Characterization and Industrial Performance Evaluation of Duplex-Treated AISI H21 Die Steel during Hot Forging Process

Die/tooling failure during the hot forging process is mainly due to repeated varying mechanical loads, rigorous thermal shocks, as well as strong friction and erosion. These cause different failure mechanisms, such as wear, thermal fatigue, and plastic deformation. The enhancement of die service life for a hot forging industry is necessary now in order to be competitive globally. Four nitride coatings—TiN, TiAlN, TiCrN, and AlCrN—were deposited onto plasma-nitrided AISI H21 hot work die steel using the cathodic arc evaporation physical vapor deposition technique. The coatings were characterized by scanning electron microscope (SEM), SEM–energy dispersive x-ray spectroscopy (EDS), x-ray power diffraction (XRD), line scan EDS, and x-ray mapping tests and microhardness tester. The wear characteristics were examined using a pin-on-disk method at high temperature and speed against EN31 bearing steel disk. It was observed that the AlCrN and CrTiN coatings have lower wear rates than other coatings at high temperature (550°C). Also, these coatings performed best during actual industrial practice. The enhancement in die life was reported to be 55 and 76 % in terms of parts being hot forged. The low value of the coefficient of friction for all coatings at 550°C can partly be attributed to lower hardness difference between the pins and disks, decreased plastic deformation in the contact, and, consequently, less ploughing (abrasive wear action). Predominately, wear was the main die failure mode there was; when used, dies were observed microscopically.