First of all, I think that you are mixing up differential tempering with differential hardening. Differential tempering would be hardening the whole blade and then apply heat to reduce the temper, usually in the spine. All you are doing is applying heat to an area to allow more carbon to escape from the body centered matrix of the iron crystals to relieve stress on the atomic bonds on the atomic bonds holding the crystals together. This will make the spine tougher but less strong. As in it will flex more easily but be less likely to break but the whole blade, depending on the steel, will still be martensetic.
Differential hardening allows you to quench just the edge and point areas of the blade to form martensite and allow the unquenched spine to form pearletic steel. It's the difference in the crystalline structure of the steel that causes the two sections to polish or etch differently and show a hamon or quench line. Yes, the two different size crystals will cause stress in the blade. It's that stress that can change a straight blade into a curved blade, as in the Samurai sword. The spine will, however, be less strong and more tough than a blade that had been differentially tempered from the same steel.
It's not a problem in the knife because, like the pipe welder, you will be relieving that stress when you temper the blade. If you have a problem it would probably be from delaying the tempering of the blade as might happen if one tries to harden a batch of blades one at a time then hold them at room temperature to batch them together in the tempering oven.
The main question for you is why do either. Neither technique is going to be very useful in a small blade that would not be used in chopping where the softer spine will take up some of the shock to the blade and support the harder edge. Would there be an advantage of giving up some strength in the blade for some added toughness?
Doug
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