My semi-real time heat treating thread

Kevin R. Cashen

Super Moderator
Folks probably know that there is nothing that I take more seriously than my heat treating. Because of this “heat treating” day at Matherton Forge is always an all day affair that gets my total and undivided attention. I could slot some guards or shape some handles while the blades are being borne but that would interfere with my continuous monitoring of the temperatures and process, so heat treating day is also a lot of tedious watching and waiting. At first I thought a thread like this would just be a matter of sharing my boredom with all of you, but then what on earth could be boring about heat treating?

Well the Blade show is drawing near and today is heat treating day, and because it is also “empty the tubes, repair leaks and renew the salts” day it will be a particularly long one. So since the computer in the lab is right there next to the testing equipment and a few steps from the heat treating area I thought I would share some of the day with you.

I started the day by turning on the kilns which hold the low temp Thermoquench salts and letting them get a head start in coming up and leveling off at 420F. Next I finished cleaning out the four foot high temp tube I emptied yesterday. By the way here is an tip on that that I found very effective- if you need to empty a high temp salt tube, don’t even think about doing it at temp when they are liquid, take them outside when they are cold and put a garden hose in the top and let it run. This method quickly dissolves and empties the tube in just a couple hours. Warning- only do this with simple NaCl based salts like Parks Nusal, salts with other hazardous elements like barium should not be removed in this way for the obvious reasons.

I knew I had to clean out the bottom of my tube because the last swords had a bit of decarb at the tip which I had to remove, indicating that a contaminant sludge ahs accumulated in the bottom, and sure enough I found a thick deposit and removed it. This is why I now keep my tubes completely covered when not in use to keep dust and debris from getting inside. I have refilled the tube and it is not coming up to temp to melt the salts, it will be a longer process than usual to get it all melted with the tube topped of and running well.
 
My high temp unit is working quite well today, it has been a many years process to develop it to this point but one of the best things I did was to switch over to T-Rex venturi style burners in place of the old forced air burners. Forced air is what I prefer for forges but the salt bath firing chamber works much better with venturi, provided it burns hot enough to do the job. Ignition is as simple as it can get since I machined socket sleeves to accept the burners and I just light them like a torch and insert them.

I had to meet the mailman out front about an hour ago to accept a package from a good friend who I am helping out with some sword heat treating; the timing seems miraculous but I planned this day according to when the package would arrive. I also have a batch of blades to do myself. The day goes the quickest if I have all the same steels and all the same blade styles since all the temps and times will match. But when there are different austenitizing temps and different tempering temperatures, it takes longer for the units to level off at the next temperature. Often I will do favors for friends and end up with blades that really need to be quenched in oil so seldom are these days short or quick.
 
I am now heating a test bar of Damascus to quench and then etch to be certain that the new salts are all in neutrality and leaving the carbon in the steel where it belongs and not pitting in any way. I finished it out to 600X and will shine it up when it comes from the low temp. A quick dip in the Ferric Chloride should tell me that things are just fine.

Things actually leveled off rather quickly this morning after refilling the tube, but the sword tube has so much thermal mass that it normally heats up even quicker than my 2 foot tall unit. It is purring along at 1500F now and the test bar should be ready to quench in a couple of minutes.

The low temp have leveled off at 420F. I normally set them there for the initial heating and some quenching so that the greater thermal mass will allow for a more stable setting when I drop back to 400F or a hair lower for tempering.

I have drum switches on some of my thermal couples so that I can cross check the probes and their controllers in order to be certain all are running within 2 degrees of my set temperatures. Both high and low temp are reading fine.

I let that test bar soak for an extra long time since I am checking the salts and want to be certain of their integrity. I will go quench it into the low temp salts now and after things level off for a couple of minutes I will air cool it, rinse it, polish and etch.
 
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Couple of questions since you are passing the time:

1) What steels do you always/never/maybe quench in your low temperature pot?

2) Is cross contamination between the high temperature and low temperature pots not an issue?
 
Perfect! The bar etched beautifully and gave me continual 64.5 HRC readings. This is what I have always loved about my Damascus mix by the way, I could use it for a calibration block if I wanted. Unless there is something really wrong this mix always consistently gives 64.5 HRC without fail. It is actually much more consistent than any single carbon steel I have tested. So much for all the old wives tales about not being able to Rockwell any type of damascus.

Well the first blade has gone into the salts. This first one is another job for another maker, which I really am not in the business of doing but this maker got a request to specifically use my damascus in this knife, from a customer who was adamant that it was the best available. Well, I found this so flattering that I reminded them that the steel is only as good as its heat treatment and that I would be happy to run it through with my blades to insure the customer got the most from this steel.

I took just a couple of minutes for rebound and I have now set the timer. You see when the blade goes into the salts it lowers the overall temperature. A blade this size drops the temp from 1500F to 1492F and then takes a couple of minutes to rebound. As soon as the temp reads 1500F again I hit the stopwatch I am wearing around my neck and wait for 4 to 5 minutes to elapse.

All Done! The blade went into the 420F salts and was allowed to assume that temperature before removal fro air cooling. It is cooling now. It is a good blade but there is always those little areas for improvement, this one had the tip just a little off center to my eye and I was looking forward to tweaking that. At 420F that was not problem I just put my gloved thumb on the end and pushed it into perfect alignment. I always agonize over the possibility of warping somebody else’s blade in heat treating and it is one of the reason I don’t often volunteer to do it, but it is a real treat when you can actually send it back even straighter than you got it.:cool::D
 
Couple of questions since you are passing the time:

1) What steels do you always/never/maybe quench in your low temperature pot?

2) Is cross contamination between the high temperature and low temperature pots not an issue?

Thanks for chiming in Gabe, a little chatting is what I was hoping for to pass the time.

1. any steel designated as oil hardening works in the low temp salt- O1, L6, 5160, 8670m, 52100 etc... Steels such as 10XX series, W2, 15n20 get Parks #50 here instead.

2. From low to high is a problem, an immediate problem:eek: From high to low just happens but the high temp salts solidify and flake off to sink to the bottom of the tube. Eventually I will need to empty this tube and clean out the debris as well, but it will take a while yet. Emptying the low temp is more difficult because it does not dissolve as readily in water.
 
O.K. I am into some of my own bowie blades now. They have a greater mass so the rebound goes down to around 1478F and back up. My neighbor has a pole barn with white siding that is really great to sight down the blade on when I have my slider doors open on these warm spring days. I stand at my anvil with a wooden block on top to protect the finish and just guide the blade on it, or in my gloved hands, as it hardens from 400F to room temp. Fro swords I go into the finishing shop to put the tang in a vise and sight down my light colored counter tops. But there I can hit it with intense light and have both hands free to work things.

As I age my eyes really have a harder time focusing down the length of long blades to do this work, but as long as the cataract in my left eye stays as small as it has for the last several years I should be good for a while yet.

When the low temp salts go completely solid you are into the M80% range and it is best to leave the blade alone. Before I temper I will dip all room temp blades in cold water to be certain I have surpassed M90%. If one is not martempering this may not be a good idea though and could result in cracked blades before the temper.
 
I noticed that you mentioned 8670M above. I have been thinking about trying some since I can't find L6 barstock. What is your opinion on it in relation to L6? I would be using a digital oven & 11 sec. oil for H/T.
 
Darrin, I think 8670M it is a pretty good steel as long as you know what it is and that it is not L6. I have heat treated a bit of it and have found it to be pretty consistent, even moreso than 5160 for instance. It should be tough, not as tough as L6 but it will also be easier to work with than L6. If a person has had good luck with 5160 they should do real fine with 8670M. It should harden real well in just about any oil.
 
The hardening part is now done for all knives and now I get to play the greatest waiting game of the tempering cycles. All the knives have been hardened, Rockwell tested and are now between the first two tempering cycles. The first cycle was at 375F for around 40 minutes just to stabilize things. Times in salts can also be much shorter than ovens, but once I find the hardness level I want I will give it a good long and full temper. Now I will go to 390F for a longer temper for any hunters and keen slicing blades. The temperatures will increase slightly for the bowies and even more for the sword.
 
Don't worry; I won't go all safety Nazi on you folks but something occurred again today that is probably worth mentioning. About 2 year ago my good friend Ric Furrer told me to get a CO detector and put it in my shop, something that very few of us do despite the number of us with massive gas combusting equipment. My salt baths are with my forges in a separate room from my finishing area, and in that forging area I have two barn sliders and a regular door along with a vent fan at ceiling level. My CO detector is above my bench in the finishing area with just a normal doorway connecting the two rooms. Since my vent pipe rusted through I ran without it today with all the doors open, but it still set off my CO detector in the other room. I turned on the vent fan and took care of the issue, but it is worth mentioning that my salt baths give off much less exhaust than the typical gas forge.

This is something to consider whenever we fire up a forge in an enclosed area. I myself am pretty glad my friend pressed me to get that detector. Otherwise I would still be breathing that garbage like I did for too many years previously.
 
I got even less than usual done today, because I kept sneaking in to check this thread. Thanks Kevin, it's been fun!
 
All of us suffer with trying to get as much as we would like accomplished in our shops everyday, I am glad I coud help out:D
 
I think of it as research, so in that way it's productive! :)

Seriously, this has been really interesting to me, even though I don't do my own HT. Once again, you've added a lot to my very-slowly-evolving understanding of the procedures, and I thank you for that. Coffee or beer is on me if you ever happen to be in my lil town ;)
 
Kevin, I know this is an understatement, but, a sincere "thanks for posting".
Not trying to be a suck-up, but, I've learned a great amount from you.
- thanks again.
 
The Chinese made benchtop model that I tuned up myself to insure it reads on. It has actually has done pretty well over the years. But I spare no effort on the quality test blocks to insure it does read well.

Kevin,
Being the frugal miser that I am, OK, poor guy with no cash to speak of, I end up buying Chinese (crap) quite often, surprisingly it does do fairly well, although it irks me to no end that the money I spent could have employed a hard working American. I'm not familiar with a China made Rockwell tester, mind sharing the particulars about where to find one, the cost and the the quality test blocks you use as well? After reading the "Quench Wars", I'm getting a tester, just to be on the safe side, and KNOW-not ASSUME what the hardness is of my steel.
BTW- I want to thank you for the time and effort you put into sharing your knowledge, I know it's not something you have to do, the information you provide has been priceless. Especially when I can comprehend what your saying! When you have time, could you post the sequence for Hting the basic steels like 1095, 1084, etc. I know those 2 are similar but all the info out there has different ways of doing it, one says to soak 1095, while others says not to, I guess what I'm saying is dumb it down for folks like me, a step by step procedure would be awesome. I'm sure there are plenty of guys out there (if they would admit it) that would appreciate you doing that. I'm not asking you to do every steel available, just the 10XX series, mater of fact 1095 and 1084 would be more than enough, as they are the most widely used forging steels, that's an assumption, based on the fact that that's what I use.
1084 would good enough for me, because of your explanation of Eutectoid steels I chose 1084 to be my primary steel, which I have to thank you for, it has made my learning curve so much easier. I have my HTing down pretty good for 1084, but after the quench wars, I will be finding somewhere to have mine Rockwell tested until I get my own tester, I used Canola oil for the first few months when I started and finally moved up to a real quench oil, which is the McMaster Carr 11 second quench. I would greatly appreciate your view on this quench oil, since I just bought another 2 gallons, right before I read Quench Wars! I think may have put up with the crazy company that makes the Parks quench oil. But the results on that test were weird.
It's late and I'm rambling, I would love any information, opinions or views that you can share, Thanks Rex
 
Rex, I typed you out a large full page, descriptive reply and then lost it all when I hit the wrong button:mad:

I will see what I can do for you, but right now I am too disgusted and have ran out of time.
 
Rex, I typed you out a large full page, descriptive reply and then lost it all when I hit the wrong button:mad:

I will see what I can do for you, but right now I am too disgusted and have ran out of time.

Kevin,
I know all too well how that feels, it's a sickening feeling. And that's perfectly fine, I understand totally, I also know it's probably the worsed time of the year for such a request. So, when ever you can get to it is fine, it doesn't have to be in your usual detail either, a simple list of how you do it is fine. From your past writings I understand more about HTing than I ever would have. Just a list like: For 1084-
First: heat to XXXX, the temp you start at, and the temp you stop at to quench. How long to soak if there is one, and how you quench. I like a differential HT, for my larger blades, and usually try to get a hamon. My success rate on hamons has been best doing an edge quench. Although, I still do a clay coat sometimes, with 1084 getting a hamon with an egde quench is almost a guarantee. With clay, I haven't dialed that in yet.

I prmarily use Aldo's 1084, but I have some other steels that I use from time to time, I got from Aldo also, I know he's known for his 1084(fg) but he has some fine 1095 as well, and I think the steels he's going to have available for knife makers is going to be awesome. He's busting his butt trying to get W-2 in bar stock. I got some W-2 from him in 1/2 inch bar stock that I like really well, I didn't get that sick hamon on it, but it still made a fine blade.

Looking forward to hearing back from you, at your leisure. I know with Blade coming up, your busy. So it's no rush. I appreciate you taking the time to let me know that you had it written up, even if the puter monster ate it!
Thanks again Bud, Rex
 
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