By Matthew ParkinsonDownload
When making a knife a high or medium carbon steel is needed. This kind of alloy is also sometimes called “spring steel” or “tool steel” theses terms should be understood to just be classes of steel rather than a specific alloy. When working with these steels the higher the carbon content and the higher the alloying content the more sensitive the steel will be to correct temperature ranges. Some of these alloys can be red hard (a temperature range that the steel to hard to work) or red short (a temperature range that the steel is prone to cracking of crumbling) generally these problems are more common in high alloy steels, simple high carbon steels tend less to these problems but will develop large grain size if over heated. Large grain size weakens the steel and is detrimental to the cutting ability of the finished knife.
The way to avoid damaging the steel you are working with is to know what alloy you are working with. Look that alloy up on line, or in one of the many reference books. Find out what that alloy is prone to, (if it is red short or red hard) what the hardening and temper ranges are. (you will need this info later) with any of these alloys there are a few things that should be done. First Do not soak the steel in the forge for no reason, second do not heat the steel to a higher temperature than is necessary to work it, and third as you forge closer to finished shape work at progressively cooler temperatures. And finally normalize the steel before finishing the knife (filing grinding etc) to normalize heat the steel to past critical temp, the critical temp can be found by using a magnet to find the curire point,(the point that heated steel turns nonmagnetic) critical temp is for the most steels fifty to hundred deg. Higher than the curie point. Heat around 1600 degF (200 deg past Currie point) and let cool in still air to the recolessance point (around 900DegF), do this three times ( or cycles) this will reduce the grain size break down any carbides that might have formed, and soften the steel making the grinding/filing easier.
In the USA Steel alloys are graded two main systems the first is a numeric based system (SAE, AISI) in this system there are 4 or 5 digits that determine the alloy, the first two determine the alloy content and the last two or three the carbon content, theses are called points, 100 points equals 1 percent by weight of carbon so 1050 steel would be a simple carbon steel (10=simple carbon steel) with .50% carbon content. The minimum carbon content to make a good knife is about 40 points (.40%) and the maximum is around 1%.
The second grading system is the letter number system of tool steels, theses are specialty alloys that were developed for a purpose so with in one set of steels (O series for example) there can be a total change of alloys with similar fished or working properties. Some of the more common steels in this system are O1,W1,W2 ,L6,S7 and D2. Most of these steels can make vary good knives but some can also be very difficult to work.