Wednesday, October 21, 2009

Surface Hardening is Cool

I work heat treating steels. What does this mean? I try to make steels last longer in their application by heating them up and cooling them down. That is my work in a nutshell. A small nutshell.

One way to make steels last longer is to surface harden them. There are several processes the most common being carburizing followed by nitriding. For aluminum anodizing is a surface hardening treatment. The basic process of carburizing is to diffuse carbon into the first millimeter or two of the steel. The carbon diffuses in the iron lattice interstitially and creates a compressive stress on the lattice and thus is harder than the interior of the material. So the very outer layer of the steel is hard and the interior is soft. This is used on everything from gears to bearings to watch casings.

The first time this process was applied commercially was to samurai swords hundreds of years ago. They would diffuse more carbon into the blade of the sword. The blade would be hard and the back would be softer. This allows the blade to bend a little bit when it cuts something. If the whole sword was hard it would shatter if it tried to cut something very hard. If it was all soft the blade would become dull very quickly. The balance of hard and soft steel allows a sharp and resilient sword.

We've come a long way since samurai swords. Probably over 100 parts in your car were surface hardened. The best part is that it is a carbon negative process, kind of. You take carbon, usually from some atmosphere like methane, and diffuse it into the steel where it stays indefinitely.


  1. There's an episode of Nova ( that talked about this process. Very interesting stuff.

  2. The History Channel did a segment on swords and described the process and since this is what my master's thesis is on I looked into it a little more. It's amazing what people discovered that is "basic" science for us today.