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?

 

[EdStainless]

What alloy are you using now? Are there cracks around the grazed joint? I am going to guess that the answers are 304 and Yes.

Brazing to stainless is a bad idea. You will get grain boundary carbide formation, and maybe some liquid metal embrittlement as well as some intermetallics forming.

If your cracking is choride stress cracking, then look at using 439 for the tanks. Weld a fitting to them and mechanically attach your fittings.

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Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[frothie]

Thank you EdStainless.

Yes the material is SUS304 and 95% of the cracks are in the area of brazing although they do not appear to originate from the SS / brazed boundary. As I also said there are dozens of ( non punctured ) cracks all over the hotter part of the boiler.We originally suspected the flux. There is a very high failure rate in Israel and we are currently getting the water tested. Possible effects of chlorides?
If we specify 439 should we also specify welding, annealing after forming / welding and anything else?
Once again thank you for your comments.

 

[TEV]

I agree with EdStainless. What is the hardness of the formed stainless? The higher the hardness, the more susceptable it will be to chloride SCC. Flux splatter or other surface contamination could also be a factor. it is very important to clean the surface and passivate it, if possible, after fabrication.

 

[frothie]

Thanks for all this information. I will enquire of our Chinese manufacturer as to the exact grade, hardness and process details for the SS.

 

[mcguire]

The grade which I have seen used successfully for this application is 444. Now, however, you could also use a lean duplex, such as 2003. 439 is a little marginal on pitting resistance.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

Mac, good call. LDX2101 or ASL2003 would work, but the cost goes up.
439 has about the same pitting resistance as 304. I have seen it used a lot. It doesn't last forever, but it doesn't crack.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[frothie]

Thanks a lot guys. I forgot to mention that a chronic problem is happening in Israel, hundreds of boilers failing. I am getting some water samples tested and will keep you posted as to the chemical compostion. I suspect that de-salinated water is possibly being used.

 

[rolledalloy]

Additional benefit of going to 439 or lean duplex LDX 2101 or 2003 beyond chloride SCC is resistance to liquid metal embrittlement. In our experience the ferritic and duplex stainless steels because of their structure resist the intergranular cracking caused by molten copper and silver.

 

[frothie]

Once again, thanks a lot. In an industry that is very cost concious many steps have to be gone through to specify a more expensive material ( which I assume some of these are ). Your info is invaluable to me. I will give the site credit for this and if I knew your names, you too. Please continue to look at this thread as I am still ewaiting for water analysis and material etc. and will let you know in due course.

 

[frothie]

1. Material grade SUS304
2. Hardness HR-15T 80-90
3. Thickness 1.2mm
4. Forming process - pressing
5 Cleaning process - chemical cleaning

 

[frothie]

Hi Guys

The water is

Hi in Alluminium
Hi in Chlorides
Hi electrical conductivity ( salts )
Hi in Iron
Hi In Manganese
Hi in Nickel

Please help with you final suggestions for material

 

[mcguire]

No to 304. Definite SCC. 439 is a gamble from a pitting point of view. Safe materials are
1.any duplex
2 444
It will be much easier to find a duplex than 444. 2001, 2101, 2003 will all work. 2205 is most commonly available and it's more than you need, but will make a very high quality product. Do not use austenitic.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

Since these are not fired there should not be any local scale formation. This means that 439 may be enough, but a lean duplex would be a lot better.

I have seen 304 used succesfully for tanks, but the stresses need to be very low. Use real thin tanks and external structural supports. I don't see 304 or even 316 making it in your application.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[frothie]

Thanks you all very much. I now feel that I can make a recomendation for a material change. I am going to try and get some samples made in 439, 444 and 2205.

I will also make recomendations regarding general construction, handling and cleaning. We will also look to modify the water.

Final bits of info are - The boiler capacity is approx 1L, the pressure is 4 Bar max, the temp when dry is 200C it is therefore possible for scale to get burnt onto the immersed element.

I will make sdure you guys and the site get the credit.

 

[mcguire]

Thanks to frothie for the question and EdStainless,Rolledalloy and others for the well thought out responses. That's what this site is meant to do.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[frothie]

Hi Guys

I am still have big big problems. Can anyone suggest a company / consultant who I can talk to in the UK regading this problem ?????

 

[EdStainless]

whoever is making the tanks now should be able to do it in teh other alloys. If you are looking for someone to backup the material selection, well some of us do that for a living. Offhand I don't know where to point you in the UK.
You might be able to search by looking in NACE.org This is the corrosion engineering association.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[unclesyd]

You may have a problem with "differential thermal fatigue" due to the rapid temperature change when the cold water is introduced and contacts the SS. This is not as a well recognised problem.
We have a had this problem in several instances even when the two liquid streams mixed with a temperature differential.

Your injection of cold water on the hot SS surface is the primary initiator of the cracking. What you are doing is essentially the same thing where people get into trouble when they use water to cool vapor streams.

The solution is to keep the cold water off the hot 304 SS surface. I don't think that a material change will help.

 

[stanweld]

frothie,
You might try the Welding Institute; I made use of their expertise for more than welding related problems, while in the UK.