Boiler Material

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ASME Code Boilers

Marty Know wrote:

Materials accepted for construction of an ASME code vessel.
  • Shell - SA106 B
  • Plate - SA285 grade C/SA515 grade 70/SA516 grade 70
  • Steel Tubes - SA178 grade A (welded) SA192 (seamless)
  • Copper Tubes - no Code specification
  • Staybolts - SA36/SA675
  • Threaded couplings - SA105 3M (3000#)


chet86 asks:

How much stronger is Type K over type L copper tubing? Only 0.009" difference in the wall.

From gwrdriver:

Let's assume for the moment you are looking for a boiler barrel (The barrel is the long'ish, tubular, front part of the boiler). If you run it through the applicable formlae for boiler barrels you will find that Type M is actually sufficiently strong for 100psi steam. But because there are always other forces acting on the boiler, both during and after construction, as Bill has just mentioned, I don't use Type M. All three types meet the basic strength requirements, but the extra rigidity and weight of Types K and L allow me to handle the material a little bit more like a solid and machine and work it with less concern for deformation. My personal preference is Type L for barrels and Type K for flues. I can usually get Type L locally but the Type K for flues almost always has to be ordered.


Bill Shields writes:

Pipe and HRS plate are NOT the material to build a boiler, unless the pipe is something like oil well casing which has the same specification / inspection requirements as boiler tubing (I worked for an oil company for 10 years and know this from experience). You need to get certified material that is OK for Pressure Vessel application.

Marty Knox writes:

I prefer to use rod stays (as opposed to girders). It is easier to calculate the stresses, they don't put a bending stress on the side stays, and they allow better circulation.
Seamless pipe, SA106B, would be my first choice for the shell. Flat plates should be Pressure Vessel Quality, SA516-Gr70 is the most common. Couplings and half couplings should be 3000# forged steel, ASTM A105. Stays and mudrings can be made from SA36.
10" pipe is 10 3/4" OD. In a boiler this size (10-inch diameter by 41-inch length) I usually use 5/16" plate.
I use only PVQ plate. Sa285 GrC is getting harder to find these days. SA516 Gr70 is much easier to get. Boiler plate used be sold as firebox quality or flange quality. Those terms fell into disuse in the early 70's, about the time I started building boilers. The current term used is PVQ, for Pressure Vessel Quality.
Keyhole boiler inside view. Photo by Marty Knox.

Nickel Steel

Tony Burzio writes:

There are all kinds of references to nickel steel boiler being used in real locomotives, then all of a sudden these boilers all had to be replaced at the same time. No reason is ever stated, does anyone know what went wrong with them?

Bruce Mowbray answers:

Nickel steel boilers were used on real locomotives thinking that the higher (70,000 psi) tensile strength would be an improvement over the standard 50,000 psi carbon steel. It worked for a while but due to the high amount of flexing from both firing and bouncing down a railroad track, these high strength, somewhat rigid steels began to crack more than carbon steel boilers. It turns out that the old soft steels were a better choice for a boiler with such a high rate of demand change and rough service. Nickel steels were also used in frame castings and rod manufacture.

Stainless Steel

"Duplex Steel Boilers" refers to boilers made using Duplex SAF 2205 Corrosion Resistant Steel boiler.

Doug Auburg writes:

There is a lot of talk in Australia and New Zealand about Duplex steel boilers (i.e. boilers constructed using a boiler grade stainless steel instead of carbon steel or copper). The big advantage of the use of this material is the wall thicknesses can be about 1/2 as thick. The material is almost twice as strong as mild steel and no extra thickness allowance need be made to allow for corrosion in the boiler through use. The material they are talking about is ASTM A240, A276, A789 and A790 (depending on the application).
Australian states (except Victoria) have adopted a standard for constructing model boilers from Duplex steel and New Zealand modelers are working hard to get it adopted in their country.

John E. writes:

Traditionally we seek two critical qualities in boiler steel: flexibility and weldability. When we talk about flexibility we are looking for materials that bend easily and that do not work harden to the point that they become brittle or crack sensative. In the case of Nickel alloy steels, the nickel is added to increase strength, yet, we loose ductility (flexibility) when we add more nickel. In the real sense of the word, we seek materials that are high in copper (very flexible) and very low or lower in nickle. Stainless, as a material is very strong, but is also very crack sensitive by nature of what it contains. When designing a boiler, materials that are 55,000 lbs are MORE than enough to do the job. In fact, early steels in locomotive boilers went as low as 25,000 psi. We can not even obtain such materials today.
Model boilers: The trend seems to be to go with stainless materials...which is fine. We just need to understand that what we are trying to prevent in corrosion, as a result of using stainless, we generally make up in repairs due to cracks. I believe that a well treated boiler (chemicals and the way the boiler is handled) is just as well off with a steel boiler as you would be with stainless. I have seen more steel boilers last longer than stainless steel boilers before a failure begins to happen. Even in a well treated stainless will never prevent the inevitable crack in the material.
The question becomes: A boiler that does not rust, but that cracks...or a boiler that rusts and does not crack.

Dale Dennis writes:

When talking about stainless steel, it's important to remember that stainless has a much lower heat transfer capacity than mild steel, and only about 15% of the conductance or transfer capacity of copper. While some of that might be mitigated by a thinner skin, at least than mild steel, and bring the system closer to an equivalence with mild steel, I don't see you gaining much, in either the short or the long run, by making that compromise.

Advice from Marty

Marty Knox shares some views on boiler material and construction:

I thought I would make some comments on the design of boilers and the desirable material properties. My experience with boilers started with building my first locomotive. Like many live steamers initially I was more interested in the machining side of the things. I had that locomotive running on air and needed a boiler. I wrote to a gentleman who advertised in Live Steam magazine and he quoted $600 to build my boiler. Well, $600 was an astronomical sum to a college student in the mid-70s so I figured I would build my own. I ended up building one in steel with copper tubes; I paid the best welder I knew $125 to weld it up for me. That experience grew into a life-long interest and career. My comments are based on 30 years of building, inspecting, operating, and maintaining boilers.
The first quality of a boiler material that comes to mind is ductility - you want a material that will withstand many,many heating and cooling cycles, with out fatiguing and cracking. Strength is almost secondary - the strength of a boiler is in its structure. Up to about 5 1/2 - 6" diameter and 100 PSI copper has many advantages,above that,low carbon steel is the material of choice. I worked as a boilermaker for DuPont. They have a whole department that works on material specifications. They never hesitate to buy the best material for the application. We worked with a wide variety of materials - Inconel, Hastelloy, and Carpenter 20 to name a few - yet all our boilers were low carbon steel!
Boiler plate and even the terms used to describe have evolved over the years. Back when I was first learning about boilers you still heard terms like 'Firebox Quality' and 'Flange Quality'. Today the term used is 'PVQ' - Pressure Vessel Quality. The specifications to look for are SA285 Grade C and SA 516-70. For bars and rods the specification is A-36. These are the ASTM (American Society for Testing and Materials) classes. A-36 also can apply to plate. In fact, SA 516-70 meets the A-36 spec, since it is so broad. The difference is the testing and quality control. The 516 has to meet much tighter specs of tensile strength and chemical analysis. When we ordered the plate for the floor of 464's new tender I spec'ed A-36 - the supplier sent us 516!
How do you know what plate you are getting? You should ask for the MTR's - Mill Test Reports, also known as certification papers. These will have the heat numbers which identifies that particular plate along with the tensile strength, yield strength, and chemical analysis. It is common to have multiple heat numbers listed on the same MTR, but you should check the heat number stamped into your plate and make sure it is listed. And yes, the heat number should be stamped into the plate even if you are not buying the whole plate. This is one of the requirements if you are building to the ASME(American Society of Mechanical Engineers) code - the heat number has to be stamped into each part. This is to provide traceability. If there ever turns out to be a problem you can go back to the manufacturer and prove that it is a plate that he made. For hobby work this isn't as important as in ASME work.
I tend to be very conservative in staying a boiler. I'm used to full size locomotive practice, where you figure a load of 7500 psi maximum on the staybolts. To plug in some dimensions, with 1/4" plate I will use 3/8" diameter stays on 2" centers. In 5/16" plate I use 1/2" diameter stays on 3" centers.
What we are talking about is an area of many square inches supported by far less than 1 square inch. To use a specific example with 3/8" diameter stays on 2" centers you have 4 square inches supported by .1105 square inches. At 100 psi you have a load of 400 lbs. divided by 0.1105 for an actual load on the stay of 3,620 psi. Another case would be 1/2" diameter stays on 3" centers, at 100 psi you have 900 lbs. divided by 0.1964 for an actual load of 4,582 psi. I weld the stays into the inside sheets before I assemble the firebox.
The Australian Model Boiler Safety Code is an excellent document, and I have copies of section 1 (copper boilers) and section 2 (steel boilers). I would refer to it more if I could think metrically, but I still think in inches and feet.
You can look up the boiler rules in effect where you live. Go to and click on Juridictions. You then select Canada or USA, then your province or state. When you click on Details it opens up a window , usually with a picture of your Chief Inspector and a link to their website. Most of these websites will have the Law or Rules and Regulations. Some of them will have an index, if they do you can quickly see if there is a section on hobby or miniature boilers. Others you will need to wade through the Law or rules to see what it says.
A word of caution - you want to be careful if you do talk to an inspector. In some cases the boiler division has chosen to ignore the hobby - you don't want to do anything that will make them take official notice of the hobby.It may be wise to speak to your local club or other hobbyists rather than an inspector.
Here are materials that are acceptable for construction of an ASME code vessel.
  • Shell - SA106 B
  • Plate - SA285 grade C/SA515 grade 70/SA516 grade 70
  • Steel Tubes - SA178 grade A (welded) SA192 (seamless)
  • Copper Tubes - no Code specification
  • Staybolts - SA36/SA675
  • Threaded couplings - SA105 3M (3000#)

See also:

"Boiler Design and Material Selection",

What not to use

Harold V writes:

It's not a good idea to use brass in a boiler, as it can suffer de-zincification (and failure). I expect the pieces you have are made of bronze, not brass. Bronze does not suffer the same fate.

Safe Operating Pressure

The North American Live Steamer, January 1956

Determining Safe Working Pressure for Seamless Tubing

The safe limit of pressure may be established by use of the following formula:

Tensile Strength     Thickness
of Metal in       *  of wall in
pounds per    inches
--------------------------------- = Bursting Pressure
      Radius of Tube
     (1/2 O.D.) in inches

Bursting Pressure
------------------------ = Safe Limit of Pressure
Factor of Safety (ex: 6)

For instance, copper tube for boiler:

  • Diameter of Tube: 6 inches
  • Radius of Tube: 6 inches / 2 = 3 inches
  • Thickness of wall = 1/8-inch
  • Tensile Strength of copper = 30,000 pounds per square inch (psi)
                     30,000 * 0.125   
Bursting Pressure = --------------- = 1,250 psi

                          1,250 psi
Safe Limit of Pressure = ----------- = 208 psi

Said copper tube will provide a safe operating limit of 208 psi for boiler, providing you have good joints.


Steamin10 writes:

Staybolts, or stays for short, are an integral part of a boiler, to keep distortions to a minimum when subjected to heat and pressure.
Stays are redundant, and the failure of one or two, do not mean the imminent failure of the boiler. (in the sense that it is going to blow or injure someone). Model boilers are over-designed from that standpoint very well. Over time unknowing people have allowed boilers to clog and deteriorate, and there have been very few spectacular failures, or blowouts in the hobby boiler universe. (That I know of).
The comment that stays are "redundant" means simply they are repeated in the design, and reinforce the basic materials. In small hobby boilers they are over-sized and overkill. You will not bend a 5/16 plate of 6x8 inches with 100 lbs of steam force. Do the math. They would, AND SHOULD be stayed. A single stay failure will have little effect on operation, or safety, though undesirable.

David Griner writes:

The ASME Code, Section I, Power Boilers (this boiler qualifies) requires a full penetration weld. A full depth countersink of an included angle of about 60 degrees.

Bill Shields writes:

I drilled all my stays with a 3/4" hand-held electric drill with 2 long arms on it. Not very high-tech, but what the heck, I had the drill handy and it seemed like a good idea at the time.
I chamfer the holes and bevel the bolts so that I can get a full penetration weld and call it a day and have never had a problem with one failing. My bolts are large enough so that even with the bevel, I have some 'flat' on the end of the bolt. Beveling the bolt heavily may not be a good idea if they are smaller in diameter (I typically us 1/2" diameter).
Some folks tell you that you MUST have this bit of the top of the stay still visible to be sure you don't 'burn off' the stay during the welding practice.
The big problem with welding stays is running around in your own slag with STICK (which I do). I tend to want to weld 1/2 the circle, stop, chip, then finish. We still had a couple of small leaks that had to be ground out and fixed, but nothing major and passing a 300PSI + hydro was no problem (well below tube crush for my smallish tubes).
If you are worried about full penetration, make up a dummy for test, have your guy weld it up, then saw it in half and inspect it. You should know the inspection procedures for full penetration welds - it isn't rocket science.

Marty Knox writes:

My standard practice is to use 1/2" diameter A36 rod for stays. I countersink the sheets with a 60 degree countersink 3/16" deep on 5/16" plate. I also chamfer the back 1/16" on the water side. I chamfer the stays 1/16" but make make them 1/8" longer than the dimension across both plates and the mudring. I leave the stay sticking 1/16" above the surface and run Lincoln Excalibur 1/8" 7018 hotter than normal. I don't always burn through but all the test pieces I have made have a solid weld all the way down without melting away the stay. Most of the time you can see the end of the stay, but not always. When I have plenty of time I will TIG in stays, but it's been a while since I've had plenty of time.
The first boilers I built with this technique are now 30 years old. The three that I know of that are no longer in service were all victims of scale buildup between the sheets, due to poor water, lack of water treatment, and few or no washouts. Even the one that had a 'mudbag'(burned sheet) bulged between the stays but the stays held.

From The Live Steamer, March-April 1950:

What materials are best to use for boiler stays, and why?
A. R. Allen wrote:
This depends on the material used in the boiler shell & firebox. Most small loco boilers are made of copper and for these boilers the best stay material is also copper. This is to avoid wasting away in the water space due to electrolysis. This action takes place when dissimilar metals are in contact and submerged in the boiler water.
For a steel boiler with a steel firebox the stay-bolts should also be of steel for the same reason given above.
B. W. Barnfather wrote:
Ever-Dur (silicon bronze) screws (or bolts) are favored a lot by steam builders for stays as having non-rusting and high tensil strength qualities and are easy to drill to make hollow stays if desired.
Note: Mixed metals in boiler work are to be avoided as much as possible, and brass should never be used for stays.


Dan Rowe writes:

One of the best sources of information on model boiler design is the Australian Miniature Boiler Safety Committee. (AMBSC) The four books (1) Copper (2) Steel (3) Sub-miniature and (4) Duplex Steel
Model Boilers and Boilermaking by K. N. Harris has a wealth of good design information and construction methods. Topics covered include fuel and firing and accessories. (Copper boilers)
Model Locomotive Boilers by Martin Evans is also a good source of design information and methods. It covers welding equipment, workshop equipment, water, fuel and combustion, oil and gas firing, and testing. The testing is more than a simple hydro a full engineering test stand designed by J. Ewins is described with drawings of the boiler and test results. (Copper and steel boilers)
Safety of Copper Boilers by Kozo Hiraoka this nine page article was first in Live Steam & O.R. (Vol. 40 No. 6 Nov-Dec 2006) and is included in Building the New Shay. The design rules are presented in graph form for simplicity with the ASME formulas and code designation listed in the appendix. All graphs have a metric and an imperial version. The graphs are (1) Minimum thickness for a cylindrical shell under internal pressure (2) Maximum allowable pitch of symmetrically arranged staybolts (3) Maximum allowable diameter of a circle enclosing an unstayed surface area (4) Minimum staybolt diameter. This book like all of Kozo’s books is a treasure trove of useful information for the model engineer.
These are the four main sources of model boiler information in my stack. Several other model engineering books I own have boiler chapters but the information is not a complete design system. These include works by Greenly, LBSC, Tubal Cain, and Martin Evans.
Cheers Dan