Influence of the Active Screen on the Embrittlement of a Plasma-Nitrided Edge

In plasma nitriding of cutting tools, edge embrittlement can result from both, deepest layer, and grain boundaries precipitation. Overheating and surface charge density concentration in Direct Current Plasma Nitrided (DCPN) can be reduced through an indirect source of plasma, called Active Screen Plasma Nitriding (ASPN). The removal of the oxide layer from previous processes, such as heat treatment, is required in DCPN samples. This metallographic preparation can be a challenge in more delicate locations, such as on edges. This preliminary step is not usually described in studies comparing DCPN with ASPN. In the present work, a special pre-nitriding procedure aims for edge integrity and equivalent surface state at the top and the flank of AISI M2 samples. Under standard temperature (500°C) and atmosphere (2.5mbar, $N_2/H_2:25/75$), ASPN and DCPN were conducted for 2 and 4 hours, respectively. Edge cross-sections were analyzed in an Olympus GX-51 optical microscope (OM) and in a Carl Zeiss EVO MA 15 Scanning Electron Microscope (SEM). ASPN was effective in reducing embrittlement caused by grain boundary precipitation at a plasma nitrided edge.

Keywords:
Plasma nitriding; Edge embrittlement; Layer thickness; Grain boundary precipitation