In this study, the formation, microstructure, and wear properties of Colmonoy 88 (Ni-17W-15Cr-3B-4Si wt.%) + Stellite 1 (Co-32Cr-17W wt.%) coating layers fabricated by high-velocity oxygen fuel (HVOF) spraying are investigated. Colmonoy 88 and Stellite 1 powders were mixed at a ratio of 1:0 and 5:5 vol.%. HVOF sprayed selffluxing composite coating layers were fabricated using the mixed powder feedstocks. The microstructures and wear properties of the composite coating layers are controlled via a high-frequency heat treatment. The two coating layers are composed of γ-Ni, Ni3B, W2B, and Cr23C6 phases. Co peaks are detected after the addition of Stellite 1 powder. Moreover, the WCrB2 hard phase is detected in all coating layers after the high-frequency heat treatment. Porosities were changed from 0.44% (Colmonoy 88) to 3.89% (Colmonoy 88 + ST#1) as the content of Stellite 1 powder increased. And porosity is denoted as 0.3% or less by inducing high-frequency heat treatment. The wear results confirm that the wear property significantly improves after the high-frequency heat treatment, because of the presence of wellcontrolled defects in the coating layers. The wear surfaces of the coated layers are observed and a wear mechanism for the Ni-based self-fluxing composite coating layers is proposed.

This study is a study on the cracking of the main piston surface which is generated in the injection molding machine to generate mold force. The main piston machining process consists of high frequency heat treatment, grinding and super finishing after lathe turning. Scale, defect size, and fracture texture were observed for four cracks on the surface of the piston during the tempering process after high frequency heat treatment using a metallurgical microscope. In this study, it was confirmed that the cracked structure of the piston structure was ferrite and pearlite structure. It was confirmed that cracks progressed to 480 ㎛ and scale layer of 3 ㎛ or less. Surface hardening layer and hardness were min 2.0mm/HRc 58±2 spec 1.6 mm/HRc 56.5~57.5 In addition, cracks on the surface of the piston appear perpendicular to the rolling process. Therefore, it can be assumed that the crack occurred in the low temperature tempering process at 200°C or less after the high frequency heat treatment, not the material defect. Therefore, the temperature should be maintained at 200°C or higher during tempering after high-frequency heat treatment, and the cracking defect on the surface of the piston can be prevented by setting the feed rate to 1.3 mm/s or less during heat treatment.