H. Lv, J. Wang, C. Zhai, F. Li, B. Sun, Shanghai Jiao Tong University, Shanghai, China
In order to develop protective coatings for sink rolls in continuous hot-dip galvanizing,the sprayed MoB/CoCr cermet coating was formed on a 316L stainless steel by the HVOF spraying process and its durability in the molten 55%Al-Zn-1.5%Si bath (800k) have been investigated by SEM and EDS. The immersing test revealed that the MoB/CoCr coating has much higher durability (640 hours) in 55%Al-Zn-1.5%Si bath than the conventional sprayed coatings(120 hours),such as WC-Co, WC/Co/Cr and ceramics. It was found that the failure of MoB/CoCr coating is main caused of the coefficient of thermal expansion (CTE) of top coating is not matched with the CTE of substrate.The failure procedure is that first generate crack because of heat stress,then crack proceeds and produce Scaled delamination ,at the same time molten Al-Zn will immerses into cracks and/or Al-Zn reached to undercoat and/or substrate, finally molten Al-Zn dissolves the subtrate.
Summary: In order to develop protective coatings for sink rolls in continuous hot-dip galvanizing,the sprayed MoB/CoCr cermet coating was formed on a 316L stainless steel by the HVOF spraying process and its durability in the molten 55%Al-Zn-1.5%Si bath (800k) have been investigated by SEM and EDS. The failure procedure is that first generate crack because of heat stress,then crack proceeds and produce Scaled delamination ,at the same time molten Al-Zn will immerses into cracks and/or Al-Zn reached to undercoat and/or substrate, finally molten Al-Zn dissolves the subtrate.
