Final Drive Sprocket - Heat Treatment

Haggis

Haggis

Hero member
Location
UK
Hi all,

Ooops.

I was heating my final drive sprocket and got distracted. So, it got far too hot and is now softened.

I know it's softened, as I have an identical new sprocket and did a comparative file-scrape test.

I'd appreciate your thoughts on hardening and tempering back to a suitable hardness/toughness.

Cheers, Paul
 
take it to the Bunkers Knob in Addington, near Croydon, leave it behind the bar out of sight, and in 48 hours it will be as hard as 9 million year old granite.
CLEM
 
I had mine heat treated after I had it machined and welded on with a 4mm offset. Wear was as per normal, so it worked.

Need to soften up another couple (20T 520 to go on a std Lav 530). How did you soften yours exactly - so I can machine it myself?
 
Softening the spocket is easy. Put it on the gas hob ring > start making a cup of tea > take a phone call > sit down for a good crap and read some of Billy connellys Tall Tales book > chuckle for a while. Remember you have tea brewing and return to kitchen. When getting back to the hob, quietly exclaim 'oh fucking bollocks' . The sprocket has just be turned red. Leave to cool slowly.  :)

I had to Google The Bunkers Knob Clem. Sounds divine. The sort of place you aren't fully dressed without a knuckle duster.
 
Tie a sturdy wire to it, place it on your gas hob, > start making a cup of tea > take a phone call > sit down for a good crap and read some of Billy connellys Tall Tales book > chuckle for a while.  When you return and it is a nice shade of "light cherry red", grab it by the wire and quench it in either water or oil.  Works well enough to wear out 2-3 chains before it's rife for replacement.

piet
 
There is a sliding scale for quenching as well (assuming there is enough carbon in the steel to harden) - so cherry red:

Very hard - quench in salt water
Quite hard - quench in tap water
Not quite so hard - quench in oil
Soft as shit - don't quench at all (already achieved)

The angle you insert the cherry red item in the quenching media can also cause bending, as one side cools much quicker than the other unless inserted 'plumb on'.
 
the very hard, quench in salt water is handy for you Si, and your works at Dungeness,
no shortage of that at all, if its contaminated with Kent county sewage, you can always say "this hardening is a load of shit" if it is too soft and wears fast.
CLEM
 
Hey Clem. You don't need to live by the sea. You can make salt water by mixing water with (duh) salt.

I dunno why it would provide a faster cooling rate than fresh water though. It has a lower specific heat and lower thermal conductivity than fresh water. Latent heat of vaporisation is also decreased. So sea water is less able to suck heat out of a hot thing than pore water. The only thing that may increase the quench rate is the increased boiling point of sea water, so it may have a later onset of the Leidenfrost effect (liquid in contact with a hot surface flashes to steam which provides an insulation layer).

I suspect a more effective way of increasing the cooling rate when quenching would be to use iced water at 0?C.

There's a few references in ancient Greek and medieval texts about quenching swords in urine (especially urine of a red-headed boy). That may be myth, or perhaps not. Nitriding is a well known process for case hardening of steel. These days it's done by heating the steel to its transition temperature and dunking it in ammonia (NH3) which causes some of the nitrogen in the ammonia to diffuse into the surface of the steel. Urine also contains nitrogen. If it's left to ferment for a while, bacteria will break down the urea down into ammonia, so it's reasonable to expect it to have some case hardening effect if used as a quench medium.

Case hardening is arguably better than simple quench hardening for something like a sprocket because it produces a hard-wearing surface while leaving the core metal in a more ductile state, making brittle fractures less likely.
 
FFS, it's taken me 60 years to understand why, in the movies, the Romans sat around the fire at night, drinking wine and reflecting on how they fought during the day, with their swords glowing red in the fire. Obviously we didn't see them pissing on their hot swords.

A N Neale, the inventor of annealing, is not a very Greek sounding name though???
 
No one mentioned tempering, my memory of 1st form Metalwork at Highschool was heating to carry red and dunk into the water quickly pull out and look for the straw colour to appear and then dunk fully.
 
Not sure why the wheel needs to be re-invented in this case, a sprocket is a sprocket, and remains a sprocket... it will eventually wear out, no matter how it is treated.  As already stated, basic heat treatment with quenching in water is enough to regain hardness to wear out 2-3 chains.

All valid points, of course it can be nitrided, case hardened, annealed, etc, etc, but, what for?  There's enough drama on the planet already, no need to create even more... ::)

piet
 
very nice Piet,
but you could have saved a lot of time and just said ,,,, shut the fuck up you bunch of morons
CLEM
 
In my defence I am a mechanical engineer, so I probably have an unnatural affinity with detail to many people.
I did learn my heat treatment skills as a first year apprentice in basic training, which was ahhhhhhh 1978/79 at the tender age of 16. 
 
Thanks for all the contributions.

My goal was to protect the splined gearbox output shaft. By getting the hardness in the spline area of the sprocket right. Too soft it will deform and start to chatter (and then damage the shaft), too hard and it might also damage the shaft.

A quench in tap water got the sprocket back to the same as it was (checked against the spare identical spocket). No tempering required.

It all depends on the carbon content of the steel of course, so I'm glad I had a spare sprocket as a comparator.

Thanks again.
 
Dellortoman said:
There's a few references in ancient Greek and medieval texts about quenching swords in urine
Click to expand...

Apparently, according to wikipedia, case hardening was traditionally done by "packing the iron in a mixture of ground bone and charcoal or a combination of leather, hooves, salt and urine, all inside a well-sealed box" so the urine theory sounds real.  What I always end up wondering is how on earth they figured this out, as they stood staring at a piece of iron bemoaning the softness of the metal, and someone suggests packing it in leather, hooves and salt - oh and while we're at it, lets piss on it too - I reckon that'll do it....

Just wondering  :D
 
:LOL: :LOL: :LOL:

I reckon we could ask that question of many 'inventions' and scientific techniques of old. Herbal medicine is a good example -  how many patients died refining this herbal hemlock remedy?  :o :D
 
Look at penicillin. Discovered because it was actually warm enough in Edinburgh one day that people had to open the windows to their labs, and some bread mould being grown in one lab drifted into another lab and landed in a Petri dish of growing bacterial colonies. And Alexander Fleming, not really a scientist but doing some pretty basic bacteriology studies, just happened to notice that bacteria weren't growing around the contaminating bread mould colonies, and told people about it.
It would be a very different world (no antibiotics) if Fleming had not made that one chance observation.

Ken
 
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