Quenching Explained
- Gavin Leo
In the olden days, blacksmiths quenched their tools and weapons for better performance. The concept of heating and cooling metal rapidly for hardness and strength has been around for ages.
Over the centuries, the intricacies of the mechanical and chemical transformations in the quenching process have become clearer. Owing to this, many manufacturers choose the quenching and tempering process to enhance material properties.
This article covers the basics of this heat treatment process and its application in structures and other areas today.
What is Quenching?
Quenching is a heat treatment process that entails heating iron-based metals to high temperatures and rapidly cooling them. Cooling can be in a liquid or any other medium. It is different from quenching and tempering, which means that the material has to be reheated to a lower temperature after quenching.
The result of quenching includes enhanced hardness and strength. This is due to changes in the material’s microstructure.
When Is Quenching Used?
Metalworkers use the quenching process, first, when they want increased hardness and strength in the material. During the rapid cooling process, the metal undergoes martensitic changes related to the grain structure. Its mechanical properties improve as a result.
Quenching is also applicable when the aim is to improve the heat resistance of the material. This form of heat treatment makes steel and other metals suitable for high-heat applications.
Some users are looking to achieve better wear resistance in the material. The harder the material, the more wear-resistant it is. By improving this property, the user can achieve a longer lifespan in the material.
Also for specific mechanical properties, the quenching process comes in handy. It is all about controlling the cooling rate and other aspects of quenching.
Quenching Process
The quenching process starts with the preparation of the metal and ends with slow cooling. The metalworker heats the material to the critical temperature – the point where the material turns non-magnetic.
Below are the three quenching stages that follow after this heating:
1: Vapor Stage
During the vapor stage, the hot metal comes into contact with the quenching media. A blanket of vapor covers the material. The main heat transfer method is radiation, and happens via vapor. Although there is a small conduction level, the heat transfer is largely slow due to radiation.
This stage produces soft spots in quenched steel and other metals. To counter this challenge, metalworkers use powerful sprays on the material.
2: Boiling Stage
The collapse of the vapor stage and the formation of bubbles on the hot part’s surface signal the boiling stage. A high heat extraction rate defines this stage. Of the three quenching stages, the boiling stage is the fastest. This is due to the quenching media absorbing the heat from the metal.
The cooler quenching media replaces the extracted heat at the material’s surface. To achieve an even better cooling rate, some quenching media contain additives. This cooling process continues until the surface temperature is below the boiling point of the quenching media.
3: Convection Stage
At this point, the boiling temperature of the quenching media is higher than the temperature of the component’s surface. Convection is now the main heat removal method.
The parameters to control for this process include the thermal conductivity and specific heat of the quenching media. The temperature differential between the two media is also important at this stage. Significant material distortion takes place, making the process relatively slow.
Quenching Media
Below are the main quenching media:
Air Quenching
Air quenching is the slowest means of quenching, especially if it is still air. For better results, forced air or compressed air around the material is necessary.
One benefit of this technique is that the chances of metal distortion or thermal shock are minimal. The method applies if you are seeking to make the metal tough and moderately hard.
Air-quenched materials have fairly good mechanical properties. Nitrogen and other inert gases are common features in air quenching.
Oil Quenching
For an improved quenching rate, you can use oil quenching. Mineral oil is the most prominent option in this category. Though flammable, oil is ideal for preventing cracking and distortion when the material is rapidly cooled. You can also adjust this quenching method for many applications that need a more rapidly cooling method.
Freshwater Quenching
For a quenching process that is faster than both oil quenching and air quenching, fresh cold water is used. Putting hot metal in a freshwater bath makes sense for flammable materials. In addition, fresh water is readily available, hence it is a cost effective method. You only need to control the process carefully to prevent the cracking and distortion of metals.
Brine (salt water) Quenching
An even more reliable quenching media is brine, also called salt water. This liquid does not form air globules. More of the metal surface is exposed to the quenching media making the cooling rate faster.
Polymers Quenching
Polymer quenching media is usually an alternative for materials that do not quench well with air, oil, or water. It can accommodate interrupted quenching. A noticeable advantage compared to oil quenching for heated metal is safety – you can remove the material from the medium for some time and then immerse it back without a fire risk.
Other options to consider are lead baths and cryogenic quenching among others.
How to Quenching Metal?
The process of quenching steel and other metals is as outlined below:
Step 1. Prepare your metal for heat treatment
A critical part of preparing the metal is preheating. Exceeding the recommended temperature or heating rate can crack or distort the material.
Step 2. Heat the metal
This is where you heat the metal completely as required. You then hold the material at this high temperature for some time. This gives the material enough time to change the crystal structures.
Step 3. Select the quenching media
Depending on the desired results and the specific metal, choose a suitable quenching medium.
Step 4. Quench the metal
When all is set, perform the quenching process. The aim is to return the metal to room temperature.
Step 5. Control the cooling rate
It is important to control the cooling rate because this rate can affect the eventual hardness, toughness, and other elements of the material.
Commonly Quenched Metals
Aluminum alloys
You can quench specific aluminum alloys to enhance their strength and hardness.
Copper alloys
Copper alloys can also undergo a quench hardening process for enhanced final hardness and strength. The most common quenching media for copper alloys is oil or water.
Steel
Steel is perhaps the most widely quenched metal. The high carbon steels are common in this process, which results in martensite grain structure.
Titanium alloys
You could also quench titanium alloys to increase their strength. With improved mechanical properties, these alloys play critical roles in aircraft and military vehicles’ manufacture.
How Does Quenching Affect the Properties of a Metal?
Microstructural Changes
The microstructure of metal contains grain structure. Quenching aims to alter the microstructure from the crystal structure level. The changes in the crystal structure determine the final properties of the material. Because of the increased brittle microstructure, it is important to optimize the cooling rate.
Mechanical Properties
Quenching increases the strength and hardness of a metal through martensitic transformation, as the carbon atoms are trapped. The level of these changes depends on the quenching method and the chemical composition of the metal.
Residual Stresses
Like any other manufacturing process, quenching can create or reduce residual stresses in the material. The extent of these stresses depends on the type of quenching media, proper agitation, part handling, contaminants, and quench temperature, among others.
We have covered the basics of quenching as used in heat treatment to alter crystal structure. This process presents countless advantages including increased strength and heat resistance.
Contact Aria Manufacturing today if you have any more questions about this and other processes of heat-treating materials.