Effect of red heat on 316 and 321 stainless tube.

[WindyParticipant @windy30762]

My flash steam generator is made of a 6 metre length of Ø0.1875″ x 24swg and a 6 metre length of Ø 0.25″ x 22swg tubing tig welded together.

 

I use 316 and also 321 stainless and they are at red heat most of the time.

 

The Ø 0.25″ gets the most heat and sometimes bursts at the hottest end of the generator.

 

After a number of such bursts the tubing needs replacing as it gets too short.

 

I am intending to put in a new 6 metre length of Ø 0.25″ tube and after cleaning the Ø 0.1875″ tig weld them together.

 

My query is after the Ø 0.1875″ stainless has been at red heat when running the engine will the metal structure be so altered so that when welding it would be weak?

 

 

 

[WindyParticipant @windy30762]

[adyParticipant @ady]

There’s all sorts of questions here.

 

Are you not applying too much energy to create your steam?

Superhet has been and gone and was an amazing form of high energy resource for ships, and ran 24/7 for days on end.

The main issue for seagoing superhet was leaks, because they could only be heard, not seen (and caused terrible injuries) but the noise in the engine room masked the sound of leaks.

 

In my opinion, and its a pretty inexperienced opinion, if the unit is bursting then you are putting too much energy in…and not taking enough energy out, the unit isn’t balanced and you’re getting the equivalent of internal boiler failure.

I used to spend days supervising a heater condenser gauge during tank cleaning and it had to have a certain level of condensing or it would have taken the front end off the accomodation block.(boom)

 

[WindyParticipant @windy30762]

Thanks ady for your reply please remember this is an experimental high speed piston engined power unit. running between 11,000rpm to 15,000rpm.

There has only to be a slight or major mechanical fault on the engine or fuel and water pumps plus extra loading on the prop through design changes or variations on the boats position on the water and the pressure can go sky high then pop.

Rules are strict regards weight otherwise I could fit sensors and various control systems.

I would still like an answer to the question is the structure of the 316 or 321 steel altered with the constant red heat that makes a fresh Tig welded joint possably weak?

 

Windy

[Anonymous]

Hmmmm, the boiler failures may be due to stress corrosion cracking (SCC)? Austenitic stainless steels are particularly prone to SCC involving chlorine; do you treat your water before use or just use tap water?

 

It may well be that the structure of the tubing has changed enough to alter the strength of the subsequent weld. Of course the prep and welding techniques can also affect the structure and strength of said weld.

 

Some interesting information in this link:

 

http://www.npl.co.uk/upload/pdf/stress.pdf

 

Regards,

 

Andrew

[mgjParticipant @mgj]

The thing to do might be to look at the fractured face.

 

If it has a beach mark then its fatigue (at high temps)

If it has just bust its probalby brittle failure, and the likelihood is either poor choice of welding rod, or chlorine/hydrogen.

If its bulged, then probalby its just got too hot stretched and burst in an ordinary tensite failure under pressure.

 

(What pressure you runng at, what is the wall htickness so what sort of stress (hoop0 loading are you running at and have you looked at a stress strain plot for your SS, to determine if you are operating in safe limits, for that temperature, because if not you should before going further )

 

(Waht about the welds- any undercuts on the edges, because if so, there is the probable culprit before going too deep)

 

You could also be seeing a phenomenon known in the gas turbine world as creep. Under a constant load and elevated temps the metal slowly stretches (and in your case will thin), but in a gas turbine in tight clearances causes fouling. Either way disaster follows.

 

To answer your question directly – no it shouldn’t,depending – big point – on the temperature, but yes it most certainly can.(or it may be a latent consequence of welding)

 

I would suggest though that you are getting into a realm where you are beyond the advice of the average interested amateur. You need proper metallurgical advice, once you have determined the nature of the failure. Start at the beginning – how/why it broke, and take it from there.

Edited By mgj on 10/11/2011 10:18:51

[adyParticipant @ady]

A change in design?

 

If you put the tubing through the wall, then weld a flange outlet with a small outlet neck onto it.

Then when you pull the tube back the flange comes up against the boiler wall and weld the rim of the flange to the boiler wall.

 

You could try a few and see how reliable they are compared to your standard welds.

 

 With bigger pipes you would never weld the end of a pipe directly to a hole in a wall, too many stresses.

Edited By ady on 10/11/2011 11:32:08

[Ian S CParticipant @iansc]

I use 316 stainless bored out to form a cup shape for the hot end of a number of my hot air engines, the engines are unpressurised and have had no problems, first one about 1998. Latest SS hot caps have been made of unidentified SS with an end TIG welded on, no trouble. Hot ends built to the same dimentions from plain steel have regularly failed indicated by bulging, and on horizontal engines the cylinder droops. Can’t help with the welding of old heated SS, Ian S C

[Richard ParsonsParticipant @richardparsons61721]

Windy Hello again. The problems with stainless steel and high temperatures are legion. I have copied a bit from Mr Palmer’s piece on welding stainless steels

He says

“Stainless steels Achilles heel: All metals are crystalline materials that have specific crystal structures that are dependent on temperature. These structures are referred to as phases and are given names such as austenite and ferrite. A block of metal is very similar to a block of salt. A block of salt is really a bunch of grains of salt all fused together. These grains are oriented every which way, and the interface to the next grain of salt is called the grain boundary. As you would expect, the grain boundary is weaker than the grain itself. The crystalline structure of metals is exactly the same in this respect. (By the way, if you would like to see a metallic grain structure, go look at an aluminium street-light pole. That mosaic you see is the grain structure.) Because the grain boundaries within a metal are the weakest sites, heat and corrosion usually affect these areas first.

The corrosion resistance of stainless steel depends on the chromium. Austenitic stainless is a supersaturated solution of chromium and nickel in iron. It is actually a very high temperature phase that has been quenched (quick-cooled) to preserve the distribution of elements.

Austenitic stainless does not like middling-high heat. It performs well up to 600 °F (315 °C), but higher temperatures in the range of 800-1600 °F (425-870 °C) cause atom diffusion and change the metal’s properties. Such high temperatures allow the chromium to diffuse away from the grain boundaries to form chromium carbides, its preferred crystalline structure at that temperature range. If exposed to these high temperatures and chromium diffusion occurs, the metal becomes sensitized and prone to cracking. The diffusion of chromium away from the grain boundaries results in non-stainless grain boundaries surrounded by stainless steel. This situation soon leads to localized corrosion and rapid cracking of the grain boundaries. To correct this, the metal must be heated to at least 1900 °F (1040 °C) for a period of time in an inert gas atmosphere and then quenched to retain the austenite crystal structure. Unfortunately, doing this heat treatment to a welded keg would result in considerable warping and distortion. It is better to get another keg and start over.

Welding is a local melting-freezing process that creates high temperature gradients in the metal around the weld. This heat affected zone (HAZ) is the region where unwanted atom diffusion can take place if it is hot enough, long enough. Time/temperature curves describe this phenomenon, and the curve for alloy 304 is shown in Figure 1. The figure shows that for type 304 stainless (nominal carbon content of 0.08%), 5 min at 600 °C (1110 °F) or higher will cause chromium diffusion that will later cause cracking in service. Type 304L stainless – “L” denoting less carbon (nominal 0.03%) – is more weldable and can spend about 6 hours at 600 °C before becoming sensitized. Most kegs (in North America) are made from 304L to facilitate welded construction.

Caution must be taken when heating stainless steel equipment. I know of one home brewer whose cut-off keg boiler began cracking at the bottom. The cracks appeared at the flame line where the flame of the cajun cooker-style propane heater met the keg. This shows that he was running the flame too hot and that, over time, chromium atom diffusion was taking place.

Diffusion is cumulative.
Once this type of cracking occurs, there is no economical way to correct it”.

You can read the full article here http://www.brewingtechniques.com/library/backissues/issue2.6/palmer.html

When I was messing about with turbines my second flash steam boiler was stainless 304. It failed in short order lasting no longer that the Mild steel I used before. It cost twice the price. When I grumped at the supplier ( Accles and Pollock) the technical sales man mad a sort of “oinking” noise and told me the answer was not to use it again.`

Later I mentioned this problem to the Materials engineers who suggested that I should try the Nimonic alloys used in jet engines. He suggested that if ii did I should win the lottery first.

[WindyParticipant @windy30762]

Thanks all it is not the welded joint that fails when the tubing bursts.

The joint is at the cool end it is the 12″ where it goes into the engine that bursts like a balloon, a very hot part under a lot of pressure.

 

The generator is like a spring, water in at one end and steam out of the other end fed by a pump.

 

If I do what some of the full size steam cars do and feed water into the hot of the generator I do not get the power for the engine.

 

It would probably do 100mph all day set up like that.

 

To get 115mph to 120mph you are pushing the limits.

 

Like my old motor bike days with Nitro burning supercharged Triumphs reliabilty came second as long as it lasted the distance then explodes.

 

Windy

 

[mgjParticipant @mgj]

Windy – seriously – look at the joint and determine HOW it failed.

 

You will get nowhere until you know that. . It may well be that there is absolutely nothing wrong with the materials – just a matter of stress and wall thickness. But it could also be brittle failure due to the chromium content of the weld playing games, allowing transformation to a martensitic structure at a lower temp than expected. (Brittle failure) You just don’t know, and there is no way that we can tell by post as it were.

 

So, what was the mechanism of failure. Then you get a pointer as to where to look.

[Richard ParsonsParticipant @richardparsons61721]

Windy I would not be so certain that it is cool. I have lit my pipe from my turbine inlets. Took a little time but it did light my baccy pipe

It also is probably at the point of maximum vibration. The boiler vibrates and the engine vibrates and the pipe joining them vibrates. . I put a stop to this sort of thing on my neighbour’s microlite’s fuel pipe by splitting a length of mild steel tube in half and bonding it to gather (with araldite) over the fuel pipe.

[Raymond AndersonParticipant @raymondanderson34407]

Windy, Could you not substitute the stainless steel for either Inconel 600 or Hastelloy X ,

both are nickel based heat resistant alloys that present no TIG welding difficulties.

expensive yes, but you would have no more of the problems that you are having with the Stainless steel.

Raymond.

Ps if you go along the Inconel route make sure it is 600 and not any of the 7** series.

[mgjParticipant @mgj]

Richard – vibration is a very good point- but the indication should be the beach mark of fatigue.

 

If you pick up that beach mark in a magnifying glass, you know that it has been flexed and a crack has propagated. It could be stress, but it could equally be vibration induced (or both).

 

BUT – you have to know how it has failed.

 

[Sub MandrelParticipant @submandrel]

I think Windy is making a different point – he knows why it fails – he’s pushing it to the limit and more! Every time it fails, he cuts off the end and starts again with a slightly shorter

boiler.. but now it’s too short.

 

Can he cut out the short thick end and weld a new one on to the thin end, or will teh thin end be degraded and unlikely to weld.

 

From the replies above I think the answer is suck it and see (while standing at a safe distance).

 

Neil

 

P.S. I think its a shame speed hydro and tether car racing seem to be fading away.

[WindyParticipant @windy30762]

Spot on Niel but it is very interesting to hear all the various comments.

 

I have had 2 seasons with no bursts but after a certain length of time the generator pops.

 

Maybe the tubing degrades with the constant changes in hot and cold at the furnace end

 

I have some spare short generators from previous bursts and would like to reclaim them.

 

One popped when testing a De-Laval nozzel on a test for my turbine.

 

It keeps the gray matter active.

 

The Hydro section is very active in the Midlands and South with a number of competitors up North as well.

 

Windy

[Ian S CParticipant @iansc]

I had a chance, and missed it at getting some surplus combustion chambers from Avon, and Goblin engines, If I got another chance I think that they would supply material for the hot end of my hot air engines.

Sounds to me that the flash boiler is in a very highly stressed situation with pressure, high temperature, and vibration. I think the problem is a mix of all these. ian S C

[mgjParticipant @mgj]

Not necessarily an because in a jet engine , the air is split into two streams. Some, a relativeley small amout is mixed with fuel in a stoichometric mix. The rest passes around the combustors and keeps the hot gases off the walls, so the combustion chamber walls are not quite as hot as one might suppose.

 

the place where the special materials such as single crystal nimonics appear are in the turbine blades, which are taking direct heat, and continuous heat (unlike an I/c engine) but even they are cooled by air bled along the length of the blade though internal drillings (often either cast in or spark eroded I think)

 

One quick point – i haven’t been told how it(this boiler bit) fails. I have simply been told that this bit breaks. I want to know HOW – in what MODE – it broke. Then you will start to know why.

 

Pushing something to the limit? Well yes- obviously, butr what limit? Tempreature, fatigue, stress?

 

Its generally quite difficult to fix a problem if you don’t define it?

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