Boiler tubes.

I've yet to come across anything on this, but is it normal practice for the ends of boiler tubes to be annealed, or not, prior to fitting and expanding them?

 

You open here something of a can of worms.

It's certainly not uncommon, some suppliers do it as a matter of course.

A lot of people don't bother, and it generally doesn't seem to make a difference.

I think modern tube generally comes in a normalised state, this wasn't necessary the case in the past, so historically it may well have been more important. One factor to consider if you are relying on the state the tube is supplied is the extent to which it's been cold worked (and thus work hardened) by swaging or swelling at the tube ends (again some suppliers do this hot, others cold, some aneal afterwards, some don't).

What is poor practice (because it leads to excess thinning of the tube wall) is using plain tubes where they should be swelled, and just winding them out to fit with roller type expanders.

If you're curious about this sort of thing, the HRA guidance note is probably worth a read - although all it says on this topic is "the tubes must be normalised before fitting" (which would imply an expectation that this would be done by the supplier - not many boilersmiths would sensibly be able to generate the controlled conditions required to normalise a full tube)

 

As John says, some railways do this as a matter of course. It is something I've never done and never had any problems in not doing so, and this includes industrial boilers, as well as locos. If boiler tubes are supplied to spec, which they should be, then the specs require them to be normalised and the supplier should provide such after any swelling of the tube ends has been carried out. Whilst normalising and annealing are not the same I would suggest that annealing a normalised tube is not necessary and it should be possible to expand the tube without problem.

 

I've always made a point of annealing the ends of tubes before fitting, and would expect anyone doing the job for Me to anneal the tubes.

 

Why?

 

Well, in simple terms I suppose that if you are working/stretching or otherwise manipulating metal, of any sort it makes more sense, to Me at any rate to do it to 'Softer' rather than 'harder' material, particularly given that the materials will 'Work harden', another point is that it may be that a 'Softer' tube end had better 'frictional qualities' when expanded into a hole than 'Harder' material. I supose that You wouldn't expect a farrier to make a horse shoe from cold steel, would You? To Me it just seems like common sense.

 

If the tubes are supplied normalised, as the spec requires, they are relatively soft. It is the act of expanding them that will work harden them. Annealing them in air will result in them getting a layer of oxide which then needs removing, which is more work.

 

Who said anything about annealing them in air?

 

So in a vacuum or inert gas?

 

Wouldn’t cause any oxidation of the steel that way