The name you mentioned " whirl cutter " I believe is a trade name for that specific brand of cutter.
Before this, I had never heard of whirl cutters. Your link showed them being used in the dental field.
Check out Scientific Cutting Tools website, www.sct-usa.com
We buy our thread mills or thread hobs as we call them from them.
We are using solid carbide spiral fluted thread hobs with TAILN coating in 15-5stainless with good results and tool life. These work well in stainless so they should work well in your hardened steel. Our tools are set in shrink fit holders.
You never mentioned what size threads you were cutting?