View Full Version : Diff. heat treat with salts??


David R
11-23-2004, 11:02 AM
I hope this is the correct location for this thread. I've been forging and heat treating knives for about two years. I've attended the ABS intro and damascus class. I'm a part time maker. I've been researching everything I can on high temp salt setups. I feel like I have a fairly decent amateur understanding of metallurgy for bladesmiths. I sure like the idea of quality control of temps for normalizing, thermal cycling, and heating for quench.
However, I know the ABS is big on differentially heat treating. For those of you who use salts, are you full quenching and then tempering followed by drawing the spine. Or are you heating in salt prior to quench and then edge quenching? I am looking to fine tune my temps and still produce a quality differentially heat treated blade. What do you pros do? Any thoughts would be greatly appreciated.
I am mainly working with 1080, 5160, and 1080/15n20.

David Rice

David R
11-23-2004, 11:10 AM
I guess I should let you know that I currently ht with my forge by placing a round pipe in to try and even the heat and then work the blade in the pipe to get up to quenching temp. Would a pyrometer inside the "heat treating pipe" be a good alternative to salts?

Jesse Frank
11-23-2004, 12:30 PM
As far as I'm concerned, having the ability to know exactly how hot you are is paramount to getting consistant results. The pyrometer will be a necessity for either setup. :)

Ed Caffrey
11-23-2004, 03:20 PM
I personally think that salt tanks are the ultimate for heat treating. When set up with a pyrometer, you can dial in exactly the temp you want. As for differential heat treating, the salt pots are great. Since you have a ground blade the edge section is going to be thinner, and therefore heat up more rapidly than the rest of the blade. You can use the salt pot to heat just the edge by placing the blade into the molten salts for a specific time to just heat the edge to critical, while not allowing the rest of the blade to get to critical temp. Pull it out and edge quench in pre-heated oil!
The times are going to vary depending on the size and thickness of blade you heat in the salts (This is where proper grinding geometry will help you out)........but as an example, a blade the size for the JS/MS test (10" blade & 15" OA) takes about 30 seconds for the lower 1/3 of the blade to reach critical, while the spine remains below critical.
I recently taught a class with Lyle Brunkhorst where one student heat treated his test blade in this manner. When we got to the bending portion, his blade was super tough to get to 90 degrees, and returned to about 10 degrees of bend! The student wanted to "break" the blade to see the grain structure........it took 12 180 degree bends before the blade fatigued in half. The grain was some of the best I've seen! (this was a 5160 blade) If I'd had any doubt before, this situation reinforced that heat treating in molten salt is where it's at!

David R
11-23-2004, 08:51 PM
Thanks for the response! Please keep them coming. I searched previous threads and ordered the digital pyrometer from Baileys Pottery that Ed suggested. I thought I would start with monitoring my forge temperature while I plan my salt baths. My "shop" is currently in my garage. I am getting ready to build a 12x16 shop in my backyard. Is there a way to use salts without rusting out everything in your shop? I spoke with a JS here in Missouri who advised the salt bath needs to be seperate from the rest of the shop or it will rust everything else. Now to try and find stainless pipe.

Ed Caffrey
11-23-2004, 11:38 PM
Depending on the type of salts used, the fumes can be very corrosive. I don't seem to have much problems with it, but then again, here in Montana it's a pretty dry climate, without much moisture to get on stuff.
The bigger problems with salts (at least I think so) is that they are hygroscopic (spelling?) which means they will suck up moisture out of the air, and sometimes you will have to take thw water off the top before you use them. This reminds me of a very important safety tip for using molten salts............ before they cool down, after use, insert a tapered iron/steel rod all the way to the bottm and let it cool in there. If you don't do this, the next time you fire up the salt pot, you will likely have a volcano on your hands. The salts in the bottom of the pot/tank will melt first and will expand to the point that it will literally blow the unmelted salt plug out of the top of the tank. I made this mistake ONCE....thank goodness I was outdoors and only had to conted with a couple of small grass fires. :eek:

Jesse Frank
11-24-2004, 11:52 AM
Ed,

Do you think that the grain structure had more to do with time and temperature than the medium used to heat it?

IMHO, the salts work very well for what we do, but I wouldn't consider them to be the ultimate in heat treating. I would think that the induction furnaces and quench routines ect. used in industry are the current ultimate.

Ed Caffrey
11-24-2004, 03:06 PM
I think it had to do with the ability to control those factors.... that the molten salts offered. The control of the temp, combined with the knowledge of the time required was , without a doubt paramount.

twistedneck
12-05-2005, 10:33 PM
The bigger problems with salts (at least I think so) is that they are hygroscopic (spelling?) which means they will suck up moisture out of the air, and sometimes you will have to take thw water off the top before you use them. This reminds me of a very important safety tip for using molten salts............ before they cool down, after use, insert a tapered iron/steel rod all the way to the bottm and let it cool in there. If you don't do this, the next time you fire up the salt pot, you will likely have a volcano on your hands. The salts in the bottom of the pot/tank will melt first and will expand to the point that it will literally blow the unmelted salt plug out of the top of the tank. I made this mistake ONCE....thank goodness I was outdoors and only had to conted with a couple of small grass fires. :eek:

Ed, i'm in the process of creating a salt pot. :eek: :eek: :eek:

I am interested in why the tapered rod helps to stop air pocket exploding salt.

From what I've read, some put the rod in and hit it with a hammer and remove it before using the pot. Can you explain this? Is the rod a conductor to leave in while heating?

Edit: I noticed some folks use carbon rods to minimize decarb in the salt bath, some also place carbon on top (powder?) to combat fumes and insulate. Is this necessary?

Thanks. Jeff

Brad Humelsine
12-06-2005, 07:09 AM
I agree with Ed. Salts are top of the line when it comes to control of your heat treatment. I have both high and low temerature salt baths. Both are electronically controlled and are very stable.

Using the salts I get far superior results then I did with any other method. The quality of the blade is much more consistent, warpage problems became a thing of the past.

Like Ed said, safety is a concern. I too had an episode of the exploding salts. My high temp pot had two burners now, and I use a mandrel. I run only the top burner during the initial heating, then run the pot with only the bottom burner. As for being hygroscopic, that is huge problem here in South Florida. My heat treat rig is on wheels, when I need it I roll it outside, when not in use it has to be stored in my air condtioned shop.

If you are willing to expend the time and money to build a rig, you will not regret it. However like everything else salt pots do have a learning curve.

Good Luck!

Jesse Frank
12-06-2005, 08:02 AM
" Salts are top of the line when it comes to control of your heat treatment."

Salt pots are great! They just aren't top of the line. I've got a buddy that works in the aerospace industry, and they do not use salt because of control issues. They do work very well for what we do, though.:)

Be carful not only with the high temp pot, but the low temp one as well. One of the main ingredients is potassium nitrate.


Boom.

Darren Ellis
12-06-2005, 11:34 AM
To remove some of the concern about corrosiveness, built them so they are portable and roll them out of the shop on a clear day to heat treat. I put mine on one of those garden carts available at Home Depot for something like $85.

One of these days I'll put up a step-by-step tutorial for building mine...just don't have the time to do it right now with running the business and doing my ph.d work (uugh school!) :)


-Darren

J.Arthur Loose
12-06-2005, 12:04 PM
I have a very effective "venturi," draft set up on my high-temp salt tank.

http://www.jloose.com/siteimages/salttanks.jpeg

The heating chamber vents out of an elbow pipe in the back of the rig, where it opens into the 4" stove pipe. It passes the opening of the salt fume collector and induces a very strong draft. You can watch the fumes get sucked right in... though while heating up I have a cap (pictured,) that fits the stainless pipe with a hollow handle fitting that further directs fumes straight into the collector.

I don't use the tapered rod, though it's a good idea. (It's the law in CA!) The rod works by providing a release for any pressures if one were to heat the bottom / middle faster than the top. It seems to me that either removing it or leaving it in place would work, provided that it is evenly tapered and fairly smooth. I'd leave it in place, as all it has to do is bump up slightly to release pressure, and I'd be afraid of damaging the salt tube by hammering on it. Instead I have the two burners pictured above; I simply start with the top one until my thermocouple reads about 500 F, then turn on the lower one. Take note: that 500 mark works in *my* tank and yours might work differently...

I also have a small hole in the top of my D.Fogg style forge into which I can drop a thermocouple probe. For small knives I sometimes heat treat in this manner. I can set the forge itself to 1475, with a reducing atmosphere.

Darren Ellis
12-06-2005, 12:18 PM
Jonathon, that salt fume collector looks like a great idea. I think I can picture everything that you are doing, but do you have any close-up pics of that part of the assembly by chance? ...you knew somebody had to ask, right. ;)

:)

-Darren

twistedneck
12-06-2005, 06:04 PM
Hey you guys are some bad assess!! those tanks are sweet as can be.

Jesse so if they dont use salt for heat treat, what do they use? oil? we use oil for vehicle spung parts all the time, along with aquious polymer and air.

Thanks for the info on the salt rod J Loose, no point in taking it out until some molten salt builds up. salt has a low specific heat anyway.

This tank i'm buidling.. welded 316L steel from stainlessandalloy.com, 26" x 4" sched 40 pipe with a threaded end, std. cap, picked up a fuji dxr4 (can use it for my espresso machine too), two ssr's, shielded thermocople K type, two ceramic band heaters 900w each. 26" chamber. I'll be sure to program heatup with the top burner first. Now.. how long to soak 4130 at 750F?..

Jesse Frank
12-06-2005, 06:49 PM
Glycol for quenching, and induction furnaces for austenitizing.....

From what I can ascertain, they can do some really gnarly stuff with that setup...... I think they austenitize and quench in the same tank.

I don't mean to down salts at all.... they work great! They WERE the ultimate in heat treating, but there have been some advancements made since they started using them, so I have a hard time with the idea that they ARE the ultimate. But they still work great:)

Darren Ellis
12-06-2005, 07:22 PM
Glycol for quenching, and induction furnaces for austenitizing.....

From what I can ascertain, they can do some really gnarly stuff with that setup...... I think they austenitize and quench in the same tank.

I don't mean to down salts at all.... they work great! They WERE the ultimate in heat treating, but there have been some advancements made since they started using them, so I have a hard time with the idea that they ARE the ultimate. But they still work great:)


ok, then how about we redefine it to say "they are the ultimate for home and small shop heat treating." ...don't know too many people who have built their own induction furnaces for home use! :)


:)

-Darren

Jesse Frank
12-06-2005, 07:31 PM
:101

RJ Martin
12-08-2005, 08:11 AM
What makes induction furnaces and salt pots so good is the rapid heat-up rate and precise control of temperature. I once saw an induction HT setup that heated a 5" diameter x 12" long dowel pin to HT temp in less than 1 min. Then, the whole thing was flooded with coolant. The pins were used in building Caterpillars-I guess that is part of their longevity, the deep case hardening of their pins.

Either setup has it's dangers, both have advantages.

J.Arthur Loose
12-08-2005, 09:10 AM
Here's a pic of the venting bits... the pipe coming out of the heating chamber is aligned with the direction of the swirl, it comes up past the three-way connection, creating suction as it passes by the elbow which is the actual collector. You can also see the draft inducer. After the air passes through the inducer it goes outside where there is about another 20 ft of pipe for natural draft.

http://www.jloose.com/forumpics/venting.jpg

Hey Twistedneck,

Be careful counting on just threaded pipe to cap the bottom of your salt tube... I'd recommend a professional welding job. The repeated heating & cooling could cause a leak and the molten salt is really viscous... you'd hate to have 1500 F salt leaking out & causing little steamy explosions with the moisture on the floor...

Darren Ellis
12-08-2005, 11:24 AM
[QUOTE=J.Arthur Loose]Here's a pic of the venting bits... QUOTE]

Thank you for posting that, it is perfectly clear now, and a very good idea! I appreciate you taking the time to photograph and post that pic and description.

-Darren

twistedneck
12-08-2005, 03:36 PM
Hey Twistedneck,

Be careful counting on just threaded pipe to cap the bottom of your salt tube... I'd recommend a professional welding job. The repeated heating & cooling could cause a leak and the molten salt is really viscous... you'd hate to have 1500 F salt leaking out & causing little steamy explosions with the moisture on the floor...

Very Very true..

When you weld, there is also a risk even with 316L stainless. I decided on this route - but as you know i can easily have a plate welded in. I will watch it very closely. Is there such a thing as very high temp pipe tape?

One big advantage is i can clean the salt pot easily by simply removing the lower cap and pounding out the salt plug. That is the part i'm really happy about.

Thanks for the heads up, i'll let you know how well it works. Still can't find a good TTT diagram for 4130. Need to get that lower bainite - i'll be quenching for at least 30min. With PID control i should be able to tune it very close to MS temp (710F for 4130).

J.Arthur Loose
12-08-2005, 05:11 PM
Hmmm...

I'd guess that you wouldn't be able to pound the plug out, as it seems to get a pretty firm grip on things... In four years of using my salts I haven't had to clean the tube for any conceivable reason. I did make a ladle with a very long handle after I decided to alter the salt composition & replace half of it.

I really need to replace my stainless tube. I consider it expensive, but disposable.

twistedneck
12-08-2005, 05:15 PM
Any thoughts on high temp pipe tape, that can handle 1000F salt? :eek: :eek: :eek:

There is only one way to find out if the threads hold.. we both know what that is :smokin

316L pipe aint cheap.. so true.

twistedneck
12-09-2005, 10:34 PM
I decided today to weld the bottom instead of threading the cap. Thanks for the help. I will post pics when the stainless steel arrives. It was well over $500 bucks to custom fabricate, weld, and build this thing 316L beast, even with a 304 stainless stand, lid, etc.. :(