hardening alu

[sigurd]

I made a chainwheel for my bike in al. can I heat and quench it to make it harder?

[apex1]

No Sigurd. Although it is possible to harden the 6000 / 7000 range of Al alloy, you would need a perfectly (to a single degree C) controllable oven and to heat it up to about 500°C and to reduce heat in a controlled temp pattern.
Not possible "at home"

Richard

[Ad Hoc]

oooo..this is a very simple question but terribly complex to answer in one or two lines and without a fair degree of understanding in metallurgy and lots of diagrams too.

Short answer is no, but it also depends upon the alloy type as noted above by Apex.

To make it "harder" you really need to know whether it is a heat-treatable or non-heat treatable alloy, ie 5000 or 6000 series

[wardd]

perhaps having the teeth plated will do

[tkk]

Don´t you trust Campagnolo or Shimano? :D

[sigurd]

Ok thanks, long time no see.

I made it because I bought one of these (http://www.tartybikes.co.uk/product.php?product_id=548&category_id=1), and fitted a rubber drive belt arond it, to soften the blow to the crank bearings when it bashes a rock. The diameter then is just right to protect a 27t chainwheel. Those are rare, all I could find was a titanium one for 100 dollars on the net. That, and I wanted it *now* - it took a fair amount of time to make and probably will quickly worn out.

[CDK]

Sigurd, the only thing you can do to prolong the life of your chainwheel is anodize it, so the surface you've worked on is covered with a very hard Al-Ox layer.

The simplest way to do that is cook it in oxalic acid for approx. 10 minutes.

[viz]

Sigurd,
I was researching this question myself a couple of weeks ago. Some good explanations of the process were available online. Some of the common alloys have a standard heat treatment.

The first stage, called "solution heat treatment" requires something like 532degC followed by water quench. In a documentary I saw Mr Britton do this for the Britton motorcycle crank case in his wifes pottery kiln then the swimming pool, after realising his bucket was only 1/2 full.

Anyway I chickened out on this when I discovered that the minimum charge for a proffessional job was quite low. Was NZ$40, about US$25. Cost otherwise is NZ$ 7/kg. These include the artificial ageing.

The second part of the treatment, the artificial ageing at 204degC seemed more viable as a DIY, but you get that as part of the package.

Cheers
Gregg.

[Jimbo1490]

Hardening is only possible if the right alloy was selected at the start of the project, as others have mentioned. If this was a sort of 'scrap bin' type project, you're probably out of luck. If it is a hardenable alloy, send it to a professional that does this sort of thing. Such facilities are not really that hard to find. There are several here in central Florida, which is not exactly a very industrialized part of the USA.

You can probably avert most wear simply by anodizing the wheel. The anodized surface is extremely hard (it's basically like a thin layer of sapphire) and will reduce wear from contact with the chain. Anodizing a small part is well within the capabilities of a competent DIY enthusiast, and there are many books, websites and suppliers for the DIY anodizing market.

This will not help with the teeth bending over, which eventually happens to all aluminum chain wheels, even those made in factories of the proper alloy and heat treated.

Jimbo

[Ad Hoc]

You need to asceretain what alloy it it first, then what its chemical composition is, since this affacts the solid solubility, which is related to which alloy you have...and so on and so on.

VIZ,
what you have partially mentioned is for heat-treatable alloys. However the temp at which one solutions treats and more importantly "ages" is related to which alloy group and again, its chemical composition, in addtion to what effects you want, or tempers. Since "age" hardening and it effects, ie increase in strength depends upon several factors. The most important being the rise in temp, ie at what temp it will be held at, and the amount of time being held at the temp.
If held for too long for example, all benefits are lots.

That is the very simple explanation!..

[viz]

It's been mentioned or infered several times that the viability of heat treatment is dependent on the alloy.
Did anyone ask Sigurd what alloy he used?

When I was interested in this I spoke to a proffessional at a heat treatments business. We discussed the alloys I was using, the alloys that they heat treated and the details of the process. The conclusion that I and he came to was that we could probably try if we had the right kiln and were really excited about the ideas. However the proffessional process was quite cheap, and I have many other things to do.

Cheers
Gregg