An Intro to Heat Treating, Part VI

Differential Heat Treating

As mentioned way back in the first article of this series, there are two basic qualities of steel. Hardness/Brittleness, Toughness/Softness. A harder steel will be less tough, and a tough steel will be less hard (capable of holding an edge). 

But what if you could get the best of both worlds? This is the thinking behind differential heat treatment.

If the edge is harder than the spine, this should increase performance a good deal.

However, a disclaimer is required. Tempering is still very much necessary even with a very tough spine. Most fully hardened steels are too brittle to hold up as an edge; no, they will not bend and dull, but it can chip. Tempering (though possibly at a lower temperature than normal) is still required. 

 There are two main ways to do a differential heat treat. One is in hardening, the other in tempering. 

Tempering is fairly simple, and I touched on this in last week's article. Differential tempering is done by heating the spine, while leaving the edge cool. Usually this is done with a torch, carefully heating the spine to a nice blue color or so (around 600 Fahrenheit). It's risky to do this in air however, as the heat can very easily creep down towards the cutting edge, ruining the temper. 

The other way is by differential hardening. There are several ways to do this. Now, the basic principle is this: if a steel must be cooled from a certain temperature in a certain timeframe, it will be hardened. To keep a portion of steel from hardening, you simply need to either slow down the cooling, or not get the steel hot enough. 

So, firstly, we can heat only the edge to critical temperature. This can be done with something like an oxyacetylene torch; heat up the edge, test with a magnet, heat some more, and quench. 

A second method would be to do an edge quench. This is done by heating the entire blade, and only holding the edge of the steel in the quenchant. With experience, this can be done with great results, but beware too much heat creeping down from the spine. 

A third method would be to quench the entire blade, keeping in mind cooling rates, temperature, and mass of the blade, and remove the blade quickly enough. With enough mass on the spine, and little enough thickness on the edge, it is possible to use timing and calculation to produce an "autohamon"; where the edge is hardened and the spine remains soft.

The fourth and most famous method is to use an insulating clay in a "clay quench". This method, used in traditional Japanese bladesmithing, is to coat the spine in an insulating clay, heat up the blade, and quench. The edge hardens normally while the clay slows the cooling of the spine. This leaves the spine soft while the edge is hard. With careful controlling of temperature and especially the clay, absolutely insane hamons can be produced.

Hamon 刃文

When a blade is differentially hardened, there will be a section of soft steel and a section of hardened steel. If the steel is properly polished and/or etched, the border between the hard and soft steel while become visible as a line of varying appearance. This visible line is called a "hamon", and is highly prized. It can appear either as a dark line separating a gray section from a silver, or it itself can be a ghostly white flame flitting through the steel. Collectors of Japanese swords deal quite extensively in the shapes and characteristics of hamons.

Bringing out a hamon again is based on a fairly simple principle: one area of the steel is hard, another is soft.
There are two standard ways then, to bring out a hamon. The first is the traditional Japanese method: polishing. Learning to do this perfectly takes years and many expensive natural abrasive stones, but it can be replicated by hand to a lesser extent. Exact methods are beyond the scope of this article. However, the idea is using various abrasive stones to rub away at the surface of the steel: the softer steel will wear off at a faster rate than the harder, and proper application can bring out some very stark contrast. 

The second method is by acid etching: acid eats away at the softer steel at a different rate than the harder, and a different rate at the border line itself (hamon). The ideal acid is diluted ferric chloride (circuit board etchant), but boiling vinegar or even lemon juice will work. Everyone has slightly different steps in etching; a black oxide layer forms on the surface, and it must be cleaned of periodically throughout the etching process. The surface finish pre-etching, longer etching, more or less frequent cleaning, acid concentration, and cleaning type will all have some effect on the appearance of the hamon. 

Deep-hardening and Shallow-hardening steels

In the world of differential hardening, there's an important factor to keep in mind: steel. Basic carbon and tool steels can be divided (so much dividing in this article!) into shallow and deep hardening steels. The difference is based on required cooling rates. 

For example, we take a made-up steel, Steel D (D for Deep hardening), in order to harden, must cool from 1600 F to 400 F in five seconds. If it takes six seconds to cool this far, the steel will not harden. However, five seconds is quite a long time. A plain blade in oil can cool this entire rate in three seconds. So if we apply clay, which slows the cooling of the spine by one second, we still have a blade completely cooled in four seconds. Thus, the clay has no effect. The blade has hardened through and through. 

Steel S, (for Shallow hardening) takes three seconds at the same temperatures to cool. If we retard the cooling by one second using clay, the surface underneath the clay takes four seconds, exceeding the upper limit required for hardening. And so, this area remains soft. A thicker chunk of shallow hardening steel, even if quenched without clay, may differentially harden due to the extra mass and residual heat, or the surface may even harden while the core remains soft. This last point is what is referred to when we say deep or shallow hardening.

Whether a steel is deep or shallow hardening depends on its chemical content. As a general rule, the more complicated the makeup, the deeper hardening the steel is. Some elements may extend or shorten the time required to harden. This is why one steel, such as W2, more readily takes a hamon, while another, such as 1084, is a little tougher to selectively harden. 

So there you have it, the basics of differentially heat treating. This process can be broken down into two main branches: differential tempering, done usually by heating the spine post hardening, and differential hardening. Differential hardening in turn can be broken down into either selectively heated edge quenchingmono-temperature edge quenching, creating an "autohamon", and clay quenching
The Hamon is the visual effect of a differentially hardened blade, brought out either by polishing, acid etching, or a combination of the two. 
The type of steel will affect whether it is possible to selectively harden a blade, given the same processes. Some steels are shallow hardening, requiring a short window of time to harden, and some are deep hardening, allowing for a longer window of cooling rate to harden.

This is the sixth and second to final article on basic heat treating; next week I'll do a basic run-through of the subjects we've covered as well as some anomalies that didn't fit into the subject titles we've named so far.