The method of electric contact heating for nodular cast iron was based on the application of the contact resistance heating between the electrode and work piece to change the characteristics of the surface. The effects of processing parameters on the width and depth of modified layer were analyzed, and among the experimental factors, the electric current had the largest effect than the others. Optical microscope was used to describe the microstructure transformation and identify the phases in the modified layer. Results showed that a cementite-martensite microstructure in the melted zone and a martensite-ledeburite-ferrite microstructure with graphite nodules in the hardened zone have been observed; especially two typical hardened shells around graphite are achieved in the hardened zone, which are ledebruite-martensite shell and single martensite shell around graphite. The effects of the changed microstructures were additionally verified by microhardness measurements in the modified zone. The microhardness of the nodular cast iron was found to be significantly increased after electric contact heating.
Use of electric-contact heating in processes and apparatus of food industry
