Selective tempering... is it really make knife tougher?

[kuraki]

Unless, you start looking into tempered martensite embrittlement (TME). Theoretically, if you tempered out the spine, you would have a transition area that would be more brittle than your edge. I have no idea how that plays out in real life but it is metallurgical fact.

That is one of the questions I have. How much impact does this have? Can it be mitigated? (staying under 500f? if that's the case is it worth doing?) Etc.

 

[joedhiggins]

That is one of the questions I have. How much impact does this have? Can it be mitigated? (staying under 500f? if that's the case is it worth doing?) Etc.

I think the effect is minimal. My understanding is that TME is a process that occurs as a steel is held in in that range (~450-550F) for a rather extended period of time (wikipedia says it takes an hour to happen FWIW). When tempering a spine, the duration any part of this steel may be held in that temp range is probably seconds or a minute at most. I don't think that this is practically a concern. Furthermore, the transition zone is very small, and when done properly should be over an area that is relatively thin. Because of the thin cross section, the steel will be less prone to failure for a given deflection.

Remember, the whole point of the selective tempering is to allow the blade to bend more without critical failure. As that is a function of the hardness vs the thickness at any point (with the understanding that cracks tend to propagate from the edge or spine, not appear at the center), I can't see how this process, when properly performed, could have an overall negative effect on toughness.

What we know with a relatively high degree of certainty is that differential tempers allow blades that otherwise would not past the ABS bend test to pass. I am not particularly enthralled with this test as I don't think knives are pry bars, but I suppose it does illustrate a point. Practically, in every use case I can think of, properly drawing back the spine of a blade will only make it tougher and more resilient. That being said, The number of knives for which it will make a practical difference is likely very small.