The arithmetic is one line
Mass times specific heat times temperature rise. A band 36 inches long, 8 inches wide and 3/4 inch thick is 216 cubic inches, which at 0.2836 pounds per cubic inch is 61.3 pounds of steel. At 0.110 BTU per pound per degree that band holds 6.7 BTU for every degree F, so a rise from 60 to 250 costs 1,280 BTU.
Twelve hundred and eighty BTU is nothing. A 400,000 BTU per hour rosebud at 35 percent efficiency is putting 140,000 BTU an hour into the steel, which does the job in 33 seconds. Anybody who has actually preheated a plate knows it does not take 33 seconds, and the gap between those two numbers is the useful part of this page.
Why the floor is not the answer
The stored heat figure assumes nothing escapes while you are putting it in. Everything escapes. Heat conducts sideways into the cold steel either side of the band continuously, radiates off both faces, and convects away into the shop, and the hotter the band gets the faster all three happen. A rosebud on a heavy plate spends most of its output feeding losses rather than raising temperature, which is why the real figure is a multiple of the floor rather than close to it.
There is no honest way to predict that multiple from a form, so it is a field. At four times, the default band takes 2.2 minutes rather than 33 seconds. Time one part with a temp stick, divide by the floor the page prints, and put your own number in — it will be stable for that part on that bench and useless for anything else.
The propane is not the cost
At four times the floor, the torch runs 2.19 minutes and burns 14,600 BTU, which is 0.68 pounds of propane, or a dollar and two cents at 1.50 a pound. The fabricator standing over it for those same 2.19 minutes costs 2.38 dollars at a 65 dollar shop rate. Time is 2.3 times the fuel. That ratio does not depend on the size of the part at all, which is worth noticing: both lines are the same minutes multiplied by a different rate, so the part cancels out and only the shop rate, the torch output and the price of gas move it.
Six parts a day is 13 minutes of torch, 6.10 of propane and 14.27 of time, which is about 5,100 dollars a year in a shop that never thinks about it. The lever is not the price of gas. It is heating more than one part at a time, or putting the heat in with something that does not need a person holding it.
Where the model is wrong, in both directions
The band is treated as a rectangular slab all at one temperature, and real preheat is a gradient — hottest at the joint, tailing off to shop temperature somewhere past where you were aiming. Taking the whole band at full temperature overstates the stored heat. Ignoring the heat that conducts out past the band understates the loss. Neither error is small and they point opposite ways, which is a reason to treat the floor figure as a scale rather than a prediction and to lean on the measured multiplier for anything that matters.
Specific heat is also treated as constant, and it is not — it drifts up as steel heats, so the figure here is a room temperature value applied across the whole rise. Over a couple of hundred degrees that is a small error. Over a rise to a stress relief temperature it is not.
Questions people ask
How many BTU does it take to preheat steel?
Mass times specific heat times the temperature rise. Carbon steel is about 0.110 BTU per pound per degree F, so a 61 pound band of steel going from 60 F to 250 F takes 1,280 BTU. The number is small — a hundred pounds of steel through a 200 degree rise is 2,200 BTU, which a rosebud delivers in well under a minute if none of it escapes.
How long does it take to preheat a plate with a rosebud?
Several times longer than the energy arithmetic says, because heat runs out of the band as fast as you put it in. The stored heat floor for the default band is 33 seconds at a 400,000 BTU rosebud and 35 percent efficiency; the realistic figure at a loss multiplier of four is 2.2 minutes. On a heavy plate with a lot of cold steel around the joint, multipliers of eight or ten are ordinary. Time one part and measure your own.
How much propane does preheating use?
Torch output times the runtime, divided by the energy in a pound of propane. A 400,000 BTU per hour tip running 2.19 minutes burns 14,600 BTU, which at 21,600 BTU a pound is 0.68 pounds, or about a dollar. Fuel is almost never the significant cost of preheating — the person holding the torch is, by a factor of two or more at typical shop rates.
What temperature should I preheat to?
Whatever the procedure the job is being run to specifies, from whoever is responsible for the weld. This page states no preheat temperature and has no view on whether a section wants one at all — it takes a temperature you were given and converts it into energy, fuel and minutes. Preheat requirements depend on the steel, the thickness, the process, the restraint and the conditions, which is why they are written into procedures rather than looked up.
Is electric preheating cheaper than a torch?
Work both modes on your own numbers, because the answer depends on your electricity price, your propane price and above all on whether somebody has to stand there. Electric pads put a much higher share of their rating into the steel and do not need holding, so the labour line usually collapses; the rating is far lower, so the heating time usually goes up. That trade is the whole comparison and it lands differently in every shop.