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Up in smoke


LarryHouse

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After gathering parts and materials for several years I finally assembled a salt bath heat treating system. I had purchased some heat treat salt from a known supplier, and some from another maker. I charged the vat with it. The salt was part "Nu-Sal" and part another name brand, I will have to find the receipt. I heated the salt to 1480 degrees F. and inserted the first blade, one of Damascus. After a few minutes I moved it to the low temp tank. No problem. I then inserted a second blade, forged from a truck spring of unknown origin, but one that forged as expected. After a couple of minutes the salt started to boil violently. When the eruptions were over the sword was completely consumed. Is this a normal reaction? I would have thought in all the years of my gathering of info about salt baths that if it could dissolve your blade I would have heard about it. Am I the only one that didn't know about this?

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You mention submerging the blade for a couple of minutes, do you have a temperature control on the high temp salt setup? If not, the temp will continue to rise above the working temperature of the salt and become unstable(eruption)usually that happens around 1800 degrees F. Way too hot for heat treating simple carbon steels.

 

 

Peter

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I believe the issue is that the unknown steel sword blade is gone...gone from sitting in the salts for a few minutes.

 

Sounds very odd to me..very odd indeed. If it happened as you say then I am a bit clueless, but sometimes we think we see something when, in fact, something else is happening.

 

 

A few things:

 

1) Most salts come pre-mixed...I do not know why one would mix two heat treating salts. At that point you have two unknown chemistries with heat energy to react...maybe not a good idea.

2)Unknown steel is unknown...but assuming it is steel I see no reason why it would corrode fully in an hour or two much less a few minutes.

How were you holding the blade in the salt?

Did you go fishing for it in the tube? Maybe it is in the bottom?

3) Do you have another bit of the unknown steel to spark at a grinder? It would be interesting to see what that spark was.

4) What is your containment tube made from? If the unknown salt mixture did indeed "eat" the unknown steel blade then I would be concerned about it also eating through its containment vessel.

Have a plan for that...in fact everyone should have a plan for that before lighting the burner on the salt pot.

 

Ric

Richard Furrer

Door County Forgeworks

Sturgeon Bay, WI

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To all the posts. As to temp control, I am using a electric Kiln and an Omega "K" thermal couple inserted into the salt directly. The digital read out read 1480 at the time of the eruption.

 

The sword blade was forged from a truck spring and had been heated much hotter than the heat treat temp while forging with no difficulties noted.

 

I did not need to fish for the blade as the eruption mostly emptied the pot and you can see to nearly the bottom, nothing there to see but salt.

 

I may still have some of the steel to check, will have to look.

 

The pot is a 6 inch dia, 1/4 thick wall 304 stainless tank. It seems no worse for the ware.

 

No one was hurt. As I had planned for many possible disasters, but for a blade to dissolve in the pot was not on the list of possible failures.

 

As to why I mixed the different salts, I have reviewed many different salt formulas and could not see a major difference in the compounds. I had tried to find salt for some time and purchased what I could find rather than mix my own as many have done in the past. I would have thought this was safe to do. I did have one blade act as expected in this recipe.

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The only possible (and I think this is far-fetched) explanation I can come up with is if you somehow mixed the NuSal with nitrates and got it that hot. I have no idea what would happen, but it wouldn't be pretty, I don't think. I have never used Nu-Sal, and have always used Heatbath high temp. salt.

 

I have been using salt pots to heat treat blades for almost twenty years, and experienced most everything that can go wrong. In no circumstances was it ever possible to dissolve a steel blade by chemical reactions nor temperature. I find this very difficult to believe. I suppose anything is possible, but I dunno how you could achieve that.

 

Every time I have ever boiled the salt out of the tube violently, it was due to moisture being introduced somehow. Either by immersing a non pre-heated blade in high humidity conditions, r adding salt that was not really dry, in spite of the fact I would have sworn to it.

 

I have lost tongs with blades in them, but they always just fell to the bottom and wrecked the sword point. I have dropped blades into the pot, and had to dump it out to retrieve them. Never had one disappear though. :huh:

 

I'll never go back to life without salt to heat treat. It enables things not easily nor well done other ways to be very precise and predictable and consistent. Yes, there are some dangers to be aware of, but it is not as though we don't play with fire, and power tools, and dangerous forms all the time doing this. It is merely a different one, it must be respected, but so must power tools of any kind. Indeed even hand tools can injure one grievously if you don't pay attention whilst working them. Done it both ways, myself. :rolleyes:

Edited by Howard Clark
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The only possible (and I think this is far-fetched) explanation I can come up with is if you somehow mixed the NuSal with nitrates and got it that hot. I have no idea what would happen, but it wouldn't be pretty, I don't think. I have never used Nu-Sal, and have always used Heatbath high temp. salt.

 

I have been using salt pots to heat treat blades for almost twenty years, and experienced most everything that can go wrong. In no circumstances was it ever possible to dissolve a steel blade by chemical reactions nor temperature. I find this very difficult to believe. I suppose anything is possible, but I dunno how you could achieve that.

 

Every time I have ever boiled the salt out of the tube violently, it was due to moisture being introduced somehow. Either by immersing a non pre-heated blade in high humidity conditions, r adding salt that was not really dry, in spite of the fact I would have sworn to it.

 

I have lost tongs with blades in them, but they always just fell to the bottom and wrecked the sword point. I have dropped blades into the pot, and had to dump it out to retrieve them. Never had one disappear though. :huh:

 

I'll never go back to life without salt to heat treat. It enables things not easily nor well done other ways to be very precise and predictable and consistent. Yes, there are some dangers to be aware of, but it is not as though we don't play with fire, and power tools, and dangerous forms all the time doing this. It is merely a different one, it must be respected, but so must power tools of any kind. Indeed even hand tools can injure one grievously if you don't pay attention whilst working them. Done it both ways, myself. :rolleyes:

 

 

Thank you, if it had not happened to me I would not thought this was possible either. When the reaction first started I thought a small amoutn of rust or moisture may have entered the pot and just steped back and watched. But as the smoke began to roll out of the pot I choose to open all doors and get out of the building. When the smoke began to clear I went to the pot to remove the blade and get it clear of the carnage. To my surprise all I had was a tang. I tried to attach some before and after shots.

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I'll try again to post a picture. This is a before, an after and the blade that vanished on top of the billet that survived. Remember I made 2 swords, one automotive rear spring and one with Chris Marks Damascus. The last one is the survivor.

100_2205.JPG

100_2215.JPG

100_2141.JPG

100_2238.JPG

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A couple of issues - first regarding the salt:

 

Your furnaces look nice - well put together it appears.

 

It sounds like you had a great deal of moisture present. Or you had a T/C fail - you should have an excess temperature T/C wired separately to shut off the heating elements in the even of a thermocouple failure. It sounds like you had a considerable amount of nitrate salt present. This can cause a violent exothermic reaction - maybe enough to cause the blade to be consumed. The discoloration of the salt leads me to believe that this is the case. I would really be interested in finding out the other brand of salt used, and its name.

 

Second, in the first picture, you have a solvent can directly adjacent to the salt bath. There is also a lot of paper and other debris present which is a BIG fire hazard. There are also a lot of aerosal cans and other flammable solvents and similar near to the furnace - not a good idea.

 

I am glad you are not hurt - or that you didn't lose any major property. It could have been ugly.

 

Scott

Edited by kb0fhp

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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A couple of issues - first regarding the salt:

 

Your furnaces look nice - well put together it appears.

 

It sounds like you had a great deal of moisture present. Or you had a T/C fail - you should have an excess temperature T/C wired separately to shut off the heating elements in the even of a thermocouple failure. It sounds like you had a considerable amount of nitrate salt present. This can cause a violent exothermic reaction - maybe enough to cause the blade to be consumed. The discoloration of the salt leads me to believe that this is the case. I would really be interested in finding out the other brand of salt used, and its name.

 

Second, in the first picture, you have a solvent can directly adjacent to the salt bath. There is also a lot of paper and other debris present which is a BIG fire hazard. There are also a lot of aerosal cans and other flammable solvents and similar near to the furnace - not a good idea.

 

I am glad you are not hurt - or that you didn't lose any major property. It could have been ugly.

 

Scott

Busted. These guys were supposed to be in their own building and were in storage. This was a last minute rush to do this heat treat and here we are. Still I am bewildered at what happened. Nitrates can do this? The low temp salt is all nitrates or nitrites. One recipe is 50/50 sodium nitrate, potassium nitrate. The other chemical was a pink powder with lumps of a white powder. Don't know yet what it really was.

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Larry,

 

Glad you and you shop lived thru it.... Like everyone else I've never heard of this either... The blade isn't stuck in a up in a rafter is it?laugh.gif

 

It would be good to get to the bottom of this cause it sounds very dangerous....

 

Too bad about the blade .. is that a pic of it? was looking good...

 

Also please resize your pictures before you post them ... I'm sure your read about they use too much bandwidth bla bla.... Thanks

 

Dick

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Nitrate salts are for low-temp use only :excl: . Most explosives and all accelerant powders are nitrate-based, to give you an idea of the dangers involved with getting them too hot.

 

Dunno about HT salts, but food-grade curing salts (potassium nitrate and sodium nitrate) are pink.

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Busted. These guys were supposed to be in their own building and were in storage. This was a last minute rush to do this heat treat and here we are. Still I am bewildered at what happened. Nitrates can do this? The low temp salt is all nitrates or nitrites. One recipe is 50/50 sodium nitrate, potassium nitrate. The other chemical was a pink powder with lumps of a white powder. Don't know yet what it really was.

 

The only Pink salt I have is low temp...its a Heatbath product...name is..um...marquench "B" I think?

Marquench 305 deg F (152 deg C) 350-1100 deg F (177-593 deg C)

MARQUENCH Low-cost quench salt with slightly higher melting point and narrower working range than Low Temp Draw Salt and Thermo-Quench salt. It is as fluid as lower melting salts above 400 deg F offering equivalent quenching and drain off. (330 Parkettes)

 

My high temp is Uni-hard

 

It appears you have mixed high and low temp...not a good idea to put low in the high.

I'd gather all that salt up, keep a small sample in a jar, dispose of the rest.

 

Still do not understand the steel going bye-bye.

 

 

Ric

Richard Furrer

Door County Forgeworks

Sturgeon Bay, WI

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The only Pink salt I have is low temp...its a Heatbath product...name is..um...marquench "B" I think?

Marquench 305 deg F (152 deg C) 350-1100 deg F (177-593 deg C)

MARQUENCH Low-cost quench salt with slightly higher melting point and narrower working range than Low Temp Draw Salt and Thermo-Quench salt. It is as fluid as lower melting salts above 400 deg F offering equivalent quenching and drain off. (330 Parkettes)

 

My high temp is Uni-hard

 

It appears you have mixed high and low temp...not a good idea to put low in the high.

I'd gather all that salt up, keep a small sample in a jar, dispose of the rest.

 

Still do not understand the steel going bye-bye.

 

 

Ric

That is where I am at now. Start over with all new chemicals with known pedigree. Get the equipment moved to its own shelter and try again.

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That is where I am at now. Start over with all new chemicals with known pedigree. Get the equipment moved to its own shelter and try again.

 

I think you will need a new tube as well. i do not know a way to remove all the previous salt...I am not sure even sand blasting will do.

 

Ric

Richard Furrer

Door County Forgeworks

Sturgeon Bay, WI

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Well really, about the only possible thing with salt pots more dangerous than mixing a chloride based high temp salt like Nu-Sal, or Uni-Hard with a low temp nitrate salt and running it at the high end, is mixing the nitrate salt with a high temp case hardening bath that has potassium or sodium cyanide compounds in it. It goes directly to boom by all written accounts and precautions. I feel no need to test that one, nor to have the cyanide based salts around at all. Even I draw the line somewhere. :rolleyes:

 

If you did put the pink stuff in with the high temp salt that explains the violent reaction perfectly, but the missing blade is still a mystery to me. I'd look in the rafters as Richard suggested, or for an exit hole through the roof. :blink:

 

You were very fortunate that this is all that happened bad, really. The two salt families are not to be mixed. Even in small amounts, low temp nitrate salt in the high temp salt can cause a great many other problems with surface chemistry and reactions on the steel, though I never had enough to get this kind of reaction.

 

I don't know if I'd scrap the tube, but I would sand blast it for sure, and maybe cut the end off first and re-weld it after.

 

Glad you are all right. :)

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dunno if it helps at all, but I've cleaned my pot out by running a garden hose in the open end while laying on the ground outside for several hours. Got most of it out then just a bit of mechanical agitation. Then one could dry and sandblast i suppose. I"m using Nu Sal by the way. This was all suggested by Kevin Cashen some years ago. Also don't know what it does for your dirt chemistry, or environemnt.

 

Dan

Dan Pfanenstiel

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Busted. These guys were supposed to be in their own building and were in storage. This was a last minute rush to do this heat treat and here we are. Still I am bewildered at what happened. Nitrates can do this? The low temp salt is all nitrates or nitrites. One recipe is 50/50 sodium nitrate, potassium nitrate. The other chemical was a pink powder with lumps of a white powder. Don't know yet what it really was.

 

Absolutely nitrates can do that - they have an exothermic reaction that is really impressive. High temperature salts for austenitizing are all chlorides. Martemp salts are nitrates. If it was pink, then it is likely that it was nitrates.

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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dunno if it helps at all, but I've cleaned my pot out by running a garden hose in the open end while laying on the ground outside for several hours. Got most of it out then just a bit of mechanical agitation. Then one could dry and sandblast i suppose. I"m using Nu Sal by the way. This was all suggested by Kevin Cashen some years ago. Also don't know what it does for your dirt chemistry, or environemnt.

 

Dan

 

That isn't a real good idea either - unless you spend a long time drying out the furnace. The best way and also the most tedious, is to chip it all out.

 

Because of the exothermic reaction I really do think that the blade was consumed. Think of it as a type of thermite reaction. You are really very lucky.

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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That isn't a real good idea either - unless you spend a long time drying out the furnace. The best way and also the most tedious, is to chip it all out.

 

Because of the exothermic reaction I really do think that the blade was consumed. Think of it as a type of thermite reaction. You are really very lucky.

 

Why is the tube intact if the exothermic reaction was so great?

I do not think a 10% difference in iron content and would have saved it (304stainless vs carbon steel) if that were the reaction.

Surely there would have been a brighter glow and temp spike if there had been a thermite reaction.

 

I am not into chemistry as much as you are, but it just does not feel right.

 

Ric

Richard Furrer

Door County Forgeworks

Sturgeon Bay, WI

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Why is the tube intact if the exothermic reaction was so great?

I do not think a 10% difference in iron content and would have saved it (304stainless vs carbon steel) if that were the reaction.

Surely there would have been a brighter glow and temp spike if there had been a thermite reaction.

 

I am not into chemistry as much as you are, but it just does not feel right.

 

Ric

 

That is the only thing that really makes sense. I am not sure what the tube was made of - if it was refractory (like many are) the melting temperature is much higher that of steel. Because of the color of the salt after the "incident" I would suspect it was consumed. I do know that nitrate salts do produce a strong exothermic reaction if heated to typical austenitizing temperatures.

D. Scott MacKenzie, PhD

Heat Treating (Aluminum and Steel)

Quenching (Water, Polymer, Oil, Salt and Mar-Tempering)

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The garden hose tip work very well with the salt tube itself. Used that change out the high temp salts in a 40 inch tube I had.

My question is what alloy was the tube made from? Can we see pics of the blade tang remains?

Patrick

The tube is 1/4 wall 6 inch dia 304 stainless. I'll post a pic of the tang today. The salt formed a concrete hard rock in the bottom of the tank. Still working on chipping it out.

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That isn't a real good idea either - unless you spend a long time drying out the furnace. The best way and also the most tedious, is to chip it all out.

 

Because of the exothermic reaction I really do think that the blade was consumed. Think of it as a type of thermite reaction. You are really very lucky.

As far as the drying out of the tube, that will not be a problem as All I have to do is put it in the kiln and cook for a few hours at a high temp. I burn out investment castings in a similar kiln. I have no doubt that the blade was consumed. Exactly the mechanics of the event I may never fully understand, but it was a learning experience I will make all attempts not to repeat. By the way, I took a friends advice and used peanut oil heated to 400F for the low temp tank. seemed to work very well

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