Martensite formation is just one specific hardening mechanism in steels. Other hardening mechanisms (work/deformation hardening, pearlite formation, precipitat

Martensite formation is just one specific hardening mechanism in steels. Other hardening mechanisms (work/deformation hardening, pearlite formation, precipitation hardening) aren’t really amenable to additive processes - what’s the point of 3D printing if you have to deform your part to get the strength or hardness you need? I read through the paper, and the as-printed parts are still fully martensitic, no post-heat-treatment needed. So the volume change is baked in at least. To me, where your last point gets super interesting is functionally graded materials, which at least some of this group of authors are working on as well. Basically you could figure out how to locally temper this martensitic steel as you print it, so that you get graded properties — strength and hardness where it’s needed, but ductility in other places. It’s sort of a pretty open research area still