By nothingbutnet, Jan 15 2018 10:06PM
Steel is a metal alloy of iron with a broad range of applications, including the manufacture of metal tools and use as a sturdy construction material. Many of these purposes require the steel to withstand significant wear and tear, sometimes requiring the material to be hardened and tempered in order to perform as intended. This process is typically performed by professionals in industrial facilities dedicated to the purpose, but you too can harden and temper steel with the correct tools and a smidgen of experience. Here's what you would have to do:
1. Start With The Right Steel
Contrary to what you may believe, not all steel can be hardened. Steel hardens when the carbon in its composition crystallizes, much like how graphite or coal turns into diamond after significant exposure to the heat and pressure beneath the earth's crust. This means that if your steel doesn't have enough carbon, there is nothing to crystallize and you cannot harden it in the manner described below.
In general, pure steel requires a carbon content of at least 0.6 percent to harden effectively. Steel meeting this criteria is commonly referred to as high-carbon steel or tool steel. In addition, steel alloys such as silver steel and gauge plate steel have enough carbon in them to effectively harden.
Medium carbon steels, with a carbon content in the range of 0.4-0.55 percent, may be hardened somewhat but must be alloyed with another material to really make an impact. Mild steels with less carbon than that cannot be hardened directly, but they can be coated with other materials to increase their hardness. This process is called case hardening.
2. Heat The Steel
First, slowly heat the entire piece of steel that you're trying to harden. Then, focus on the area of steel that you want to harden, such as the point of a screwdriver, until it begins to glow red hot.
3. Quench The Steel In A Fluid
This is the step where you dip the steel into a liquid to "quench" it, or cool it off nearly instantaneously. The liquid you use varies depending on the chemical composition of your steel. Oil is a popular option if you're not sure exactly what's in your steel, with vegetable oil, mineral oil, cottonseed oil, and whale oil all viable options. You can also use plain water, but oil cools the steel more slowly (meaning less cracking) while offering just as much hardening potential.
Most modern steels are considered complex steels, meaning that they are especially prone to cracking if quenched too quickly. If the metal you are hardening is of a modern vintage, you should definitely use oil.
You also need to ensure that bubbles do not form while you are quenching your steel. Bubbles can slow the cooling process while introducing undesirable soft spots on your metal, so try agitating your liquid of choice before they can form.
Salt water pops bubbles faster than fresh water does, making it one of the fastest quenching liquids you could use. However, its sodium content will cause your metal to corrode much faster than other options, so be sure to rinse it off immediately if you decide to use it.
Safety precautions should always be taken when using your quenching liquid. For instance, oils are prone to causing spatters, fumes, and spills that may quickly turn into fire hazards if left unattended.
Glycol polymers may be mixed into water to create a quenching liquid acting as a happy medium between the corrosive potential of water and the fire hazards of oil. How fast it cools your steel depends on the amount of glycol you mix into the solution. The ratio must be monitored closely to ensure consistent results, though.
Finally, cryogenic quenches may be used to eliminate the possibility of soft spots forming while simultaneously ensuring that your steel becomes as hard as possible. This is only an option for high carbon steels.
4. Clean The Steel
If you used something other than water to quench your steel, water may be used to rinse it off. Otherwise, an emery cloth or comparable light abrasive may be used to clean it.
5. A Second Heating
The second heating is not as intense as the first one and serves primarily to temper the steel against the brittleness created by the hardening process. The heating may take place in a furnace or chemical bath (oil, lead, and potassium nitrate are all suitable). Your steel will become softer but tougher the more you heat it in this stage.
Your steel will change color as it heats up, starting with light straw at around 200 degrees Celsius up to purple at 300 degrees Celsius. These colors serve as a useful guide for when to take your steel out, but remember that certain alloys must reach higher temperatures to achieve each color.
6. Cool The Steel
You can cool the steel quickly or slowly this time, but metals subject to becoming brittle after tempering should only be cooled slowly. The method you choose will impact the final properties of your steel.