The crack after the quenching is the biggest defect of graphite mold in the process of heat treatment, it will make the processed graphite mold scrap and the production and economic loss.
The causes of the crack: there is serious network carbide segregation in graphite mould material ;there is mechanical or cold plastic deformation stress in the graphite mould; graphite mold heat treatment improper operation (heating or cooling too fast, quenching cooling medium selection improper, cooling temperature is too low, cooling time is too long, etc.) ; the graphite mold has complex shape, uneven thickness, sharp corners and threaded holes, which makes the thermal stress and the tissue stress too large ; graphite mold quenching heating temperature is too high, overheating or over burning ; the graphite mold quenching after tempering is not timely or tempering insulation time is insufficient ; graphite mold repair quenching heating, without intermediate annealing again heating quenching ; graphite mold heat treatment, improper grinding process ; there are high tensile stress and micro cracks in hardened layer after edm.
The preventive measures: controlling the internal quality of graphite mold raw material strictly ; improving forging and spheroidizing annealing process, eliminating mesh, ribbon, chain carbide, improving the uniformity of spheroidizing structure ; the graphite mold should be stress annealing after the mechanical processing or cold plastic deformation (>600 degrees) after the heating quenching ; For complex shape graphite mould, asbestos should be used to block thread hole, wrap dangerous section and thin wall, and adopt graded quenching or isothermal quenching ; repair or renovation of graphite mold, it need to be annealed or high temperature tempering ; the graphite mold should be preheated in quenching heating, take pre-cooling measures when cooling, and choose the appropriate quenching medium ; controlling the quenching heating temperature and time strictly to prevent graphite mold overheating and over burning ; the graphite mold should be tempered timely after quenching, holding time should be sufficient, high alloy complex graphite mold should be tempered 2-3 times ; Choosing the correct grinding technology and suitable grinding wheel ; improving the graphite mold EDM process, and stress relief tempering.
