Steel alloy choice for wood fire service?

[wnicoll]

Hi,

I'm building a maple syrup evaporator and need load bearing metal bar stock that will withstand thermals with direct exposure to wood fire (Google tells me this is around 600degC, but not sure the accuracy of that). I tried mild steel bar stock that was fine under ambient temperatures but deformed severely at temperature. What would be an affordable choice of metal for this application? Would 316 stainless be okay?

Thanks

 

[metengr]

What is the configuration of the bar stock for this design? Because it deformed suggests it may not have sufficient thickness to accommodate loads at elevated temperature. You may want to look at wrought iron grates for support.

 

[EdStainless]

or it may have been too thick. With high heat applications, especially where the heat may not be uniform you really need to allow for deformation. If you make it rigid enough to not deform the differential thermal expansion will tear it apart.
If you are getting a lot of scaling and metal loss then a stainless might make sense.
In reality this more of a mechanical issue.
Your main supports should be out of the direct heat. The rest need to have some 'give' to them.
I have melted Al in a wood fire, that is 660C.

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P.E. Metallurgy, Plymouth Tube

 

[wnicoll]

I'm going off memory but I think I purchased 1.5" x 0.5" bar stock and it has about a 26" span over the fire. I had 3 bars, although 2 were probably taking the brunt of the 75 pound load. They sagged a good 4-6" when at fire temperature.

I see I can get carbon or galvanized steel bar grating for not that much money. But not sure if that will hold up better than what I used previously.

 

[ornerynorsk]

Galvanized would be a waste of money, it'll burn off with the first fire and produce a not-so-pleasant smoke.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metengr]

You had inadequate support over a 26" span. Get carbon steel bar grating with added support legs to reduce the 26" span. You do not need to waste your money on stainless steel for this application.

 

[EdStainless]

You are supporting a tub or tank?
Why does the support need to be in the fire?
Can you support or hang it from the upper lip?

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P.E. Metallurgy, Plymouth Tube

 

[wnicoll]

If I was building from scratch I would not have the supports in the fire. But I am utilizing an existing stone/brick firepit with a built in chimney. The firepit area has a row of bricks on each side for supporting some kind of grate, with the aforementioned 26" span. The level of the grating is only about 2' above the base of the fire pit so a roaring fire will easily be directly contacting the grating/bars.

As an interim solution I just piled a stack of bricks in the middle of the fire pit to make a midpoint support. This greatly reduced the amount of sag as I can actually support a good portion of the weight of the maple sap tub/pot on the bricks instead of the steel bars (which I re-checked and found to only be .25" in thick not 0.5" steel). Low tech solution.

The downside of this is that it's taking up a good bit of space in the firepit, where I could otherwise be stacking logs for maintaining my boil. And it impedes the airflow. So I may still look to getting steel bar grating to replace these somewhat flimsy bar stock & brick combination.

 

[EdStainless]

You could build a support pier out of fire brick that takes up less space and still offers some support.
The important thing for the grate is that the bars spanning need to be taller.
In order to keep the bars from twisting is is common to weld small rods across them at the top and bottom.
This still lets them flex a bit but adds stiffness.
Mild steel should be fine.


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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

You have a bar supporting a tub of maple syrup, and the bar is bending due to getting too hot. Of the hundreds of ways to do what you want, it seems you have found the one that does not work. How are you not scorching your syrup?