S7 Tool Steel

[scottyd]

Gday Gents.

Im just getting stuck into some S7 to make some punch inserts for metal / woodwork punches and im trying to nail down the hardening proceedure. The datasheet suggests that quenching should be done by air (2 bar pressure) or inturrupted oil. Neither of these options are as nice in my workshop as water is, If I quench a relatively thin section, will the quenching in water damage the part? Im assuming that the damage would come from surface cooling significantly faster than then internal metals and hence be under stress from the expansion within the part, or is is because water will simply make the part too brittle for service use?

Basically...does anyone have any experience with controling the hardening of tool steels and want to share some knowledge?

I have to say though, this stuff machines very well in the annealed state, cuts almost as cleanly as 12L14 on our little hercus lathes :U

[BobL]

According to Heat Treating Toolsteel - Tips - Techniques & Useful Information (http://tidewaterblacksmiths.net/2.html) you can just use still air, pressure is not needed for S7.

I would definitely not use water.

[Ropetangler]

Absolutely no experience in this field ScottyD, but in the link here Speedy Metals Information for S7 Tool Steel (http://www.speedymetals.com/information/Material12.html)
they say that for sections smaller than 2.5" to air cool, and I'm guessing that your workpieces are much smaller than that in cross section. While there would be some leeway built in to the system, I would think water quenching might be taking it too far. A large spare wheel off say a Landcruiser may be all you need for a small part, if you have no compressor, but if air is not possible for some reason a small knapsack sprayer, (of the kind that you pressurise by pumping up before turning the valve open) filled with water, so that you could lightly mist the S7 part, instead of dunking it might be worth a try. Hopefully someone who really knows what they are doing will put us both right:D, but if no-one else chimes in, that might be worth trying.
Rob

[barkersegg]

I occasionally harden punches and cold chisels I've made using the odd bit of tool steel and find that generally the cherry red then quench in oil method (I use old gear oil) works most of the time and seems to hold the hardness quite well for normal use...I haven't measured the hardness, but aside from the odd bit getting brittle and fracturing which I put down to my poor efforts, you need to be careful to draw the heat toward the point as in my experience tool steel heats unevenly and that effects the end result for me.............but then my set up is pretty rural at the least

....ditto about not using water to quench tool steel...gets real ugly :no:.......just my two bobs worth...... ..Lee

[scottyd]

Cheers guys, just the sort of thoughts I was after. I do have a compressor, but im uninclined to use it because ill be doing the hardening as part of a demonstration to a class at some stage and im pretty certain high pressure air will make them a rowdy mess to manage afterwards. I might go with the interrupted oil quench and see if the smoke is maneable, otherwise ill go with some sort of air setup.

Id love to do some testing and answer the question for myself, but the rod is 25mm dia. and doesnt like deep cuts. It sure ain't cheap either!

[BobL]

You don't need compressed air or oil - look at that link I provided. Just still air and a fire brick. That's a biscuit tin and a fire brick. What could be simpler?

If you wanted to use compressed air then a compressor is not really needed on the spot. What you'd need is a chamber that could withstand 2 atmospheres (30 PSI) and is big enough to place a half a fire brick inside it, and a old gas bottle with compressed air in it connected to the chamber with a valve and pressure gauge. Put the heated S7 on the fire brick, seal the chamber and open the valve till the gauge reads 30 psi.

For a chamber something like a piece of 4" water pipe with one end welded shut and the other end with a hinged door and an O-ring seal on it that could be clamped shut should work.

Personally I'd go the biscuit tin method. :)

[scottyd]

Ahh, ive got my wires really crossed then, I assumed two bar meant that flowing air at 30psi (as opposed to still air). So that really means that I just need to increase the surrounding pressure to 30psi. Ill have to have a kick around the storeroom and see whatll do the job.

[Ueee]

Just to add my 2c worth.....
I would quench in oil (clean) but i would not quench the whole punch only the first bit...it really depends on the size as to how much. Then remove the punch from the oil, polish the tip with an old file and wait until the tip reaches straw yellow. The whole punch can then be quenched. This will give a hard tip with a softer body, and also temper the steel appropriately. The other thing to keep in mind is not to heat the steel over the currie point when hardening. if you overheat and then let the steel cool to the currie point and then quench you will encourage crystal growth which will make the steel softer. You can also pack the tips of punches and chisel to break the crystals down further but this is well into the realms of forging and not machining.(as i take it you are doing)

[BobL]

Ahh, ive got my wires really crossed then, I assumed two bar meant that flowing air at 30psi (as opposed to still air). So that really means that I just need to increase the surrounding pressure to 30psi. Ill have to have a kick around the storeroom and see whatll do the job.

I checked out the McMaster-Car guide to Heat treatment for steels and this is what they say about S7.

"Hardening: Preheat thoroughly to 1200°-1300° F and raise to an aus- tenitizing temperature of 1700°-1750° F in a furnace for 30 minutes per inch of greatest thickness. Sections up to 2 1⁄2" thick may be cooled in air. Sections 2 1⁄2" to 6" thick should be quenched in oil until black (1000° F) then air cooled to 150° F. Sections larger than 6" should be oil quenched to 150° F and tempered immediately. Material should be held at least 2 hours at the tempering temperature before air cooling."

"Annealing: Heat to 1500°-1550° F in an inert material. Hold at tem- perature for 1-11⁄2 hours for each inch of greatest thickness. Cool slowly at a maximum rate of 25° F per hour to below 1000° F, then air cool to a hardness of 187-223 Brinell."