hang with me guys on tempering

wmhammond said:

I am also learning a lot from this thread. I take it from what's been said that two quench tanks are necessary for a proper set-up. A Parks 50 tank (fast) and say a Canola tank (slow). Is that generally correct? Thanks,

Wallace

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Maybe not “generally” or “proper” but, if you really want to split hairs (no pun intended), then yes… Or, cater (tweak the other variables) to the one you have. Also, canola is more comparable to a fast petroleum based fluid than a slow one.

Lots of good info here :happy:

On the topic of quench oils, would anyone please comment on how to evaluate unknown quench oils in order to "rate" them against a known oil? Parks 50, and most other oils generally referred to on the forum are available in the USA, but they are not generally available in many other parts of the world (like in South Africa where I live...). How, in your opinion, would one go about testing a local oil (with no manufacturer's specifications), or various brands of canola oil for example, to rate it's quenching speed to compare with e.g. Parks 50?

You can make coupons of different thicknesses and different steels, say from 1/16 of an inch through 3/8, quench and test them for hardness. Keep in mind though that speed is just part of the equation. Ideally, the least amount of stress to achieve full hardness is best. In other words, use the slowest least stressful possible medium to achieve full hardness. This is especially true with knife blades, because they are prone to warping and cracking. Also, just because there aren't any cracks visible to the naked eye, doesn't mean the steel hasn't been damaged on a microscopic level.

Since knife blades present certain geometric problems, thermal gradients, long and thin etc, and they are generally high carbon/alloy steels, it makes good sense to test on similar geometry, long and thin, wedged or diamond shaped... or better yet, knife blades and years of field testing.

Aside from that, speed wise I'd put canola somewhere between Parks#50 and AAA. If you want a fully hardened blade out of something like 1095, it's probably best not to go over about 1/8 inch thick at the thickest point, with canola. Differentially hardened blades are another story.

To put it bluntly, there just aren't any "ideal" quenching mediums for knife blades. An "ideal medium" would have to be able to cool the thin parts and the thick parts, the whole macrostructure, at the exact same given speed.... a "smart medium".

Canola oil will work well on a very wide range of steels because there are fundamental metallurgical differences between it and the engineered petroleum based fluids, due to different cooling mechanisms/mechanism. There is no "one size fits all", but canola probably comes the closest. I look at this as a *plus* for canola, but I'm sure others will hate it for that.

EdCaffreyMS said:

To me, scale is just a by product (something that happens) during heat treat. If the atmosphere is correct inside the heat source you're using (forge, oven, etc) then scale is minimal.

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Wow I'm finding that to be absolutely true these days. I use to HT with an open forge, probably over heating a few blades along the way etc.,.. since making my mini fire brick forge I've seen very little scale at all. It's soooo much easier to clean up. A very fine layer of black color is all that's left of the blade. I'm using 1084 pretty much exclusively these days after reading a lot of what Ed and a bunch of other you pros have said about steel types. I quench in peanut oil and I've found it to work pretty well so far. I was told by Paul Lebatard to heat the quench oil up with a hot piece of scrap steel and to check it with your finger. If it's just hot enough that you don't want to keep your finger in it, you're good to go. And one knife at a time so you don't get it too hot.

Unless you are in a real hurry, it's best to heat the vegetable oils with a hot plate or a stove. It only takes a few minutes. Quenching pieces of steel in it to heat it up oxidizes the oil (alters the properties) and can cause you to have to change it out more frequently. The main disadvantage with the vegetable oils is they are less stable than the petroleum based fluids and don't last as long. In the long run, vegetable oils tend to cost more because of this and you will have the problem of disposing of it afterwords. I must have about 15 gallons of spent canola in buckets out by my forge... maybe I can get money for recycling it or something.

You can also get a candy thermometer at the grocery store to spot the temp. Either way works. It won't make a huge difference.

The only real advantage to peanut oil is that it has a higher flash point than the others, but it's quite a bit more expensive than canola. One reason canola is used more than most of the other vegetable oils, is that it has slightly better stability.

Canola = CANadian Oil Low Acid.

I do use a hot plate to keep it at temp...it just takes it forever to heat up. I change out the peanut oil about every six months, so that's maybe every 7 or 8 knives, so 14 to 16 actual quenchings. But my wife's stupid cat likes to drink the stuff too. It disappears faster than it should. Cat's gonna die a horrible death one of these days.

You should be able to get quite a few more blades out of it,... depending on the volume.. 5 gallons should do at the very least 100 "average sized" knife blades... if you take care of it, store it properly etc. Changing it out more often won't hurt, but may not be necessary.

These work good for heating it up quick. They hook up to a standard propane tank.

http://www.walmart.com/ip/Buffalo-Tools-Single-Burner-Cast-Iron-Stove/19240077

Ok I'm the one that started the thread and have an up date. I applied ATP 461 to each blade,3 coats. I re hardened the blades to 1550 and had the quench very close maybe one and a half seconds from oven to quench. Cooled and retested 3 of the 4 read 57 to 58 RC the other read 53RC for some reason. I retested that one 4 times coming up with the same reading each time. Any ideas? The main question is do I have to temper since the RC is where I want them at least on 3 of them? If so what temp? I had a little pitting which surprised me. Thoughts.
Thanks

On the ATP, I've been using only one very thin coat, and get really good results. No decarb, no scale. Even at 1650°F.

Do you know if I should temper and what temp?

Groucho. If in doubt, at least do a flash temper - maybe at 325. This will take some of the stress out and give you some time to make other decisions. It will not give you what you want for a well performing blade, but it mights prevent cracks and delamination. How thick is your blade by the way - and where are you taking the rockwell readings?

There are 2 blades .085 and 2 blades .125. There is a 4.25" piece of 1084 handle welded to a 15n20 - 1084 Damascus blade. I'm taking the reading half way of the welded piece of 1048 handle

Well, that takes out the possibility of a blade just too thick to harden completely. It does raise the question of bubble jacket. Even with agitated quench tank, I often see pieces that are harder toward the tip end than the butt end. The answer to that is aggressive entry into the quench medium and aggressive agitation - stabbing and slicing but never side to side.

You've seen lots of suggestions in this thread that differ greatly - and that's just an indication that there is more than one way to get it done. Not suggesting anyone is wrong. FWIW, I would be using 1475 - 1500F - putting the blades in a pre-heated temp stabilized oven - allowing 5 minutes to reach temperature and a three to five minute soak. On the topic of oven temps, I'm guessing your 53 blade was last out and that you had all the blades in the oven together. On subsequent blades, it is not enough that the oven has returned to temperature. It is necessary that the workpiece has returned to temperature. That can take minutes, and by that time, the later blades have soaked too long. I never do more than two carbon blades at a time for this reason.

I agree that anti-scale is a great idea and that one coat should be plenty - three might insulate. You may have to patch a few spots that were left open when the ATP641 'beads up'.

Lastly (or not) much has been addressed about the rockwell testing. If there is anywhere against the anvil (side opposite to the penetrator) that might have scale, contamination or a burr, readings will be off. A tiny burr is the usual culprit. A slight blade warp can make a similar difference, especially with larger anvils. Try testing the other side to check for that one. Also test again later. It should not happen, but I have seen tests go from 57 to 65 just by waiting a couple hours. Waiting has its risks, but can be illuminating.

OK I know I said 'lastly' but one more thing needs to be addressed. I already mentioned that you are testing away from the performance end of the blade and it may be harder towards the tip. It is also true that you are testing different steel - 1084 vs damascus - and that makes any presumption of similar outcome something of a stretch.

I'd also like to congratulate you on your efforts. There are a huge number of makers who presume that if they "follow the recipe" then they know what the outcome is. You and I know that simply isn't true, and I hope some others will be enticed to start a regular habit of checking.

Rob!

What medium did you quench in, hydraulic fluid again or canola? With canola there is no "bubble jacket" or "vapor jacket" or vapor phase to slow it down, and because of this (everything else being comparable, such as viscosity) it is faster than *unaccelerated* petroleum based fluids in the initial part of the quench (getting around the pearlite nose). It works well with a "still quench" and agitation/circulation is not as much of a necessity, (if at all), in most case. The *convective current* takes care of it by starting fast and ending slow. This also helps reduce the chances of warping. 2-3 gallons should be plenty. I usually get it on sale for about 7-8 dollars a gallon at the supermarket.

Welding a tang on can burn the steel. It may also be a matter of where you are taking the readings. If too close to the weld or if the section you are testing is much thicker than the blade, that could also be part of it.

Thanks guys