stainless steel bushes and stays

[jetex]

I understand why it is not a good idea to use brass for the bushes in copper boilers and that the preferred material for bushes and stays is bronze but is there any reason why 303 stainless steel should not be used for these parts as it is cheaper and stronger than bronze

 

[xo18thfa]

From a construction standpoint, bronze materials silver solder a lot easier then stainless steel. Even thought stainless silver solders, it can get a mind of it's own sometimes. I would not risk doing all the fabrication work on a boiler, only to have trouble soldering it together.

There may be some chemical reasons to avoid mixing the two metals, but I am not aware of any.

 

[lordedmond]

In the deep dim recess of my brian cell I seem to recall a problem with stainless steel and silver solder if the joint is damp then it will just fall apart



There are two other aspects to be considered when silver soldering stainless steel. The first is the occurrence of a phenomenon called ''crevice corrosion''. This shows itself by the joint literally falling apart after exposure to water. After failure, the joint appears not to have adhered to the steel. The water does not affect the steel or solder but attacks the molecular bond between the two.

maybe that why all the federations do not like stainless steel in boilers


Stuart

 

[xo18thfa]

There is a different flux available for stainless steel. All other metals use the same flux. I have silver soldered stainless with the non-stainless flux and it works, but it's scary. No idea if the stainless flux would work with other metals. My guess it maybe would, but again, I would not risk it.

 

[Florian]

Hi Everyone

Well, the problem with stainless steel is intercrystalline corrosion. The thing is, that stainless steel gets oxidised when its being heated to soldering (or welding) temperature.
If you don't remove every single bit of the oxidised area then it will start to corrode between the grains.
The worst thing of this kind of corrosion: you cant see it with the eyes!!

It is possible to make stainless steel boilers, but they have to be welded by a certified welding operator which has a certificate for welding pressure vessels and also stainless steel.

After welding, stainless steel needs to get acid cleaned to remove all the oxides.
The problem is that the acid used for this process affects also copper and other boiler material.
So you see the problem is not the material itself but more the oxides from the heat.

Another aspect is that the thermal extension of stainless steel and copper is quite different!
Whenever possible, you shoud use the same Material on the whole boiler. (Except ferrules, they can be made from bronze such as CuSn8 or red bronze (= gunmetal).

Oh and of course you can use boiler steel for steel boilers.

Florian

 

[SBWHART]

Ok Thats knocked silver soldering bushes in the boiler on the head, but what about screw fitting to the finished boiler valves regulator etc, ?.

Any thoughts

Stew

 

[lordedmond]

They will be OK as you have not silver soldered them so no crevice corrosion

 

[SBWHART]

Hi Stuart thats what I thought.

I have in stock some very good quality high UTS ally bronze I thought of using for the screwed fitting nuts bolts etc.
Being ally bronze it can't be soldered, don't ask how I know,

Any thoughts on this

Stew

 

[lordedmond]

Stew

ali bronze can be silver soldered

here is the info from cupalloys " get 3 teaspoons of your flux powder and mix in 1 teaspoon of table salt " mix as normal works fine I have tried it

the chlorine ions in the salt remove the aluminium oxide


look here at the bottom of the page
http://www.cupalloys.co.uk/fluxes-c59.html

have fun

Stuart

 

[SBWHART]

Thats a great bit of Info Stuart

Thanks for sharing

Stew

 

[lordedmond]

No problem that what the forum is for sharing good knowledge

Stuart

 

[Dan Rowe]

Florian,
Thank you for your explaination, it gave me enough information to find this on the web.

http://www.azom.com/details.asp?articleID=1177

Quote:
Intergranular corrosion is a form of relatively rapid and localised corrosion associated with a defective microstructure known as carbide precipitation. When austenitic steels have been exposed for a period of time in the range of approximately 425 to 850°C, or when the steel has been heated to higher temperatures and allowed to cool through that temperature range at a relatively slow rate (such as occurs after welding or air cooling after annealing)

Dan