How to Harden Mild Steel? Check Out Different Solutions!

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If you ever came across to a mild steel, then you probably wondered how to harden this piece of metal. It’s very interesting to see mild steel that you can give whatever shape you want. Things get interesting when it’s about hardening it.

There are various ways to harden mild steel that doesn’t require industrial techniques. In this article, I’m going to discuss different ways to harden mild steel.


You will deal with fire, metal, and oil. Therefore, taking safety precautions are a must. Make sure your working area is well-ventilated. Work using protective clothing such as heavy-duty gloves, eyewear, etc.

Also, have a fire extinguisher standby in case of an emergency. Oil catches fire easily, therefore be very careful when you are about to immerse red-hot steel in the oil. So, having a fire extinguisher ready will minimize the damage if there is an accident.

If the oil catches fire, make sure to drop the steel into a heavy duty bucket and then use the fire extinguisher.

Method 1: Harden Mild Steel With Salt Water

It is one of the oldest methods of hardening mild steel. You no longer require any fancy tools to do the job, and the things you will need are easily available. The interesting part is, you don’t need to deal with any toxic chemicals with this method.

Things you will needs

  • Wholemeal flour
  • Water
  • Salt

Cut-off Saw Cutting Metal With Sparks

1. Prepare A Hardening Paste

First, you need to prepare a hardening paste. To do that, take a glass bowl and add a spoonful of whole meal flour and two spoonful of salt. Slowly add water into the mixture until it turns into a paste.

The amount of hardening paste you need will depend on how much surface of steel you want to harden. If you see any shortage of paste, follow the same procedure to produce a sufficient amount of paste.

2. Heat the Area before Applying the Paste

Before coating the paste in the area you want to harden, make sure you heat that area first. Heat the surface slowly; it will help the paste to stick to that surface securely.

Again, the temperature you need to achieve to heat the metal will depend on its size and the temperature of your location. But the rule of thumb is to heat slowly and frequently check how well the paste is sticking to the metal surface.

3. Apply the Paste

When the temperature reaches to the degree that will make the paste stick, you will need to cover the areas you would like to harden.

The way paste will lay on the surface; it will harden the object to that shape. Therefore, make sure you carefully apply the paste.

4. Heat the Area Again to Harden the Paste

Now it’s time to heat the area covered in paste. Heat it in a temperature of 1500 degrees Celsius until it turns into bright red color. The heating process will take about several hours.

5. Temper the Steel

Immediately after the surface reaches the appropriate temperature, quench the steel into the cold and clean water. It will instantly harden the steel.

Leave the steel in that water for several minutes. It will further complete the hardening process.

After the hardening process, you will need to temper the steel. When you heat and quench the steel, the extreme and rapid change in temperature will make the steel even harden. But, it will make the steel becoming brittle.

Therefore, you need to temper the steel to avoid it becoming brittle.

To further temper it, heat the hardened steel until a blue heat appears around it. The visible blue heat indicates that the steel is in appropriate temperature. Now, let the steel sit and slowly cool at room temperature.

A grinder on steel

Method 2: Harden Steel With The Motor Oil

Using motor oil is another way that you can harden mild steel. Normally, pure steel is soft for many applications. Therefore, if you want to put a hard layer on steel, you need to make sure the carbon is fused at a molecular level on the top centimeter of the steel.

Now let’s see how you can harden a steel using motor oil.

1. Heat the Steel and Dip it in Motor Oil

First, heat the steel by using a torch. You can also use a furnace with bellows. Keep heating the steel until it turns to red-hot.

When you are finished with heating, pick the steel using tongs and immediately immerse into the motor oil.

Let the steel stay in that position in oil for about 60 seconds. The carbon that is in the motor oil will bond the top layer of hot steel molecules forming a tough outer surface on the steel.

2. Wash and Reheat the Steel

Now, pick the steel from the oil and wash it properly with dishwashing soap ensuring that there is no oiliness on the surface. Be careful when picking up the steel, because if you drop it or it gets hit, it will shatter like a glass as it is brittle at this stage.

Further, heat the cleaned steel until it turns to blue-hot. This color appears just before becoming red-hot.

Therefore, do not heat it for too long. It will ensure that the steel doesn’t shatter like glass and avoid brittleness.

3. Cool it With Water

Pick the blue-hot steel with tongs and immerse immediately in a heavy duty bucket filled with clean water. Allow the steel to cool in room temperature water.

Your hardened steel is ready. The outside layer of your steel is now at least 40 percent harder than it was in the previous state.

Person handling steel with sparks and protective equipment

Final Words

It was fun playing with red-hot metal and oil. Now, if you need to harden any metal object, can know what to do.

The size of the metal will influence the number of materials you will need to harden it.  You might save some time for hardening the steel if you have already removed its plating or have already removed its coating.

Make sure no kid is playing near your working area. And do not underestimate wearing hand gloves and eyewear when you work.

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