Newbie temperature question - not related to hardening/tempering/quench

[jimwyma]

Hey all! new member, longtime lurker.
I've been casual forging for a few years now - blades, decorative, mechanical, etc. What little I know, I have gleaned from great forums like yours, random u-tube, and that one show... lol
I've not been able to find an answer to one really basic question though, and thought y'all might have an easy answer.
When working carbon steel (5160, 1095, whatever it may be), I know that cooling too fast is tough on the steel, and should be avoided. At what point as it's cooling naturally is it alright to stick a blade in water to get it to where you can handle it? Here's a common scenario in my shop -- when nearly done with "rough" shaping, I like to hold a blade in my hand a bit, flip it around, maybe carry it around the shop for a while, just to see how it feels. I get my last few hammer strikes in, and now I have to wait for a half hour before I can handle the blade. I assume that there is a "critical" stage where quick cooling is a really bad idea, but is there a point where it is cool enough to drop in my water bucket without adding undue stress? I figure at some point it is probably OK, but I have no idea how long to wait, so I wait until it naturally cools to the touch. definitely a safe approach but as you all know, shop time is precious and I hate wasting it sitting around waiting for things to cool!

 

[Stacy E. Apelt - Bladesmith]

Several things metallurgically:
The blade needs to cool down to around 900°F in one of two ways:
1) The one we use to harden a blade is to quench it so the blade cools rapidly down to below 1000°F fast enough to miss the pearlite nose and remains austenite. More on this stage later.
2) The blade cools slowly and hits the pearlite nose and does not harden. It does not remain austenite and converts to pearlite and other soft structures.
If you don't quench the blade, the drop form 1500°F to 900°F in still air takes 20-30 seconds - a minute at most.

With either method - once below 900°F, cool the blade evenly until it passes around 400°F. This takes around 5 minutes by hanging it in still air.

1) At 400°F the quenched austenite blade converts to martensite which is the hard structure we want in a knife. It should cool evenly in still air until below 200°F, where you can hasten the cooling in a bucket of water. That takes a couple more minutes. Most folks just let it cool to room temp, which takes about 10 minutes.

2) The pearlite blade never converts at 400°F so you can cool it in water any time after it reaches 400°F.

The one thing that can affect these simple explanations is some steel with sufficient alloying will harden to some degree even with a slower cooling. 1095 needs a fast quench, so it is not a concern ... but 5160 can cool much slower and still harden a bit. My suggestion is to wait 5-10 minutes then cool in a bucket of water.

There is no such thing as wasted time in a shop. While the blade is cooling, sweep off the anvil, put away tongs and hammers. sweep the floor around the anvil, vacuum around the grinder ........

 

[jimwyma]

Thank you for the detailed explanation, Stacy! This will save me a lot of waiting. I do generally use much of the "wasted" time on other projects, cleanup, etc. I think the problem is that I enjoy getting touchy-feely with the steel (more than necessary), and a "stolen" hour or two ends up meaning a very short time swinging the hammer!
Much appreciated!