Oil Hardening vs Air Hardening- What’s the Difference?

Heat treatment is an essential process in the manufacturing of steel products. It is the process of heating and cooling metal to alter its mechanical and physical properties to make it more useful for its intended purpose. Hardening is one of the most common heat treatment processes, where steel is heated to a certain point and then rapidly cooled. There are two main types of hardening: oil hardening and air hardening. This blog post will discuss the differences between these two processes and their advantages and disadvantages.

What is Oil Hardening?

Oil hardening is a method used to harden ferrous metals such as steel and iron. To oil harden the metal, it is heated up to an austenitizing temperature (a temperature at which the metal becomes malleable), quenched in an oil bath, and then tempered. The process works by causing martensite, a very hard form of iron crystal, to form as the metal cools down quickly due to its contact with cold oil. This change in crystal structure increases strength and hardness while still retaining some flexibility for ductility.

Oil hardening is a process of hardening steel by quenching it in oil after it has been heated to a specific temperature. The process of oil hardening allows for control of the cooling rate, which can impact the properties of the steel. The cooling rate must be consistent and adequate to ensure that the steel is hardened evenly.

One of the benefits of oil hardening is that it is less likely to cause warping or cracking in the steel. The process also produces a harder, more durable steel with good wear resistance. However, oil hardening can also result in more brittle steel than air-hardened steel, making it potentially more susceptible to cracking under impact loads.

What is Air Hardening?

Air hardening is a heat-treating process that allows a metal object to cool slowly in normal atmospheric conditions. It helps to give the object an extremely high surface hardness and often causes significant changes in its internal microstructure. The process begins by heating the metal until it reaches its critical temperature, which depends on the alloy being worked with. Once heated, it should be cooled very slowly at room temperature. A slow cooling rate ensures that desired metallurgical transformations happen and brings out maximum potential from the material’s properties for enhanced wear & tear resistance and structural strength.

Air hardening is another process of hardening steel by cooling it in the air after it has been heated to a certain temperature. This process relies on the cooling rate by the surrounding air to harden the metal. The thickness and shape of the steel being hardened can influence the cooling rate.

One of the benefits of air-hardening is that it results in less brittle steel than oil hardening. This type of hardening also produces steel that is easier to machine, grind or cut. However, air hardening can cause warping, cracking, or distortion in the steel due to the uneven cooling.


Comparing oil hardening and air hardening depends on the application of steel, shape, size, and mechanical properties required. Oil hardening is the preferred process where durability is required, such as in knives, drill bits, and hammers. Air hardening might be preferred where resistance to deformation is required, either for machining, materials handling and in dies and moulds manufacturing. Agriculture equipment and wear parts in industries like mining require high impact and wear resistance, making oil hardening ideal in those cases.

Advantages and Disadvantages

Both oil-hardening and air-hardening processes have unique advantages and disadvantages. Oil hardening results in hard, wear-resistant steel with less distortion but tends to make the metal more brittle. On the other hand, air hardening ensures less brittleness but might result in warping and distortion of the steel during the hardening process and may not achieve the required steel hardness in some scenarios. Like any other heat treatment process, it is important to consider the tradeoffs between cost, time and the impact of the properties on the end-use before selecting between the two.

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In conclusion, the choice between oil hardening and air hardening depends on the type of steel, its intended application and the desired properties. Both methods have their advantages and disadvantages, and it’s up to the manufacturer to weigh them before selecting a method. With proper consideration and tradeoffs to the properties of the metal, the end user may get the expected performance from the material. So, understanding the differences between the two methods is essential. We hope this article has been informative and useful in selecting the right hardening process.

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