HT oven cooling

[dbalfa]

How do you fellows cool your HT ovens down for use as a temper oven after HT?

Mine can take a few hours if left to itself. I tried blowing a fan into it and got it there in 20 mins or so but as soon as the fan was turned off, the heat rose back up 100 degs - radiating off the bricks. I've thought about a more forceful air current like a shop-vac backwards or the like but don't want to crack my bricks....???

Any tricks to this or should I do as my FIL said and "build another oven"? Or is 30-60 mins not to long to wait to temper after a plate quench or oil quench? Would leaving the blade in the warm oil help or hurt during the oven cool-down time?

[Al Polkowski]

Use your kitchen oven with a digital thermometer.

Al P

www.polkowskiknives.com

[Doug Lester]

I second the ketchin oven for heat treating. I actually use an old toaster oven. I also use a pan of sand to bury my blade in. It serves as a thermal mass to buffer the rise and drop of temperature as the heating coils cycle. I would not delay getting the blade into the tempering oven after hardening. The steel is under a great deal of stress and more than one smith has heard the dreadded "tink" from a hardened blade as he was heating and quenching another. I don't delay getting my blades into an oven any longer than it takes me to quickly scrub the oil off the blade under hot water. Also, have the tempering oven up to temp before you start to harden your blades.

Doug Lester

[pipecrafter]

Most of the suggestions I've heard are to use the kitchen oven or a decent toaster oven for the first temper, then use your HT oven for any successive cycles.

Up until now I haven't had an electric HT oven, so I've been using my forge for heat treat, then tempering in the toaster oven - actually a really nice one that my parents gave us as a house-warming present when we bought this place. I've tested it with a thermocouple, and it stays within about 10 degrees.

I like Doug's idea of using a sandbox. That would really help, I think.

But, I think your problem with getting the oven cool might be central to your floor brick. Using the hard firebrick on the floor made it really resilient, but also raised the thermal mass considerably. Those are probably what is holding all the heat for all that time.

For what it's worth, I like your FIL's suggestion.

[Ray Rogers]

You've already received all the good advice you need for the basic answer to your question but I'll offer another alternative just for the record.

First, the harder you cool your oven the greater the chance you'll damage it so I wouldn't go that route. A toaster over can be had for $20 so that's a very viable alternative. Of course, buying an additional heat treat oven is ideal but very expensive. But, on the possibility that you might consider buying an additional heat treat oven I'd suggest you consider buying a cryo tank instead. An liquid nitrogen dewar of adequate size will cost as much as one of the less expensive H/T ovens but it will also add that extra ability to your knife making.

When i quench my blades they stay in the oil until they fall below 125 F (the usual procedure for most all of us on most any steel). At this point we want to temper but the darned oven is still very hot. However, it is also at this point that cryo is appropriate so instead of tempering my blades I put them in cryo. They stay in cryo over night which is longer than necessary but doesn't hurt anything. The next day, the blades come out of cryo and go into the cool oven. I do this with stainless blades which benefit greatly from the cryo process but also with carbon blades which benefit less (some say not at all). Either way, cryo doesn't hurt the blade and it allows me to delay the tempering time. BTW, I think cryo adds significant toughness to carbon blades.

Anyway, it's something to consider if you feel inclined to spend the money for another oven or want to push your heat treating results up another notch .......

[dbalfa]

Ray, for the cryo route, do you think dry ice and acetone mix would be a close second to liq N?

Thanks BTW for everyones thoughts.

Yeah Kurt, I was cursing the hard bricks over that too....but you can't have it all they say!

[Ray Rogers]

Yes, dry ice and acetone (or kerosene) is supposed to get you into the minimal range for effective cryo. LN is far colder but the dry ice method is a lot cheaper to set up.

I've never had a blade crack due to a delay in tempering when I used the LN so, if it happens with the dry ice I guess you'll know it's not cold enough. My opinion is that it would be worth trying .........

[spaknives]

I've always been told to cryo carbon blades after the first temper, and cryo stainless after the quench. Is that right????

I went to seminar at Ed Fowlers shop. He taught us to elimenate as many variables & unknowens as posibble to get consistant results. I think using to different ovens for the temper would possibly add another uneccasary variable. After the quench I put my blade in the freezer until the over comes down to temp. Do everything the same everytime. If you leave your blades just laying around in the shop the temeture may be different one day than another. Do everything the same until the blade is finished with the heat treating process and eliminate as many variables as possible. Like using a vegtable oil is an unknowen becouse we don't know the actual quench time of the oil. So use an oil that has a knowen quench time. Hope that helps wome what. Thanks, Shane

[Ray Rogers]

You can cryo any blade at any time but the results differ. The usual point of the cryo process is to continue the transformation that was begun by the quench. Obviously, doing the cryo after the first temper would not accomplish this but perhaps it might add toughness or some other desirable characteristic.

In fact, as I mentioned before, doing cryo on carbon blades may not have an effect on the transformation at all. I think it will have more effect on alloyed carbon steels like O1 than on very simple carbon steels. The good news is that cryo doesn't do any harm to a carbon blade and it does allow you to postpone the first tempering which is why I brought it up

[CWKnifeman]

When doing cryo you need to do at least one temper on your material prior to doing cryo to reduce the stress risers within the material. I have been doing sub-zero quenching for over 12 years now with very good results. I use Dry Ice and 93% rubbing alcohol in a small Plastic ice chest. After a few times of going through the dry Ice the only thing left is 100% alcohol, once the dry ice is put into the alcohol the alcohol actually freezes and will maintain a true minus 109 degree F. I place it then in my freezer which actually has a vacum cycle on it. This will keep the dry ice good for several days use if needed.
You need to leave your materials in the mix for six(6) hours for any arstinite to convert to marstinite sorr for the spelling errors. After speaking to several metallurgist over the years as well as several from crucible steel, the same thing was said If it is going to convert is will do so within or over six(6) hours. the next step is to temper for a second time again for reduction of stress risers in the material, and a third time to add strength and duribality to the material, and to give it a true through tempering of the material. Some may say that you don't need to go this far but I differ. My reasoning is that I can put a blade of 440C steel and match or even out cut a blade of ATS-34 of the same RC. I hope that this is of some help to all who are interested.
Thanks for reading.
Curtis Wilson

[Ray Rogers]

Manufacturers do recommend a snap temper or a full temper prior to cryo. Remember though that their recommendations are usually based on a 1" cross section and complex shapes which are much more subject to stress risers than most blade designs. I've never had a stress failure on a blade in cryo in 10 years of doing it this way. I do make sure not to include characteristics in my blade designs that might be susceptible to stress risers though, no point in taking chances.

If you have the chance to do a snap temper or a full temper then by all means go for it prior to cryo. But, if you have but one oven and it won't cool as fast as you need it to then go straight to cryo. With reasonable care you won't have any trouble and you can temper when the oven cools down. I do agree that six hours should be enough for cryo to do its thing but leaving your blade in longer won't be a problem as long as the temperature is maintained....

[Wade Holloway]

Ok let me see if I got this straight Ray. Just quench, wash, and straight to the freezer. Then temper at least 6 hours later. Am I understanding correctly? Thanks.

[Ray Rogers]

Yes, Wade, that's what works for me. To be more accurate though, I don't wash the blade, I simply wipe off the excess oil with a paper towel. The oily film that remains doesn't affect the cryo and it burns off later when the tempering is done so I don't worry about it. I point this out now because Curtis brought up the subject of stress on untempered blades and that's a very real consideration. It occurs to me that washing the blade, even in hot water, might provide enough temperature change to induce a stress problem. Maybe I lucked out and never saw this problem because I never washed the blade or maybe there wouldn't be a problem either way but it is worth considering...

[Doug Lester]

Ray, a little quick education please. I have my copy of "Metalurgy for the non-Metalurgist" packed away in a storage shed right now. What does the cryotreatment do for the steel and is there types of steel that this works better on?

Doug Lester

[Ray Rogers]

I'm not a metallurgist either Doug but I can give you the high points in extremely lay terms as I have gathered from basic reading.

You already know about the transformation that takes place when a blade is quenched, the purpose of that quench is to 'freeze' the steel while the good beneficial crystaline form that is brought about by the high temperature is still present. The quench accomplishes this by solidifying the metal before the molecular form can revert to what it was before BUT we know that not all of the steel was completely converted to the desirable form (it never is but we get enough). Turns out, extreme low temperatures can continue this transformation . The net result then is more of the desirable molecular form in the steel and less of the other not so useful forms. A blade put through cryo will usually gain about 2 and sometimes even 3 Rockwell points over its 'as quenched' hardness.

In short, more of the steel will have a finer grain structure and it will be harder. Generally, edge holding will be improved and many steels show improved toughness as well.

As for what steels benefit the most I say the industry experts would say stainless steels, hands down. Pretty much any cutlery stainless will benefit but some more than others. S30V and BG-42 show significant improvements when cryo is used. 440C shows improvement but less than the others. Carbon steels may not show much gain in hardness and edge holding but often do show better toughness (which is why this process is used on gun barrels) . The simpler steels will generally benefit less than the more highly alloyed steels. In any case, cryo won't cause any detrimental effect to the molecular structure of any cutlery steel even if it doesn't benefit that particular steel. Of course, if the blade has a defect in its physical structure (stress riser) then cryo could make that defect show up pretty quickly .

I think I managed to say all that without using a single technical term, probably a first for me .......