Question about Oil Quenching...

[JohnKSa]

Yeah, yeah, this is a question generated by Forged in Fire. You can run through your list of standard epithets, then take a couple of deep breaths. Now that you're over it, here's the question.

I have noticed that it seems some of the competitors are in a big hurry to get their blades out of the oil. To the point that I've seen at least one competitor screw up his blade by pulling it out before it had cooled sufficiently.

What is the rush to get the blade out of the oil? Is there some kind of a problem that would result from leaving the blade in too long?

 

[Valkman]

I've only watched that show a couple of times but have no idea why any knife maker would hurry the heat treat of a knife, arguably the most important part of making a knife. I don't know - maybe they figure the quench is done after a few seconds but I used to leave it in the oil a good 30 seconds before cleaning it up and tempering it. Hard to believe they don't have 30 seconds, but if I had too I'd make up time on the grind or handle rather than heat treat.

 

[pintler]

There are some alloys where you try to avoid cooling all the way to room temperature, because of cracking. You might cool to 200 degF, for example, which is cool enough for it to harden, and then immediately temper at 400, or whatever.

 

[CraigC]

Part of it might be the way the show is edited. 30secs isn't a lot of time in the competition but it's a long time to sit there and watch it.

 

[redneck]

There is a Time, Temperature, Transformation ( triple T) chart for just about any alloy of steel you want to work with, and it maps out the necessary speed of your quenching procedure. As a very broad generalization, if you don't get the steel below 800 degrees in a couple of seconds you risk forming pearlite instead of martensite.

Martensite is the hardened form of steel. All the carbon is dissolved and trapped in solution in the steel. Pearlite has the carbon distributed differently and it will not be hard enough to make a useful blade. This distribution is actually what gives a katana its curve. The hardened edge is martensite, which has a taller crystal shape than austenite, pearlite, cementite, etc. So when you differentially harden, the edge gets longer, the back stays the same and the blade curves ever so slightly.

People do funny things trying to get special results, and there are procedures that call for an "interrupted" quench, where you put the steel in the oil or water, and take it out, and then put it back in. For the most part though, for the average bladesmith, the best bet is heat to just the right temp, and put the blade straight in the oil, then leave it there until it is only a couple hundred degrees and can be handled easily.

There is a slight grace period where you can straighten a blade that is still warm from the quench with less risk of breaking it, and that may be why these guys are rushing the quench. They want to eyeball the blade and see if it needs straightened while they still can. Anytime you see one of them pulling a blade out that is fully engulfed in flames and not just smoking a little bit you can figure they pulled the blade out too soon to be safe. It may have hardened but they are taking a big gamble that it hasn't.

 

[JohnKSa]

Wow, lots of good information. Of course, I kind of expected that would be the result. Thanks for the responses!

It's also nice that the information was provided snark-free. I know it's just a stupid TV show, but I like it.

 

[Old Dog]

It's a fun show. I tell my wife it's the guy version of "Chopped" or "Top Chef" that she watches on the cooking channels ...

 

[mdThanatos]

I find the show entertaining and it is bad for me because I keep wanting to venture into bladesmithing.

 

[bodam]

I'm guessing that pulling the knife out quickly might cause a fireball? If so, it adds to the dramatic effect for tv?

 

[JohnKSa]

You're probably onto something.

In addition to the excellent information provided here, I was able to find some videos on youtube made by past FiF competitors. So far all of them indicate that although nothing is scripted, there is a TON of video taken and only a little bit of it makes the show. One past competitor mentioned the fireball specifically, making a comment along the lines that if a bladesmith does 5 quenches and only one creates a fireball, the fireball quench will be the only one to make the show.

 

[Raybj]

As far as straightening a quenched blade I personally have heard a "klink" and then had 2 pieces of knife blade. I hated that .
I want to see those pros straighten ten in a row, then I'll believe it can be done.

 

[hso]

I want to see those pros straighten ten in a row, then I'll believe it can be done.

If you're referring to Forged in Fire...most of what you see aren't "pros". The real pros I've been to Hammer Ins with have never tried to straighten anything but the most minor warping.

 

[JohnKSa]

The real pros I've been to Hammer Ins with have never tried to straighten anything but the most minor warping.

That sounds right. However, I can see why someone might risk it if they have an absolute deadline with time running down and no time to reheat and try again...

 

[hso]

A competition show like Forged in Fire will cause all sorts of risks to be taken to correct flaws all the rushing caused in the first place.

 

[JohnKSa]

Yup. Gotta have drama.