Questions re: overheating tip when HT, sairset, clay speeding up cooling?

[Fitzhugh]

I have a few questions I haven't been able to find answers to by searching.


1) I'm attempting to heat treat a hand plane blade - in this case much like a wood chisel in form. I'm using a two brick forge with MAPP, seems to swirl OK but I'm having trouble getting the overall front portion hot enough without the tip getting too hot. This is just intuition - the tip gets to non-magnetic much sooner and seems closer to yellow (I'm colorblind so I'm really not sure). The blade is 3/16 with a 29 degree bevel on the end.
What should I try? I think it is simply because the tip is thinner, not because of a hot spot. I do move it around a lot. Does it help to decrease the forge temp and let it take longer to bring the blade up to heat? What about coating the blade to even out the heating? Oh - can you still see the color of the steel through the clay? I assume the clay would be similar enough and that color maps to temp regardless of material, right?

I'm happy to experiment but I don't want to wantonly screw up steel if I can help it, so I guess my next question is: how much can I screw up the steel in experimenting? Does it sound like I'm reaching temps where decarborization or other damage is done? How much of a problem is it if it takes a lot of time and reheatings to actually forge something because, say, one has no clue what they're doing? (looking ahead there, not forging yet).

2) I picked up some sairset. It was only a few dollars for a small jar and available locally. Is it OK for coating blades? I have a second tool I want to heat treat - a woodworking float - that is quite asymmetrical and coating it was recommended as a way to reduce warpage. How long do I let it dry - either coating a blade or just coating the inside of the bricks so I stop gouging them? The thought of putting something with a moist interior in a forge sounds like trouble.

3) I've read that clay coatings accelerate the quench rate. That seems counter-intuitive. Did I read it wrong or could someone explain how that works? Somewhat similarly, does heated oil really quench faster because it is less viscous in the volumes we're looking at? I can see it happening with large pieces where convection becomes a bigger factor but it doesn't seem likely at this level. I don't have a clue about the validity of what I just said - it's just my gut feeling based on nothing much at all.

As always, don't hesitate to point me to a thread or sticky if these are answered already. I searched but that doesn't mean I searched right!

 

[Grayzer86]

Clay coatings should slow the quench rate. This is why Hamons produce a differential hardening. The clay insulates the blade steel so it cools at a slower rate not reaching full possible hardness as far as I understand. Yes warmer oil quenches faster when talking about canola and such, but some commercial quenchants perform best at room temperature. My understanding is that the warmer oil is less viscous and this allows better contact with the blade during agitation, and helps lower the time spent in the phase where the blade is in a vapour jacket.

 

[Stacy E. Apelt - Bladesmith]

The whole problem is that you are using a two brick forge for HT.

Before we deal with that, a clay wash can accelerate the collapse of the vapor jacket, but that isn't quite the same as accelerating the quench. Clay coating, as in a hamon, will slow the conversion time, but that isn't the same as decelerating the quench.

Now, about the problem:
You didn't say what the steel is, but since it is a hand plane blade, I assume it is hyper-eutectoid, maybe W2 or 1095. These won't HT in a 2BF well at all. A 2BF is a very minimal device for those who are deciding if a full forge is in their future. A hand plane needs the very best HT it can get, or it will be less than optimal....which may be OK for a knife, but considered sub-par for a plane blade.

Hot spots will occur in most forges, but a 2BF has no room to move the blade to a more even area. Heat the blade, paying attention to the edge, and quench when the edge is at the right temperature. Since you are really making a guess in a 2BF, and then not being able to tell the subtle differences in the colors, the edge could be almost any temp above 1350F.

I would really recommend you have someone with a HT oven do your plane blade. Maybe someone out your way will offer a shop visit where you can do the blade.

 

[Fitzhugh]

Ah, I see I had a few misconceptions. More properly, now I know what they were.

I'm using 1084. I do understand what you're saying about sub-optimal results. Luckily I was actually aware of that heading into this, thanks to good advice I got here. I'm OK with that. I'd rather have the joy of making my own blades and refining as I learn than buying a great one or even sending off for heat treating. I'm not dealing with full with plane blade here - it's for a shoulder plane, but I hope to in the future. I may send off for a heat treat after a few just to get a good reference point. Or if I find I just suck at this! The ability to make various tools is fascinating to me. I'm starting with this blade because it is so simple, doesn't use all that much steel and is one I won't use all that often. Over time I plan on making some kitchen and carving knives, a drawknife, other plane blades and so on. I've got a joiner's float almost filed in the vise but I'm going to hold off HT on that until I have more practice.

I'd love a chance to visit someone's shop, either just to see how they do it or to ht a blade properly. Baking is another hobby, can I say I'll bring pie or does that violate the no barter policy?

With thermocouples you only test the temp of the chamber, not the blade, right? I think I should get some tempilstiks...