Heat Treat then Temper

[Rob Davis]

Last night I heat treated a blade.   It was too late to dedicate any time to tempering, so I  put it in the oven this afternoon for the two 2 hour temper cycles.   Does waiting 12 hours between completing the heat treat and beginning to temper change the end result??  Will the steel be any different than if I had begun the tempering process immediately??

[Brian Dougherty]

The delay time will not change the result appreciably.  It does, however, come with some risk of the blade cracking all by itself.  Some materials have been known to break spontaneously on the bench while waiting to go into the temper oven.  It sucks when that happens, so you will often hear people advise to get into the temper as quickly as possible.

 

Also, tempering is part of heat treating.  What you are referring to as "heat treat" you probably mean to call the quench.  

[Joël Mercier]

I will add to what Brian said that you can do a "snap" temper of around 10 minutes if you are in a hurry, then do the full cycles later. 

[Alan Longmire]

One huge variable here is what steel you used.  On some of the higher alloys, retained austenite will stabilize if left untempered for any length of time. It usually converts to martensite during the first temper, which is then tempered peoperly by the second.  If, in an alloy like O1, you leave it sitting for a few hours, it won't convert.  While usually not fatal, it results in a blade with sub-par performance compared to what it could have been.  Still better than a Walmart special, but not as good as it could have been.

[Rob Davis]

The steel came from a very old Nicholson Bastard File (made in USA).   

[Joël Mercier]

In that case I'd follow Alan's advice. That file has lots of carbon and will have a good amount of retained austenite that should be dealt with right after quench. 

 

Btw, doing heat treatments when in a hurry is my personal recipe for catastrophe . 

Edited October 19, 2021 by Joël Mercier

[Brian Dougherty]

13 hours ago, Alan Longmire said:

 If, in an alloy like O1, you leave it sitting for a few hours, it won't convert.  While usually not fatal, it results in a blade with sub-par performance compared to what it could have been.  Still better than a Walmart special, but not as good as it could have been.

This I didn't know...

[Rob Davis]

This was an unexpected lesson learned.   Thanks Alan!

 

[Rob Davis]

Yes, Brian.  You correctly deciphered my imprecise language.   I soaked and quenched then waited half a day before beginning the temper. 
I will happily update you all that the blade came out of the oven with a beautiful color of straw.  It seems to hold and edge nicely.  It does stand up to some moderate abuse without chipping or, deforming.   I'm still hand sanding but so far so good.  

Edited October 20, 2021 by Rob Davis

[Alan Longmire]

2 hours ago, Rob Davis said:

soaked

 

That file will be either 1095 or W1, neither of which need more than a couple of minutes (like, seriously, two) at temperature.  Longer than that and unless it's W2 and has vanadium in there, you'll get grain growth.  On the one hand, this improved hardenability.  On the other, it decreases toughness.  Be sure to consider your alloy when adding soak time, most of the simpler ones don't need much, if any at all.  

[Rob Davis]

Alan, if I were to let the blade soak too long (I didn't with this one) can I just let it air cool and try again or have I permanently damaged the steel??

[Jerrod Miller]

You can certainly re-normalize to refine the grain.  You do have to worry about decarb with too many cycles (especially when near the final shape), and it does get hard to balance "how much did the grains grow" with "how much did I shrink them", but you are usually pretty safe to err on the side of doing cycles to shrink them too much (better to over normalize).  

[Alan Longmire]

As usual, what Jerrod said. 

If you're using an anti-scale compound or a muffle tube with a bit of charcoal in it, decarb isn't an issue.  

 

Somewhere around here I posted some pics of what soaking and subsequent normalizing does to grain size in W1.  A five minute soak at around 1600 degrees created table-salt-sized grain; one normailzation took that to a decent size, three made it fine as silk.

 

The basic rules of heat treating straight carbon steels are few and easy.

 

1. Do not soak, no matter what you saw on YouTube. 

2. Always normalize 3x after forging and prior to quenching, and

3. Always temper immediately.  This leads to 

4. If it's late and/or you don't have time to temper at least once, don't harden your steel.

[Joshua States]

I use my Paragon oven for both the quench and the temper. This means I have to wait for the oven to cool down in between the two processes. I purchased a fairly good quality IR thermometer gun, and while i wait for the oven to cool, I leave the door open. This gives me a nice little table I can lay 2 or 3 blades on to "soak up" some heat. I simply wait until the temp of the door (right in front of the open oven) reads approximately my tempering temperature and lay the blades on the oven door. I have found this to be the equivilent of the "snap temper" Joel mentions. The process invaribly brings the blades up to around 250*F +/- and I can then put them into the oven for a full two hour tempering. If you use your forge for the quench, and find you don't have time to do the full temper in the home oven, you could always do something similar in front of the forge.

[Jerrod Miller]

12 hours ago, Joshua States said:

you could always do something similar in front of the forge.

Or heat up a block or two of steel in the forge prior to heating the blade for quenching.  You can lay your freshly quenched blade on (or sandwiched between) the chunks of steel like Joshua does on his oven door.  Or if your set-up allows for it, pre-heat a bunch of sand (like in a bread pan or roast pot) first.  You can then bury your blade in the sand and the thermal mass will keep it hot enough long enough to do a bit of a snap temper.  Just make sure whatever you use is the right temperature for your temper.  It can be less than your final temper, but still needs to be at least 250F, 300F would be better (final temper temp would be best, whatever temp that is going to be).