McMaster sells D2 tool steel that is 3/32" thick by 1" wide by 18" long for about $28. They say it is "Wear-Resistant Air-Hardened D2 Tool Steel." Hardness is specified as "B89-B102" with a maximum attainable hardness of C62-C65. I have found a local sharpening shop that will cut it into two knives and sharpen them for less than $20.

My question is whether the initial B89-B102 hardness is enough, or if they would require additional hardening. If B89-B102 is enough, then
$50 doesn't seem too bad for a custom set of knives. But if additional hardening is required, I imagine that would be expensive and extra complicated.

McMaster also have a 4140/4142 steel that they say is hardened to C28-C32. However, I have seen D2 mentioned for planer knives but never
4140/4142, so I don't know if this 4140 steel is appropriate?

Any ideas?

I've been running my molder knives up to Rockwell C56-58 using A2 steel. Cut to size, sharpen, heat treat. I've been told that my blades should run about 5,000 to 10,000 before sharpening. I doubt you will get the wear you want with anything much less. I use A2 to minimize distortion of the funny shapes I need.

Be carefull if you order tool steel of any description. The normal material is oversize to allow for finish machining or grinding to the specified size. Most of the McMaster stock is ground to spec, not necessarily true of other suppliers.