I was talking to a gunsmith / blacksmith last night. He specializes in guns from before 1800. He remarked how easily real (300-year-old) wrought iron worked compared to mild steel. He loved working with it. But it hasn't been made in the USA for many decades now.

Wrought iron was preferred for locomotive staybolts because it won't work-harden from the boiler thermal expansion and contraction. Even the lowest-carbon "cherry-picked" grades of mild steel just aren't as good at resisting work hardening. Wrought iron is also much easier to rivet over than mild steel, which is an advantage for staybolts.

Tensile stress in staybolts is limited by FRA code to a very low value, so the <=20% lower tensile strength of wrought iron (vs. mild steel) isn't an issue.

Rooting around the internet this evening, I was amazed to find that one manufacturer of real wrought iron still exists. Here is the info: http://www.realwroughtiron.com/home-12.html



Rolled wrought iron available in:
Round Bar 1/4" to 2" Diameter
Square Bar 3/8" to 2"
Flat Bar 1/4" to 3"
Puddled Plate 3/32" - 1/2"
other sizes/shapes on inquiry

- Doug Debs

So for we armchair mechanics, why is the tensile strength of staybolts limited to a "very low" value?

Steve

Wrought Iron is out there, although most of it is rerolled scrap (old bridges, etc.) and a little New Old Stock (NOS).

-Hudson

Speaking as an armchair mechanic, I'd suggest it's so that the staybolt is an intentional failure point; rather than ripping the threads out of the sheet or transmitting flex stresses into the sheet, causing work hardening and cracking of the sheet at the staybolt threads.

Michael Seitz
Missoula MT

Has anyone use wrought iron to make staybolts from in the last 50 years?

Should we?

Wrought Iron was also used to make boiler plate until the 1870s - 1880s. I'm not sure that is an indication that we should start making wrought iron boilers again.......

Speaking from experience, I have not had any difficulty beading over the ends of threaded steel staybolts we use every day....although in new construction welded stays are even more common. I'm not sure going to a softer material is of benefit. The additional margin of strength in a steel stay is nice to have, and unless there's a problem in the design, workmanship or operation of your boiler the occasional need to change a staybolt should be rare no matter what they are made of.

If the modern wrought iron is principally made of recycled stuff all puddled together.....how do we document its quality? Do the blacksmiths supply houses offering it for decorative purposes actually test it and provide pedigrees?

dave

"Stress", not "Strength".........

Does FRA code differentiate between "Iron" and "Steel" or is it simply a matter of tensile stress.

Also, I have to wonder, if there's only one place left that makes the stuff, obviously there's almost no demand for it. There must be a good reason for that?

Ah that makes sense. Low stress, less need for strength. Thanks for catching that.

Steve

Stay bolt stress is limited to 7500psi due to the potential for bolt breakage in a locomotive boiler. As has been discussed on here before, the output demand and number of thermal cycles on a locomotive boiler is far in excess of what is seen on most other fired pressure vessel applications which leads to the greater likelihood of breaking stay bolts. If one bolt breaks the load that it had been taking is transferred to the other surrounding bolts. Keeping the allowed stress low allows a margin of safety, preventing the surrounding bolts from being loaded in excess of their actual strength.

The FRA does not differentiate between stays made of steel or wrought iron, although the ASME code does not recognize wrought iron as a suitable material to be used in new construction. Like any material, wrought iron has its advantages and disadvantages. Steel excells as a staybolt material in its strength, wrought iron in its maleability and toughness. Wrought iron stays are therefore not just softer for riveting over and expanding for making water tight but also more flexible than their steel counterparts of the same size. The advantage then being that the wrought iron bolts are less likely to break (although not free from the possibility) and an unbroken staybolt is better than a broken one, regardless of strength. Wrought iron was also the preferred material for boiler tubes and other boiler braces well into the 20th century.

So if wrought iron is so good for certain things why isn't it made anymore you might ask.... It's simple, wrought iron is extremely labor intensive and expensive to make when compared with steel which can be made to have properties similar to wrought iron but able to be used in a wider array of applications. Also, puddled iron is not the same as reclaimed iron, which is the cheapest form of wrought iron production where reclaimed iron is bundled together, brought to a welding heat and passed through rolls forming it into a bar of variable quality. Charcoal iron, on the other hand is usually of extremely high quality and was the only type specified for use in boiler tubes, but again adds many steps to the manufacturing proces and obviously a great deal of expense as well.

A careful reading of their website reveal that they are not actually making wrought iron, but are rerolling it. I did not see any mention of a puddling furnace on their website.

Another point is that some of these wrought iron rerollers are getting their bar stock by shearing plate, for most blacksmithing this is not a problem, but if you are trying to machine it, you will have problems, as it is not that straight and true.

-Hudson