http://americanbladesmith.com/index.php?section=pages&id=176

heat treat section.

I've tried forge tempering on pieces to long for my oven, and had poor results. The uneven heat resulted in cracked blades on all occasions. I am using a gas forge and doing clayed blades (they have a high residual stress from the sori), so others fully hardening may have had better luck. Personally I can't get my forge to run at less than around a 1000f so it seems like it would be real touchy. Most of the information I have gathered is that the blade benefits most from long soaks, either two or three. I know many people use a torch, I haven't really tried it. If I were, I would rig up something like Wayne Goddard suggests in $50 Knife Shop. He uses a slotted piece of brass that he brings up to temp and then puts the spine of the blade in it. Personally I would be concerned that there is no way to accurately gauge the temperature(other than oxidation colors wich can vary given a number of factors) and no way to keep it at temp for any real time. All that I have read suggests two or three cycles of two hours each.

Personally I use a commercial convection oven at work, it hold within around 5deg. As soon as I can afford it I will switch to a PID controlled low temp salt pot( I have heard of people using peanut oil as well but it seems also close to the smoking point for me).

I am curious why you would want to switch from an oven to a forge or torch?

Gentlemen, I am probably not the only one who will wonder if there isn't some confusion concerning the terminolgy used in this thread. Are we discussing tempering, hardening or both?

When it comes to H/T'ing or tempering, I am a firm believer that you can't be too accurate as it is the most important part of making a quality blade. If a torch is the most accurate system that you have at your disposal then by all means use it. But in this business, consistancy is critical. There is an old adage "A reputation is a long time coming and a short time going." which applies closely to our business and I would recommend other more controlable means if possible.

Gary

To be clear, I was talking specifically about tempering, and tempering only.

Gary, I certainly agree--when I have a choice I will absolutely go for the more reliable, accurate option.

The reason I ask is that I've been hearing about competitions--make a finished knife in two hours. Well, most tempering protocols I know of take at least two hours, so I'm trying to get my head around tempering with the flame from a forge or a torch.

Conceptually, I can get my head around it no problem, and I know how I /think/ I'd go about doing it. I just wanted to throw it out to the community and see whether anybody out there happened to be doing it that way, for whatever reason.

Kevin, I would obviously love to hear what you have to say on the matter.

Thanks,

Zack

Zack, I have done it when I need to. Like at a demonstration where I have a limited time. The main steels I use for this are 5160 and 1084. You can use a propane torch or oxy/acetylene. I mainly use a propane torch. It is slower than a O/A but I think I have more control with it. I start at the ricasso and slowly move down the spine towards th tip. You have to be in good light and really pay attention, as you are watching for a straw color. Also have a water bucket handy just in case the color starts to get more than you want. I also do this twice and clean the blade between. Of course I prefer to use my oven as it is much more consistent.

I have not tried tempering with the forge, that could be real tricky. I have seen a person temper with a heated bar of steel that the blade was placed on, spine down.

Brion

In all heating processes metallurgy has an old saying that is oft repeated, time = temperature. But the proportions for equivalency are seldom discussed, and just a little bit of temperature equals a whole lot of time. A really thorough tempering does require a very tightly held temperature over a long span of time, a torch or forge can make a lot of changes in a much shorter time, but in a much less ideal way. Cranking an oven to 600F can bake a cake in just a few minutes, but you will probably notice a difference when you eat it, steel is a little more subtle but the differences are also there.

Many knifemakers have discovered the immeasurable benefits of “control’, so much so that many others have grown weary of hearing about “control”, but here is one of the best examples of control. An O/A torch burns in excess of 5,000F, a smith has to determine the precise amount of seconds (or fraction of seconds) it takes for steel to borrow exactly the miniscule proportion of that heat required for the given heat treatment. And that must be done continuously with a ½” diameter 5,400F circle over the entire length of the metal; human abilities only go so far. Now add to this that diffusion requires time, but more than a second of exposure to that kind of heat sort of kills that idea, so the smith will need to increase that required temperature quite a bit to compensate for that lack of time, so we also need to recalculate how much of that 5,400F we now need to borrow. The forge is a little better in that it is just a modest 2,000F+ in comparison but it will heat things much more evenly. But, on the other hand, it will heat things more evenly, so the smith will now have the blade shape working against him in tempering, with the edge overheating very quickly while the spine is still coming to temp. To me it is like brain surgery with a chainsaw, sure it would be very impressive if there is a guy good enough to do it, but I would definitely choose another doctor for myself and my family.

Now compare these former scenarios to a well regulated oven or forge with a thermocouple. It can be set so that nothing inside gets any hotter than the desired temperature, sure the edge may heat sooner but it will stop at the target temperature and stay there until the spine joins it. With this simple luxury the smith now has the ability to hold the entire piece of steel at temperature for any amount of time to accomplish an even and thorough heat treatment. By eliminating all of the chaos of unregulated temperature you now only have the variable of time to monitor, and you now have minutes or hours to avoid any mistakes rather than fractions of a second, that is what control is all about.

Now what would be the difference between high heat/short time and longer time/lower heat? Once again, the level of even and thorough results. Several things happen in tempering that the human eye, and even an optical microscope, can’t see. First there is a shift in the atomic stacking to a more stable configuration, trust me we want this as even and thorough as possible. Next there will be precipitation of secondary carbides that are super fine at normal tempering temperatures, overheating will get you softer steel but may not get you the same tempering carbides. You can get 61HRC by fully hardening a blade and then drawing it back to exactly that, or you can slow the quench to the point that the blade only ever reaches 61HRC to begin with. If all you focus on is the Rockwell, either one will get you there, but it is up to you which one you want. The same pretty much applies with tempering in one minute versus 2 hours. As always with these considerations- yes the ancient smiths did it by just drawing a blade trough a charcoal fire, but they also had steel that was just iron and carbon, very few of us work with that steel. These days even our simplest alloys are considerably different from that and have different time requirements.

As for demonstrations and other venues, absolutely, you do what you have to do to meet time constraints, but what you do at home when you have the time to make your best piece may be a little different. There are ways to make the most of things when faced with these time limitations. As already mentioned, using a large block of steel to more controllably bleed the heat into the spine is one, using a shallow pan of water to protect the edge is another. I have also seen “tempering tongs” that had large blocks of steel for jaws, which you heated and then selectively applied to the blade were you wanted the heat.

Thanks Kevin,

I certainly agree that, where slower and more reliable methods are available, there would be no good reason to temper by torch or forge. My interest extends only so far as unusual situations--like demonstrations, etc.--where those methods are not available.

Zack

I was going to answer.....but everything I had to say would basically be repeating Kevin, so all I can add is "DITTO" to Kevin's post!

Kevin's explanation is about as good as I have ever seen. It ought to be copied and pasted somewhere for all of us to see. I have tried to verbalize the same thing but not near as pretty.

I do emphasize "control". The more control you have over the heat and it's application, the better your steel will turn out.

|quoted:

Kevin's explanation is about as good as I have ever seen. It ought to be copied and pasted somewhere for all of us to see. I have tried to verbalize the same thing but not near as pretty.

I do emphasize "control". The more control you have over the heat and it's application, the better your steel will turn out.

All that being said, a couple weeks ago at the smelt I held at my place it came time to heat treat the blade we made from rocks and sand over the weekend and my high temp salt pot controller had died! So since we had REAL old fashioned steel, I decided to do things the old fashioned way and fired up my forge. The problem is that we are spoiled with our modern steels as far as what they will tolerate in heat, with no alloying or other elements to help control grain size you need to really keep on your toes with the heat, and soaking may not be your friend. I get to do things by eye at demos and hammer-ins all the time and it is good to keep that skill exercised so I didn't mind, but I started out with a thermocouple in the forge and all that did was keep things too dim and uneven to do the large blade I was working with, so I removed it and simply worked through the phase changes by eye. It was as if the two technology levels were just getting in each others way, and when I worked that steel the way they used to work that steel it responded. It is just one more very stark example that I have encountered that reminds me that their steel and the steel we work with today are two entirely different beasts.