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Stainless steel for hot water boilers
4

Stainless steel for hot water boilers

Stainless steel for hot water boilers

(OP)
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?

RE: Stainless steel for hot water boilers

2
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.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

(OP)
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.


RE: Stainless steel for hot water boilers

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.  

RE: Stainless steel for hot water boilers

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

RE: Stainless steel for hot water boilers

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/

RE: Stainless steel for hot water boilers

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

RE: Stainless steel for hot water boilers

(OP)
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.

RE: Stainless steel for hot water boilers

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.  

RE: Stainless steel for hot water boilers

(OP)
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.

RE: Stainless steel for hot water boilers

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

RE: Stainless steel for hot water boilers

(OP)
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

RE: Stainless steel for hot water boilers

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/

RE: Stainless steel for hot water boilers

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

RE: Stainless steel for hot water boilers

(OP)
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.

RE: Stainless steel for hot water boilers

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/

RE: Stainless steel for hot water boilers

(OP)
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 ?????

RE: Stainless steel for hot water boilers

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.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

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.

RE: Stainless steel for hot water boilers

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

RE: Stainless steel for hot water boilers

(OP)
Hi

Sorry Ed I am not second guessing your material selection. The water being heated is high in Chlorides and metals. I am also investigating filtering the water first by running it through a resin. I need to make samples of the water for test. I have been told that it is far easier to make the sample water without metals. I wanted to talk to someone regarding the effect of certain metals ( in the water ) on the 444 material and if I could leave them out or not. As I am in the UK I thought I might have a face to face with someone here. I thank you all for your help in the past and I am soon to trial samples of my boiler in 444. Any help. tips, snippets would still be greatly appreciated.

RE: Stainless steel for hot water boilers

frothie
What specifically are the big problems you are still having? Is it SCC with the new alloys? I guess we must assume that it is still the ones in the field.
You should not consider metals in the water inconsequential. Fe ions are detrimental.
Do you have your product on a web site on which you could describe the locus of failure?
Fixing units in the field may be will be tough even if you know exactly what is happening. I have worked with a desalinator company which put a copper sacrificial anode in the unit to protect the stainless. A sacrificial anode, non-toxic preferably, may stop the corrosion which initiates the SCC. It doesn't take much to move the stainless away from the pitting potential. It kills algae, too.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Stainless steel for hot water boilers

(OP)
Thanks for that Mac. I may have over dramatized. We had big problems getting 1.0mm thick 444 in China. We have just made samples. These are on test in China and I think one has failed due to cracking. although I cannot be sure. We are definitely having problems with the brazing of the heating elements and are trying silver solder with greater than 15% silver and an alloy that has a high percentage of Nickel.

If someone can tell me that it is important to leave the metals in the sample water, I will.

Also what is the exact specification of the 444 or where can I find it.

RE: Stainless steel for hot water boilers

The metals are secondary.  Chlorides and temperature are the major factors.
Is there any S in the water?  That would be critical.

TWI would be a great source of technical info.  Good call Stan.

Testing will be problematic.  I have worked with similar applications and had a lot of trouble.  We could not reliable fail units in testing, even with very bad water.  Service patterns and cycling play a major role in the temperature and stress in units.

A good spec for sheet stainless is ASTM A240.  I covers about all of the alloys.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

(OP)
Hi - there is no Sulphur in the water. Had yet another meeting regarding this boiler. The manufacturers do NOT like 444, they say it is too hard and cannot be deep drawn ib the same tools as the 444. Can I ask a simple question. If you had to choose a material that was available as a 1mm thick sheet to a) withstand CSCC and be deep drawn, what would it be and what would be the post production annealing be. Also we have specified that the heating elements are brazes in using a Nickel brazing alloy doen in an oven at 1050 deg C.

Any Comments - pleeeeeaaaaaaase

RE: Stainless steel for hot water boilers


On a first reading of your original post, I was wondering if you should not have been preprocessing the water before feeding it to the boiler. That would give you a chance to raise the water temperature to some extent, and control at least some of the contaminants.

As for the boiler material itself, mcguire may be right in suggesting duplex SS.

Hope mcguire, EdStainless, unclesyd, et al will pardon my audacity.

 

RE: Stainless steel for hot water boilers

Sorry, you won't be able to deep draw a ferritic the same way you can 304.  Tell them to get over it.
If you need pitting corrosion resistance at least as good as 304, and reasonable CSCC resistance; then you are looking at ferritics and duplexes.
To keep cost and processing reasonable with duplexes you should look at lean duplex grades.  Either 2304 or LDX2101 would be options.  AL2003 is a good material, but more expensive.
If I made these in duplex (or ferritic) there would be no post-production anneal.
In ferritics my first choice would be 439 (or a variation of it).  Neither 439 or 444 are very strong, but they don't have the elongation and strain hardening of 304 which does make them more difficult to form.

panduru, you make a good point.  For industrial applications pre-treatment is unually more cost effective than changing the equipment metallurgy.  For smaller units though pre-treatment is expensive, and often causes as many problems as it fixes.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

(OP)
As I said I am going to filter the water before boiling. My original Q was whether I could leave the metallic contaminants out. I think the answer to that is yes. Everybody has a big No NO to 4445 at the moment. I initially specified 2205 but for some reason my Chinese supplier could not get hold of it.

My spec is
Temp = 20 to 150 deg C
Pressure = 4.5 bar
Material thickness 1 to 1.2 mm

The boiler is deep drawn in 2 halves ( say 50mm each )
Welded together
Elements brazed in.

Are you all still confident in 444 or would you prefer 2205

RE: Stainless steel for hot water boilers

(OP)
Hi

I am convinced that the boiler is cracking due to CSCC as you learned gentlemen suggest. These cracks may be initiated from stress cracks already in the material due to poor post production processing.  The reason the boilers are failing is that the water in Israel is

Hi in Chlorides - the main culprit.
Hi electrical conductivity ( salts )
Hi in Alluminium 300ppm
Hi in Iron 100 ppm
relatively Hi In Manganese 5 ppm
Hi in Zinc 300 ppm

As I said I have 444 samples to test and would like to make water to the same spec so that I can test the corrosion cracking resistance of the 444 material. The water expert has told me that it is problematical to include the metals.

My simple question ( partly answered already ) is - can I ignore the metals regarding there contribution to corrosion cracking?

RE: Stainless steel for hot water boilers

If you go to the Outukumpu web site, there is a lot of technical information on SCC, water treatment, etc. I gather from reading a number of these that the metals are unimportant. Zn and Al were not my worry, iron was. But, it is reported that iron actually deposits, so will be inactive as an ion in drinking water. You can just concern yourself with anions, temperature and pH in setting up your testing.
You haven't mentioned any biological content of the water. That seems okay since at SCC temperatures biological activity ought to be negligible.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Stainless steel for hot water boilers

The Fe and Mn are only an issue as deposits.

There is no reason to test 444.  It will not CSCC, you can find that in the steel mfgs data http://www.alleghenytechnologies.com/ludlum/pages/products/xq/asp/P.73/qx/product.html#  Look at the Blue Sheet.
The same lit will show that 444 has slightly better pitting and crevice corrosion resistance compared to 304.

Your real issues are fabrication.
After the welding and such, are these tanks pickled?  Is there any chance for heat tint to remain?

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

Ed
 Are you sure 444 will not SCC when cold worked or possibly embrittled by alpha prime from brazing, as these units might be? I have only seen ferritics to be truly immune to SCC in the fully annealed state.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Stainless steel for hot water boilers

I am sure about the lack of effect from cold work, I used to be in the wire business.  The SCC tests use u-bends that are formed, and then loaded to 100% of yield.

As far as the braze, or welding, that is more problematic.  It is also one reason that I like the stablized 439 as a choice.
I have seen embrittled ferritics fail from corrosion before CSCC.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

Good point on the 439. JFE in japan produces an alloy called RSX-1 in a 1999 kawasaki tech report, #40 May 1999. It has 18 Cr and 1.5 Mo and may be available to the Chinese producer. It should be easier to form than 444 because it was made to be formed into exhaust componenets.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Stainless steel for hot water boilers

(OP)
Oh my brain hurts - I am just a poor mech eng with a general knowledge of materials - this is getting a bit specialist. I will try and look at the links you suggest.

The current ( silver ) brazing is done by hand ( torch )-ouch!. The sub contract boiler maker can and will nickel braze in an oven ( this process was not chosen by the manufacturer to save pennies !! ) we beileve this is good practise.

The boiler will ultimately be redisgned with external heating elements which we have found to be more reliable, however, it is not retro-fittable. We need an intermin solution for spares and repairs.

The water is for an ironing system and does not have to be drinkable.

I think I can leave the metals out of the water sample ???


RE: Stainless steel for hot water boilers

Yes,leave the metals out.
To solve the problem in the field you need to stop the corrosion. Pitting corrosion is a necessary condition for SCC. Ergo, stop the pitting. You can't change the materials, so you must protect them. Add a sacrificial anode to the vessel and be done with it. Cheap and retrofittable. There are companies who produce such items for marine applications.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Stainless steel for hot water boilers

(OP)
Any preference for the sacrifial anode material, shape fixing etc?

RE: Stainless steel for hot water boilers

See Frothie, you made it.  You have the information and you understand it.
When you get stummped you know where to come back to for advice.
Good comment McG, some of us over look the obvious.  Anodes are common in hot water system (except for high purity).  The only drawback to using them is that they have a finite life.  In high conductivity water like yours the life can be short.  

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

(OP)
Hi Ed

Thanks, I will still persevere with the duplex or 444, the sacrifial anode is another suggestion I can put to the manufacturer. The main point is my water sample can now be constructed to test ANY solution.

I am sure I will be back please keep your email notification checked.

RE: Stainless steel for hot water boilers

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/

RE: Stainless steel for hot water boilers

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)

RE: Stainless steel for hot water boilers

(OP)
Sorry?

Please expand " water decom issue"

Many thanks.

RE: Stainless steel for hot water boilers

de-contamination, I guess.

RE: Stainless steel for hot water boilers

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.

RE: Stainless steel for hot water boilers

(OP)
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.

RE: Stainless steel for hot water boilers

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.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Stainless steel for hot water boilers

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/

RE: Stainless steel for hot water boilers

(OP)
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.

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