Blacksmith Lessons: Choosing the Right Steel when Making a Knife
Many blacksmith students ask how to choose the steel to use for items, and many repeat advice they have gotten over time, such as car springs, lawnmower blades, etc. All of which is good advice as far as it goes, but there should be a bit more judgment in it than that,... in the example of car springs, they are no longer quenched in oil to retain the harder form, but in air, so modern springs are a harder composition, and therefore harder to work, and oil quenching can make them too hard and brittle. As for lawnmower blades, they are flexible to a point, and sufficient to hold a bit of an edge, but the overall quality isn't up to par for knife use, and many blades are getting thinner and thinner, also have hard places and even inserts that are different metals,... rebar, the iron or steel bar stock used to reinforce concrete, is questionable for the most part, as are railroad spikes, the spikes will form a knife, but the problem is even those marked on the head with a "C," which are higher carbon, and therefore steel, not just iron or mild steel can be terribly inconsistent, and really don't tend to hold a good edge. The exception in rebar is the "sucker rod" type that is used for work in pre-stressed concrete, meaning that the bars are held in tension before the concrete is poured. This rod is higher and more consistent in quality and carbon, so it will take an edge, a very fine one, and the end result is a better blade.
Another means of testing steel in the "spark test" where you take a power grinder to a bar stock and by examining the sparks and pattern, are able to tell the grade of the steel. This is a better test, but since we are practicing "primitive" work, we may not have access to either power or a grinder, and the best manner of judging the metal is from what use it was made previously. Files work, are good steel, but, as I stated in another article, require softening, as do springs, and the manner of doing so is to heat the entire piece to a decent cherry red, and then let it cool slowly, so as to allow the crystalline structure to regain it's stable, normalized (softer) state. This should be done three times before working on the item to shape it, as I have stated before, so as to ensure proper softness.
The best judgment you can make is by what the tool was before, most files today are of a consistent quality and can be used, but another ready, and easily determined tool is the drop-forged wrench, such as the one shown in the picture, the "wrench-Bowie" was, in fact, a two-ended wrench that had one end broken. The broken end was heated, stretched, flattened and formed a great shape for a knife, which was then quenched in oil along the edge only (differential tempering) to harden the edge, but retain some softness in the back and handle. Look for older heavy steel wrenches, and tools that were intended for flexible, durable use, such as jack-hammer tools (VERY hard to work) and also something nearly everyone has, which is lug wrenches, the quality and toughness of these tools is really at a higher level than any rebar or railroad spike, and much easier to come by, also, they do not have the brittleness and possibility of stress cracks that files do.
The main things to look for is age, pre-1940 if possible, as many tools were oil-quench, examine the edges and ends of the tool, if it shows deformations in the edges or twists at the tip, don't use it, because you cannot judge (short of a spark test) the quality of the metal. Look for clean, sharp edges, defined, undented work surfaces, and if there is a fracture, look for small, regular grain strcture, rather than large, irregular grain.