Quenching Bohler K340

H

h3n4

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May 30, 2006
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Ok, I have a larger knife blade made from the above mentioned steel. To give an idea of the size, the knife has a 180mm blade, 6,5mm spine and the blade is 42mm wide. OAL is about 300mm. Check this link for more info on the steel. http://www.bucanada.ca/k340.pdf.

Now, the pdf says that quenching in oil has a high risk of cracking/distorting the workpiece. As I'm not going to be able to pay for a professional heat treat, I'm going to be limited to only oil-quenching it. (Lack of vacuum-inert-gas-quenching-tooling :D).

Question is, if I make all the correct annealing, stress relieving and pre-polishing and scratch-removing of the blade and then quench in warm oil, will there still be a high risk of cracking the blade?
And what kind of oil should I use and at what temperature? (The datasheet says warm oil :confused:). Couldn't find anything that would suit my needs as the darn search function never works when I need it... But can't wait to show the finished piece and maybe a post with in-progress-pictures =).

Thanks in advance for all info!
 
I have some of this steel, but have yet to use it. From what I've read, it appears to be similar to A2, only with some extras in it(plus, it's vacuum melted). :thumbup: I would not risk oil quenching it. When I get around to using my K340 Isodur, I'll be sending it off to Paul Bos for optimum air hardening heat treat.

Since you mentioned that you're not spending the money to send it off for a professional heat treat, I would you read up on air hardening techniques. Get some good high temperature heat treating foil, wrap the blade properly. Soak it for the recommended time at the proper temp and air quench it, by removing the foil as quickly as possible, or use quench plates.

Let's see some pics of your blade, please. How do you like working with it so far?
 
Oil is usually heated to 130-140 F. For the K340 you would need a slow oil . A plate quench is a better option - usually eliminates warping . Wrap the blade in stainless steel foil and plate quench in the foil. I hope you have a proper temperature controlled furnace .Precise times and temperatures are needed to get optimum properties for a complex steel like this one.
 
Some people, Mete included have mentioned plate quenching. Does this mean simply taking the knife out of the oven and then just clamping it between two big pieces of say aluminium so that it transfers its heat *quickly* to the plates and therefore getting quenched? No quenching oils or other liquids needed? How about tempering then? And then, why foil? What does it help with? I have never heard of wrapping the blade in SS foil prior to setting it in the oven.

Sorry, I know I sound quite stupid but I really want this to go ok as it is quite a big project for me.

Danbo: The steel is absolute crap to mill, but filing and grinding goes ok. It's quite hard to drill, too. Very dough-like because it is so pure and has no sulphur in it to aid in machining I guess.
 
I must add that I will most likely be using a pottery oven that has a processor-controlled temperature-curve. It should be able to hold correct temperatures and times.

Thanks in advance anyway!

-Henri
 
h3n4, you are correct, the blade is removed from the oven and placed between two plates of (typically) aluminum, still in the foil envelope. The plates I use are 25x150x300 mm and they cool two hunting knife blades in well under a minute.

Regarding the use of foil envelopes: steels heated to the 1900*F you require will oxidize substantially and cause pitting unless the atmosphere is controlled. Thus, the object to be heat treated is wrapped in a Type 309 stainless foil pouch to minimize this. Prepared pouches are commercially available, though most knifemakers begin with foil sheet and construct their own envelopes. A folded sheet is cut to fit the blade with sufficient excess to allow the three open sides to be double-folded and the seams rolled or tapped shut to ensure good closure and airtightness.

Some people will include a small slip of paper inside the envelope to consume oxygen. I no longer use that technique, having found it provides no advantage the way I wrap the blades.

There are two types of foil available in the US. Type 321 and Type 309. I personally prefer 309 because it tolerates higher temperatures.

I also insert the sealed envelope inside another SS envelope. This yields blades with essentially the same appearance as they went in.

Hope that helps. :)
 
BTW, your blade should be very clean when you wrap it up. No fingerprints or other contaminants.
 
Fitzo is correct , 1900 F + oxidation occurs rapidly !! After plate quench temper as usual.Something like 400 F [2+2 hours] is typical. Cryo would be optional and it would add 1-2 points HRc.
 
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