Carbide Inserts Knives are 8 to 10 times longer in us than HSS ones. The hardness varies from HRA90.30 to HRA95 with US codes from C1 to C4+++ or ISO codes from K40 to K01. For different materials, we recommend different knives with different hardness and different material property. All the carbides are manufactured with advanced machines from Europe. The interchangble knives are precision ground to +/-0.0005".