The knife blade is about nine inches Tom.
Of course I can close the front. I'm using firebricks. I don't have back opening, but I could. Seems to me though since the attempt at heat treat went so appallingly bad that would reduce the heat accumulation even more.
I just don't think the torch is up to it despite being rated to be.
Before I gave up I tried heating just my pieces raw without the oven using direct torch heat. Still wouldn't get to temp.
In this instance I wasn't yet HT'ing my blade. It was two small pieces I'd drilled for a file guide. Thinking of using charcoal now.
Rick, when you say the torch isn't up to it 'despite being rated to be' I wonder if you are talking about temperature or power - elsewhere you say the the Bernzomatic torch is good for 2400 C, which makes me think perhaps temperature is what you're going by. But it's power that is more important in this application - which is why others have recommended a bigger torch. Your torch is rated at around 1.9kW max power, which is just too puny to get the heat into the refractory.
I use a Sievert 7.7kW torch with a standard propane bottle for this sort of thing -
Gas Products do a kit for under £70. Once you've got the handle, regulator and neck tube, burners going up to over 40kW, if you need them, are relatively cheap.
You don't say what refractory you are using, but that's important too - solid fire clay bricks soak up heat and take ages to get to temperature. I have used a home brew refractory using clay and vermiculite with good results, but now just use commercial vermiculite slabs (the type of thing used for lining wood stoves). It would be no problem getting a 9" blade up to HT temperature with this arrangement and the Sievert torch.
It's not surprising that you couldn't get your pieces up to temperature using direct heat from your torch - it's just not powerful enough.
Charcoal can work for sure, but it has its own problems, not least controlling the temperature - which is important for heat treatment.
Bob.
