Temper line

[Don G]

I know what a temper line looks like but I don't know how it is created. Would someone please explain? Are there any advantages or disadvantages to having one? Thanks!

 

[Chiro75]

My understanding of a temper line is that it is created on differentially heat-treated blades because the crystalline structure of the steel is different in hardened areas (along the edge) and soft areas (the spine and handle). If a knife has complex chemical structure the ability to differentially heat treat and therefore, see a temper line, is reduced. Therefore your simple carbon steels like 5160, 1095, 52100 are your best bets for temper lines, and even then only if they've been differentially treated for that soft spine/hard edge effect. Once a blade has the differential temper, it usually requires a little more work to see it well. A high polish helps, and etching the blade is the best way to reveal a strong temper line. The acid will discolor the softer metal more, so you are left with a lighter edge. Etching brings out a temper line pretty obviously. As far as advantages/disadvantages, that is debatable. The concept of differential hardening is for work knives, chopping tools, and the like. The idea is that the edge will be hard, giving it the ability to be sharpened nicely and have a good, strong edge. The rest is left softer so there is some give in the blade, making it tough in the sense that it can absorb shock and will not be brittle, crack, etc. With special knives such as khukuris, they are traditionally heat treated like this to resist impacts since they are mainly for chopping. This also transmits less shock down the tang and into the handle, so it can be used longer and more comfortably. That's the theory, at least.
There are a variety of ways to achieve differential heat treating, and thus temper lines. The Japanese use a clay-like medium that is layered on to insulate the areas they want softer. The areas covered in "clay" hold the heat longer than the bare areas so they turn out softer than the edge. You can get a nice pattern and shape to the temper line this way, too. Others will harden the entire blade and then use a torch to draw a temper line back from the spine. The heating will soften the drawn area more than the edge. Another method is to quench the blade in the quenching medium by putting the edge only in, causing it to cool faster and be harder than the spine, which is allowed to air-cool. Still another method, used by the kamis of Nepal on khukuris, is to pour water from a kettle along the edge to cool it faster.

------------------
My Custom Kydex Sheath page:
http://www.geocities.com/CollegePark/Lab/1298/knifehome.html
Palmer College of Chiropractic
On Two Wheels
agocs_s@dd.palmer.edu



[This message has been edited by Chiro75 (edited 06 October 1999).]

 

[Osbourn]

The clearest and most dramatic temper lines that I've seen have been from the japanese method, and the khukuris are next. When you harden the whole blade, then draw down the spine, the line is somewhat blurred and often almost invisible, even with etching.

------------------
Oz

"When your work speaks for itself, don't interrupt."
- Henry J. Kaiser