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Boiler steel

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MattMaie

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Hi everyone.

I just got started in model engineering and I'm taking unto it like a moth to flame. My first model (incomplete) is a small hit and miss engine detailed in "The Shop Wisdom of Phillip Duclos". That being said, after building some more small models, I want to tackle the task of building a 5/8 scale steam traction engine.

I talked with someone else who's father had built one. He said the boiler was made from 16 inch diameter, 3/8 thick steel pipe. I would like to know what sort of steel is suitable for boilers as obviously I want to do this project right and have a model at the end that is safe to use.
 

GWRdriver

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Hello Matt,
The first thing you need to do, before you do anything else, is to contact your nearest local live steam or model engineering club to determine what the state (USA) or district (UK) regulations are with respect to a boiler of that size. If you have no local live steam club (in the USA) you will want to contact the agency in your state which is responsible for pressure vessel or steam boiler regulation and inspection to determine what code and construction requirements you will be faced with. Depending upon your state, you may find that a boiler of that size is exempted from certification under the state regulations for code boilers for some reason, a "historical" or hobby boiler perhaps, but maybe not and it would then have to be built by a licensed (code) shop - an expensive proposition, but that's not for me to comment on one way or another. However, in any case it would be pointless to proceed with anything until you know if and how what you plan to do will be regulated.

As for grade of steel, in the USA most builders use SA-106B for barrels and SA-516 or SA-285C for plates.
 
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MattMaie

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Thanks for the info. I live in Washington state, so barring the presence of any clubs here, I definitely know of an organization in Oregon that can help me out. In a way I kind of grew up around steam, so building a model like this has always been something of a dream for me.

Otherwise, I will look up the washington state boiler regulations and see where I can go from there.
 

starnovice

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Matt,
I live in Portland and used to put on the GEARS Model Engineering Show. This year we had a guy from Washington that wanted to bring his steam boiler to the show. We had to turn him down due to insurance. However, if you want I can probably find his name and contact info for you. Let me know.

Pat
 

MattMaie

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Thanks. I'd like that very much. I used to go to the GEARS show every year as a little kid back when it was called PRIME.
 

Entropy455

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An advantage of regular mild steel is that it retains a lot of strength down to freezing (32 degrees F). Other advantages of regular mild steel is that it’s easily welded. Mild still is not considered susceptible to hydrogen embrittlement. It is ductile, and bendable, thus it is tough. The low carbon content prevents the formation of martensite within the heat effective zone of welds, which means post-weld stress relieving is not necessary.

Mild steel is the workhorse of the steel industry, and is used all over the word by the millions-of-metric-tons per year, for the reasons identified above.

A disadvantage of regular mild steel is that it loses strength at elevated temperatures, and loses very appreciable strength around 650 degrees F. This fact alone makes mild steel a poor choice for boiler construction.

Boiler steel is a steel alloy that’s engineered to retain higher strength at elevated temperatures. Boiler steel retains appreciable strength over 1000 degrees F. The downside to boiler steel is that it behaves opposite of mild steel. Boiler steel becomes brittle and loses appreciable strength at temperatures below 50 degrees F. Most boiler steel is susceptible to hydrogen embrittlement, thus it must be welded with low hydrogen welding rod. The higher carbon content will harden the steel within the heat effective zone of welds, which is why boiler steel boilers must be fully stress-relieved at 1100-ish degrees F, for many hours, prior to being placed within service.

That being said – you “can” construct a boiler out of regular mild steel. Just keep the operating temperature below 600 degrees F. If you’re using ASTM-A36, assume a yield strength of 15,000 psi, then subtract even more strength for your design factor-of-safety. Remember, mild steel becomes very weak at elevated temperatures. Keep the operating pressures low, and do not exceed 600 degrees F.
 
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starnovice

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Matt
Info sent in private message. Let me know how it works out.

Pat
 

GWRdriver

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What Entrpoy455 has said about the behavior of steel is true in a general sense, but because there are 100s of steel alloys many of which shouldn't be anywhere near a boiler, simply saying "use mild steel" is not enough information. I am a copper boiler builder and I don't build in steel, but the process of PLANNING a boiler, and investigating regulations, is the same. Since I don't build in steel I contacted a friend who is a highly regarded professional steel boiler builder (both model and full size) with code credentials and his material recommendations for model or small welded steel boilers are as follows:

Plate Materials - Pressure vessel grade, low carbon (ie, mild).- SA-285C or SA-516-70

Pipe - First choice: SA-106 B, 2nd choice: SA-53 (Seamless preferred but not required.)

Stays and Mudrings - SA-36

Welding - Full penetration groove or fillet welds
Root Pass - E-6010 or E-6011
Filler and Cap Pass - E-7018

I have had conversations with other professional builders of model steel boilers in the recent past and their choice of steel is about the same. You can take that for what it's worth, I will be glad to furnish contact information, but one of the concerns within the US live steam boiler building community (in steel or copper) is those individuals who will always devise a way to do a thing cheaper and/or easier. There's nothing wrong with making something less expensive or easier to do, unless safety is sacrificed in the process. None of the builders I have talked to have been willing to come off their choice of materials and say "this other stuff is a lot cheaper and just as good." If it was, they'd all be using it.
 

MattMaie

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"One of the concerns within the US live steam boiler building community (in steel or copper) is those individuals who will always devise a way to do a thing cheaper and/or easier."

I'm not interested in doing something unsafely or taking shortcuts, knowing full well with boilers that failures can be catastrophic and deadly, and most often are too. If it takes me extra time and money to ensure that the most stringent safety standards are met, then I'm fully prepared for it.
 

GWRdriver

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I'm not interested in doing something unsafely or taking shortcuts
The fact that you've begun to ask questions about how to go about it properly is evidence of that. Best of luck in the future.
 

GWRdriver

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get an Australian steel boiler code booklet, . . [snip] . . This will enable you to build a safe boiler
This is probably true. The Australian codes are well done, worthy, and useful codes in certain jurisdictions, BUT . . .
Most countries base their boiler regs on this standard.
I can't speak for other countries but the States in the country where you and I live (USA) definitely do NOT base their boiler regulations on the Australian Miniature Boiler Code for Steel Boilers. Using the AMBSC you would probably very well build an excellent boiler . . but you might not be allowed to legally steam it in WA. Again, before doing anything (including buying a copy of the Oz Code) I suggest you first determine what regulations will affect you locally.
 

Entropy455

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From an engineering aspect, I’m not aware of any ordinary strength mild carbon steel alloy that is unacceptable for low-pressure boiler use – assuming the maximum operating temperature is maintained below 600 degrees F, and the engineer has incorporated an extra factor-of-safety within the wall-thickness, to account for mild steel’s loss of strength at elevated temperatures.

From an engineering aspect, the best material for boiler construction, is actual boiler-steel alloy. Again, the problem for the majority of home hobbyists, is accomplishing a proper post-weld stress-relief (heat-soak), at 1100-ish degrees F for 12 or more hours - which from an ethical saftey aspect, is mandatory when using boiler-steel. Most boiler steels also require a several hour long minimum preheat of at least 150 degrees F prior to welding, or the metal becomes susceptible to hydrogen embrittlement cracking. Another difficult obstacle for the home hobbyist.

If you’re designing 1,200-psig superheat boilers for marine propulsion systems, you’ll likely specify the expensive high-strength boiler alloys. For low pressure boilers however, you’ll find that the “approved” low pressure boiler steel alloys are not that much different from ordinary strength mild steel. . …

There’s a group of hobbyists in Washington State that enjoy building model steam powered locomotives. These models are large enough to pull people around on tracks, and normally there are a lot of kids riding them. The models are also large enough to draw the attention of Washington State boiler inspectors. Yes, these hobbyist boilers must be licensed and inspected by the Washington State L&I inspectors.

With regard to the comments on cutting corners - the boiler is either constructed to code, or it is not. It will either pass hydrostatic testing, or it will not. It will either have a proper relief valve installed, or it will not. There is very little grey area.

I took a class on boiler design in college. The bulk of the class focused on what happens inside the boiler (i.e. the engineering art of maximizing steam production and efficiency without damaging the boiler tubes.) The pressure vessel is just that – it’s a pressure vessel.

http://apps.leg.wa.gov/WAC/default.aspx?cite=296-104

Above is the Washington State Adminstrative Code for Boilers.

http://www.lni.wa.gov/TradesLicensing/Rules/files/boilers/BoilerLawBook.pdf

And here’s a PDF of Washington State Boiler Laws.
 
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MattMaie

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The question that I have for you then, Entropy, is this. At what threshold is a boiler considered to be operating at high pressure as opposed to low pressure? I used to volunteer for an exhibition put on every year called the Great Oregon Steam Up, and the boilers there had their safety valves set to blow off at 100 PSI. What pressures to hobbyists typically run boilers at?
 

Entropy455

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Per section 296-104-010 of the Washington State Administrative Code for Boilers:

"Low pressure boiler" shall mean a steam boiler operating at a pressure not exceeding 15 psig or a boiler in which water is heated and intended for operation at pressures not exceeding 160 psig or temperatures not exceeding 250 degrees F by the direct application of energy from the combustion of fuels or from electricity, solar or nuclear energy. Low pressure boilers open to atmosphere and vacuum boilers are excluded.


Note: the second part of the above definition is somewhat misleading – in that they are describing a water heater as a boiler. In my opinion, it is inappropriate to call a water heater a “boiler” if you are not actually boiling the water. Nonetheless, that’s the definition. Thus the answer to your question is that a low pressure boiler is one which operates at 15 psig or less.


From an engineering aspect, a boiler pressure of 15 psig correlates to a saturated steam temperature of 250 degrees F. A boiler pressure of 100 psig correlates to a saturated steam temperature of 338 degrees F, which is significantly more energetic, and is classified as a “power boiler” by Washington State. Also, a 1200 psig oil-fired naval boiler correlates to a saturated steam temperature of 568 degrees F, which is 100 degrees hotter than needed to ignite wood. Note that if you add a superheat stage to any of the above boilers, very high superheat steam temperatures are achievable. Regardless if you are running superheat or not, you must control the temperature of the pressure vessel shell, as non-boiler rated carbon steels become very weak at elevated temperatures, and significantly weak at temperatures above 650 degrees F.


Helpful tip: like mild steel, Grade-5 and Grade-8 steel fasteners also lose strength at elevated temperatures. ASTM-A193 steel is used to construct bolts/studs for high temperature applications. ASTM-A194 steel is used to construct nuts for high temperature applications. Like boiler steel, these "high-temp" fasteners are alloyed to retain appreciable strength well up to 1000 degrees F. Note that ASTM-A193 and ASTM-A194 fasteners are readily available through Fastenal, and other on-line supply stores. They’re not too expensive either. For steam joints, exhaust joints, or any other high-temp joint requiring high strength, these are the fasteners to use.


With regard to the second part of your question - unless the home hobbyist is looking to maximize boiler thermodynamic efficiency (which is unlikely) the safest approach would be to run the lowest possible boiler pressure required, while still obtaining satisfactory model engine operation. What that pressure normally is, I don’t know. . . .
 

GWRdriver

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What pressures to hobbyists typically run boilers at?
Matt,
I'll confine my answer to steel model locomotive boilers which in the USA are typically greater than 6" (nominal) in diameter. 1"scale (4.75"gauge) and small 1.5" scale (7.5"ga) locomotives, with boilers of 6" diam or near that, will be a relatively light engine and have less adhesive weight on the drive wheels resulting in an increased tendency toward drive wheel slip. To minimize wheel slip operating pressures for smaller engines are kept in the 80-100psi range. Locomotives with larger larger boilers are uaually asked to do more work (passenger hauling) and tend to be run at between 100 and 125psi. However, even with often considerably more weight on the drive wheels many owners find that 125psi results in too much power for the weight at the drive wheels and are operated at nearer 100psi than 125psi. For most model locomotive builder/owners of steel boilers that I know of 125psi is considered the practical maximum operating pressure and this has less to do with the safe capacity of the boiler than with maximizing the operating behavior and power to the rails of the locomotive.
 

Jasonb

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As this question started off about traction engine boilers I'll just say that here in the UK the larger size engiens would be run upto about 170psi working pressure for the compounds. Singles which I think most of the US engines are would be more like 100-120psi. Again this is a practical pressure not what the boilers may be capable of running at.
 

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