L6 blades typically edge 58~60 HRC, back 38~42 HRC

Hanwei's latest version of the Praying Mantis features a "straight" (Sugaha) hamon, as opposed to the "wave" (Notare) style used on earlier versions.  This results from Hanwei's development work in producing a hard martensitic edge on Bainite blades. Traditional claying techniques produced unsatisfactory results and it was found that the best edge hardness was produced by microwave heating of the  edge section (Yakiba) (this takes about four seconds), followed by immediate quenching.  As the microwave technique can only be applied to an area that follows the line of the edge, a Sugaha hamon style results. While the technique is very modern, the hamon style is very traditional. 

The gray color of the blade surface is not oxidation, it is simply the coloration of the Bainite phase of L6, which has an inherently "dark" appearance. It could be polished, like the section behind the ridge line (shinogi-ji), but we would lose the hamon (the differential hardness would still be there, it just would not be visible).  As the Bainite is used primarily to take advantage of its "forgiveness" in cutting and appearance is secondary, we figured that a visible hamon would be the preferred option.

  L6/Bainite is a fairly modern creation (Bainite is a phase of L6, made by very careful control of the heat treating process).  It is very tough and resilient - it has a high yield point, so that it can be flexed strongly without taking a set, and it holds an edge very well.  Until recently was probably the best steel in use for Japanese swords (although the Tamahagane traditionalists would argue).