How to harden steel?

What Are The Treatments?

Hardening and tempering of engineering steels is performed to provide components with mechanical properties suitable for their intended service. Melbourne steel Fabrication are heated to their appropriate hardening temperature {usually between 800-900°C), held at temperature, then “quenched” (rapidly cooled), often in oil or water. This is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses. The actual conditions used for all three steps are determined by steel composition, component size and the properties required.


Hardening and tempering can be carried out in “open” furnaces (in air or combustion products), or in a protective environment (gaseous atmosphere, molten salt or vacuum) if a surface free from scale and decarburisation (carbon loss) is required (“neutral hardening”, also referred to as “clean hardening”).


Two specialised quenching options can be applied in special circumstances:


  1. Martempering (also known as “marquenching”) uses an elevated-temperature quench (in molten salt or hot oil) which can substantially reduce component distortion. This process is limited to selected alloy-containing steels and suitable section sizes.


  1. Austempering can be applied to thin sections of certain medium- or high-carbon steels or to alloy-containing steels of thicker section. It requires a high temperature quench and hold, usually in molten salt, and results in low distortion combined with a tough structure that requires no tempering. It is widely used for small springs and pressings.


What Are The Benefits?

Hardening and tempering develops the optimum combination of hardness, strength and toughness in an engineering steel and offers the component designer a route to savings in weight and material. Components can be machined or formed in a soft state and then hardened and tempered to a high level of mechanical properties.


Hardening from open furnaces is often employed for products such as bars and forgings that are to be fully machined into components afterwards. Neutrally clean hardening is applied to components that require surface integrity to be maintained; examples include nuts, bolts, springs, bearings and many automotive parts. Neutral clean hardening is carried out under tightly-controlled conditions to produce a precision component needing the minimum of final finishing.


What Sort of Steels Can Be Treated?

Almost all engineering steels containing over 0.3% carbon will respond to hardening and tempering. 8S970 and 8S EN 10083-1 and -2 (which have superseded parts of BS 970) list the majority of hardenable steels used for engineering components. (A number of other standards include hardenable steels for special applications; e.g. “S” aircraft standards, BS3111 for fasteners and BS5770 for springs)


Tool steel

Turning tools are now rarely manufactured from carbon steel, but it is still used for many other purposes. Even for turning tools, it still works as well as it did when the old-time turners made their living with it. You can buy high carbon steel in various grades from steel stockholders, some suitable for hardening in water and others in oil.


Low carbon mild steel

Ordinary low-carbon mild steel is not suitable for most cutting tools. You cannot harden steel of that kind, and it will not keep a sharp edge. To harden steel, the metal must have a high enough carbon content. The metal is harder than wood, and a cutting edge on mild steel may last long enough for a one-off job. But if the edge is thin it will just give way under pressure. You can case-harden mild steel. 


Hot forging


If you want to do any hot forging, you will need something to use as an anvil, a hammer, vise and heavy pliers. You may also need a hacksaw, angle grinder, file, bench grinder and a drill press. To work thick steel, you will need heavier tools and a forge.  If it’s hot enough, it’s surprisingly easy to bend steel using a vise or wrenches, or to hammer it into shape on an anvil. You can forge ordinary mild steel in the same way.



Some scrap carbon steel is too hard for sawing or filing into shape. You will have to anneal it to make it softer and workable. To do this, the first step is to make the metal red hot. While it is hot, you can forge it. Don’t try to work the metal when it has lost its red heat or it may crack. You have to strike while the iron is hot. After any forging, get the steel red hot again and then anneal it by cooling it slowly. 


Harden the steel

To harden steel, heat the part to be hardened bright red hot again, if possible ‘soak’ it in the heat for a bit, then quench it. It’s the rapid change from red hot to cold that will harden steel. You can use various quenching liquids, but a bucket of water will usually do the trick. Plunge the red hot metal straight in, and swirl it about to cool it as rapidly as possible. If the steel warps or cracks when quenched, try using oil instead of water, or use different steel. If using oil, fire precautions are necessary. Use a metal container, not a plastic bucket. The hot steel will heat and ignite the oil, so have a metal cover handy to extinguish the flames.



The freshly hardened tool will be brittle. If you were to use it in this state, the edge could chip or it could shatter. For most purposes, you must heat it one more time to temper it before use. Tempering takes away the brittleness. It makes the metal tougher, but softer. The higher the temperature reached during tempering, the softer and less brittle the steel will become. Each tool has its own optimum compromise between hardness and toughness.


How to Flame Harden Steel


Flame-hardening steel involves heating the steel and then cooling it. This first part of the process changes the molecular structure of the steel and makes it hard, but brittle. If dropped or struck hard, it could actually shatter. The second part of the process, known as annealing, involves reheating the steel and re-cooling it. Once this second part of the process is complete, the steel is hardened and yet malleable enough that it can still be worked.


Using your blow torch or a furnace with a bellows, heat your steel until it glows red-hot. You will need to watch your steel carefully as it goes through several different color changes until it passes blue-hot and finally becomes red-hot.


Using your tongs, pick up the red-hot steel and immediately dunk it into a vat of room-temperature water. This is called quenching. Once you remove the steel from the quenching water, do not try to work it. Your steel will now be hard but extremely brittle and could shatter, almost like glass.


Reheat your steel using your torch or furnace with bellows. Watch the color changes as your steel gets hotter and hotter. When your steel is blue-hot, pick it up with your tongs and dump it into the vat of water.