How to Heat Treat Mild Steel

This guide on how to heat treat mild steel is for professionals who are qualified to heat treat metallic low carbon steel parts. And for people who wish to understand the science and process of how to heat treat mild steel. This process includes 3 stages called carburising, hardening and tempering.

Equipment Needed

Mild Steel Part

This process is for low carbon steel only, also known as mild steel.

Carburising Box

This thick steel box is used during the carburising stage.

Carburising Pellets

These carbon pellets are used during the carburising stage. Wear gloves when handling these pellets.

High Temperature Furnace

Your furnace must be able to reach maximum temperatures of around 950°C.

Oil Quench Tank

An oil quench tank is a steel container filled with oil.

Long Metal Tongs

Used to transport your hot part from the furnace to the oil quench tank.

SAFETY PPE REQUIRED

Face Shield

Welding Overalls

Gauntlets

Welding Gloves

Temperature Resistant Safety Boots

STEP 1:  PACK CARBURISING THE STEEL

• Wear full PPE when handling hot parts!
• Fill a carburising box with charcoal pellets, just under halfway. 
• Put your mild steel into the carburising box.  Ensure your steel is in the centre of this box.
• Make sure the charcoal pellets surround your steel.
• Place this box into the middle furnace.
• Set temperature to 940°C.
• Switch the oven off after 6 hours.  Then allow to air cool for 3 hours with the furnace door slightly open.  Place a warning sign on the door to alert people of the hot part inside.
• Remove your steel sample from the box, once fully cooled. 
• Throw away the pellets that are inside the carburising box.    

What is Carburizing?

This is the process of adding more carbon to the surface of low carbon steel. It works by heating the low carbon steel to an ideal temperature range of 930 to 955°C, whereby carbon then seeps into the surface of steel. Imagine the atoms at the surface of the steel getting more excited and more spread apart, carbon atoms then have the opportunity to enter the steel structure. This is known as diffusion. There are other methods to carburise steel too, such as vacuum, gas, plasma and liquid bath carburising.  This procedure shows you how to carry out Pack Carburising.

STEP 2:  HARDENING THE STEEL

• Wear full PPE when handling hot parts!
• Place you steel part in the middle furnace.
• Set temperature to 850°C.
• Quench your part in oil, 15 mins after it reaches 850°C.  You must use long tongs to transport steel parts at this temperate.  Do not try and transport using your gloves.  Be careful not to rush the process!   

What is Steel Hardening?

To harden steel, heat the part to a bright red.  Allow the heat to soak in for a few minutes.  Then quench it. Why does this harden the steel I hear you ask? Well, when you heat steel up to a certain temperature range its atomic structure changes from:

Body Centre Cubic (BCC)

to

Face Cantered Cubic (FCC), known as austinite

In order for steel to stay in the FCC state, the temperature must be suddenly reduced by quenching it in oil. Quenching basically freezes the structure in the FCC state, a structure that is much harden then BCC. 

Ideally, you should use oil to quench your steel after heat treat. You can use water, but you risk warping or cracking during the quench.

You can easily test the hardness of your part after by using a file. Your file should slide off it without cutting. Please be aware that you part is probably too hard to be used as a useful tool or component. Therefore, you must carry out the following tempering stage.

STEP 3:  TEMPERING THE STEEL

• Wear full PPE when handling hot parts!
• Place the steel in the middle furnace at 350°C.
• Once the temperature has reached 350°C, allow to air cool.
• Your part is now ready to use.  Or to be ground via a surface grinder.

What is Tempering?

Your freshly hardened part will be brittle. If you were to use it in this state, chips or shattering could occur.  Tempering is where you heat the steel after the hardening and quench process.  Tempering takes away the brittleness. It makes the metal tougher, but softer. It does this because carbon atoms that are trapped within the iron structure become dislodged, allowing strain to be reduced.

The higher the temperature reached during tempering, the softer and less brittle the steel will become. Each steel part has its own optimum compromise between hardness and toughness.