Why harden just the notch and not the whole cutting area?

[dks]

Most pliers based multitools have an 1cm cutting area, but only the first 3mm (the notch) is hardened enough to cut hard wire, SS nails etc.

I understand that the notch is placed there since this is where you would get the maximum torque and thus the maximum cutting force.

I also realise that you could not harden the whole head as that could make it more fragile (less elastic) and more likely to crack if it gets hit or twisted.

What I wonder is, why not harden the whole cutting area, the full 1cm.

What are you going to cut with that, less hard, area that needs it to be softer than the notch?

Is it to protect it if it gets twisted?

Is it so that the manufacturer limits the size of hard metal you can cut, thus protecting the tool?

This is even more bizarre in the supertool 300 which has replaceable cutters, made from a harder metal and still has that specified hard wire notch.

Any explanations?
 

[jzmtl]

The entire head are of the same hardness, the only difference between regular cutter and hard wire cutter is the regular cutter has a thinner edge thus cut through softer wires easier but cannot withstand high pressure, whereas the hard wire cutter has thicker edge/more obtuse angle to prevent the damage.

[dks]

 
That makes sense, though it is not clear in the tool's manuals.

I allways assumed the notch was hardened differently

[Mercury]

It is very difficult to heat treat just a section of a casting, let alone a section that is in the middle.  jzmtl makes a very good point.  Look at the notch on the hard wire cutters, the cutting face is much thicker than the soft cutters.  Hence the difference in cutting ability.