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Post by Nigel Bennett on Jan 22, 2008 12:59:17 GMT
Nobody seems to have actually mentioned experience of dezincification. I've seen it in a 1946 boiler of about Juliet size. It was my Dad's engine - his dad had bought it for him in 1948 - and I was having a look at it with a view to getting a boiler certificate for it. All the major boiler joints were "brazed" - it was some kind of brass - and all were sound except for one - the dome literally fell off in my hand. We'd had it in steam a few years before, too! The clacks were in brass and mounted low down on the backhead. When it came to removing them for re-use on the now boiler I made, they just snapped off like carrots - leaving a reddish, spongy stump in the boiler bushes. I've always made my own boiler fittings - and since then I've always used bronze.
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Myford Matt
Statesman
There are two ways to run a railway, the Great Western way, and the wrong way.
Posts: 621
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Post by Myford Matt on Jan 22, 2008 20:58:30 GMT
Oh my gawd:
Clearly not a MECH reader, check out eBay no. 190192830512
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Post by spurley on Jan 22, 2008 21:54:00 GMT
And it's dented
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Post by baggo on Jan 23, 2008 0:57:55 GMT
Someones bought the two 45" lengths! Probably going into mass production of boilers
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Post by the_viffer on Jan 27, 2008 11:48:34 GMT
Oh my gawd: Clearly not a MECH reader, check out eBay no. 190192830512 Well the buyer says he is a bagpipe maker and assuming he is not as incontinent as Aggie The Pew (or should that be Aggie the Phew?), my rat, I think he'll be fine.
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stevep
Elder Statesman
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Post by stevep on Jan 30, 2008 9:33:47 GMT
I may be wrong, but it always been my understanding that the recommendation not to use brass for steam fittings was based on the fact that most of them are screwed into the boiler using fine threads. As some of these could be 40 TPI, it doesn't leave a lot of metal, and if dezincification occurs, these fine threads could fail and the fitting pop out.
When I built my first engine, I used (as recommended) 40 TPI fittings, but I have found that when I need to remove the fitting, the "crud" built up on the threads made them really difficult to remove. So now I don't use anything finer than 32 TPI, which also means that there is more thread depth if one should choose to make the fitting from brass.
Cheers.....Steve
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Post by havoc on Jan 30, 2008 11:08:34 GMT
I doubt that is a reason as the threads are not so much in contact with water/steam as the rest of the fitting. Except the bit sticking out into the boiler perhaps.
The few failures I heard of were like nigelbennett experience. The fitting breaking off when trying to remove it.
Just another tought on the issue: how much of the dezincification has been brought by the soldring/brazing of the fitting? I noticed that when brazing brass it turns out copper coloured after cooling. Also the flame turns greenish. So I wonder how much of those failures started at the brazing stage instead of being due only to water/steam influence.
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Post by GWRdriver on Jan 30, 2008 13:35:11 GMT
That poses a very interesting question. I have never noticed the greeenish flame but I'll watch for it next time I solder brass. I presonally have never had fitting break or crumble but every account I've heard was the same, broken or crumbling.
'driver
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Noddy
Statesman
Posts: 672
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Post by Noddy on Jan 30, 2008 14:16:14 GMT
Now to put some asbestos gear on ready for flamings if I'm completely wrong!
I think we could be looking at several vaguely related problems:
If I can split alloys into 2 kinds;
-those where there is a "solid solution" and only one phase
-and those with 2 or more phases present
I suspect that the alloys most subject to problems of leaching of one metal will be those where there are seperate phases in the solid, for example the brasses with >30% Zn, which are the brittle ones.
That's not to say that the ductile brasses (<30% Zn) will be immune, as there will probably be a greater concentration of zinc around the edges of the crystal grains, which solidified last, but I suspect that they will be less prone.
An analogy would be with Stainless steels, where the ferritic and (as long as no work hardening has taken place) austenitic steels, which have only one solid phase present, are more resistant to corrosion than the martensitic steels, which have multiple solid phases present.
Either of these alloy structures could give rise to the observed crumbly de cemented grains.
That is one possible problem
The other is stress corrosion.
Where a part is stressed, and in a hostile environment, chemical reaction can lead to local stress concentration after which both stress and corrosion go hand in hand propogating a crack.
The classic example of this is cracking of brass cartridge necks (work hardened 70:30 brass) during the rainy season in India.
I think that cartridges with annealed necks are less prone, but this was not possible with the long spaghetti strands of cordite which required loading before the case was necked down.
Sorry there's no definitive answer here, but it is interesting..... Keith
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Post by alanstepney on Jan 30, 2008 16:03:36 GMT
Irrespective of the reasons, for which one probably needs degrees in chemistry and metalurgy, there is more than enough evidence, plus experience, to indicate that brass isnt a suitable material for boilers, tubes, or "primary" fittings. By "primary" I mean those fitted in the boiler, although I realise that some designs suggest it for turrets etc, and seem to get away with it.
From a model engineering viewpoint, we dont need to know any more.
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Post by havoc on Jan 30, 2008 18:01:42 GMT
I disagree on this. It is model engineering. Part of the job of engineers is to make sure that a design works. Just using something "because it always has been done like that" isn't engineering. To make sure that materials are suitable, loads are taken into account, environment is taken into account, lifetime and cost are balanced etc is the job.
I hate it when I see nice plans where a complicated solution is used when far simpler solutions exsist. Or when sound modern construction techniques are discarded for sake of tradition. Or when the appearance of a model is spoiled by using overdimensioned stuff "just to make sure".
Engineering is more than making a part to size. It is knowing why it has that size and just as well when and why that size doesn't matter. You don't need a degree in whatever science applies to your problem. There always are model engineers around that have that knowledge because of their studies/jobs (experience). Also knowledge of the 1:1 world is applicable to models.
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Post by the_viffer on Jan 30, 2008 19:48:50 GMT
Noddy as ever is correct that the problem is due to the differences between alpha and beta brasses.
Alan is, I think, correct, because you can't tell whether you've a "good" brass or a "bad" brass by looking at it. It would be a bold boiler inspector to pass a brass bush in a boiler when a gunmetal one is clearly (don't get me started actually!) OK.
Havoc is right IMO that learning is of itself a good thing and a reason why we do ME and that an intellectually satisfying model (oh how I hate that word!) is so much nicer than the ones I make. One of the great things about the web is that an electronics klutz like me can get sensible advice from people who know about these things. Thanks goodness Noddy is around to deal with the chemistry points!
I've seen a few real world dezincifications btw and they have been of the snapping carrot kind.
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steam4ian
Elder Statesman
One good turn deserves another
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Post by steam4ian on Jan 30, 2008 19:56:25 GMT
G'day all. I'm with Havoc on this topic, we do need to understand not just know. I think Noddy is on to something, I have had a little experience with corrosion protection but don't claim expertise just awareness. Corrosion resistance in metals comes from one two factors. Either the metal is inherently resistant to attack eg gold ,platinum or it has an oxide or similar chemical layer which renders it inert. Aluminium, zinc, copper, chromium, stainless steel, etc are in this latter category. If the oxide or chemical layer is durable and impervious then the metal is protected, damage the layer and the metal corrodes. Once corrosion starts it generates local areas where the oxygen is depleted and the oxide cannot reform, hence pitting generally and the rapid destruction of stainless steel in some environments. Regarding brass, with crystals of zinc and copper it is a galvanic cell waiting to spring into action, it only requires an anoxic environment.
Regards, Ian
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abby
Statesman
Posts: 925
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Post by abby on Jan 30, 2008 20:24:06 GMT
Noddy is quite right with the alpha /beta brass stuff , but I love gun-metal anyway , it machines beautifully and looks , well looks like gun-metal. If you want any cast sticks for bushes , bearings etc give me a pm. I can't roll or draw it yet but soon..........
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Post by the_viffer on Jan 30, 2008 20:26:16 GMT
Oh and while I'm at it the reason why stainless boilers are not acceptable in the UK is that you can't tell easily by inspection whether the material is liable to chloride stress corrosion cracking. The good ones would make admirable boiler shells (but not fireboxes or tubes on account of the poor thermal conductivity: good stainless makes good boiler fitting spindles ) the bad ones are real disasters waiting to happen.
**** vermin. How can one expect to type nearly right with a rodent running around the puter?
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Post by alanstepney on Jan 30, 2008 21:52:09 GMT
Havoc, I dont agree with you.
For anyone BUILDING a boiler, there is a simple rule. Dont use brass. That is all one needs to know.
The precise reasons why may be interesting, but they dont affect the basic premise that brass isnt suitable for that purpose.
In the same way, some people have tried hexagonal (actually polygonal) wheels on locos, but I am happy to accept the experience built up over generations and stick to round ones.
That is far removed from being, or not being, an engineer. Even the best engineers accept evidence /formulea etc that has been devised by others before them.
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Post by Shawki Shlemon on Jan 31, 2008 8:14:24 GMT
All the information above are very interesting and nice to know but the bottom line is we have boiler code/s and stipulate No brass and No stainless steel in copper boiler construction and that is that . One can build a boiler to the code and get it certified and can be used in public or build one to his/her own code without certification . The choice is clear . I stick to the code . This is not say that i am not interested in the information ,on the contrary I am very interested and read the threads but not every modeler can understand the fine details of the properties of alloys etc...
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Post by havoc on Jan 31, 2008 11:02:35 GMT
Don't generalise codes! Our code permits stainless (has been added in latest rev) and in germany brass is the prefered material for small boilers (at least in those parts I have seen of it).
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Noddy
Statesman
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Post by Noddy on Jan 31, 2008 11:36:33 GMT
I was actually thinking more about corrosion on cutlery and kitchen sinks when I mentioned stainless, oh well, Noddy finds yet another tangent ... Keith
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abby
Statesman
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Post by abby on Jan 31, 2008 20:28:44 GMT
Good tangent Keith , the question of suitability is entirely down to to the grades used, being able to identify them and the method of joining. Nuclear power stations have stainless steel boilers , I would be very worried about corrosion problems if I were a nuclear engineer , welds in stainless steel use Niobium stabilised filler rods to combat preferential weld decay, cutlery grade stainless is a different type of steel requiring hardness to keep an edge unlike sinks which need a deep drawing quality for cheap manufacture. My personal choice would be plain old mild steel which is cheap, easy to obtain and was good enough for the real thing , and with no oxygen in the water it doesn't rust , but both copper and gun-metal (bronze) are affordable (just) easy to obtain and work , but most importantly can be identified and are of a known standard and performance which has become essential in this world of litigation and insurance. I believe there are copper products on the market which did not originate in the west, although god help us we sold them the scrap , and although probably perfectly OK they have no ISO "whatever it is" certification and so cannot legally be used in products for sale , Check out how many foreign electrical products sold on ebay have no CE mark , use them at your peril. can you have parallel tangents ?
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