A Good Cut – 3

How Metallurgy and Heat Treatment Make the Blade

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy. ~ Shakespeare (Hamlet)

The same could easily be said of types of steel and methods of heat treatment.

There is no one best steel. Not all steels are appropriate for knife making – and of those that are, there are many good steels that would be destroyed by hand forging – these steels are for stock removal only.

And even if today there were one best steel, new steels are fabricated every year. For a revealing list of blade steels, look at Gator’s excellent interactive chart (ZKnives.com). Which brings us to:

Elements of Steel

Steel is simply Iron (Fe) with a small amount of Carbon (C) added. Not much carbon. Even “high carbon” steel has less than 2% carbon. Knifemaking steel can have a lot of other trace elements in it as well. Silicon, Manganese, Molybdenum, Vanadium, Phosphorus, Tungsten, Sulfur, and Chromium… the list goes on. A single element can have many effects on the steel. Some affect how the steel hardens, or how tough it is, or how wear-resistant it is.

Chromium is worth a special mention. When you get up around 14% Chromium you get into classic “stainless” steel. Some folks turn up their nose at stainless because the early stainless steels were pretty soft and would not keep an edge. That has changed. There are modern stainless steels that (if properly heat treated) hold an excellent edge. Heat treating stainless is an exacting science, best left to the professionals in that art.

Hard or Tough?

OK tough guy – are you hard enough to pass the test (or vice versa)?

That is essentially the question that the American Bladesmith Association (ABS) asks applicants for it’s Journeyman Stamp in the “Performance Test.”  The performance test requires a blade to be hard enough to hold a hair-shaving edge after chopping wood – while still being tough enough to take a 90° bend without cracking.

Hardness is the resistance to deformation. If the edge of a knife is hard it will not fold over, and it will generally retain its edge until it wears down or chips.

Toughness is the ability to sustain a blow or a bend without breaking. If a knife blade is tough it will not break easily.

There is a trade-off in steel between hardness and toughness. If steel is hard then it has a tendency to be brittle. If it is tough then it tends to be ductile, not hard. However, skilled heat treatment with appropriate steel can produce a blade that embodies both hardness at the edge and toughness in the blade as a whole.

Rockwell

The Rockwell scale (Rc) is used to measure the hardness of steel. A Rockwell in the high 50s an into the 60s will give you a hard edge. An edge having a Rockwell under 55 will tend to roll over before it will chip. Which is better? For a machete or an axe, a Rockwell in the mid 50s is good. A machete might even be in the upper 40s. For a camp knife, mid 50s to low 60s depending on the steel and the expected usage. For a chef’s knife, upper 50s to 60s.

But Rockwell is not the whole story of a knife that “holds its edge.” A lot of work has been done with grain structure and exotic carbides so that a stainless steel like S30VN can keep a nice, sharp edge at an upper-50s Rockwell due to the small, uniform grain and the presence of Vanadium carbides.

The Short Version

The link at the top of this page to Gator’s (ZKnives.com) steel charts gives an idea of the array of steels available. A great knife can be made from any decent steel if the proper heat treatments are used for that steel. It is easier to make a bad knife by messing up the heat treatment than from choosing a poor steel.

There are many steels that are excellent for stock removal knives, but would be ruined by the process of forging. But that leaves a wealth of steels – including a few stainless steels – that are good forging steels.

For my top-level view of what’s involved in heat treating steel, see my “Steel Phase Basics”.