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Yes, you can repeat tempering as often as needed until the hardness you want is dialed in. On mystery steels, start at a lower temperature and advance the temperature slightly and repeat the cycle...

Mikew: I like those thermocouples. K type with iconel sheaths is what I use and the prices are fair for what you get.

Gregg...maybe less soak time (5 minutes) will do the job at 800C.
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A razor is pretty small compared to some of the industrial sized parts that would take some time to heat into the center of the mass. I don't think you'll need to. I think this situation would be...

Correct, not long at all.

Your normalizing process should have refined the grain in your bars/blades. Thermal cycling is basically the same process, as I understand it, but you are quenching into oil from 800C rather than...

Papa, the subject of cryotreatment is interesting.

I don't think this problem will require it. It adds a set of variables that will only distract from the set of tools Gregg has to work with...

In this case, clearly yes. The steel mills produce bars. They don't necessarily control the temperatures to do so. There are several of us who have been fooled by materials we purchased from mills...

I agree. Ahem, some of us colonials can speak Centigrade at need, LOL. Good reference Mike! At some point the steel bar was above the coarsening temperature and that's where grain growth is going...

Okay, no worries. Sharp corners can be the focus of some of the stressful movement during heat treatment or a sudden drop in an untempered blade. It's sad, that looks like it had everything going...

Mikew: that's a good example of fine grain. The oxidized bit at the top? Is that a plunge cut from grinding exposed to the forge or a crack?

I am going to define your concern as "how many times before I ruin the steel?" Any time steel is heated, the possibility that carbon could be lost from the surface to the environment occurs...

I agree Shooter but let's allow gregg71 to get his feet under him first. If you're going to photograph the broken blade, take a picture of the cross section not the profile, we want to see the end...

Thank you for your help with my question. I understand what you are doing now...

First, the bars of O-1 are likely just fine from the steel mill, but that you have added the insurance of the...

I'm not sure the paints will be accurate enough. The range on the multi change paint is quite wide and the single change paints have odd set points that may not be good for heat treating steels. ...

My first thought is that the grain/particle size is too large and yes, that is related to HT preparation. But help me refine my answers. Are you forging these to shape? If so this may be the...