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1/14/2017 8:11:35 PM
Posted: 8/30/2008 3:01:05 AM EST
[Last Edit: 9/1/2016 1:57:27 AM EST by 22zSteve]
This was a reply to another thread but I figured Why not let everyone read it.

I will try to be as thorough as I possibly can. I realize heat treating terms are confusing and it seems everyone uses alot of the terms interchangeably. I will however back up my info with other sources as I find them and see fit. OK, lets go.

Austenitizing: Forming austenite in carbon steel by heating to the austenitizing Temperature. 4130 is 1600deg. This is the initial heating of the steel. Some good Info

Soak: Time at desired temp to allow structural change. A half hour is probably sufficient for a receiver. My kiln will start clicking on and off, thinking it is up to temp about 1 hour before everything inside is actually up to temp. I say this to remind every one to make sure you know what the temperature really is. It is easy to under fire if you are not carefull. Do some testing with Pyrometric cones

Quench: Rapid cooling through the critical zone to create desired properties. For our purposes it is usually done in water or oil. I have never used water, but I am curious how it would work in a jig.

Tempering: Also called Drawing, it is raising the austenitized and quenched steel to a temperature that draws the hardness back, increasing toughness and reducing brittleness. 700deg will give a hardness of around 41 on the Rockwell C scale, Which is suppose to be around spec for a 4130 receiver. I have heard of guys doing it in their kitchen oven at 500deg or so and getting around 45-47Rc, I guarantee your ejector wont dent at this range. Tempering will also require a 30 to 45 minute soak. Also it is best to let it cool to room temp before tempering. After tempering remove from oven and cool at room temperature, There is no need to cool slowly. Heat Treat Terms

(Update 12-2-08) Over The Thanksgiving holiday I quenched a receiver in soapy water and Tempered at 550F in the kitchen oven, It seemed to come out fine. Without a hardness tester we will just have to report after a few thousand rounds.

OK That is the order it is done, Austenitize, Soak, Quench,and Temper. Now lets talk about Oxidation, At elevated temperatures you will completely destroy a receiver if it isn't protected from oxygen. One way to do this is to use a controlled atmosphere oven. This is an oven that has a constant flow of inert gas purging the chamber. I suppose with a little research This could be rigged up to any heat treat oven. There are also things like neutral salt baths where Molten salt is used to submerge the part. This is not even in the scope of our needs. The best method I believe is the purged foil wrap. Stainless Foil is used to completely envelop the receiver. Use glove s, it cuts like a razor. Leave enough to get three folds on all 3 sides, I use a piece 12"x 18" and lay the receiver longways. Crimp each fold real good. and Just before you fold the last end purge the bag with inert gas,(Mig,Tig welding gas, Argon is best) I believe these gasses are heavier than air so hold the open end up, make sure you unhook your wire feed mechanism or you will fill your receiver with wire. Turn on the welder and poke the stinger in and filler up. Your guess is as good as mine as to how long to purge, I just do it until I say to myself "Man that has to be full". To be honest you may not even need to purge it. The amount of oxygen in a receiver might not cause excessive oxidation but if you have gas available I would do it. I have never done it, but putting something like paper or apiece of wood will burn up the oxygen. I've wondered if the extra carbon could cause some form of Carburizing. Another Stainless Foil Link, And Another

OK This first picture is of the jig and its first receiver. As you can see I used washers to hold every thing together. and while the receiver didn't warp, there were some slight deformations (waves) where the bolts squose so tight. The jig is a piece of mild steel 1/4 X 1 1/4 strap with long nuts tac welded to the top. If your receiver has stepdown rails you may have to mill the edges of the jig. The jig slides right in between the top and bottom rails. Also make sure you have finished fitting your trunnions, magazine, Safety selector, and trim the ejector, because they don't file nearly as well after heat treating.

This next pic is of the clamping frame I built for the jig. It does 2 things. The first thing it does is puts even pressure along the entire top edge of the receiver instead of just under the washers as in the last pic. It also covers the receiver therefore holding more heat in during the transfer to quench. It takes time to razor knife the foil off and get it in the oil (although I have gotten pretty fast at it), and if you take too long it can cool too slowly through the critical point. So the extra steel helps hold the heat in. And it works very well. You will also notice in the next pic that I left openings in the bottom. These are to allow oil flow during quenching. It also made it easy to build. It is just 3 pieces of strap welded across. It is also wider than the receiver at the bottom by about 1/16" That way the only place it touches the receiver is along the top at the rails. One more thing I would recommend is design it so that all the bolts have exposed nuts that can be cut or ground off when they gauld and seize. I was lucky with the first one that used 5/16" bolts into the long nuts. When I built the clamping frame I had 1/4" treads tapped in the side of the thing and I had one seize, oh what fun that was. So bolts and nuts from then on. Also use #8 hard if you use 1/4". I had a 1/4" mush bolt brake just before I put it in the oven. Had to unwrap redo etc. what a pain. You should be safe with 5/16" mush bolts, that is what I used on the first job. When things get red hot they tend to relax a bit, so the bolts need to be snug (1/4"#8 - 12 ft-lb 5/16"#2 - 13 ft-lb) Let me also add, that by using the clamping frame you don't even need the long nuts on the jig.

The receiver in these pictures is for demonstration, it should actually have the rails installed.

One last thing, If you build a heat treat oven (Kiln) make sure it is big enough! These foil wrapped receivers get kinda big when all the clamping accessories and foil folds add up. I say that because mine is just barely big enough. I have to put it kitty corner diagonally and have to be careful not to short out the elements and not hit the thermocouple. But I cant complain it does a great job. Here is a link to building a heat treat oven. You can Buy one as well. Well that about sums it up untill I get some questions. I hope it has helped and I would love to here of all your progress and successes. Happy AK building, Steve
Link Posted: 8/30/2008 9:24:13 AM EST
Sticky! This needs a pin!

Add the "how to build a kiln", sources, and it is perfect.

Link Posted: 10/4/2008 7:39:29 PM EST
To build a electric heat treat oven.Hmm ! seems I saw one on British Blades.com Forums .Would have posted details but not sure how to work controls here yet,hope someone can .Nice fixtures by the way.You have had good results with this method?
Link Posted: 1/3/2009 1:00:08 PM EST
do the rec warp during the ramp cycle or during the quench
Link Posted: 1/3/2009 4:58:47 PM EST
Originally Posted By maxpowers:
do the rec warp during the ramp cycle or during the quench

There is NO warping, if a jig is properly used. However it has been my experiance that warping in thin materials happens mostly during the quench. Hope This helps.
Happy Ak Building, Steve
Link Posted: 3/3/2009 9:02:50 PM EST
What is the proper procedure for quenching in order to prevent warping? Do you drop it in the oil / water fast or slow? ( I have read of some who submerge the receiver about 1 inch per second) Do you place the reciever in vertically or horizontally? Do you stir or agitate? Or just hold it still?

Link Posted: 4/23/2009 1:23:04 PM EST
To avoid warping you want to cool the metal evenly so put the receiver in your quenchant as quick as possible. Use salt water or something else (liquid soap was mentioned before and i think that should work) if you use water. Introducing the hot receiver to the water causes hydrogen gas to accumulate on the surface which in turn causes non uniform cooling and possible warping. The salt/soap breaks up the bubbles quicker and allows the metal to cool more evenly and faster. When I treated mine I did not make a jig as labeled above and it turned out fine. Anything that you bent will relax if you don't have a jig to hold it in place.

I didnt use a jig because it introduces too many variables that may or may not be an issue. If the receiver cools quicker than the inside jig, your part will be out of spec because it will cool to the size that the expanded jig is in. The expansion from the jig may be negligible but I didnt want to take the chance.
Link Posted: 5/27/2009 6:12:17 AM EST
[Last Edit: 5/27/2009 6:20:51 AM EST by twistedneck]
22zsteve, Nice Jig for the quench, that is the best one i've seen.

Jigs like that are normally only used during intensive quenching when you have extreme agitation. Most of the time you can get around warping by a combination of pre stress relieving and even immersion. actaully it would be ideal to normalize these before heat treatement to get rid of bending and welding stresses, however heating up to 1600F and air cooling would cause too much decarburization.

Also be very careful tempring 4130 at 550F oven temp. its going to be about 47Rc hardness, that is super hard but OK. the only other issue is tempered martensite embrittlement also called 500FE (500 Fahrenheit Embrittlement, or blue embrittlement) - since that is about the blue oxide temperature for 4130. 750F to 800F is more like what the soviets would have used to hit 32-36 Rc.

And that link to eng-tips.com is a very good resource. I worked with NickE one of the mods there for the last two years before our company started shrinking due to this auto industry slump. however its my experience that you dont need anywhere near 30min soak time at 1600F+ - the decarb will be too great. with thin sheet you only need just a few seconds at that temp since it takes a while to achive it in a convection kiln. unless you plan to use molten salt?

I did not like how foil worked out. it slows down the quench so much that the properties were compromised due to slow quench.

As for how quckly to immerse these in quench fluid? a lot of heat treat places will have them fall of a conveyor right into the quench. usually they fall in a vertical orientation. if you want to make a banana, try quenching it horizontally, i've done that a few times. all swordmakers will tell you vertical is the only way to go unless you are trying to get a curved blade - i.e katana.
Link Posted: 9/30/2009 9:54:28 AM EST
What material did you make the jig out of? What kind of weld wire did you use to weld it up?
Link Posted: 8/9/2010 8:55:10 PM EST
Another way to create an oxygen free environment within your foil wrap is to put some wood chips inside the wrap with the receiver. The heat within the kiln will be high enough to make the chips smolder as long as there is oxygen within the wrap. The wood chips will stop combustion after all of the oxygen has been consumed. This will create an oxygen free environment during heat treating. I know this was mentioned above but, it is a good way to create an oxygen free wrap. I have never tried it on an AK receiver, but I use this technique when heat treating knives. The same principals apply.
Link Posted: 1/13/2011 11:29:17 PM EST
have any of yall thought about using an oil for the quench instead of a water based solution? it has a higher vapor temp. so you could avoid a "vapor sheild" which could compromise the quench?

just throwing that out there, thats why oil is often used in blade making.
Link Posted: 3/20/2011 12:16:48 AM EST
I have thought of putting a receiver into my wood stove using the stainless foil, I dont know if the foil would hold up, I have put metals in the wood stove for a few days to make them soft enough to machine on my lathe. Do you think the foil would melt?
Link Posted: 6/10/2011 10:45:12 PM EST
[Last Edit: 6/10/2011 10:50:11 PM EST by ar_daddy]
Just baked a receiver at 500 for 1hr and wrapped it in 3 layer of foil and just turn off the oven and let it cool down that way , had blue and green tint
to it and no warp age at all ,this next build going to tack weld on the lower rails this time and see what happen .

Eta the receiver came out mostly black kinda liked had a blued finshed to it.
Link Posted: 6/11/2011 5:35:21 AM EST
Originally Posted By ar_daddy:
Just baked a receiver at 500 for 1hr and wrapped it in 3 layer of foil and just turn off the oven and let it cool down that way , had blue and green tint
to it and no warp age at all ,this next build going to tack weld on the lower rails this time and see what happen .
Eta the receiver came out mostly black kinda liked had a blued finshed to it.

I hope this isnt all you did. If you did not heat to 1600 deg. and quench prior to a 500 deg bake (Temper) then you have done nothing to the hardness of the metal.
Link Posted: 6/11/2011 5:44:26 AM EST
Originally Posted By Sniperhandle:
I have thought of putting a receiver into my wood stove using the stainless foil, I dont know if the foil would hold up, I have put metals in the wood stove for a few days to make them soft enough to machine on my lathe. Do you think the foil would melt?

They do make a high temp foil that would probably hold up. I have seen video of pakistani gun builders drag gunframes out of a fire. I would be a little leary about the consistancy of it all. Most modern alloys are a litttle more tempramental. Happy AK building, Steve
Link Posted: 6/11/2011 12:04:07 PM EST
Originally Posted By 22zSteve:
Originally Posted By ar_daddy:
Just baked a receiver at 500 for 1hr and wrapped it in 3 layer of foil and just turn off the oven and let it cool down that way , had blue and green tint
to it and no warp age at all ,this next build going to tack weld on the lower rails this time and see what happen .
Eta the receiver came out mostly black kinda liked had a blued finshed to it.

I hope this isnt all you did. If you did not heat to 1600 deg. and quench prior to a 500 deg bake (Temper) then you have done nothing to the hardness of the metal.

I fire up the trigger and hammer holes and the ejector and quench them in salt and soap water bath .
Link Posted: 3/25/2012 11:34:56 PM EST
[Last Edit: 3/2/2013 11:55:11 PM EST by mg34ss]
Hi Guys, It's been years since I've posted in this forum. So lets get down to business.

#1) First, you have to understand the nature of the material, 4130 steel. 4130 is a steel
built specifically to be heat treatable with the simplest of technology. Unlike 4140, which
requires pyrometers, soak times, and annealing to specific temperatures for proper results,
4130 requires only heat and water. As you can see, ( 40 v.s. 30 ) it has only enough chrome
to barely get hard. It requires a violent quench to harden at all. If you use oil, you will get a lead
soft receiver. So don't do it. Use cold water. Also, if you use oil, you will NEVER get a nice blue/black
oxided finish. The oil will saturate to surface and cause the chemicals to bead up and the result
will be the ugliest splotches you have ever seen on a gun. Don't even go there.

#2) There are several conditions to this steel. Your flat is in a condition called NORMALIZED. It is
kind of half hard. There is also annealed, and hardened. If you want those sharp edges, those 90
degree bends on your corners, you will have to anneal your flat ( soften it ) before you bend it. If you do,
you will get the nice sharp folds the pro's get. The gun will work either way, it just depends on how
much you care. To anneal the flat, heat it bright orange and let it cool slowly. Slower the better.

#3) Scale. We hate it. So how do you avoid it ? Some will say, use foil, Well, that will help if you
put some carbon in the foil to deal with the oxygen. You guys with big shops can also use an
oxygen-free atmosphere in the furnace, like argon or CO-2. You don't need to. Scale requires
time to form. Preheat the kiln and drop the receiver in just long enough to get bright orange hot, say
ten minutes or less. That will eliminate 90% or more of the scale. What is left you can wire brush off.

#4) Warping. Well. what about it ? Your trunnions, front and rear will hold it in place, If it's worse, just
bend it back. If you annealed it before you bent it, you probably will have little problem.

#5) The quench. Cold water and movement. Swish it when the receiver hits cold water. Be sure not
to admire the orange color before you plunge it into the water. You have only about 2 seconds maximum
to get it from the furnace to the water. More, and you risk your hardness. The quench MUST be violent.
Do not lower it gently into the water, plunge it NOW. Fast.

#6) Jigs. Well, I'm going to upset people here. I don't think you need the jig. The heavy sections to
the jig will wreck the violent quench required for hardening. The jig can warp too. It may help some, but for the
casual builder, it's not a necessity. I've built a dozen AK's without the jig. You can too.

#6) Don't freak out. This isn't hard if you have access to an ordinary ceramics kiln. You cannot do this
with a torch. Too much oxygen to make scale and the heat is too uneven. Don't even bother. You'd be better off with a
hot charcoal grill.

#7) Have fun. If you're a total newbie or are prone to disasters, get a couple of flats. They're
cheap enough. This is all about first hand knowledge. My first receiver was an RPK receiver. This was BEFORE
you could buy them. I made one from a sheet of 4130 steel. No Tapco flat, no Cold Steel Solutions 80%.
Just a flat piece of 4130 sheet metal. I destroyed about 15 on my way to building one good one so I know what I'm
talking about.

#8) Look through the archives here and see years of information. It's all free and the more time you spend looking through the
old posts, the better informed you will be about what you are attempting to do. I know it takes time and you're in a rush to build your
AK. Just do it. You and your AK will be better for it.

#9) Enjoy your freedom. In most of the world you would be jailed for doing this. Vote for your freedom and be politically responsible.

Link Posted: 3/30/2012 10:48:21 AM EST
Look Up The Dark Night on YouTube he also has some excellent info on full receiver heat treatment
Link Posted: 6/3/2012 6:09:55 PM EST
I annealed my Polish 80% receiver in a ceramic's kiln last night. I set it up to rise to 500 degrees (f) over the course of two hours, hold at 500 degrees (f) for two more then cool. Here is the result:
Is it normal to be so blue?
Link Posted: 12/23/2012 10:45:39 PM EST
Tho I never posted on this particular forum before, have done a bunch of builds. Concerning the heat treat process, 22zSteve has covered the basics of heat treating conventional chromoly pieces. Whats not taken into account is the thickness of material used. These receivers are only 1mm thick, and dont require any of the duration and saturation time or need for foil wrapping. Also wont have most of the issues compared to thicker materials getting hardened.

Now mg34ss is almost spot on pertaining to the heat treat of these receivers. I had nearly identical results as was described.

First thing is a ceramic kiln is is ideal of using. Mine is an old dinosaur that has no provisions for temp control. I just plugged it in using both heating elements. My last batch consisted of 5 receivers needing heat treat. After letting the kiln preheat until at near max operating temp, I simply dropped scrap pieces of sheet metal in and checked with a magnet for the correct temp needed for hardening. As posted its just above 1600 degrees. I believe its right at 1625, but not important, since you just check for when it is no longer magnetic, then time to remove. Do NOT for any reason let the kiln heat up with your receivers inside. It will leech out all of the carbon and leave you with a mere shell of a receiver. You will actually loose about 20% thickness of sheet metal doing so. After the kiln is preheated just drop the receivers in and wait until they are glowing. You will check with a telescoping magnet. Its really that simple. When no longer magnetic time for next step. This part is critical. You literally have no time from removing to quenching. It be be right out and right in bucket of water. The thin material will not hold heat at all once removed from kiln, again not at all. You need to remove and quench immediately.

Lets go ever everything before proceeding further.

Preheat kiln. Check with scrap sheet metal pieces for when material is no longer magnetic. At this time you can drop your receivers in and wait a few minutes. It doesnt take very long for them to reach proper temp.

When receivers are no longer magnetic, remove from kiln and quench in water. Again I want to stress the importance to have everything ready, right next to kiln to transfer with out any delay at all. You need to have proper safety gear and even practice on scrap to find out any problems not expecting to encounter. I have very thickly padded and insulated Asbestos gloves to protect my hands, but they still got extremely hot just reaching them down into kiln. I was using extended needle nose and channel lock pliers to remove receivers from kiln, and hands still felt the heat thru gloves. Imagine wearing a pair of tight fitting blue jeans and backed up against a nice fireplace to warm up after being out in the cold. Just about the time your legs start feeling the warmth pass thru the denim, it becomes too hot almost immediately to the point you find yourself stripping out of them asap.

I have never used a jig to hold the receivers from warping. What I have encountered is some warp, some dont. Some a little, some a lot. But ALL of them warped because of the rails will cool at a different rate than the rest. You have thicker material, and so will not be able to achieve complete uniform cooling. You may think that quick as it is plunged in water there is no chance of happening, it does and isnt a big problem. I simply bent them back afterwards. They are very easy to straighten with a little patience and common sense.

I have never used foil either. Its simply not needed. The receivers will not oxidize or even discolor. After they are quenched, there will be little difference in appearance than before. A little duller, but thats about it. You may see some very light clear bluing where hydrogen formed a barrier in the water in some areas.

For drawing the temper, I unplugged the kiln and simply waited until it had cooled down below 800 degrees. Then set receivers back in and wait until kiln is cool enough to remove them by hand. Now the receivers will be a dark blueish purple in color, but still very clean. The hardness will now be in the low 40's Rockwell, and good to go.

I have normally plug welded the rails into place. I made several attempts of building a spot welder using a microwave oven transformer or two, but did not get the results I was after. Spent a lot of time developing a very nice fixture to spot weld rails on, but was unable to get enough amps from the home made efforts to achieve decent penetration. So went to plan B on them using a mig welder. It was not that important to me that they had original appearing dimples where spot welds were.

Next step is to parkerize after everything was finished and assembled. Since the receivers are now a little harder than what most of the EU ones are, your finish will not be as dark compared to other parts getting parkerized. The welded spots holding rails turned black, but receiver was only a dark grey. If you were to draw the temper down to the 30's range on Rockwell scale, then should get a more uniform color. I didnt care about the slight differences in shades myself. The goal was to harden material so would withstand years of use without failure, not a beauty contest.

Anybody trying the localized method of heat treating, it wont work. Heating only small spots with torch and quenching wont do any good. I have tried all kinds of ways in hope to reach some results, but never got the areas any harder than before trying. I did have some ugly no longer flat areas that got heated tho. The surrounding area is dissipating the heat too fast for any chance to maintain high enough temp before quenching. Now with that said, I have also built up plenty that never got heat treated at all. Unless you are in the sandy desert and shooting full auto, its not going to really affect the outcome of your build if not heat treated. If you go thru cases and cases of rounds, then a definite wear pattern in prone areas will become obvious in time.

Now you can try just heating the shell ejector on left rail until becomes non magnetic, then quench immediately might get some reduction in wear. This is the only spot I had any signs of success. I would not even waste the time to draw the temper after either. On early attempts of heat treating the whole receivers with success, I did not bother to draw the temper on them either. After nearly ten years, they are still going strong with no issues at all. This is before I was achieving any level of consistentcy or control of temps.

Just for the sake of curiosity, I have a couple more builds that will be done a little differently. My objective is to first heat treat the receivers as normal. Then heat treat the rails separately. I will plug weld them into place after the fact. I want to find out if this eliminates the hour glassing affect with rails already welded. This way will not have any affect on the durability of receiver. I have repaired other ones that suffered broken rails and they went on to have a long service life. One junk receiver that did not have rails welded suffered no warpage at all. It was just s test part early in trials stage, so I could work out the handling of these and what issues I would encounter doing a sizable batch at one time. Anyway, these are the cliff notes of results and issues encountered and documented, As usual, your results may differ. So I am offering thing without any implied warranty what so ever. I have no idea of your skills level and understanding of machinery and other equipment similar to what I used. If you want to bash me and tell everyone I have no clue about what Is posted, thats OK too. I dont have thin skin and you wont hurt my feelings either. If this post helps you in any way, then great. Anyway, have a nice day!
Link Posted: 2/22/2013 3:07:58 PM EST
What about salt bath heat treating as used in knife-making?
Link Posted: 2/22/2013 6:49:29 PM EST
Originally Posted By CBR900:
What about salt bath heat treating as used in knife-making?

I've had a bunch done that way. Still working on getting it perfected. I use the services of a professional heater though. I don't know how you would do it your self.

Link Posted: 2/22/2013 11:22:47 PM EST
Originally Posted By ratfink57:
Originally Posted By CBR900:
What about salt bath heat treating as used in knife-making?

I've had a bunch done that way. Still working on getting it perfected. I use the services of a professional heater though. I don't know how you would do it your self.

A friend sent me this:

All About Salt Baths
> I actually got an idea to write an informative post about salt baths about a
> month ago, when I saw some very puzzling post/ false assumptions regarding
> them, but the notes I started got set aside. However with the amount of
> recent posts regarding them I though it may be worth while to finish the
> writing and post the information that I have.
> Many of us would love to believe we are some sort of innovator or pioneer
> because we use salt baths, or that we were using them before others. The
> truth is that salt baths are VERY old news in industry. Records go back to
> their industrial use in America and places as distant as China to the turn
> of the 20Th century. By the 1930’s they had been around long enough to be so
> commonplace as to not even be notable.
> The first account I have of a custom knifemaker in America describing a use
> of salt baths is in 1984 at the first ABS hammer-ins in Wyoming, where
> German smith Heinrich Frank explained how he used a little table top salt
> bath to heat treat small blades after he had engraved them. By all accounts
> the mention was pretty much ignored by most knifemakers who found it too
> esoteric. The first full scale demonstration of salt baths, high and low
> temp, in use, that I am aware of, was many years later at the New England
> Bladesmith Guild Ashokan seminar, where smiths such as Dan Maragni, Phil
> Baldwin and Tim Zowada were quite familiar with the wide scale industrial
> uses and recognized the potential for knifemakers. Tim Zowada had designed a
> stackable columnar kiln to be built by the Evenheat Kiln Company in MI
> specifically for salt bath use. Folks like Al Pendray may have been working
> with the ideas in their shop at this time, but I believe Ashokan was the
> first full public demonstration by custom smiths.
> It wasn’t too long after this that I got set up with a high temp unit of my
> own due to my increasing work with swords. If none of my blades would have
> ever exceeded 10” I doubt I would have developed the motivation to take on
> the added expense and maintenance of the necessary equipment. For those who
> not familiar with my work, I have been using these tools for many years, I
> have helped many others, from individual knifemakers to large production
> companies, get set up with and properly use them. I didn’t invent anything
> about them, I am no innovator in their use, but I have plenty of experience
> and a good understanding of their use in knife/sword shops.
> What are salt baths anyhow?
> They are not brine solutions, brine is a very fast quenchant achieved by
> mixing salt (around 9%) with water. Salt baths are a different beast from
> the normal bladesmiths heat treating entirely. When steel is heated for the
> quench several problems arise at the necessary temperatures. Common issues
> in forges or ovens are times required for thorough heating, scaling and
> oxidation, decarburization, uneven heating and over heating of thinner
> parts. Most of these issues are connected to the atmospheres. Air is an
> insulator so heating is slow. Air contains oxygen so oxidation and scaling
> will always be present, even if you eliminate the oxygen in the heating
> chamber there will be momentary exposure when the part is removed. Most of
> the same applies to decarburization, and forges are very prone to
> overheating tips and edges.
> Salt baths were industry's (and eventually some knifemakers) answer to many
> of these issues. A salt bath is a volume of superheated molten salts that
> replace the atmosphere and heating chamber of the traditional oven, kiln or
> forge. The high temperature salts that most knifemakers would use are
> primarily NaCl based and begin to melt at around 1275F and have a working
> range up to around 1600F, but can go higher with other elements added to the
> chemistry. Most knifemakers find a columnar, tube type design to be the most
> convenient so I will focus on that. Picture a tube filled with crystalline
> NaCl salts that is sticking out the top of an enclosed heating chamber
> (either electric kiln, or gas fired). Heat is applied until the salts melt
> and then the liquid can be adjusted to any temperature you like within their
> working range.
> The blade is then placed not into an evacuated or gaseous open space, but
> immersed into this superheated liquid. Because it is a liquid (a conductor)
> the first thing you will note is the speed with which the steel will come up
> to temperature, indeed the only thing I have seen heat quicker is induction.
> The next thing you will notice is how evenly the heating will be. Unlike
> radiant heat traveling through an insulative space where thin sections can
> overheat and thick areas still not reach temperature, the totally even and
> conductive nature of the salts will not allow this. The cycling overshoot
> and undershoot nature of electrical elements is overcome by the conductive
> and convective effects of the mass of salts.
> Then there is the total absence of oxygen or other atmospheres under the
> surface of the salts. There is no scaling nor oxidation whatsoever because
> there is no oxygen involved in the heating. That is not to say that poorly
> maintained salts cannot tear up a blade, they are superheated NaCl salts
> after all. This is why proper salts prepared for, and intended for, the
> purpose are important along with regular monitoring and maintenance being
> essential. Contaminated or tired salts that get out of neutrality will begin
> to pit and decarburize steel on levels that will make you long for a forge
> or kiln again. However if you know what you are doing and pay attention to
> proper maintenance, salts pretty much eliminate oxidation, scaling or
> decarb.
> I take all of my blades to the final hand rubbed polish BEFORE heat
> treating, and could even sharpen the blades if I wanted to without any
> concerns that the blades would be just fine afterwards. All I do is a quick
> rub down with 800X on damascus blades before proceeding to etching, with no
> concerns about scaling, decarb, or overheated edges or tips.
> But why don’t you get decarb and scale when you pull the hot blade from the
> salts, you may ask? Because the blade is wet with a thin protective coating
> of salts, now if you left it exposed long enough you may have problems, but
> that length of time would also negate your entire quenching operation as
> well.
> The inherent benefits should already be apparent, but no salt bath should be
> complete without a digital controller to give the hand free, unequalled
> precision. Wired to a relay or valve to control the electricity or gas
> supply to the heating device is a PID electronic controller to which you
> attach a thermocouple probe which goes into the salts and sends temperature
> readings back to the controller. The controller uses this input to control
> the heat source and regulate the temperature of the salts to with 1F. or 2F.
> of the set temperature, to give one control over the austenitizing process
> that you may have never dreamed of.
> I have seen it fallaciously implied that things like salt baths can have
> negative effects on things such as grain size; this is totally false to a
> level that causes me to seriously consider the motives of anybody who would
> suggest something so erroneous. Serious steel industries turned to this
> technology to achieve controls that are still unparalelled in many
> applications. In the ranges for the steels we work, temperature is the key
> factor in final grain size, and with salt baths even someone new to heat
> treating with a few keystrokes can dial in any size they would like to
> maintain with temperature accuracy not possible with a forge or a torch. It
> is just a fact that I can punch in any number in the liquid range of the
> salts and hold it within 1-2 degrees for as long as I please.
Link Posted: 5/29/2013 1:46:11 PM EST
Does anybody know where to buy small scale salt bat furnaces? All i have come up with is huge furnaces.
Link Posted: 6/18/2013 11:26:42 AM EST
I think the knife-building forums might have found them.
Link Posted: 7/12/2014 8:08:03 AM EST
[Last Edit: 7/12/2014 8:09:54 AM EST by hvymax]
Wow. That is a great setup and by far a best practice but way beyond home builders. At my build party we did the ejectors with a torch before installation as well as the holes. You would make Ihzvesk proud. Most home builders have no concept of metallurgy but fortunately the AK is a very forgiving platform.
Link Posted: 10/11/2015 9:08:05 PM EST
Anyone know if an old wood stove could be used for heat treating? Obviously the temperature wouldn't be as controllable as using a kiln but would it work?

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