Can someone explain this? Iron oxidation and heat

[ichthos]

Hello,
Can someone explain to me why using heat makes it easier to remove a bolt that is rusted? I know that the iron and oxygen forms iron oxide. That means mass would increase in the bolt thread area. Heating the bolt would make it expand. Does the heat just cause the iron oxide to be compacted, making more space again? Any thoughts would greatly be appreciated.
Kevin

 

[dklawson]

I never stop with just heating a bolt or nut. I'll quench the fastener with a light oil like WD-40. I've always had good luck combining oil and heat. My assumption is that the rapid cooling of the expanded parts allows the oil to wick into the threads better.

I've never tried heat alone but I'd say your explanation is good.

 

[JPSmit]

my understanding has always been that when heat is applied, the two things (bolt and nut or bolt and block or whatever), expand at a dissimilar rate hence breaking their bond from each other. Rapid cooling also encourages the pieces to move relative to each other.

 

[spritenut]

My best guess is that the heat expands the metal, or rust.
then it cools a bit but because it has expanded, and then shrunk a little.
That little bit is usually enough to free up a stuck bolt.

 

[Guest]

I never stop with just heating a bolt or nut. I'll quench the fastener with a light oil like WD-40. I've always had good luck combining oil and heat. My assumption is that the rapid cooling of the expanded parts allows the oil to wick into the threads better.

I've never tried heat alone but I'd say your explanation is good.

I do the same.