Correct me, if I'm mistaken but I remember seeing someone post somewhere that differential HD of 52100 does not produce a hamon but I've definitely seen something that looks like one on 52100 blades. Can anyone tell me what the deal is here?
Firstly let's establish what Hamon "is".
Hamon is a Japanese word used to define the pattern that the hardened edge of a properly made sword. A common misconception is that Hamon refers to the hardened area, this is incorrect, the hardened area is known as Yakiba.
In modern knifemaking terms however Hamon has come to define the hardened portion of any knife which displays a differentially hardened edge.
There are many ways to achieve a differentially hardened edge but they all center around the same concept, getting the edge hard while keeping the back soft.
I use Satanite clay to insulate the spine area of my knives, thus preventing the spine from cooling fast enough to form Martensite (hard steel), while allowing the edge to harden. I use Parks #50 metallurgical quenchant with all of my blades but similar results can be achieved with any decent quenchant.
This tutorial assumes that you are using appropriate steel for creating a Hamon, a shallow hardening carbon steel is preferable.
1075, 1095, W1, W2 all do really well, 1084 is a little picky, O1 and 5160 are technically possible but a real bear.
Creating a good Hamon starts in the hammering and shaping phase. It is very important to have an understanding of what each heat is doing to the grain boundaries in your workpiece. If you are doing stock removal you should be pretty well set to go, if not you MUST make sure to properly normalize your work before HT.
true hamon happens during quench and temper has little to nothing to do to it then
in a nut shell unless you need a knife to be able to be bent to a set there is no real use other then to look nice (tradition trickle down from swords)
now then on a sushi knife that is single bevel being able to straighten a blade as you hone it would be mayber the one time a soft back is useful
shock absorption? If the entire blade is as hard as some honyaki blades are, wouldnt it be helpful to have that softer steel to help absorb shock from cutting?
I got to agree with Butch that hamon on a honyaki gyuto (57-60RC) is all about look. Not sure about yanagi. I was under impression that you can't straighten honyaki, so not sure why it has a soft spine.
I do it because I like the way it looks nothing more. A differentially hardened gyoto will bend 90 and pretty much stay there whereas a fully hardened and tempered 52100 blade will bend 90 and srping back to straight even at 62-63 rch.