Q&A for How to Harden Steel

Heat the area you'd like to drill just until it starts to glow red, then allow it to cool slowly. This answer was tested with a utility knife blade and it drilled through with ease.

This is not advisable. Hardening will cause it to be more brittle, which could cause the hammer to chip and cause injury.

No. The tempering reduces the amount of martensite and thus makes the steel softer. Heating it again to a lower temperature will not introduce additional martensite. The only way to do that is to heat it until the metal de-magnetises, quench it again, and then temper to the desired hardness.

0.25 percent. A36 is a low carbon or mild steel, and as such cannot be hardened. It can be case-hardened, however, which just means using a chemical treatment with heat to add a thin layer of tough material around the soft steel core.

Just use the file as is. It's already hard and tough. Recycled old files make great knives and shop tools, like three corner scrapers for deburring. You can buy file "blanks" - files formed into shape but before the teeth have been cut - that are easier to start from than an old file where you have to grind the teeth off before shaping the knife.

That is normalizing the steel, which is mostly done when you have a high carbon steel in order to make it easier to shape or form. Already hardened steel can be quite a pain to work.

There is no perfect oil for quenching. Used motor oil or vegetable oil works fine. Heat the oil by dipping a hot piece of metal in it; this helps the quenching process.

Yes. Watch "How It's Made: Screwdrivers". They use induction to heat only the tip.

No chemical can be added to water to convert mild steel to carbon steel. Your question is like asking "what should I add to the cream to convert my banana sundae into a strawberry sundae?"

You could use almost any oil you like. Just check for additives in the oil before quenching, because modern fluids contain some additives that may give undesired results or can be hazardous if they reach a certain temperature.

No. There is a reason they are soft. Hard and brittle make excellent projectiles.

No external chemical will harden steel throughout. You either need to use a different alloy or harden and temper it as the above method describes. Quenching in oil can add some extra carbon to the steel alloy and harden the surface, but this is only useful if what you're looking to harden is a fine cutting edge. Excessive carbon or hardness can make the metal more brittle though, so be sure to find the right balance for your application.

It crystallizes the carbon in the steel making it harder but more brittle. It must be tempered to reduce the brittleness.

Using cold water to quench is possible, but it often results in the steel cracking and shattering. The water can shrink the molocules in the metal too fast, which leads to a blade becoming brittle. Oil is a much safer option.

Use medium-thin metal (any kind) and use peanut butter. (Yes it sounds silly, but it works.)

Primarily hardening is done to increase hardness, wear resistance and yield strength, while toughness and malleability are lost as the metal is further hardened.

Leave it until it is hardened and cool enough to handle.

Heating cherry red and quenching in oil works best for hardening. You can get your parts tested at bigger weld shops of foundries.

Do a spark test with a shop angle grinder. Look up a chart of photos of the colors and trajectory/spark descriptions of the different alloy and carbon contents. It's easy to do and great if you are reusing or salvaging old hunks of steel you come across on old machines and in construction.

Oils with a lower flash point and free of additives are best.

You shouldn't do this. Cooling the metal too quickly can make it brittle and cause it to shatter.

No. Cold oil is thicker and does not cool metal as quickly. The best temperature for the oil is around 130 degrees Fahrenheit. Heating the oil will make it thinner, heating the metal more quickly.

If they are made of steel, I would think so. Tungsten oxidizes rapidly above 1000 degrees Celsius (1832 F).

No. Graphite is not metal-carbon composite. Quenching applies to steel and some other metals. Graphite is flat structured carbon and there is no simple process (like quenching) to get it to harder crystal structure like diamond's tetrahedrons. This requires lots of pressure and very high temperatures. The process is known as HPHT and is used to produce tiny synthetic diamonds, or to improve imperfect natural diamonds.

The oil should be at room temperature.

As a rule of thumb, it's 100 degrees for every point of carbon. Preheat and slow cool if it's an alloy frame.

They are usually made from mild steel, so they don't have much carbon in them.

If it's music wire, just wrap it around a form, then put it into an oven at 400 degrees F for a half hour. Guitar strings make nice small springs.

Run a file along it and see if it cuts. Be careful to know the difference between a file skidding and cutting. You may think that it isn't hardened if it cuts, but this isn't 100% the case (though it is 95% of the time). You may just a have a much harder file than what is attainable with the steel you hardened.

That's what I would do. However it got bent, just do bend it in reverse to straighten it out.