Oil hardening steel refers to steel that must be quenched by oil. Typically, this is the cold-work group of steels, including the O family of tool steels such as O1 and O2.

Pure steel often comes in a supply that is too soft for application. Therefore, quenching is the process needed to harden steel to its required strength and hardening properties. Although this article discusses hardening steel by oil, other methods are applicable depending on the grade of steel you have and if quenching has not already been performed before purchase. For example:

Oil is one of the most popular quenching methods worldwide and involves heating your steel to the recommended temperature before cooling it in oil to gather extra hardening properties. The oil contains properties that allow for slower cooling when analysing the results of water and brine. However, when looking at the analysis of air, oil cools much faster.

Properties of oil hardening steel

Oil hardening steel grades offer excellent wear resistance, and due to this, the O1 tool steel grade has grown to be one of the most popular worldwide. The ability to use oil for quenching compared to water or air adds extra properties that allow for tools to be produced that are less likely to crack. 


How do oils harden steel?

Hardening tool steel in oil requires a quenching process to be used, which involves a range of steps. Once the steel has been heated, the toolmaker should place it into oil for quenching. When first submerged into oil, the heat from the treated steel and the quenchant form a layer of vaper, a process called firm boiling. 

It’s essential to research the grade of steel you’re hardening and the properties of the oil it will be submerged, as the properties can massively change the time taken to cool and, more importantly, the ability to set.

Next, as the steel cools below the quenchant’s boiling point, the process moves into nucleate boiling. As mentioned before, the properties of the oil used to harden steel play a significant role in cooling speed. You can find a basic overview of quenching steel in oil below:

As the temperature continues to cool, it moves into the liquid stage, and cooling begins to become much slower than the previous stage (again, dependent on the oil and steel used). This process helps develop the properties within the steel to reach total hardness and reduce cracking and distortion of the finished tool. However, this is only true if the hardening of the steel has been performed correctly.

Benefits of oil quenching

Using oil to achieve the desired hardness of your steel after heat treatment has a range of benefits.

The main benefit is that there are many oils to choose from, which allows you, as the toolmaker, to have complete control over the quenching process. Each oil will achieve different outcomes, and having the ability to control this aspect can be more difficult with other hardening methods such as air and water. Furthermore, for even more control over the result, additives can be added to the oil.

Selecting the correct hardening oil

To select the correct oil for your quenching purposes, you first need to research your steel grade as different oils become more applicable depending on the steel type. For example, some oils are less likely to cause cracking to the steel as they maintain a consistent heat between the core and surface temperature – these are referred to as hot oils.

For further information relating to quenching oil and oil selection, please visit Engineering 360.

Vegetable oils

A popular choice for toolmakers is to use vegetable oils for their steel hardening needs. They offer extensive benefits such as cost and are relatively positive for the environment compared to some quenching oils.

Although vegetable oils and other cooking oils have significant benefits, they also contain a disadvantage. The disadvantage is that the level of hardness they can achieve compared to industrial lubricants and quenchants is much lower. So, when choosing if vegetable oil is the best quenchant for your project, you need to weigh up the cost, environmental impact, and hardness.

Industrial quenching oils

As the name suggests, industrial quenching oils have been designed with the sole purpose of oil hardening your steel. They offer significant benefits as you can find specific oil for the grade of steel you are quenching and even the properties that wish to be achieved.

On the other hand, these oils come with a high price tag and are usually much more challenging to find than other general oils. Don’t expect to see these in a general hardware store, but if you’re serious about the properties you wish to achieve, and the quenching speed needed, commercial quenching oils should be your first choice.

Other oils

When quenching your steel in oil, you can use other quenchants such as motor oil. Although motor oil is a cheaper, more readily available option than commercial oils, the risk of toxins due to the added properties when quenching can cause health and safety risks. Furthermore, you risk damaging the steel or causing an unwanted finish when using this quenchant.