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Dan Jack

tempering after quenching - timeframes

19 posts in this topic

Is it critical to temper right after quenching? Can you quench, say a knife blade, in oil and them come back a few days later and throw it in an oven to temper it?

 

Thanks!

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For some alloys and geometries that is possible, others not. It is never recommended. You should always temper as soon as possible, even at some temperature below where your final temper is going to be; use boiling water if you have to.

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Ok, Why is that? Does it change the crystalline structure formation? What are the ramifications to the final product - does it result in an untempered blade, or a blade that's too soft?

 

I'm very new at this and trying to understand what's going on inside the metal and different stages, So thanks for everyone's patience!

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If don't take at least a minimal temper ASAP after the HT, you risk breaking the blade. When you quench you are creating a hard, glasslike structure called Hardened Martensite. It's hard, but not very resilient. You've also created a ton of stress in the steel. If you leave it in that condition, it often breaks under the stress. Tempering relieves the stress, and causes the hardened Martensite to transform into tempered Martensite. Tempered Martensite is tougher (more able to resist shock, twisting, shearing, and chipping). You give up some hardness for toughness by tempering,

 

Tempering temperatures are generally between 200 F and 800 F. You can adjust your temperature to dial in the hardness/toughness you want or need in a blade. Razors and small scalpel-like blades can be pretty hard, temper at the low end. Axes and hammers and such need toughness, temper in the 500+ range.

 

This is when you know what kind of steel you have. If you are working with mystery steel, then you need to test to get the most out of it. One good test for an edge is to flex it over a brass rod. If it flexes and stays that way, you are too soft and will need to start over. It it chips, it's too hard. Take another temper run 50-100 degrees higher.

 

This works for most low alloy, carbon steels. If you are working something else, you'll need to research specific info.

 

In short (something I'm really good at) you want to temper right away, with most steels, most of the time.

 

Clear as mud,

 

Geoff

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I think years back someone hardened a large Seax and left it over night on there anvil without tempering, in the morning the inner stresses had broke it apart...

 

My sword kiln holds heat well and takes several hours to come back down to tempering temperatures, what I do is to leave the blade in the hot quench oil, normally about 200 f after a hot blades been in there in till I can do a proper temper

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Classic Luke...

 

There is another friend here who shall not be named that has a habit of leaving blades untempered for months at a time. Especially for mono-steel this isn't a huge issue in and of itself, except for the fact that you have fully hardened blades laying around the shop. I'd prefer to not take the risk of someone coming into the shop and start hacking at a stump with a full hard blade. :blink:

 

What I'm saying is, why not temper asap?

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Thank you for all the replies! It was just a timing issue for me.... I quenched before I really thought about it and then had to go out so the blade cooled. I didn't get back to it for a few days.... I did toss it in the oven for a couple of two hour blocks 3 days later. It's coil spring steel and it's certainly hardened, I just don't know if it tempered properly. I guess I'll throw it through a few stress tests and see... it's one of my first blades, so I don't mind breaking it. It's all part of the learning process.

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Classic Luke indeed!

 

From personal experience, I break enough blades already, so taking that new and exciting way of adding a few more is well worth the trouble of having a tempering setup standing by for post quench use. Even if it's just a quick and dirty way of relieving some of that internal stress before a more calculated tempering cycle, I do what I can. For anything I quench in oil, I leave a little residual on there and use the flash point as a rough measure of how hot to get the steel.

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I use an old electric kitchen oven to temper most of my stuff, everything that will fit in it. When I go to HT, I turn the oven on to let it heat up. By the time I'm done normalizing and HT'ing the pieces, the oven is hot and I can just pop them straight in. Even if they go overnight, it won't hurt anything as long as the oven will hold temp.

 

Geoff

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i try to never harden a blade while under time constraints, but the few times i have i used a snap temper. Stick a 1/2" or larger piece of stock in the forge. After quenching touch the glowing hot steel to the side of the quenched blade just to see some straw colors run just a tiny bit. And throw it in the oven when i have free time!

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Good to know... I use an old kitchen oven myself.... holds pretty good up to 450 degrees

 

I like the hot stock idea too - so in a pinch the idea is just to allow the stresses in the steel to relax a bit until you can temper properly?

Edited by Dan Jack

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I had a nice snap temper not too long ago. Actually it was a snap WHILE tempering, so not the same thing. I had just quenched a blade and cleaned the oil off, meanwhile the tempering oven was dropping from 1500 to 400F. If you have an electric oven, you know the probe is reading the air temp. With the door open the air temp can drop pretty fast, but the bricks retain heat much longer. So I laid the blade with the spine down on the edge of the brick to slide it into the oven. As soon as the blade picked up some heat from the brick, it snapped exactly where it touched the brick. This had never happened before with the same process, but now I put the blade in a cold oven and heat it up WITH the oven, instead of sticking it in there cold.

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When using ovens, it's important to know that they work on a mean temperature. The temp controls turn on when the measured temperature drops too low, then heats up to a certain cutout and shuts off again. The average of those two temps is what you set, and can range more significantly than you might think, depending on the oven. Something I do to prevent this is taking a baking tray the blade sits in and fill it with sand. Heat up the oven, let the sand come up to temp, then bury the blade into the centre of it. This acts as a fantastic thermal damper, and can help keep the temperature the blade 'sees' within a degree or two of what you set rather than a dozen or more in either direction.

 

John

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On ‎22‎/‎11‎/‎2016 at 4:25 PM, John Page said:

When using ovens, it's important to know that they work on a mean temperature. The temp controls turn on when the measured temperature drops too low, then heats up to a certain cutout and shuts off again. The average of those two temps is what you set, and can range more significantly than you might think, depending on the oven. Something I do to prevent this is taking a baking tray the blade sits in and fill it with sand. Heat up the oven, let the sand come up to temp, then bury the blade into the centre of it. This acts as a fantastic thermal damper, and can help keep the temperature the blade 'sees' within a degree or two of what you set rather than a dozen or more in either direction.

 

John

This is a great tip. Thanks John

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I normally add a large peace of steel while bringing it up to temperature and leave it in there. Also monitor the temperature externally. Lucky my oven has minimal fluctuations only the temperature inside is lower then it reads on display.

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I know that people say to get the blade right into the tempering oven as soon as possible after quenching but remember that you can get it into the oven too fast.  If you get it into the oven before the Mf point you can halt the conversion of the austenite to martensite and cause the left over austenite to convert to something like bainite.  Let the blade cool to where you can hold it in your hand before putting it into the oven.

Doug

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