Cliff Stamp wrote:Impact toughness is measured by hitting a block of steel with a hammer and seeing how much the hammer slowed down after the block broke. The slower the hammer the higher the impact toughness of the steel. It is a direct measure of the resistance of the steel to sudden gross fracture, i.e., a very heavy load comes on the steel so fast that it can not bend to absorb it.
For knives the impact toughness has a directly significant connection to what happens if you chop into something very hard as you need enough toughness to prevent fracture. It also has a secondary benefit for all chopping because if the toughness is very low then the shock of even wood chopping can cause blade blowouts.
That is how it is hardened for the cutting competitions, it is under soaked to reduce the alloy that goes into solution, hence you see hardness levels which are significantly below how this steel is hardened in industry.
This maybe why people are finding that M390 has greater wear resistance in use as they are comparing it to undersoaked M4 and thus if you look at the composition sheet it is a bit deceptive.
It would be like for example comparing ATS-34 at 54 HRC to 440 C at 60 HRC and then noting that the wear resistance was higher for 440C and thus it had more carbides, etc.. The main influence there would be the large hardness change.
If you don't want stainless as a property it is always better not to have it simply because it costs to get. If I was getting M4 in a blade I would not want it undersoaked I would run it as it was designed for a cutting tool at 66+ HRC. But unfortunately I can not convince Sal to just make knives for me, though that would be nice.
The way they are currently ran is to provide a mix of toughness and wear resistance and prevent the kind of dramatic failures you can see on blades which are that hard as they are not overly forgiving if you are a bit ham fisted with them. No manufacturer want to see pictures/video's of knives which big pieces missing out of them.
Now interestingly enough, the main reason it is promoted in the cutting competitions is the claim that M4 allows thinner edges than steels like say 52100. This is a rather interesting claim and for a while I just called shenanigans on that because there is no materials data to support it and what data is available would argue the opposite.
For example comparing M4 (vs 52100) with both at 60 HRC :
-impact toughness is lower
-edge stability is lower (deformation on the micron level)
-compressive strength is the same
Thus how can the resistance to deformation laterally be higher?
But recently a few makers who I know don't hype have said their experience seems to be the same, though I wonder again if it is not just retained austenite which they are seeing as this is very prone to be a problem with low tempers vs high tempers and people also do all kinds of crazy things to 52100 to harden it.
Torsional strength curves would measure it directly, however few manufacturers actually do that.
Cliff, I almost always learn something new from your posts.
Thanks.
