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?

 

[frothie]

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.

 

[mcguire]

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/

 

[frothie]

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.

 

[EdStainless]

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.

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Rust never sleeps
Neither should your protection

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[frothie]

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

 

[panduru]

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.

 

[EdStainless]

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

 

[frothie]

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

 

[frothie]

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?

 

[mcguire]

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/

 

[EdStainless]

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

 

[mcguire]

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/

 

[EdStainless]

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

 

[mcguire]

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/

 

[TVP]

The Kawasaki technical paper can be freely downloaded by using the following link:

http://www.jfe-steel.co.jp/archives/en/ksc_giho/no.40/e40-023-029.pdf

JFE is the new company formed from NKK and Kawasaki. They have many of their technical papers available for download.

 

[frothie]

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 ???

 

[mcguire]

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/

 

[frothie]

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

 

[EdStainless]

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

 

[frothie]

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.

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