A continuous galvanizing line (CGL) has a zinc pot, which is filled with molten zinc for zinc coating. In a zinc pot there are pot rolls to guide steel strip. Usually WC-Co thermal sprayed coatings are used for protection of the pot rolls from severe corrosion by molten zinc. Authors analyzed WC-Co coatings used in a zinc pot of a CGL for 33 and 56 days. On the surface of a WC-Co coated roll, many kinds of deposits were observed including top dross, Fe2Al5 inter-metallic compound, which might induce dross defect on the surface of galvanized steel. Diffusion depth of zinc into the WC-Co coating used for 33 days was only within 10µm but some areas were severely attacked along cracks within the coating layer. Usually molten zinc contains small amount of aluminum about 0.12 - 0.2%. Through SEM study, we observed that not only zinc but also aluminum diffused into the WC-Co coating after service in the zinc pot for 56 days. Al-Fe rich layers were observed on the surface of the spray coating for some cases. The phase of those layers might be Fe2Al5 since their chemical compositions are similar to Fe2Al5 top dross.

Interactive Computational Code is presented that provides a high-productivity analysis of the complex detonation spraying technology. It is based on exact gas dynamics theory taking into account chemical reactions for description of the gaseous detonation phenomena. It deals with specific features of the process like non-homogeneity of gaseous mixture compositions due to gas filling operation and powder injection into the barrel. Acceleration and heating of powder particles by the detonation product flow were simulated including particle melting, fragmentation and vaporization. Calculations for different fuels and powder materials were accomplished. Optimum conditions (gas mixture, barrel length, powder injection point, etc.) to provide highest velocity and appropriate temperature for typical metal and cermet powders are discussed. The conditions for coating formation by the detonation gun «Ob» were established and samples were produced. Experimental data on detonation coating properties are presented to prove the theoretical results.

Amorphization induced by sliding wear and consequent wear resistance have been investigated in relation with the microstructure of Fe-Cr-B alloy spray coatings. The Fe-Cr-B spray coated layer exhibited much higher wear resistance and significantly lower friction coefficient in comparison with that of low carbon steel substrate thanks to the amorphous surface film formed during the dry sliding wear. Electron microscopy on the cross-section of the coated layer exhibited intra-particle segregation associated with rod-shape Cr rich (Cr,Fe)xB particles in the matrix of Fe-Cr solid solution phase. From the observations using TEM and EDS, Fe-Cr solid solution phase with super-saturated B and Si content was confirmed to be the phase which mainly contribute to the crystalline-to-amorphous transition induced by sliding wear. The formation of oxide inclusions seems to impede the crystalline-to-amorphous transition by lowering the solute content in Fe-Cr solid solution phase. Keywords : microstructure, wear-resistance, Fe-Cr-B alloy, amorphization, detonation gun

The role of surface roughness in coating adhesion mechanism is studying for detonation spraying. Roughness was produced by conventional grit blasting, D-gun blasting and was formed as a result of spraying of high-adhesive thin layer of detonation coating. Cermet and alloy powders were sprayed by detonation gun Ob. The coating bonding strength measurements show the WC+25Co adhesion to be above 200 MPa independently of a substrate surface preparation. Contrary, NiCrSiB coatings are very sensitive to surface conditions their adhesion varies from 180 MPa to zero. As-sprayed alloy particles fail in adherence because of insufficient energy to fuse substrate material at a flat surface. Only developed (wide scale) roughness may be fused partially by these particles for their bonding to the substrate. Otherwise, high heated cermet particles do not need special surface preparation (except cleaning) for fusion of substrate material to provide high bonding with it. The wide scale and ball shape roughness, which is similar to the self-reproduced coating roughness, provides the best conditions for the coating bonding and it is recommended as the purpose of surface treatment before thermal spray coating.

Mechanical properties of WC-Co D-Gun coatings produced from various powders were determined by using a four-point bend test equipped with a special device for strain measurements. The MOR(Modulus of Rupture), elastic modulus, fracture strain and toughness were measured from stress-strain curves using the four-point bend tests. The fracture strength values were increased in the order of clad, sintered, agglomerated(no densification), blended and cast & crushed powder coatings. The Co content blended with WC-Co cermet powders significantly increased fracture strain values and decreased elastic modulus values. It was found that the type of powder more significantly influenced the mechanical properties of D-Gun coatings than the composition of powders. The toughness of thermal spray coatings was increased by using WC-Co powders (high MOR) blended with self-fluxing alloy powders (high fracture strain).

To maintain surface roughness of process rolls in cold rolling steel plants, WC-Co coatings have been known to be effective ones. In this study, a high pressure/high velocity oxygen fuel (HP/HVOF) process was used to obtain WC-Co coatings. To get the best quality of coatings, WC-Co coatings are sprayed with numerous powders made by various processes. These powders include agglomerated sintered powders, fused-crushed powders, extra high carbon WC-Co powders and (W 2 C, WC)-Co powders. After spraying, properties of coatings such as hardness, wear resistance. X-ray diffraction, and microstructures were analyzed. For coatings produced by agglomerated-sintered powders, hardness of the coating increased as power levels and the number of passes were increased. In case of the coatings produced by fused-crushed powders, a very low deposition rate was obtained due to a low flowablity of the powders. In addition, the WC-Co coatings sprayed with extra carbon content of WC-Co did not show improved hardness and wear resistance. Also, some decomposition of WC was observed in the coating. Finally, the coatings produced by (W 2 C, WC)-Co powders produced higher hardness and lower wear resistance coating.