I think there is two parts to what actual HAZ is. HAZ is just a heat affected zone so you have to consider what’s actually going on. The total area that experiences heat might be 1/8”-1/4” but that’s just heat. You will see some discoloring like tempering colors But that’s a non issue. What’s really important is the area that was actually in direct contact with the liquid steel. Plasma and laser do not require the steel to be preheated like a cutting torch so material heating is very limited. I don’t have any personal experience with laser cut blades Besides have blanks cut for knives years ago. But a plasma uses a jet of superheated air to vaporize the steel. This jet of plasma is anywhere from 60-90psi and coming out of a hole thats around .020-.050. So if steel is in contact with this jet it gets removed very quickly. There is a skin between the vaporized steel and the solid steel that will get hot enough To harden But it’s not very thick at all. But the thickness is not consistent from top to bottom. It starts at just about zero thickness at the top of the cut and gets slightly thicker as it gets twords the cut exit on the back side of the sheet. Then right at the exit edge it jumps out in thickness. This is from the slag on the back side that sticks and heats that bottom edge. In knives this is a non issue as that area gets ground away when doing edge bevels.
Some alloys experience HAZ more then others. We cut a ton of AEBL and the HAZ is very shallow to the point of being completely removed once you clean up the plasma finish on a grinder. Where we actually see the most HAZ is on carbon steels like 15N20. This steel is just looking for a reason to harden. We find that knocking the slag off of 15n20 can take chips out of that trailing edge. But what’s interesting is you would think that plasma being over 20,000°C would really blow out the grain. I have done quite a bit of tests under the microscope and the grain is very fine in the HAZ area. But I have only been able to observe it visually in these carbon steels that harden easily. Funny thing is I can use my metal scissors (that I cut foil envelope open with) to cut plasma cut aebl with zero damage to the edge. The slag that sticks just does not get hot enough to transfer enough heat to the edge to harden it like it does on alloys like 15N20.
If anyone’s interested I can take some pictures next time I’m looking at some HAZ under the microscope. I really wish I had a dedicated camera for my microscope it would make this kind of thing much easier to share.
