# Thermal expansion of steel

I was playing with putting a cross inside a small ring yesterday and found out that steel expands and shrinks when heated quite a bit more than I
thought it would. Made the cross fit and hold by heating the ring and making it shrink around the cross.
Then today, I was drawing out the ends of a rod to equal lengths by marking the center and then measuring how far I drew each taper out so I was sure they were the same. Again, I was surprised to see how different the measure was between when it was hot and cold.
So I just went out to the shop and did a little more careful testing with digital calipers and a 1/2" square rod of mild steel.
What I found was that a 10" rod will lengthen about 1/8" when heated to a bright orange (around 1000C or 1800F). That's a simple enough number to remember and adjust for.
That value matches the documented thermal expansion numbers of steel I found on the net.
It never occurred to me that it would stretch that much. I assumed the expansion was more like a few thousands - something that would be hard to measure with a ruler and below the level of accuracy a typical blacksmith project would need to care about (aka one tap with a hammer to adjust). But it's a lot more than that becuase we are working with such large temperature changes. At these amounts of change, trying to make hand forged parts a specific length (like fitting a hot scroll inside a cold frame) becomes a fun challenge if you don't keep in mind the difference in length between hot and cold. Make the inside part snug when it's hot, and it will be very loose once it cools if you don't adjust for the change correctly.
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Curt Welch http://CurtWelch.Com /
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Both the ring and the inside expands on heat. Same linear expansion of heat.
naturally some volume expansion also.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net "Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
On 5/31/2010 6:23 PM, Curt Welch wrote:

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Yes, that's very true.

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Curt Welch http://CurtWelch.Com /
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i remember sliding a carrier bearing on a drive shaft that i froze with CO2 (- 110 F).the press fit no longer applied, until the drive shaft warmed back up to room temp. I think you will find that not only will a hot bar or rod of steel get longer it will also be heavier ! have fun, mark
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Ah, you've found the nugget of truth in flame bending / straightening. Thermal expansion will take place along the unrestrained planes of the part. If you have a chunk of flat stock laying around, play with your torch on it for a bit!
The principles behind flame straightening / bending is pretty simple. Perfecting the practice is most definitely a hard-earned skill.
1: Localized rapid heating and thermal expansion, 2: Cooler surrounding metal restrains direction of expansion 3: Plastic deformation in the unrestrained direction occurrs if the expansion is greater than the metal's yield point. 4: When it cools, the metal will contract in all directions, causing it to change shape.
A good resource is "Welding Fabrication and Repair" by Frank Marlow, PE from Industrial Press. I pretty much copied the above straight out of his book.

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Tin Lizzie
"Elephant: A mouse built to government specifications."-Lazarus Long
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In pattern making for casting IRON, I think they use 1/8" per foot. That's how much larger that pattern has to be than the finished product. For casting STEEL, they use 3/16" per foot because the steel melts at a much higer temperature than does cast iron.
Pete Stanaitis --------------------
Curt Welch wrote: <snip>

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