Tools and Process: Heat Treating


Welcome to Tools and Process. A new series of short videos in which I will
talk about the tools I use and my process for accomplishing certain goals. Today, I will cover some aspects of heat treating. Heat treating is the process of hardening
carbon steel to a specific hardness. It requires heat and controlled cooling of
the steel. My heat source for long swords is an Evenheat
KF 45 electric kiln. For smaller knives, I normally use the forge
to bring the steel up to critical temperature. As preparation I remove all vertical grinder
marks from swords by draw filing. Another important tip is that the sword needs
to be on its edge instead of lying on the floor. Putting the sword on the floor of the kiln
leads to uneven heating and warping of the blade. Since my high carbon steel is 1075, I program
that Rampmaster to bring the temperature to 1500F (815C) and hold it there for 6 minutes. To find the critical temperature of high-carbon
steels, you need to refer to a table that you can often find on the Internet. Curiously when searching for 1075, you will
find at least 4 different temperature and 3 of them are completely wrong. The first step in my heat treating regime
is to normalize the blade. Since steel is in crystal form, the repeated
heating in the forge causes the steel to form larger and larger grains that make it brittle. Slowly cooling it down from its critical temperature
reduces the size of these grains. This is also a good opportunity to check for
warping which may occur when the bevels have been ground unevenly. I was also hoping to show you some recalecence
which happens right when the carbon in the iron matrix transitions from austenite to
ferrite. That transition releases visible heat. You have to check my spear video where you
can see that clearly. Here, I unfortunately stepped into the video. The next step after normalizing is hardening
the blade. Steel can only be hardened when it has enough
carbon content. Generally, the more carbon the harder you
can make the steel. As I had mentioned before, the steel has a
crystal form and the position of the carbon in the iron matrix determines the hardness
of the steel. There are three carbon arrangements you should
be aware of: ferrite which is soft, martensite which is hard and austenite which is the carbon
arrangement when the steel is at critical temperature. When working with pattern-welded steel, the
pattern becomes visible under heat due to color differences in oxidization. As you can see for the sword here. Now, it’s time to heat the blade back up to
critical temperature and then quench it quickly in a high speed quenching oil. The oil will cool the blade down rapidly and
cause the austenite to convert to the hard marentsite. When quenching in oil, I alward wear full
face protection. If the oil gets too hot, it can rapidly ignite
and flames can spurt up into your face. Before quenching the blade, I quickly check
it with a magnet to verify that it is no longer magnetic. When I insert the blade I also move it up
and down vertically to prevent a rapid transition of hardness in the tang. Coming back to the quenching oil, I am using
Parks #50 which has a high flash point and my quenching tank is also large enough so
that the whole tank does not become too hot. Other oils with lower flash points are much
more dangerous to use. So be careful. After quenching the blade is brittle but the
pattern so much more visible. If the blade has been ground unevenly, heated
unevenly or quenched unevenly, it may develop a warp and as a result may no longer be straight. However, I will show you a trick that you
can use during tempering to straighten blades. Before we go there, let’s quickly check the
hardness with a file. On the hard part of the blade, the file will
skid and on the unhardened parts it wil bite.