"salt bath" to soften metal

I was talking to my metalworker today and he mentioned that when he was younger they used to soak sheet metal in a solution he just knew asa "salt bath" that would soften metal enough to mould with your hands. It is how they used to make wing tips on aeroplanes just by forming around, then it would harden as soon as dry. Would work on aly and steel. It can also be used if a material has been tempered wrong to bring it back to its softest state.

Wandering if anyone here knows what this black magic is and whether it can be utilised in the home workshop?

Cheers

Hi Thomas

never heard that one before. But would be interesting to hear what other have to say about it

Dave

Molten salt. Nasty stuff. Avoid.

His name wasn't Uri Geller, was it?

(Very) hot baths of molten salts are used in the heat treatment of metals, particularly during hardening. I expect your friend was describing annealing, and in particular the reference to wingtips makes me think of the way in which sheet aluminium alloys get pretty pliable when heated and allowed to cool, but tend to work harden as they are bent ionto shape.

Andy

Well Thomas,
Have been an old engineer for nigh on 50 years (now happily retired) but have never heard of this
Bob

You might find some interesting stuff at this site **LINK** as they are a salt bath manufacturer (they used to be linked with ICI). My knowledge of salt bath treatment covers "austempering" spring steel parts and requires the parts sunbathing (in the furnace at +800 deg) first before plunging into nasty molten salt (around 300 deg) as a quench & temper medium! There is a bit more to it than that but the Ajax web site should explain.

Mark

PS. you are unlikely to be doing much of this on a DIY scale - it's way too dodgy!

Salt baths are used by hardening and tempering because of the even temperatures, The one time I tried it in a DIY setting was no succes to say the least, Niko.

Where I did my apprenticeship Saunders Roe/BHC, they used salt baths to anneal and otherwise heat treat light alloy. The sheet metal grade most used was L72, what that is in todays terms I don't know. Anyway the process went something like,,,,,

1 Required part was cut to developed profile, including holes and tooling holes.

2 Part annealed in salt bath. Usually had a time stamp in ink, to indicate the working life of the heat treatment, from memory it wasn't long, 15 minutes rings a bell.

3 Part rushed to wherever it was due to be bent...fleet footed apprentices came in handy here. Most stuff was bent (formed) on a rubber die press, but the simpler parts were formed by hand, using bending bars etc. An inspector was standing by to ensure that the parts were formed within the allowed time.

Salt baths are unpleasant places and should be reserved for the professionals.

cheers

Bill

Definitely not good things to have at home. We used these for heat treating Al Alloys for aircraft use.

With an operating temperature topping 800 degC should any water, oil, or other contamination come into contact with it some serious reactions will take place.

We had some photos on the wall of an incident of a bath exploding due to contamination and it removed a large part of the factory roof and I believe killed and injured a number of people.

Just trawling the net I found **LINK** just for info.

Lofty

.

Edited By Lofty76 on 19/06/2013 07:36:37

I think the fact that it is a 'salt' bath is irrelevant - the important thing is the temperature the salt melts at is consistant, self-indicating (because it melts!), and transfers heat to the metal efficiently. It is the temperature control which is doing the softening while the salt itself is inert.

On a small scale you can use a sand bath provided you have some other method of measuring the temperature. In the past the change in colour of a piece of polished steel (used as an indicator) was used but modern electronic wizardry gives us cheap remote IR sensors as a much better substitute.

Chris

Ordinary sea water will soften iron given long enough. I recovered some (I think 16th century) cannon balls off the sea floor at Fowey and you could cut them with a knife. They eventually just fell apart.

Brian

At school if I remember correctly for tempering aluminium for forming we would rub on hand washing soap and gently heat until it went black it was then soft enough form. But it was a long time ago I wonder if modern soaps would work.

Olly

Sure, at 800 deg C Al would be very soft..... you could pour it in any form you like!

Regards, Frank

We used Cyanide salt baths for case hardening mild steel. That was nasty as well. We softened Al at Vickers Weybridge in huge ovens. There were two processes. Soloution heat treatment and Precipitation.

Soloution - heat to 900 - 1000deg F for 20 - 60 mins and quench in water. This 'dissolves' the alloys into the aluminium and makes it softer.

Precipitation - Age hardened the softened (anealed) aluminum alloys - about 300deg F (Don't know the time, but some alloys precipitate at room temp !!

BobH

Did they have a silver thre'penny bit inside?

Cast iron, salt, water, oxygen, time, rust, and a possibly a very sharp knife dont seem to make sense in the same sentence.

Ian P

Hi, many years ago, back in the late 70's where I used to work in maintenace, we had dewatering pumps in the quarries. In one of these quarries the water was natuerally acidic and once the company had to hire in a land based diesil driven cast iron pump. To cut a long storey short, this pump broke down after a week or so of cotinuous use and I had to assist the sevice engineer from the pump company to assess the situation. When the service engineer opened up the pump to find the impeller all but worn away was no surprise, but he was puzzled by the grey black look of the inside of the casing and suction cover. When I said to him "just scrape it with your fingure nail" he was astonised at how soft the metal was and that he could just scratch it away to a mush.

What had happened was that the acid in the water had just desolved all the iron out of the casting and left the carbon behind in the exact shape of the original casting. Needless to say the whole of the pump casing and associated vacuum chamber where rendered useless which our company had to pay for.

Regards Nick.

Edited By Nicholas Farr on 19/06/2013 14:51:32

Hi all,Perspex sheet can be softend by adding sugar to water and heating .can get over 120c.

I have not tried salt but I think it would work. There is more even heat so you avoid bubbles.

I know this is not salt bath ,molten salt (sodium chloriide) melts at 801 c not nice!

Maurice.

Maurice,

The salts involved are nitrate salts rather than table salts, and they melt at a much lower temp than that.

Mark