heat treating questions

[munroe47]

I am not totally new to heat treating metals, but I don't have a forge or an oven. I do have oxy-acetylene for heat and a recently-purchased BBQ grill which I have heated to a temp of 700+°F according to its built-in thermometer. I know this equipment is inferior to the forge and/or oven, but it is what I have to work with.

I have made 2 blades, one from a Nicholson file, and the other from a non-laminated chainsaw bar. I annealed them both to a working hardness by heating and allowing them to cool slowly, and have finished shaping them; now I am ready to re-harden and temper them. I have been successful in the past (making small chisels from hacksaw blades, etc) by heating to cherry red, quenching, polishing, then tempering using the color-change of the metal as a guide to when the correct temper is reached, then quenching again. I plan to do this with these blades, BUT I have been warned that whether to quench in oil or water must be determined first.

Which medium should I use for quenching each material? Should I depend on the BBQ grill's thermometer when tempering, or would it be better to use the color-change method?

Since I am not using first-class materials or methods, I am not expecting a first-class blade, but I would like it to take an edge without rolling, and I don't want to crystalize the edge during hardening or tempering!

Answers to these questions and any additional advice will be appreciated; thanks in advance.

 

[Stacy E. Apelt - Bladesmith]

Simple answers are:
Yes, you can use a torch to do HT.
No, the BBQ is a bad choice for tempering.

More complete answers are:
Doing HT with a torch is poor at best, and pretty much restricted to 1084, 1070, and 5160. It requires some skill and practice, and even then you are guessing.
A kitchen oven is quite suitable for doing the temper. Even a toaster oven is better than the BBQ grill.

Sending the blades out for HT or going to visit another maker and doing them at his shop is a far better plan.

 

[Andy Lewis]

Welcome to Shop Talk.

I understand wanting to get the job done with the gear on hand, but using an O/A torch and mystery steel will most likely act in concert against you. Doable with the right experience but probably not easy for a new maker.

Some things to consider:

1. The file blade you have might be high-carbon throughout, or it may have been only case hardened originally. As in a practical dead end. Who knows what your CS bar is composed of? Even with temperature control, you can't hit HT target specs that you don't know. Guessing and testing some of your drops/offcuts may help answer the general question, "is it hardenable?", but known steel is cheap and readily available for your next efforts (Aldo's 1084 or 1075 would be a much better choice IMHO but you got what you got).

2. O/A burns in the 6K F range IIRC. Almost twice as hot a flame as propane/air mix. Even with a large rosebud and patience, the chance of overheating some sections of your work is likely as you develop this skill. At least two notorious US makers still heat treat their reportedly decent blades this way though. Obviously blade points and thin edges are both most vital and most vulnerable. Overheating these by a few hundred degrees before quench will not likely leave you an edge you can count on - even if everything else is about perfect. I would suggest building a basic propane HT forge ASAP and also learning about decalescence in steel however you plan to heat your blades for now.

3. For quench medium, you aren't likely to damage a fast-hardening steel by quenching in oil when it may require water or P50 etc. While it may not harden, it is unlikely to quench crack if all else goes well (temps and times), then you can simply try again with a faster medium if your initial oil is too slow. Many here will advise canola oil, Parks 50 or brine/water, maybe in that order, but then they don't know what your steel is either.

4. Tempering by color can work well enough for some small tools, but knowing the actual drawing temps is critical to consistency on working blades IMO. Many things can affect oxide layer colors (steel alloy, surface cleanliness, polish level, etc). They are not reliable, so chase down a few candy thermometers or the like to keep company. After quench, clean your blades with brush, hot water and dish detergent, then wrap in foil or bury in a tray of sand and allow time in oven to come to temp. Draw at 375 - 400F X 1hr, then sharpen and test edge. Temper at 400 - 425F X 1hr again if it is still chippy once thin/sharp. I place a type-K TC probe in between the cutting edges and wrap the bundle loosely in Al foil for tempering. I can tell you that 25F either way makes a significant difference in edge performance/durability in the steels I use.

I think your expectations here are realistic enough but homework will help. In the end, study the details and learn to make the most out of what you've got to work with. We all are doing the same, just some have a few more toys and less adult supervision than others.

And have fun!