Zong-de Liu

Corrosion behavior of Hastelloy C22 coating produced by laser cladding in static and cavitation acid solution

Abstract The Hastelloy C22 coatings on Q235 steel substrate were produced by high power diode laser cladding technique. Their corrosion behaviors in static and cavitation hydrochloric, sulfuric and nitric acid solutions were investigated. The electrochemical results show that corrosion resistance of coatings in static acid solutions is higher than that in cavitation ones. In each case, coating corrosion resistance in descending order is in nitric, sulfuric and hydrochloric acid solutions. Obvious erosion-corrosion morphology and serious intercrystalline corrosion of coating are noticed in cavitation hydrochloric acid solution. This is mainly ascribed to the aggressive ions in hydrochloric acid solution and mechanical effect from cavitation bubbles collapse. While coating after corrosion test in cavitation nitric acid solution shows nearly unchanged surface morphology. The results indicate that the associated action of cavitation and property of acid solution determines the corrosion development of coating. Hastelloy C22 coating exhibits better corrosion resistance in oxidizing acid solution for the stable formation of dense oxide film on the surface.

Microstructure and mechanical properties of interface between laser cladded Hastelloy coating and steel substrate

Abstract In order to improve the corrosion resistance of carbon steel, Hastelloy coatings were prepared on E235 steel substrate by a high power diode laser with laser scanning speeds of 6 and 12 mm/s, respectively. The interface between the coating and substrate was firstly exposed by dissolving off the substrate. Its microstructure, composition and mechanical properties were systemically studied. Special “edges” along the grain boundary were found at coating/substrate interface. These “edges” consisted of intergranular corrosion area and real grain boundary. The interface of coating mainly displayed austenite structure ascribed to the rapid solidification as well as the dilution of Ni during preparation. Additionally, Hastelloy coating and its interface prepared at the speed of 12 mm/s showed higher hardness than that prepared at the speed of 6 mm/s. Grain boundaries had higher friction coefficient than grains at both coating/substrate interfaces. Moreover, the interface at higher laser scanning speed exhibited smaller grains, lower dilution rates of Ni and Fe as well as a better tribological property.

Microstructures and properties of TiCx-reinforced metal matrix composite coating on TC4 alloy prepared by laser cladding

Abstract TiCx-reinforced metal matrix composite coating was prepared on TC4 titanium alloy by the laser cladding method. The microstructure and phase compositions of the coating were analysed by means of scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffractometry. The TiCx exhibited a dendritic microstructure, which was homogeneously dispersed in the metal matrix. With increased ingredient supercooling, temperature gradient and cooling temperature, the dendrites displayed a finer morphology with longer primary trunks and intensified side branches in the dilution zone. The hardness of the coating has improved about three times compared to that of the substrate because of the hardness of the major phase.

Preparation and characteristic analysis of TiCx-NbC/Ti composite cladding on pure titanium substrate

Abstract TiCx-NbC-reinforced Ti-based composite coatings S1 and S2 were successfully prepared on pure titanium plate by diode laser cladding. The microstructure and phase compositions were analysed by means of scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. The micro-hardness was measured by the Vickers hardness tester. The main phase of the clad coating consists of fine dendrites TiCx, thick dendrites TiCx, granular TiCx and dilute fine dendrites TiCx and most of the NbC has aggregated together and formed larger particles. And with the increasing of carbon powder, the contents of TiCx increase, and some intermetallic Ti4Nb and CuTi3 appear. The Ti proportion decreases from the substrate to the surface of the coating, whereas the carbon increases. The Vickers micro-hardness presented a linear increasing trend from the substrate to the sub-surface. The average hardness of S2 is higher than that of S1.

Microhardness homogeneity analysis of thick amorphous composite coating prepared by TIG cladding

Abstract Amorphous coatings play an important role in surface modification owing to their inherent properties of high strength and abrasion resistance. A ∼4 mm thick layer of Fe based amorphous composite coating was deposited on the surface of a boiler waterwall tube by tungsten inert gas cladding. Microstructure analysis demonstrated that the coating consisted of amorphous and nanocrystallised phases. The longitudinal and transverse microhardness measurements were evaluated systematically by Vickers. The results showed that the microhardness up to 1400 HV0·3 varied under different loads and the more obvious microhardness fluctuations were observed under smaller loads. On the basis of the Hall–Petch rule, the uneven distribution of amorphous and nanocrystalline phases in the small indentation area may be responsible for the microhardness fluctuations. A careful study was undertaken of the reason for the high hardness of the coating and the microhardness fluctuation phenomenon, which may lay solid foundations for achieving a nearly three-dimensional homogeneity in the amorphous composite coating, with a large thickness.

Thick Ni based amorphous composite coating with good erosion wear resistance

Abstract A thick Ni based amorphous composite coating was prepared by the laser cladding technology. The microstructure, phase composition, hardness and erosion wear resistance of the coatings were characterised by means of scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and erosion wear experiments in this study. The results showed that the amorphous and nanocrystal composite coatings had dense structure, low porosity and metallurgical bonding with the substrate. The addition of TiB2 played a significant role in improving both the amorphous content and the mechanical behaviours of the coatings. With the increasing content of B, the hardness, erosion and wear resistance of the coatings increased. The overall performance showed better for the Ni based coating containing 20%TiB2, due to the superior combined efforts of amorphous and nanocrystallines. The hardness of the coating was about 6·5 times better than that of the 304 steel, and the erosion rate, especially at high impact angles, is superior to the commercial NiCr/Cr3C2 coating.

Thick Fe based amorphous composite coating deposited by laser cladding

Abstract A 2·5 mm thick Fe based amorphous composite coating is deposited on the steel pipe using laser cladding method. The microstructure, phase composition and microhardness are characterised using scanning electron microscope, X-ray diffraction, optical microscope and Vickers hardness tester, respectively. The results show that the laser cladding coating mainly consists of amorphous substrate and dendritic nanocrystals. No obvious defects such as pores are seen in the coating, and the bond between coating and substrate is metallurgical bond. The microhardness values of the coatings are much higher than that of the substrate. A comparative electrochemical corrosion study of the coating and substrate was performed by linear polarisation method and electrochemical impedance spectroscopy in 3·5 wt-% NaCl solution. The unique microstructure contributes to the superior microhardness, corrosion and wear resistance of the coating. The relation between microstructure and properties is discussed in detail. The millimetre sized Fe based coatings are promising wear-resistant structural materials in modern industrial applications.

Effects of Tungsten and Laser Re-Melting on the Properties of Hastelloy C22 Coating

Hastelloy C22 coatings with (H1) and without tungsten (H2, H3) were prepared on Q235 steel substrate by laser cladding. Coating H3 was also re-melted by the same laser process. The effects of tungsten and laser re-melting process on coating properties were studied in terms of the microstructures, mechanical properties and corrosion resistance. The results show that coating H1 exhibits higher micro-hardness and better corrosion resistance than that of coatings H2 and H3. It is mainly ascribed to the solution strengthening and stable chemical property of tungsten added in coating H1. Coating H3 after laser re-melting displays more compact microstructure than coating H2, which would be conducive to form dense oxide layer on the coating surface. Thus more positive corrosion potential of −0.415 V and lower corrosion current density of 1.149 ×10-6 A/cm2 of coating H3 indicate better corrosion resistance than coating H2.