Can heat weaken a bolt?

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Mike Z

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Hey there,
97 TJ Sport, 4.0, 5spd. Was replacing my rear sway bar bushings and bushing brackets and had a hell of a time getting the bolts out. Sprayed penetrating oil and let it sit for a few days...re-sprayed multiple times. Was able to crack three of the four loose with a breaker bar, but the fourth was seized. Had to heat it about 5 separate times to finally crank it out.

Not an engineer, so wondering if heating that bolt that many times to red hot fatigues the metal? Thanks for the help!
 
Got it. Posted this in For Sale Finds. Sorry.
 
He did. Thanks. That's what I get for reading For Sale Finds and then deciding to post about something on my mind and not remembering that I was still on that forum. It's hell getting old :oops:
 
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It does weaken the bolt significantly, though technically not through fatigue. High strength bolts are typically heat treated to have a very small grain structure, which gives them good strength. The smaller the grains (which are essentially small crystals of metal atoms), the fewer aligned slip planes in the metal, and the higher the isotropic strength.

When you heat a bolt, those atoms begin to realign themselves with neighboring grains, resulting in fewer and larger grains. Each grain is a crystalline structure of metal, meaning all the atoms are neatly aligned. This means that the atoms can much more easily slip along each other, resulting a weaker (but more ductile) metal on the macroscopic scale.

There are ways to restore a tighter grain structure, such as quenching (heating and then rapidly cooling to break grains into smaller ones), but unless done very carefully, it will likely result in other changes, such as precipitation of alloying elements, embrittlement, or other defects. There is a whole lot I'm leaving out, but you get the idea.

In short: replace the bolt.
 
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Honestly, I didn't understand roughly 50% of what you shared, but I greatly appreciate the information. Rather than just saying, "replace the bolt" you gave the science as to why it's important to do so. Thanks for taking the time to give that explanation.

In short: I will be replacing the bolt ;)
 
Steel City 06 said:
It does weaken the bolt significantly, though technically not through fatigue. High strength bolts are typically heat treated to have a very small grain structure, which gives them good strength. The smaller the grains (which are essentially small crystals of metal atoms), the fewer aligned slip planes in the metal, and the higher the isotropic strength.

When you heat a bolt, those atoms begin to realign themselves with neighboring grains, resulting in fewer and larger grains. Each grain is a crystalline structure of metal, meaning all the atoms are neatly aligned. This means that the atoms can much more easily slip along each other, resulting a weaker (but more ductile) metal on the macroscopic scale.

There are ways to restore a tighter grain structure, such as quenching (heating and then rapidly cooling to break grains into smaller ones), but unless done very carefully, it will likely result in other changes, such as precipitation of alloying elements, embrittlement, or other defects. There is a whole lot I'm leaving out, but you get the idea.

In short: replace the bolt.
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Do you think he got it hotter than the exhaust does the manifold or collector bolts?
 
mrblaine said:
Do you think he got it hotter than the exhaust does the manifold or collector bolts?
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HSLA components will begin to anneal at temperatures exceeding around 900°F. Metals will begin to visibly incandesce starting at around 975°F. So any HSLA bolt heated to the point that it visibly glows has been permanently altered.

Most exhaust fasteners are designed to use various grades of stainless and other alloys designed for use in high temperatures. There might be some grain structure change, but generally the alloy itself is designed to make up for the lack of proper heat treatment, or to withstand repeated high temperature cycles. Occasionally standard fasteners can be used, but the design has to account for a significant loss of strength and possibly even loss of preload due to heat.
 
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Steel City 06 said:
HSLA components will begin to anneal at temperatures exceeding around 900°F. Metals will begin to visibly incandesce starting at around 975°F. So any HSLA bolt (such as SAE Grade 8) heated to the point that it visibly glows has been permanently altered.

Most exhaust fasteners are designed to use various grades of stainless and other alloys designed for use in high temperatures. There might be some grain structure change, but generally the alloy itself is designed to make up for the lack of proper heat treatment, or to withstand repeated high temperature cycles. Occasionally standard fasteners can be used, but the design has to account for a significant loss of strength and possibly even loss of preload due to heat.
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I'm aware but generally we lack the ability to get bolts hot enough to glow in place. While I'm not against replacing the fastener for other reasons, we rarely hurt them enough with heat and render them unable to work as intended. I won't use them because I don't like discoloring the plating.
 
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IMHO any bolt that sticks or looks funny on a 20+ year old vehicle gets replaced. Cheap insurance. And then either red, blue or antisieze depending on the location.

-Mac
 
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Steel City 06 said:
It does weaken the bolt significantly, though technically not through fatigue. High strength bolts are typically heat treated to have a very small grain structure, which gives them good strength. The smaller the grains (which are essentially small crystals of metal atoms), the fewer aligned slip planes in the metal, and the higher the isotropic strength.

When you heat a bolt, those atoms begin to realign themselves with neighboring grains, resulting in fewer and larger grains. Each grain is a crystalline structure of metal, meaning all the atoms are neatly aligned. This means that the atoms can much more easily slip along each other, resulting a weaker (but more ductile) metal on the macroscopic scale.

There are ways to restore a tighter grain structure, such as quenching (heating and then rapidly cooling to break grains into smaller ones), but unless done very carefully, it will likely result in other changes, such as precipitation of alloying elements, embrittlement, or other defects. There is a whole lot I'm leaving out, but you get the idea.

In short: replace the bolt.
Click to expand...
Wow. Someone who knows metal living it Pittsburgh. It’s not a myth. Lol.

Thanks for the explanation!
 
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