Stainless steel for hot water boilers 4

[frothie]

Maybe you learned gentlemen and ladies could help me out.

My company produce ( via China ) a product that uses a 1mm thick stainless steel boiler ( approx 0.8L ) with an immersed element. It cycles between 1 and 4 bar and say every 10 minutes 0.1 litres of cold tap water is pumped into a 300 deg C ( virtually empty )boiler. The boiler elements are silver soldered and there is evidence of localised heating ( no sign of zone depletion ) . The boiler has many formings and not very deep drawings. I think the boilers are not annealled.

We have a very high failure rate for the material cracking, there are numerous cracks,some 30mm long. Can anyone advise the correct grade to use?

 

[mcguire]

My calculations indicate that a surface area of copper equal to about 10% of the area of the stainless will protect it from attack. For zinc the area required would be much less, but mass loss will be at a greater rate so life will be limited. Any further advice comes under the category of consulting.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[DALESUTCLIFFE]

with a cscc issue try stabilising with a water decom issue or anode if ness. This will at least clean up the issues that are present (eliminate them)

 

[frothie]

Sorry?

Please expand " water decom issue"

Many thanks.

 

[panduru]

de-contamination, I guess.

 

[robsalv]

I don't have the depth of knowledge regarding the recommended grades of stainless, but our company literature clearly recommends against any austenitic stainless in sevices >65C and >50ppm Chlorides particular in highly stressed situation. The boiler would appear to have the temperature and Chlorides and thermal related stress due to rapid temperature changes.

In addition to water conditioning and a different material choice [a duplex would be our automatic response], how about a preheater to reduce thermal differential? I think you mentioned a different design of heating element, so hopefully that reduces the risk of failure from straight out thermal fatigue/stress.

By the way, I've always operated on the conventional wisdom that once Austenitic SS has CSCC, it can't be repaired. In thick sections you might be able to blend grind, but don't think that's possible here.



On the topic of water contaminated with metal ions, the heating elements and boiler surfaces will most likely get coated in a scale. This scale will ultimately flake off and start clagging up any small bores down stream of the boiler.... I suspect this will probably be an issue in an "ironing system".

Good luck with it.

 

[frothie]

People are not keen on duplex - too expensive. I have tried boilers in 444 but the manufacturer complained that it was too hard to form ( 1.2mm thick sheet metal pressing ).

We have changed the brazing from silver solder to nickel done in a controled oven at 1150 degrees C. This obviously anneals the SS.

I currently have a sample in 304 and 316L on test.

 

[EdStainless]

316!!!! The lean duplex will be a lot less expensive than 316.
It sounds like your forming shop is telling you a fish story. The 444 should not be any more difficult to form. It will requre a different allowance for springback, but hte forming forces should not be much different.

Furnace brazing has a couple of advantages. You don't have the local variations in heat input, and you don't have agressive flux residues. The only thing to watch is how the tanks are cooled from the braze temp. You want them cooled uniformly, which probably means slowly, until they are cold enough to be strong.

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[mcguire]

I have had the same experience with forming shops and 444 when making water heaters. The issues that they are unfamiliar with cause them to blame the material for their lack of knowledge. The issues are die abrasion from the carbides which is solved by die material choice, different springback characteristics, and anisotropic behavior...just like carbon steel. It just doesn't behave like austenitic stainless. It more resembles HSLA steel.


Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[frothie]

Thanks

You are preaching to the converted here. You know what its like the technical Director knows best. Its not my reputation on the line if the 316 takes a dive.

I take your point re the 444 especially when its a Chinese manufacturer - this one lies through his teeth !!

Will keep you posted as to test results.