Hi there! So i still have to learn some of the basics... yesterday I learned that to go through steel i needed specific drilling bits (my poor "metal" ones didn't stand a chance against an old soften file) With the good bit and a good drill press, i drilled a hole trough the files like if it was butter But then i needed to drill holes in some old circular saw blades... and... it didn't go so well Are they usually hardened and i need to anihilate them before attempting to drill a hole, or do a need a specific bit for that kind of steel? Thanks! Olivier
Hi Olivier, As a rule, hardened steel needs to be annealed (raised to critical temperature and very slowly cooled, like overnight or more) before any machining is done on it. When you buy tool steel or cutlery steel from the steel suppliers, it comes annealed. If you get in a spot where you need to make a hole in hardened steel, then carbide is an option. Really you need something that is harder than hardened steel. Use plenty of lubricant and say a few prayers to the gods of friction. As for regular bits like HSS, titanium coated and cobalt the are easily damaged by heat. The heat destroys their hardness (like re-tempering them) and then they stop cutting and the friction increases and so on. If the bit tip has gone blue on the end it's not much good for any more drilling, well maybe re-grind and use on wood. To answer your question, anneal the saw blade first if you can before drilling. Bon chance! Dan
Thanks a lot for the answer! i'll be sure to build some kind of Altar to the god of friction, if not i have everything I need to anneal steel ^^
Olivier, Some tips for annealing... I have an electric heat treatment oven. I put the file or rasp into the oven and go up to about 900°C, then I turn the elements off. It takes a long time to cool, like hours. When they come out they are ready to grind and drill. If you have a gas or coal forge, bring the steel to a medium to bright reddish colour and then press deep into the middle of a 5 gallon steel bucket filled with wood ashes and let sit overnight. I understand the vermiculite (sold in bags at the local garden centre) works the same way but I have only used wood ash from our fire pit. Oh, be sure to get us some photos. It sounds like you are having fun! Dan
Cat litter (unused, of course ) also works to hold the heat in steel for annealing, and it's easier to find if you don't have ashes on hand.
Just a follow-up questions from a newbie on the same topic. So what happens if you heat to the 900°C and just air cool it. Also, what is the significance of the wood chip or cat litter or vermiculite?
Whew, metallurgy is not really my thing, but I'll see if I can explain this one. If you let the plain high carbon steel cool in air it will soften the steel somewhat but not soften it fully. This heat treatment is called normalizing and is usually used for refining the grain and relieving stress in the steel. Stress that is often the result from beating on it with a hammer. This may work ok for you if your steel is not very hard to start with. I don't know how hard a saw blade would be to start with, but a file or rasp should be Rockwell C 60 or more. When the steel cools over a long period of time it's called annealing. The long cooling time of annealing gets the steel into the lowest hardness state (most ductile) for machining, drilling or grinding. This will help us save on bits and abrasives and of course time. Think of annealing as a "reset" for the steel. Steel that has been hardened in the past can be reset and made more workable. For annealing, the material that you park the hot blade in needs to be a good thermal insulator to slow the rate of cooling way down. If you can imagine, a bucket of wood ash/vermiculite will render a farrier's rasp too hot to touch with your bare hands after sitting in there overnight. The hardness of the steel after annealing will be low around say Rockwell C 30 and fairly easy to work. On a side note, some makers use torches to lower the hardness of specific parts of the knife, say the tang or along the spine. This gives the steel more flexibility but less hardness, so a part of the knife would be less likely to snap under hard use. You would not want to do this on the cutting part of the blade as the hardness would be ruined and the edge would roll and require a lot of sharpening. Generally bad performance for a knife. Of course if you are working with an air cooled alloy, the normalization process is actually the hardening process! Hope this helps. @John Noon is into this stuff and could probably add to or correct me here. Dan
Lots of good info and you are right trying to anneal any air-cooled steel is different on the order of 20F temperature drop per hour from the 1500-1900 range down 600F. This is where it really helps to look up a particular steel and find a copy of the CCT and TTT diagrams, or spend the $400 on the ASM heat treating book. Oh and there is a phone app for this as well as a free download following are examples Slow cooling in air is lets say 100F per hour which will have you hit the nose of the above diagram and as you go into the zone with Auestenite+Ferrite+Carbide and back into the Zone that gives you Martensite some refining of the grain takes place. Slower cooling in the 40F per hour range from 1500F down to 600F is 900 degrees divided by 40Fequals 22.5 hours or 81,000 seconds or solidly in the Pearlite zone on cooling. Hopefully I got this somewhat right
If you have some rainy day reading time, I'd recommend stopping by Kevin Cashen's website. http://www.cashenblades.com/heattreatment.html
Also look up hypefreeblades forum, Kevin cashed hangs there. Huge piles of very good info.. Not a regular free for all forum and thought I read they must have seen your name other places before accepting a join request. Not trying to take from this forum since visiting both will teach lots.
In simple terms, placing the hot steel in a container of wood ash, vermiculite or cat litter makes it retain the heat for much longer, reducing the hardness of the steel to a greater extent (annealing). It's a simple method to make previously hardened carbon steel workable.
Thank you, John. I didn't know knife making was going to be so interesting. I will check hypefreeblades.
I got a few bags of vermiculite from my local homehardware store, enough to fill a metal garbage can. I put a layer of pink foamboard insulation in the bottom and have another I use as a lid. With the can about 1/4 full of vermiculite I put a in a 10lb chunk of mild steel that I've heated to 2000F, cover it to half full and then put in whatever is going to be annealed, fill the can up and put the cover on plus the metal lid. It´s still hot nto the next day. This setup is light to work with and move around. Wear a mask and work outdoors if you can, vermiculite isn´t friendly to lung tissue. As for drilling hard stuff I set my drill press to the slowest speed and use lots of oil, keeps the bit from biting and stalling the motor, and keeps the sound levels down too. cheers, Rob
