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Brian Dougherty

Heat treating with an oven

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Ok, this is going to sound really dumb, but I have been struggling to figure out how to use and oven for heat treating :unsure:

I’ve been making a number of smaller blades lately that would easily fit in the small heat treating oven I happen to have.  (This was bought for non-bladesmithing purposes)  It seems like a natural thing to use the oven rather than my forge as it would allow for much better control of the temperature, but as I went to do this the first time I realized I have no idea how to heat treat in an oven.

In the forge I do the following:

  • Normalize by moving the blade in and out of the forge (propane) until I see the shadows from the decalescence stop. (Or at least move out of the region I want to harden) Then I allow to air cool while hanging vertically.
  • After the first normalization step I look carefully and correct any warping before repeating the same heating and cooling process 2 more times.
  • Then I heat the blade up the same way one more time and quench

I have always thought that an important part of the process is to not have the blade sit at high temperatures for any longer than necessary to keep the grain from growing, so I don’t dill-dally in this process.  Perhaps this is where I am wrong.

If I set my oven to 1500F for a normalization run, and let the blade soak in there to make sure it is up to temp, won’t I be letting the grain growth happen?  If so, how do you minimize the time in the oven, but make sure you are getting to temp since you can’t see what is happening?

Obviously I am making this too complicated, but I’ve gotten so used to watching for the phase change that I don’t know how to do anything else.  How do you guys that use ovens do this?

 

 

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As I understand it, temperature is a much stronger factor than time for grain growth. You can have your blade sit in the kiln for quite a while at the correct temp without any noticeable grain growth. 

The first normalisation run at higher than critical is mostly to equalize the grain size and remove stresses, so I wouldn't worry much about gain size at this stage, as it is going to be shrunk later. 

Again, Jerrod will correct me if I'm wrong :lol:

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Posted (edited)

I wouldn't change anything in your normalization process. Normalizing is most efficiently done in the forge. You might want to start using a baffle tube to get a more even heat and avoid the whole moving the blade in & out business. I usually only normalize after forging. Then I do all of my rough grinding and go for hardening and tempering. Rarely do I do another normalization prior to H&T.

Now for the hardening & tempering processes, you have a choice.

1. Continue to use the forge to bring the blade up to MS temp and quench. Use the oven only to temper back.

2. Set the oven for the correct MS temp for the steel you are using, bring the blade up to temp and quench. Let the oven cool down and reset the temp for tempering and put the blade in.

 

 

Edited by Joshua States

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Brian,

For me it is much easier and quicker to normalize my blades immediately after forging.  This way you already have a hot heat source (your forge) and you are only removing the forging scale one time.  This is about the only time that I use borax anymore and I will use it while normalizing for anti-scale.

There is nothing wrong with using your  oven but normalizing can be done effectively with your forge as well.  I would recommend for your second & third (always do three) normalizing cycles that you reduce the heat each time.  By the time that I  do my final normalizing I only go to a dull cherry color.

 

If you have other questions get with me at Central  States and we'll discuss it further.

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Basically what they said, except that the Ms is when martensite starts forming upon cooling, which is usually around 400 F.  Not going to do much good to quench from there.  What Joshua meant was the austenitizing temp.  But you knew that already.  

I'd also add that one may want to do a normalization cycle after mechanical stock removal (grinding or machining) if heavy work is done.  Meaning really big /aggressive cuts, not just lots of material removed.  If you use a file and remove half your starting stock, that is pretty gently and shouldn't need another cycle.  But if you were to mill your bevels in one pass, that aggressive machining may have caused stresses in the metal.  I would think this need is not very common, but if anyone is experiencing problems this may explain it a little.  

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Thanks all.

I agree with  the idea of normalizing after forging.  I do that to get rid of the odd hard spots that tend to wreck drill bits, but my process has been so disjointed and beginnerish that I just got into the habit of doing again right before quenching.  I can probably start refining that a bit as I am not quite as disjointed as I was a couple of years ago.

The oven has really come about as an option for the folders I have been working on.  It is an attractive option with these little ~3" blades as I can set them in the oven, and just pick them out with a hook on the end of a wire and quench.  Aside from the forging involved in making the pattern welded bar stock, these have all been ground to shape.  I normalize the original parent bar when I am done forging, but primarily to eliminate any hardness, so I can't say I have refined the grain much.  As stated above, I should start doing a proper normalization at that point to save myself time and fuel.

From what you all have said, it sounds like I could do a series  of normalization runs at 1550, 1500, 1450 F and just leave the blade in the already heated oven for ~10 minutes each time to make sure it was up to temp without worrying about grain growth.  Then go back to 1550 for the quench cycle.  (all my blades are a 50/50 mix of 1095 and 15N20)

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25 minutes ago, Brian Dougherty said:

Then go back to 1550 for the quench cycle.  (all my blades are a 50/50 mix of 1095 and 15N20)

I'm not sure I understand. Do you use 1550° as the quenching temperature? If so, you are overheating. You should use 1475. 

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That's a fair question Joël.  In my forge I don't try to measure the temperature, I just look for the decalescence.  I Guess I don't really know what my quench temperature has been for quite a while now. It's never mattered because I have always let the steel tell me when to quench rather than the temperature.

I had 1550F stuck in my head all this time because I heard that as a decent quench temp for simple carbon steels. For all I know the same source probably told me to quench in virgin lamb urine while the blade was pointing north!

This points out to me that before I start blindly using temperatures I need to dig Verhoeven's book out and read it more thoroughly.

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You can also use Cashen's website as a reference or here. I think most common carbon steels are covered. 

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