In-situ annealing? 1

[AshleyK]

Would heating up a cap-head until it is cherry-red and then letting it cool to room temperature reduce the hardness?

We were attending a breakdown up high on a crane the other day and found ourselves having to drill out a sheared caphead bolt (I'm a maintenance fitter currently doing my BEng in my spare time). As we already had oxy-propane equipment nearby I suggested that heating it up to cherry red and letting it cool in its own time would reduce the hardness and make it easier to drill. Obviously I got ignored because I'm under the age of 30, but does anyone know what the chances were of this helping at all? I'm happy to concede that I was wrong as long as there's a decent explanation behind it. As it turns out we ended up leaving the crane isolated and waited for some cobalt drill bits to come in.

Cheers in advance,

Ashley.

 

[swall]

Without knowing the bolt alloy, it is hard to say with certainty, but in many cases your plan would soften the bolt enough to make it easier to drill.

 

[AshleyK]

Thanks, swall.

If it's any help this was just an M16 mild steel cap head, with 12.9 marked as the tensile. Regular HSS drill bits didn't show a lot of interest and it was sheared off under flush so we couldn't use a centre-pop to unscrew it.

I did wonder about sticking this thread in the, 'materials', section, actually.

 

[TMcRally]

I'm not an expert on this but have a fair bit of practical experience.

Socket head cap screws to my knowledge are all high tensile, not mild steel. 12.9 is tough to drill. Heating it up to cherry red may have softened it but I'd say it might just as likely made it harder.

HSS drills with a small rake will drill them with difficulty and you will need a number of drills or access to a grinder to re-sharpen them. 16mm is big enough to weld a rod to it then bend the rod and unscrew it that way. It depends on why it broke.

 

[AshleyK]

I understand there's plenty of ways to remove the caphead, I was just unsure whether my suggestion was any good.


How would heating it to a cherry-red and letting it cool in its own time make it harder? I don't understand that at all.

 

[ornerynorsk]

For a little quick field improvisation, you can sharpen a masonry (carbide) drill in order to drill out the bolt for an extractor or EZ-out. Doubtful you will have much luck annealing in-situ. The surrounding plates and structure will rapidly suck the heat out of it, and you risk doing other damage.

Use Kroil or other good penetrating oil (not WD40!!!)to make the job easier.

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.

 

[redpicker]

How would heating it to a cherry-red and letting it cool in its own time make it harder? I don't understand that at all.

It has to do with the cooling rate from "cherry-red". Steel will become hard if it cools fast enough. Hard and fast enough are emphasized because these are relative. How hard hard is depends on the carbon content of the steel. How fast fast enough is depends on the alloy content of the steel. A 12.9 screw has been hardened and tempered, so it has enough carbon and alloy to allow it to become harder than it currently is (tempering softens steel, so it had to be harder before it was tempered).

The point is that in the field, you are aren't likely to have much temperature control ("cherry-red" is not a well-defined temperature) and you are not going to be able control cooling very well. The result is that you can't control the process very well, which is why TMcRally said you are just as likely to make it harder. We can't tell if you will make it harder or softer, but you are just as likely to do one as the other since you can't control the process very well. If you do get the capscrew hot enough to affect the hardness, you are likely to affect the surrounding material as well.

rp

 

[TheTick]

Perhaps if you insulate it as best you can and let it cool slowly. Maybe apply flame and slowly reduce flame over time.

 

[tygerdawg]

The old adage is [blue]A test is worth 10,000 opinions.[/blue] It would take you two hours of your spare time to determine this with certainty.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[AshleyK]

Yeah, maybe a test is on order. Cheers for your opinions, guys.

 

[mfgenggear]

as said before the mating threads could be damaged.
heating the bolt could make it also more difficult to remove.
heat seized threads. the opposite would be said if it where a nut.
Heating it would result in easy removal while it's warm.
due to thermal expansion.
drill, & easy out is the standard practice.

when a bolt is difficult to remove. using a punch with a blunt tip
and smack it with medium blows to loosen the the threads that have seized.
This has saved my ass many times. & prevents breaking the bolts or nuts.
The object is to loosen not yield the threads, thats why mild blows & not get carried a way with hammering. :>)

Mfgenggear