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Joël Mercier

Mechanical properties

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I've had a discussion with some smiths who told me the harder the steel, the more yield strength it has. It was a bit counter intuitive to me at first, but after explanation it made sense. 

I already knew two blades of the same geometry but different steel or hardness had the same flexibility but what I didn't know is that the harder blade would actually bend further before the steel underwent plastic deformation. There's a direct correlation between the steel hardness and it's yield strength. But, because harder steel is more brittle, plastic deformation results in a snapped blade. 

So let's say two given blades of the same steel and geometry are tempered to 55 and 62 hrc. The one at 55hrc will begin to stay bent at a 60° but the one at 62hrc will completely spring back until it reaches 72° and will then snap. The 55hrc one will stay bent but won't snap at 72°. 

So that's basically how it was explained to me. Is this accurate?

Edited by Joël Mercier
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Yes, it is totally correct.  Though it may be hard to see the difference between them.  Obviously your example is just theoretical numbers (60° and 72°), when in reality it may be more subtle, or more drastic.  But in principle, that is all correct.  

If you want to learn more, search for stress strain diagrams.  

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4130-stress-strain.jpg

This is from a project I worked on a little while ago and might be a helpful visual for this topic.

These stress-strain curves are for 4130, which is lower carbon than the steels in knives for the most part (though still hardenable to the mid 40s), and shows what some of this stuff looks like in a qunatitative sense. The unhardened samples are very ductile and show a higher toughness, but much lower yield strength. The quenched and tempered sample has a fairly similar overall behavior, but with roughly double the strength and a lower elongation at the break.

The untempered sample had too high of a strength for the tensile testing machine (actually, too high of a cross-section) and maxed out the load cell, so I unfortunately I don't have a full curve for it. However, you can see the substantial difference in strength from the tempered sample . By the time the test maxed out, it had already stretched almost 1.5x as far as the lower strength tempered sample at yield almost three times as far as the unhardened samples.

 

Edited by Aiden CC
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