Laminated blades

[J-san]

I just bought a Spyderco Caly3 with the laminated ZDP/420J blade. Very nice knife! How are the two metals joined? I assume they are "welded" together under heat and pressure. How are they heat treated? ZDP and 420J are steels on different levels so how does the manufacturer account for differences in thermal expansion, optimizing hardness for the core steel, etc.? I am told they harden the core steel to a high hardness for edge retention, while tempering the side laminations to be softer for toughness. I assume they are unable to heat treat the steels separately before laminating, as the high heat would alter the original hardening process.