Air hardening steel refers to steel that undergoes the process of using air as a quenchant. This natural quenchant is often used on steel in the A group of tool steels, such as A2, A3, A4, A6 and A10. However, it has also been known to be used on steel grades such as D2.


Quenching is the process used to harden steel and to alter its strength and hardening properties. Quenching steel can be done using a range of different mediums, such as oil and water. Although, we will be looking at cooling metal using air or air with additives such as gas for this article.


When using air as a quenchant, it’s recommended to note that leaving the steel out to cool in the natural air is the least controlled method. Instead, it would help if you placed it in an atmosphere or vacuum furnace to ensure complete control over the quenching process. 


Importance of air hardening steel

Air quenching, in general, is an essential process used to heat treat steels that have been identified as needed to be hardened by air. These steels that are not quenched in the air will not contain the additional strength and hardness properties that allow them to be used in challenging environments. For example, end products such as shear knives, gauges, dies and punches would not last without this heat-treatment process.


When quenching by air, your steel manufacturer will recommend if the process involves simply allowing the metal to cool to room temperature or if you should blast it with compressed air. If the manufacturer has recommended blasting with air, there are many ways this can be performed, including a simple can of compressed air or using machinery specifically designed for controlling air.


Properties of air-hardened steel

The properties of tool steel classified as air-hardening includes steel with carbon content in the radius of 0.5% to 2%. The ability to harden by air means that additional properties such as molybdenum, chromium and manganese are found within these steel types.

AISICMnSiCrNiMoWV
A21.001.000.505.000.301.000.15-0.50
A31.300.600.505.500.301.401.40
A41.052.200.502.200.301.40
A60.752.500.501.200.301.40
A101.502.101.502.051.75

Advantages of air quenching

The advantages of this quenchant medium include the ability to have complete control over the cooling speed and the end hardness result. For example, adjusting the pressure and exposure of the steel to air means that you can control the rate at which it cools and thus the desired properties. This is important for toolmakers that want to use the product in environments that require extreme toughness and durability, as it is known that air quenching creates solid and durable metals that can withstand even the most challenging environments.


One of the main benefits of using air as a quenchant is the cost. Air is free, and if expensive environmental chambers are not being used, you can significantly save on the cost by using air to harden steel.


Disadvantages of air quenching

When hardening steel by the use of air, it is common to experience problems with the metals if not performed correctly. The main issue that may occur is cooling fractures due to the rapid cooling of the steel part. However, you can easily prevent this issue by using vacuum furnaces and proper hardening techniques.