The process and priciples of quenching

thanks fo rthe further explanation stacy....i am real new to all of this and alot of it honestly goes right over my head....i appreciate the response.....ryan
 
I am really glad to see that this thread has been made a sticky. It will make a great reference resoure and will save a bunch of furure rehashing of best principles.
 
bladsmth said:
James -
You are sort of on the right track.
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"Sort of on the right track" is the story of my life. :D Thank you for clarifying that viscosity issue. I think I will refrain from making any more statements in this thread and just ask questions. :o
 
When we harden steel we want to quench fast enough to get the full martensite structure. If we quench too fast we risk cracking.Looking at a typical TTT diagram the steel should be brought rapidly past the 'pearlite nose ' then more slowly through the martensite transformation. Commercial quenchants do just this. We can also marquench by quenching rapidly to just below the start of the martensite transformation .Then air cool from there.Third option is to do an "interrupted quench" where the steel is quenched rapidly to get the steel below the pearlite nose then removed from the quench. This may require a bit of experimentation to get the timing right. I have also done an iterrupted quench where the steel is water quenched for a few seconds then oil quenched to room temperature. For more complex shapes these procedures become more important.
 
Don, good to hear from you, long time no see. Mete covered it pretty well but I have a couple of minutes here to add my input. As previously discussed in this thread the idea behind hardening steel is simply not to make any soft stuff as it cools. The thing is that if you avoid any soft stuff above 900F, from 900F to 450F (assuming folks like carbon as much as I do), things will still be totally austenitic with continuous cooling. But that austenite will be very unstable and not want to hang around at lower temperatures. The hardened form of steel that we know so well, caused by martensite, only occurs when a certain temperature is reached, that temperature is known as Ms (martensite start) and it is unique to each steel chemistry since it is heavily affected by alloying. But for the sake of our conversation I will stick to just carbon in the effects on Ms, the higher the carbon content, the lower Ms. A steel with around .5% carbon will have an Ms perhaps around 550F while one with .9% carbon or more could be at 400F. This is due to the stabilizing effect that the carbon and other elements have upon the austenitic matrix, requiring more cooling to initiate the shearing strain necessary for the martensitic transformation.

As we also have discussed many times before, that strain is related to the massive expansion that takes place within the metal as it shifts from the compact stacking of FCC (face centered cubic) to the distorted BCT (body centered tetragonal) configuration. The most vivid example of this is a katana going into the quench straight and the edge martensite expanding so much that the blade abruptly curves from the expansion. With all of this movement potential much of our distortion occurs while martensite is being made. If all of this does not happen evenly and gets enough out of hand we then get the dreaded “ping”. Also in higher carbon steels there are certain conditions that occur which will results in much lower impact strength and true microfracturing. When I say microfracturing here I mean truly microscopic, as in within the grains, but less than desirable all the same. With all this stressing and straining going on you can begin to see why the less shocking we do to the steel from Ms to room temperature the better things could be for the steel. Water doesn’t offer any mercy in this range and that is one big reason we so often get cracking when we mess with water.

The martensitic transformation is not time dependant, it happens almost instantly once initiated but instead is entirely temperature driven, so if we stop the cooling at any point we stop making martensite. So while cooling too fast is overkill, cooling all the same really must be continuous.

If we cool fast enough to avoid pearlite, or any form of bainite, we now have accomplished all we needed in the fast cooling and it would be better at this point to slow down. A very good quenching oil will do this but it is possible to go even slower for the maximum gentleness, this falls within the techniques of marquenching or martempering. The idea of these techniques is to equalize the entire part being quenched at a temperature above Ms and then proceeding slowly and evenly through the martensite formation. This does a few things, first of all it is obvious how it can eliminate much of the distortion through equalization, but it will also eliminate large percentages of the stresses involved to typical quenching. However one of the biggest benefits is what is know as the autotempering effect, when as much as 40% of the martensite forms at temperatures capable of tempering it but remaining within that range long enough for some tempering effects to actually occur.

Marquenching, is properly done with low temp salt baths or specially designed martempering oils, and thus works best with deeper hardening steels, but the technique can be approximated by interrupting a normal quench just above Ms an allowing the steel to air cool. With salts you simply remove the equalized part and allow it to air cool but with a normal quench you have to get good at judging the cooling progress to interrupt correctly. If you interrupt too high you could form undesirable products, and if the edge should dip slightly below Ms before arresting the cooling action the heat from the spine could autotemper it at too high a range. On the other hand if the interrupt is too late you will kind of lose the whole point of what you are doing. Also I personally advise against getting creative with the cooling after the interrupt, it really should be continuous and not too slow, so a simple air cool is just fine. If you time things correctly you should have a light coating of oil on the blade with some light wisps of vapor coming off. The blade should not immediately look dry, nor should it flame. You should not be able to hold it with bare hands but you should be able to hold it for short times with lined leather gloves.

The leather gloves are good for the benefit of most interest to blade makers. You see since there will be a large percentage of austenite still in the blade, you can sight down it and see if there are any warps starting and if there are any all you have to do is easily push it straight with you gloved hands. How do you folks think I keep rapier blades straight? So if you do things correctly you can have a fully hardened blade, that has already automatically been through a snap temper (you could see perhaps 2 HRC points less as hardened, but that is not lost hardness it is autotempering effects), as nice and straight as you want to keep it, and with a significant gain in some impact toughness due to the low stress method of forming the martensite.

Be it from an interrupt, of from marquenching in salts, every blade I make is done this way so I must admit to being rather biased toward these techniques, but they have worked wonderfully for me for many years.
 
Does Parks publish the temp at which Parks 50
enters induction phase...?

ie.
(is Park 50's transition point, from boiling phase to induction phase,
at an appropriate temp for use as a reference for interupted
quenching..?)
 
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Kevin R. Cashen said:
I have recommended the book “Quenching and Martempering” by ASM, but until those interested can get their own copy I will touch on some of what is in that book for starters.


To be continued…
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Kevin

I have searched for this book on both the ASM website, and Amazon, but cannot find a book with that exact title.

http://asmcommunity.asminternational.org/portal/site/www/template.PAGE/Search/?javax.portlet.tpst=92b757d8693a892142690c1007e110a0&javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetokenqueryResultsTop&externalPost=y

Can you give us an ISBN #, or link, or more info to find that ?
 
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My copy of "Quenching and Martempering" came from Borders
Used Books, and doesn't have an ISBN #.
 
Mete and Kevin,
Thanks guys, I finally got back to check this thread, and after reading through your descriptions for the 4th time ,am getting a clearer understanding of how important it is to select the proper steel(s) for the function and designs of your (my) knives.
I want to thank Kevin for the time that he has spent on the phone answering my questions, and talking me through the mystery (to me! )of heat treating.
I'm just glad that I didn't know any of this stuff before I started cutting all those holes in my blades!
DonBell
www.Bellknives.com
 
Find a place that sells home brewing supplies. You can get stainless steel 5 gal tanks that are used to keg home brew. thats what I have my quench oil in right now. as a bonus, it has a lid that you can put on when not in use. On the top of the keg are also in and out gas fittings that can be modified to circulate oil through.:thumbup:
here is a link http://homebrewheaven.com/5-gallon-cornelius-keg-used-ball-lock-.htm

ddavelarsen said:
That sounds fantastic - almost too good to be true. :D

I'm thinking of building (or preferably finding) a tank with dimensions like Fred described - narrow, long and the right depth to hold 5 gal comfortably. It would need a pump to direct a stream of oil toward the blade, but I haven't quite figured out the right "direction" for that stream to flow. It'd also need a heating element and ideally a thermocouple to maintain temp. Should be stainless steel with a sealable lid, if I'm going to dream up something totally silly and ideal.

Anyone have any ideas what an existing container of appropriate dimensions might be? I'd prefer not to have to have it fabricated if possible. ;)
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Stacy E. Apelt -your elephant analogy was great --one disturbing thought that arose in my mind was I wondered what twisted course his sex talk took with his poor children. I would imagine they could suffer from that one for a long time ..Steve
 
I have been looking for information about various quenches over the internet for the last few days. I have compiled a google document with alot of information about all of the quenches I have come across. There is still alot of information missing....I left it open for editing if anyone else has more information they would like to add(inserting cooling diagrams would be great if anyone has them, or possibly suppliers for the various oils, or anything I left out).

http://spreadsheets.google.com/ccc?key=rHQbl5EXrksvteEEYMBkR7Q

Hope someone finds it useful!
 
That's excellent! I appreciate your 'open source' kinda attitude too. :thumbup: I don't have anything to add immediately but I've sure bookmarked it for future reference and contribution. Thanks for your work!
 
ddavelarsen said:
That's excellent! I appreciate your 'open source' kinda attitude too. :thumbup: I don't have anything to add immediately but I've sure bookmarked it for future reference and contribution. Thanks for your work!
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Thanks!

I spent about 3 days digging through the internet trying to find as much info as possible, and got various things confused with one-another and overwhelmed. So I started throwing stuff into a spreadsheet, and figured why not put it on google and open source it to flush it out better. Seeing as this very forum is where I learned about most of the quenches and got a fair bit of the information from, I figured this would be a great place to put it and see what happens. Its in pretty rough and inconsistent shape at the moment. I hope that if more information gets added I will be able to go back and standardize the information on the various quenches so its easier/more understandable.

Any information contributed is very appreciated, by me, and anyone else that happens to benefit from finding my spreadsheet. Feel free to completely alter any part of the spreadsheet(esp the parks sheet, it needs alot more info) to make it look nicer or be more informational.

[edit] Feel free to link the sheet to any other metal working forum or give it to anyone you think would find it helpful. The more people looking at it, the more likely someone with some knowledge will help flesh it out better.
 
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I should also have mentioned - I'm a spreadsheet kinda guy too. I use spreadsheets for everything. :D
 
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