Knob Cutter Concave Cutter Tian Bonsa Tools Master Quality Stainless Steel 205 Mm (8") Quality: master's grade, handmade by the master worker from TianBonsai, have the best quality in our tools. Material detail: the Knob Cutter, manufactured via 5Cr15MoV stainless steel. Hardness detail: Knob Cutter was differential heat treatment by the high frequency machine, the working edge's hardness is over HRC58. Quality Assurance: 3 Years, in normal use, if the tools have any problem, we will resending a new one to you without any fee, you just need to take a photo to us to show the problem.