Phase:
After starting the quenching, since the metal is at a high temperature, the quenching coolant evaporates immediately, and a continuous steam blanket wraps the surface of the object.
Now that there is no liquid in contact with the metal surface, heat escapes from the hot surface very slowly by radiation and conduction through the water vapor layer to the liquid-vapor interface. Since the vapor film is a poor conductor of heat, the cooling rate is relatively slow.
Phase B – Batch Contact Phase (Liquid Boiling Phase):
Heat is rapidly evaporated at this stage, as indicated by the steep slope of the cooling curve. During this phase, the steam blanket is intermittently broken, allowing the coolant to contact the hot surface for a brief moment, but is quickly pushed away by the vigorous boiling action of the steam bubbles. The air bubbles are carried away by convection and the liquid touches the metal again.
The rapid cooling at this stage quickly brings the surface below the boiling point of the quenching medium. Then, the evaporation stopped. The second stage corresponds to the temperature range from 100°C to 500°C, where the steel in the austenitic state transforms fastest (≈front of the CCT curve). Therefore, the cooling rate at this stage is very important for the quenching of steel.
Stage C – direct contact stage (liquid cooling stage):
This phase begins when the temperature of the surface of the object drops below the boiling point, or the quenching medium. Steam will not form. Cooling is due to convection and conduction through the liquid. The cooling rate at this stage is the lowest.
Quenching steps
First, the alloy is heated to 30-50°C above the critical temperature. We don’t want to stay at this temperature for a long time because it may cause grain growth.
If you are working with alloys that are sensitive to oxidation, you may need to heat the alloy in a vacuum. Some furnaces can be heated under vacuum, but a simpler (small scale) method is to encapsulate the alloy in a quartz tube that has been evacuated or filled with an inert gas such as argon.
Alloys require rapid cooling. The main way to control the cooling rate is to use different quenching media. Brine is usually the fastest practical quenching medium. Liquid nitrogen is a relatively slow quenching medium due to its low thermal conductivity and specific heat.
If the alloy cools too quickly, it may crack. If it cools down too slowly, you may not get much metastability. The best way to determine the optimum quenching rate for a material is to use a time-temperature-transition (TTT) phase diagram.
Most of the metal in the quenching process is heated between 715 and 900ºC. During the heating process, the material must be heated at a constant temperature. Heating at constant temperature leads to the realization of the desired properties of the metal.
The second thing you have to do after heating is to get wet, or we can say soak. Immerse the material or heated workpiece in a medium such as a vacuum or air. The workpiece must be soaked in salt or sand for 6 minutes, and the ambient temperature must be constant during the soaking.
Some of you may think soaking and cooling are similar. But both the soaking and cooling processes are different. So, after soaking, it’s time to start cooling.
During the cooling process, the workpiece must be kept in the quenching liquid. Use water, and oil as quenching medium. Using water as a quenching medium has a disadvantage, such as it can cause multiple cracks in the metal surface, or it can deform the metal surface. One thing to note is that oil cools much more slowly than water.
The quenching process can also be carried out in the presence of inert gas. Inert gases such as nitrogen, helium, and argon can be used in the quenching process. In this heat treatment process, the quenching medium plays a vital role. If the cooling rate of the quenching medium is lower than required, then you will not be able to obtain the expected properties of the output metal. If the quenching medium cools faster than necessary, cracks can develop in the output metal.
After the quenching process is complete, you may notice that the material you get may be very brittle, or it may be much harder than normal metal. This is due to the abundant presence of martensite in a given material. Therefore, you must temper such metals. Tempering reduces unnecessary hardness. To temper, you have to heat the metal below its critical temperature, then you have to cool this metal in natural air or the environment.
Common quenching media are:
1. Water:
Water is probably the oldest and most popular quenching medium, meeting the requirements of low cost, universal availability, ease of handling, and safety. As the temperature rises, the cooling characteristics change more than oil, especially when the temperature rises above 60°C, the cooling capacity drops rapidly due to the increase of the vapor layer stage. The best cooling power is when the water is between 20-40℃.
The cooling capacity of water is between brine and oil. Although water provides a higher cooling capacity near the tip of the curve to avoid transformation to pearlite or bainite, as shown in Table 6.11, the biggest disadvantage of water is the cooling rate in the temperature range of martensite formation high. At this stage, the steel is subjected to both structural and thermal stresses, whose additive effects increase the risk of crack formation.
2. Saltwater:
About 10% (weight) sodium chloride aqueous solution is widely used in industry and is called brine. They provide cooling rates between that of water and 10% NaOH in water. They are corrosive to equipment but, like corrosive solutions, are not harmful to workers.
An explanation for the higher efficiency of brine, caustic soda, or aqueous solutions is that in brine or caustic soda, the heating of the solution on the hot steel surface results in the deposition of NaCl/NaOH crystals on the hot steel surface. This layer of solid crystals breaks violently with a slight explosion, throwing out a cloud of crystals.
3. Sodium hydroxide:
Usually, 10% (by weight) of sodium hydroxide is added to the water. These solutions rapidly extract heat from the steel at the moment the steel is immersed in the coolant, and do not exhibit the relatively “inactive” state of the initial phase (a phase) of water. Therefore, this is useful when the required cooling rate exceeds that of a water bath.
4. Oil:
Oil, as a group, is between water at 40°C and water at 90°C in terms of cooling rate. Considerable variation can be produced by using animal, vegetable, or mineral oils, or mixtures of two or more types of oils, in the oil quenching process. The vapor pressure of the oil is especially important as it determines the thickness of the oil vapor film produced on the hot steel surface, which limits the rate of heat removal. However, the oils commonly used have high boiling points.
Oils have a much lower quenching capacity (maximum cooling at about 600°C) and are relatively slow in the range of martensite formation compared to water or brine, which minimizes the risk of crack formation. The cooling power near the front end of the steel’s CCT curve can be increased by vigorously stirring the molten pool or part of the molten pool.
5. Emulsion (water and oil):
The rapid cooling of the water (near the top of the CCT curve) and the slow cooling of the oil at a later stage (in the Ms – Mf temperature range) lead to the development of emulsions – mixtures of water and “water-soluble” oils in varying proportions. A 90% oil and 10% water emulsion have a lower cooling rate than oil. An emulsion consisting of 90% water and 10% oil is also inferior to oil because it cools faster than oil when martensite is formed at around 300°C, increasing the risk of deformation and cracking.
6. Polymer media:
These are new entrants in the field of coolants and approach the properties of an ideal quenching medium (6.3), i.e. rapid cooling to Ms temperature followed by a rather slow martensite formation.
These synthetic quenchants are high molecular weight organic chemicals, usually based on polyalkylene glycols, or polyvinyl alcohol, although the former is more commonly used as a quencher. These are water-soluble materials, therefore, by varying the concentration of organic additives, quenchants with widely varying cooling rates can be obtained. When the addition of a quenching agent is 5%, the surface hardness of the quenching agent is similar to that of water at 60 ° C, and the risk of cracking is the smallest while quenching non-alloy steel. The quenching agent with a 15% additive has the same cooling performance as oil, and there is no fire hazard.
7. Salt bath:
For steel with not too large a cross-section and good hardenability, a salt bath is an ideal quenching medium. Table 6.12 gives the composition of some salts and the applicable temperature range for each mixture. The recommended incubation time in the salt bath is 2-4 min/cm slice thickness, with lighter slices having a shorter incubation time. A bath like 100% NaNO3 needs 400-600°C. The cooling capacity is as high as about 400°C, and then decreases as the temperature of the steel continues to decrease.
Therefore, the cooler the bath, the greater the agitation and the better the cooling capacity. If contaminated, the cooling efficiency of the bath will be reduced. The stirred tank keeps impurities in suspension and attaches to the parts being cooled, reducing heat transfer. Adding 0.3-0.5% water to the salt bath keeps the surface of the salt bath in a state of steam, and the cooling capacity is almost doubled.
8. Air:
Compressed air or still air can also be used if the steel has high hardenability, i.e. high alloy steels such as air-hardening steels; or light sections of low alloy steels. Due to the slower and more uniform air cooling, the risk of deformation is negligible. The surface of the steel is always oxidized during cooling.
9. Gas:
Of the gases, hydrogen and helium are more efficient for cooling, but nitrogen is often used for hot-working steel and high-speed steel because hydrogen and helium are expensive to use and can explode.
Gas quenching can make thick-section parts with complex shapes and different section thicknesses cool more uniformly, to obtain more uniform mechanical properties. There is minimal risk of cracking or deforming. The fast-flowing airflow directly contacts the austenitized steel in the air chamber to dissipate heat quickly.
10. Flowing layer:
It consists of alumina particles in a retort fluidized by a continuous stream of air blown upward from the bottom of the retort. These particles move like a fluid. The use of nitrogen creates an inert atmosphere.
Mainly used for quenching high-alloy steel, cold-worked steel, hot-worked steel, high-speed steel, air-hardened steel, etc. Fluidized bed cooling is slower than water or oil, 10% slower than molten salt quenching, but significantly faster than air. Fluidized beds can operate at any low temperature. There is no residue on the part and no post-treatment is required. No hazards of smoke and pollution.
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