Duplex stainless steel locomotive boilers

Loco112 · **Posted:** Wed Feb 22, 2012 8:35 pm

The old problem of chlorine destroying SS boilers foiled the dream of a long lived minimum maintenance boiler long ago, but now that promise is renewed with the use of this new Duplex steel formula that is usable for steam locomotive boilers and the ASTM has a code for its use (the FRA might be another matter all together).

Duplex SAF 2205 Corrosion Resistant Steel boiler for the Darjeeling B-Class engine;
http://sonadaworks.com/duplex_stainless_boilers/

I know that its much stronger and thus you can use a much thinner boiler barrel (and all the other sections also) to contain the same pressure.

One potential problem that I keep thinking about is if a thinner boiler barrel might cause problems even if it is made out of a much stronger material. The thinner Duplex boiler barrel not being strong enough to stay "round" while supporting the weight of the boiler resting on the cylinder saddle. Even though the steel is stronger, the structure (resting on the cylinder saddle) might not be as strong as a thicker common steel boiler barrel now in use, because of its much thicker plate.

I also suppose stay bolts could be machined out of Duplex also, and that might be a godsend to boiler longevity, who knows.

So tell me what you guys know about Duplex as it pertains to steam loco boilers.

daylight4449 · **Posted:** Wed Feb 22, 2012 9:06 pm

Is there supposed to be some sort of picture here?

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

What THE CODE says:

Section II D
See note G34

G34 For Section I, use is limited to PEB-5.3. See PG-5.5 for cautionary note.

PG-5.5 The use of austenitic alloy steel is permitted
for boiler pressure parts that are steam touched in normal
operation. Except as specifically provided in PG-9.1.1,
PG-12, and PEB-5.3, the use of such austenitic alloys for
boiler pressure parts that are water wetted in normal service
is prohibited. See Note 1.

Note 1 Austenitic alloys are susceptible to intergranular corrosion and stress
corrosion cracking when used in boiler applications in water wetted
service. Factors that affect the sensitivity to these metallurgical phenomena
are applied or residual stress and water chemistry. Susceptibility to
attack is usually enhanced by using the material in a stressed condition
with a concentration of corrosive agents (e.g., chlorides, caustic, or
reduced sulfer species). For successful operation in water environments,
residual and applied stresses must be minimized and careful attention
must be paid to continuous control of water chemistry.

whodom · **Posted:** Sat Mar 03, 2012 4:34 pm

Very interesting article and an interesting idea. There's a brief entry on wikipedia about this material:

http://en.wikipedia.org/wiki/SAF_2205

According to the article, this alloy is ferritic-austenitic, which is apparently where the "duplex" name comes from. There's a separate entry under stainless steels on duplex stainless steels:

http://en.wikipedia.org/wiki/Duplex_Stainless_Steel

The Code quote Matt posted (I'm assuming from the ASME Boiler & Pressure Vessel Code) specifically addresses austenitic alloys, which this apparently is not. From what the wikipedia entry says, duplex stainless was specifically developed to counter the problems that occur with austenitic stainless steels in these sorts of applications. The article mentions that these steels are more cost-effective than conventional austenitic alloys in many applications as the overall alloy content is lower.

The question now is what does the Code say about materials that are not specifically addressed in the Code (assuming it's not addressed somewhere else)? At any rate the article is about a boiler in New Zealand and apparently it follows a New Zealand model boiler code version specifically written for duplex steel boilers.

Loco112 · **Posted:** Mon Mar 05, 2012 2:18 am

I didn't read anything about "Fire Touched", so that doesn't read too well for the prospect of a 2205 boiler. Hummm.

060 Hogger · **Posted:** Mon Mar 05, 2012 3:51 am

No comment on the metallurgy, but some intuitive observations are " too good to be true ".
That raises some suspicions...
..." no appreciable sludge was collected in the blowdown catch containers. As the local water was full of all sorts of minerals, it was expected that calcium could be a problem, but no buildup was detected on internal inspection of the boiler after the cyclic tests. "

whodom · **Posted:** Mon Mar 05, 2012 7:29 am

060 Hogger wrote:

No comment on the metallurgy, but some intuitive observations are " too good to be true ".
That raises some suspicions...
..." no appreciable sludge was collected in the blowdown catch containers. As the local water was full of all sorts of minerals, it was expected that calcium could be a problem, but no buildup was detected on internal inspection of the boiler after the cyclic tests. "

Good point. The boiler material should have little to do with what shows up in the blowdown.

I guess the main thing to consider is that boiler corrosion using "conventional" materials shouldn't be a problem with proper water treatment, so there wouldn't seem to be a great need for this material for locomotive boilers and fireboxes. Perhaps there are other corrosion-prone locomotive parts it would work well for?

daylight4449 · **Posted:** Mon Mar 05, 2012 8:24 am

whodom wrote:

I guess the main thing to consider is that boiler corrosion using "conventional" materials shouldn't be a problem with proper water treatment, so there wouldn't seem to be a great need for this material for locomotive boilers and fireboxes. Perhaps there are other corrosion-prone locomotive parts it would work well for?

Thinking of the higher strength seen in stainless steel, and the corrosive resistance of said material, I thought of some parts I think that this duplex material could work well (if said parts were composed of the duplex metal):
Steam Dome
Sand Dome
Components in the valve gear, like eccentrics and other moving parts

dinwitty · **Joined:** Wed Oct 22, 2008 8:18 pm **Posts:** 2226

this prolly needed to be invented/discovered in 1940 when steam devopment was in its height.

I figger there needs to be a steam revolution to happen to prove it outdoes the diesel.

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

The Fine Print

Heat Resistance of Duplex stainless steel

The high chromium content of Duplex stainless steel that protects against corrosion, causes embrittlement at temperatures over about 300°C.(572F)

At low temperatures duplex stainless steels have better ductility than the ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C.

Fabrication of Duplex stainless steel

Fabrication of all stainless steel should be done only with tools dedicated to stainless steel materials. Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolor the surface of the fabricated product.

Heat Treatment of Duplex stainless steel

Duplex stainless steel cannot be hardened by heat treatment. They can however be work hardened.

Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.

Machinability

Although machinable, the high strengths of Duplex stainless steel makes machining difficult. As an example, machining of 2205 is around 20% slower than for 304.

Machining can be enhanced by using the following rules:

· Cutting edges must be kept sharp. Dull edges cause excess work hardening.

· Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material.

· Chip breakers should be employed to assist in ensuring swarf remains clear of the work

· Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities.

Welding of Duplex stainless steel

Duplex stainless steel have good weldability. All standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. The recommended filler material for 2205 stainless steel is 2209.