I'll be watching this one closely, as I think I tend to stumble my way through the quenching process as best I know how, but I'm sure not as good as it could be done. I've edge quenched with metal plates clamped to the sides (which left an interesting quench line), and full quenched in both horizontal and vertical quench tanks. I understand that moving the blade during the quench (front to back, not side to side) is good to do? As for quenchants, I've only used Canola and/or Peanut Oils and a friend's pan of "goop". I'm going in with a few other guys and it looks like I might be able to end up with a couple gallons of Parks 50 sometime soon-I really want to try hamons at some point and understand it's pretty difficult to get a good one without using the best quenchant.

One question I do have is how long do you leave your blade in the quenchant? Does it depend on whether you're doing clay, edge, or full quenching? Is there a point a short time into it where nothing else is to be gained or do you leave the blade in for a while? I've seen videos of guys doing interrupted quenching and have no real understanding of how the process affects the steel.

Thanks for posting this topic-I imagine I'll learn quite a bit.

Jeremy

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Discussion on Quenching Techniques

I have seen several posts on the ABS Forum and have had several recent discussions with members who had questions about proper quenching techniques. I think that it would be productive to have a general discussion about quenching techniques, quenchants, and the principles surrounding them. Please join us in this discussion.

Wow, that sounds great, but rather ambitious, kind of like condensing “War and Peace” into 20 words or less <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//biggrin.gi f' class='bbc_emoticon' alt=':D' /> . I will take up your challenge, but I am going to need more than 20 words <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' /> .

It thing one needs to understand why we quench in order to understand the best ways to approach it. Not just the obvious “well to harden the blade Kevin, you fool”, but what processes in the steel are we actually affecting. Quenching is immediately preceded by a timed heat, what this does is dissolve carbon within the steel and creates a solid solution if carbon and iron; at room temperature the two want to be separate in the form of segregated structures. If we just air cool this solution the carbon and iron will separate again and the steel will be same as before- soft, among other things.

But the separation process takes time so the faster the steel cools the less separation there can be. Now I said it takes time, I just didn’t say how much time and the truth is that it is not much at all. It only takes fractions of a second to form the soft stuff so cooling has be much quicker than air will allow. If you use the cooling properties of a liquid you may be able to beat the process of iron/carbon segregation. If you do this the carbon trapped in solution at room temperature induces incredible strains on the inside of the steel and results in it being much stronger and harder, even to the point of it being brittle like glass. And there you go, hardened steel.

But how fast is fast enough? This a very big question, and the answer is determined by the chemistry of the steel. With simple carbon steels, meaning alloys with basically iron and carbon and little else, it has to be VERY fast. And this is why W1, W2 1080, 1084, 1095 and others have been designated as “Water hardening” in the larger parts that industry uses; it is a little different with thin stuff like knife blades. With the addition of other chemical elements to alloy the steel the time it takes for iron and carbon to segregate gets extended by all these other odd atoms in the way, so steels like 5160, 52100, O1, L6 and others only require oils, even in thick sections. Some modern alloys have so much extra chemistry that they don’t even need liquid cooling and can harden in the air, but I will also exclude them from this conversation sine they are not all that fun to forge.

As already stated, hardening involves inducing tremendous strain and stresses within the steel, the trick to getting it just right is to get just enough to make the steel as strong as it can be without going overboard and introducing other weaknesses. Obviously a cracked or broken blade is less than weak, it is useless. Many new knifemakers fall prey to the “if a little bit is good, a whole bunch has to be great” trap. All you need in cooling is to trap as much carbon as possible without stressing the daylights out of the steel, nothing is really gained by that extra stress aside from warping and perhaps very tiny cracks that can come back to haunt you.

Yes the Japanese quenched swords in water, but their steel is even simpler yet. I work with bloomery iron (the western equivalent of tamahagane) and it is so quick to make the soft stuff that no oil is fast enough to completely harden it in sections greater than around 5/32”; this is not the case with say 1084 which has extra chemistry. Applying clay is another factor which rearranges all the rules by introducing many new variables, and making oils a little different than water. So to keep this post smaller than “War and Peace” I will confine this conversation to bare blades with no clay.

If there is one thing that is already becoming apparent it is that one quenchant cannot properly meet the needs of all steels. Remember that the secret is to quench just fast enough to avoid all the soft stuff but not unnecessarily stress the steel. This also why I always confine my discussions to engineered quenchants that were formulated with specific steel groups in mind and have data to refer to and compare. Also they are consistent, whereas I can’t predict the effects of another guy’s homemade quenchant and thus be able to give accurate advice.

I will assume that most reading this are not working with molten salts or polymer additives, so that leaves us with oil, so forgive me if it seems “oil” centric. Most quench oils can be broken down into two groups, medium speed and fast quenching. The vast majority out there are medium speed since industry developed oils for the new alloys that would more thoroughly harden. 5160, 52100, O1 and L6 will all do well with medium speed oils. Kind of as an afterthought industry developed the fast oils to handle smaller parts (like knife blades) made from steels such as W1, W2, 1080, 1084, 1095 so these oils are not as common.

What happens if you use the wrong oil for steel? Nothing too catastrophic unless it is too fast and you get warping or even cracking, which is rare, but instead you will just be limited by how thick a piece of steel you can fully harden. This limitation will result in the blade only hardening fully in areas closer to the edge where the blade thickness is small enough. This often results in a hardening line that looks like you clayed it even though you used no clay at all. But for those who lose sleep over the oil being in control and not them, not that I know anybody like that, such variables could be disturbing.

How to get the most out of your quench

So much for “War and Peace” in 20 words, so I broke it down into two posts. For most of this I would refer folks to my other post:

http://www.americanbladesmith.com/ipboard/index.php?/topic/14-a-few-words-on-quenching/

Many oils benefit greatly in speed by slight heating (around 125F- 130F) to reduce their viscosity and facilitate convection, but some of the fast oils actually work better just above room temperature. Keep your oil clean and covered when not in use, removing scale and debris introduced in quenching doesn’t hurt, but above all avoid burning it! Although it is a very common practice that causes no alarm among bladesmiths, flames on a well formulated quenchant is the kiss of death to its consistent performance and will reduce its predictability to any other homemade mix. If edge quenching, you are better off burning an inexpensive vegetable oil.

There are three to four distinct phases to how a liquid behaves in a quench. The first is when a red hot blade instantly creates a vapor blanket surrounding itself. Vapors are much better insulators than conductors. Remember that soft stuff we want to beat? Well it forms in factions of a second from 1200F to 950F, just when the vapor blanket can be at its worst, so you can see how you want a quenchant that resists this behavior. But there are things that you can do to help this along as well, and number one is agitation.

I know many bladesmiths a scared to death to move a blade in the quench for fear of causing warping, but trust me on this, if you give proper agitation a try you will find much less distortion due to more even cooling when you break the vapor jacket and keep cooler liquid uniformly flowing over the blade. It is true that improper agitation will not be good, so you never push one flat side of the blade into the oil, always use a cutting motion from spine to edge or tip to tang.

If you want to avoid flames always get your blade entirely under the surface, only vapors combined with oxygen burn, liquid does not, so if you are below the surface there will be no flames. One caveat to this is to remember that your tangs can be hot steel as well and I often see students overheat tongs and still flash oil regardless of where the blade is.

The next phase of the quench, which is the most violent and fast cooling, is when the vapor jacket collapses and heated liquid is rapidly carried away from the steel by convection. The more evenly this occurs, the more evenly the steel will quench and this takes us back to good agitation. This is the growling vibrating phase in water where the scale gets blown off.

The last phase is the continuous cooling phase that brings the steel to the same temperature as the quenchant; it can be very fast as well and needs to be carefully considered. You see, while you avoided the soft at 1000F the blade still is not hardened. A new series of mechanisms, driven entirely by the cooling, are waiting to occur inside the steel when the temperature is low enough to initiate them. For most knives this is between 400F and 500F degrees. This is where the tremendous forces within the steel make it hard, so any unnecessary additions to this will result in warped, cracked or broken blades.

Water is a very fast cooler in this phase and forms a huge vapor jacket in the first phase, so you can probably now see why modern steels tend to self-destruct in water based quenches. The reason cracking and distortion are greatly reduced in oils is because of the smaller vapor jacket and slower cooling from 500F to room temperature.

When is the quench finished? That seems more obvious that it actually is. The chemistry of the steel actually determines at what temperature it is fully hard. The simpler the chemistry the higher the temperature it begins to harden and the higher the temperature it finishes. So with 1080 as soon as it is the temperature of the oil, you are ready to temper. Due to extra carbon 1095 is just a little bit behind that but not enough to mention. Steels that have enough alloying to air harden are often not even done at room temperature and that is why they benefit from extra cold treatments.

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...One question I do have is how long do you leave your blade in the quenchant? Does it depend on whether you're doing clay, edge, or full quenching? Is there a point a short time into it where nothing else is to be gained or do you leave the blade in for a while? I've seen videos of guys doing interrupted quenching and have no real understanding of how the process affects the steel.

Thanks for posting this topic-I imagine I'll learn quite a bit.

Jeremy

Jeremy, the time you leave the blade in the quench is only a huge factor if you are attempting an interrupt, a slightly more advanced technique that is not entirely necessary for everybody. If all you want to do is quench your blade fully keep it in the oil until it is the same temperature as the oil and then temper as soon as possible. IF you have clay on the blade you will want to leave it in a couple minutes longer.

The interrupted quench is an improvised version of what industry calls mar-quenching. Actual mar-quenching involves quench mediums engineered to handle being heated to 400F+ and still handle heat extraction; most oils drop off drastically in cooling ability above 250F and like to burst into huge walls of fire when quenched into at those temps. Marquenching takes advantage of the fact that steel does not actually harden until it reaches 400F to 500F and no longer needs the severe cooling speeds to avoid the soft stuff. In my heat treating video that the ABS offers, I demonstrate the principles behind the interrupted quench.

The idea is that since you no longer need to fear the soft stuff, you can stop the fast stressful cooling before the blade starts to harden and allow it to cool in the air instead. The advantages of this are that the blade cools more evenly,without added stress, and you can also spot any warps as they begin and gently guide them back straight before the blade cools. The slower cool allows the blades own thermal mass to get a head start on the tempering process with the new hardened structures within the blade providing some gains in toughness.

However, as I said, it is a more advanced technique that does have some pitfalls to be avoided. It is important that you interrupt as close to the hardening start temperature for your specific steel as possible without under shooting. This requires practice with an oil, that you are familiar with, which has consistent performance over time. Clay will add at least three to five seconds to the time before interrupt. If the interrupt is too high the auto-tempering effect can be overdone and if it is too low you lose many of the benefits or even create some full hardening issues with more alloyed steel. The cooling should also be continuous after the interrupt to avoid the same hardening issues.

Thanks again for such good information in a format that even I can understand it. Quenching and all that goes into it still seems a bit complicated, but with folks like you helping and some practice, I'm hopeful <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//biggrin.gi f' class='bbc_emoticon' alt=':D' /> .

Jeremy

Kevin you are a wealth of information! Thanks for taking time to explain here and also on all the forums I see you on. If you don't already you might could put all this in a big word doc so you can cut and paste it. Just a thought. Thanks again!

Kevin,

I have seen 52100 quenched in water and only imersed long enough (seconds) to get past the time temperature curve to the martensite start then allowed to air cool for final martensite formation. He gets great relults, consistent Rc 58. Could you elaborate on this. Is it his techique, time, or is it steel specific, etc..? Overall it is fantastic. What do we need to know here?

Thanks Tom.

|quoted:

Kevin,

I have seen 52100 quenched in water and only imersed long enough (seconds) to get past the time temperature curve to the martensite start then allowed to air cool for final martensite formation. He gets great relults, consistent Rc 58. Could you elaborate on this. Is it his techique, time, or is it steel specific, etc..? Overall it is fantastic. What do we need to know here?

Thanks Tom.

Since I personally have only seen it on video, you actually may be more qualified to assess this than I am for I must assume that you have had an opportunity to personally examine the results thoroughly to find them both great and fantastic. I am a pretty analytical kind of guy and couldn’t make any judgments before a complete examination of the product, both inside and out.

I often advise people to view any test as but one piece on a large puzzle if one wants to accurately see the entire picture of the puzzle. Rockwell for instance can only tell one that a piece of steel resists penetrative deformation to a specific degree, but it can’t tell you why that is, it isn’t until I have a sample cross sectioned and under a microscope that the story behind that Rc number begins to make sense. A knife bent in the vice is assumed to tell us its toughness, but there are so many things in play that it is impossible to actually get that information reliably. There are tensile, compressive, as well as a smidge of shear strength actions going on and in the end much of the results are determined solely by the thickness of the blade. For me, in the end, the true toughness is only added to the puzzle after I have done Charpy impact, or similar tests, as sudden loading is an entirely different beast that is not hidden under all the other factors.

Despite only having two significant alloying considerations, chrome and extra carbon, those two are enough to make 52100 much more complex in its behavior than most of the other steels mentioned so far. One will often see smiths using methods that deviate from the normal straight forward approaches when working with this steel due to the curveballs it can throw at you. The best person to ask about what metallurgical principles guided them in developing a particular method is the maker himself. I can say that I have my goals in heat treating, based upon my own testing, studies and analysis and that they are very different from a few other guy’s. I feel a responsibility to keep my advice to others based upon sound and well proven principles, even though I often have very strong beliefs and opinions, my assumptions are only that until I have overwhelming data to safely state otherwise.

So I guess, after all my verbosity, what we need to know here is that we need our own personal verification of the results, in order to make accurate assessments, sorry, I know that is not much but that is all I got to work with.

Kevin,

Thanks.

T.

Hello Friends!

I understand some of the aspects of quenching and have much more to discover and learn. From what I gather there are two basic ways to strive to minimize adverse effects of the vapor jacket, which raise a couple implementation questions for me regarding equipment and/or procedures.

The main method of side stepping the vapor jacket seems to be quite literally stepping out of it via agitation of the quenchent.[list]

Good questions Phil

Of course excellent agitation can be achieved by moving the oil with pumps, impellers and “J” tubes, but it is not always practical in a knife shop. Many folks are hesitant to build such elaborate rigs and probably feel that it sort of gets way from the whole handmade knife thing. If one does go that route, it is important to once again avoid the “little bit is good so a whole lot must be great” fallacy. I have personally seen very fast oils slowed dramatically by introducing air by over-agitation. Once again it is best to streamline the movement, with the oil moving parallel to the length of the blade, this is where “J” tubes and such come in quite handy.

I believe the vast majority of smiths handle agitation by moving the blade in the oil. For this it is also important to remember VOLUME. Some people are irritated by quench oils not being offered in quantities less than 5 gallons, but I don’t think the stuff should be sold in lesser amounts. The 5 gallon minimum is doing knifemakers a favor by ensuring that they have enough quenchant to effectively do the job. You need enough oil to handle the quench without overheating and still provide convection, but you also need the volume required to be able to move the blade through the oil. A 5” hunter in a mason jar does not have a very good shot at the best results.

You are correct on the gentle heating. Heating is no different than quenching, things that cause the oil to flash or flame are oxidizing and will change its chemistry. For hunters, and knives of a similar size, I have found the large roasting pans, so often use to keep food warm at potlucks, to be very good for a horizontal quench (this becomes a pretty tight squeeze for bowies). I drop a thermometer in to verify the thermostat (which is normally not so accurate on those things) and set the roaster to the temperature I want. These usually come with a pan within the roaster, so I put some oil in the roaster itself and then set the pan in so it is surrounded by that conductive layer of oil, this makes a faster and more even heating of the main body of oil.

If you are quenching large volumes there could be a need for oil cooling and one sword making operation here in Michigan that I worked with to set up their heat treating dept. included a radiator style cooling assembly in the line of their circulation pumps. But if you are working with a decent volume of oil the average one man shop should be able to control the heating of the oil without such elaborate methods.

So rigging an aquarium type pump in the oil pan (a large steam table tray I got from salvage) isn't really needed? I did that and set it so the flow goes in the direction of the blade.

I know its a little off topic but doesn't the design also affect the bending test, by both how the steel hardens (thickness mostly I guess) and the actual design in how it redistributes stress?

I have a large tank for my oil and I use a therometer to measure the oil temperature. As described I move the blade up and down since it is plunged into the oil tip first. I recently had a muscle spasm in my back and saw the athletic trainer who gave me some ice. While I was in the training room a couple of the trainers were working on some student athletes. One was massaging a leg muscle. The trainer had a vibrator with a vibrating unit on the back of hand. It was worn kind of like a glove and caused the hand to vibrate as the massage was applied. That made me wonder if holding the tongs while quenching with this type of vibrator would be the way to go for agitation. This would avoid the big movements with tongs and introduce a controlled agitation. Any thoughts?

Dan

|quoted:

I have a large tank for my oil and I use a therometer to measure the oil temperature. As described I move the blade up and down since it is plunged into the oil tip first. I recently had a muscle spasm in my back and saw the athletic trainer who gave me some ice. While I was in the training room a couple of the trainers were working on some student athletes. One was massaging a leg muscle. The trainer had a vibrator with a vibrating unit on the back of hand. It was worn kind of like a glove and caused the hand to vibrate as the massage was applied. That made me wonder if holding the tongs while quenching with this type of vibrator would be the way to go for agitation. This would avoid the big movements with tongs and introduce a controlled agitation. Any thoughts?

Dan

I wonder if it would change the speed of the quenchant as ultra sonic tank vibrators do... even though they delete the bubbles.

Mike