Institute of Fundamental Technological Research

Metallic coatings are often applied on steel tubes in power generation boilers to improve their performance and extend the lifetime. Besides the high-temperature corrosion and erosion protection, the coatings should manifest good adhesion and cohesion strength, and relatively low residual stresses. In this study, three processing techniques: high velocity oxygen fuel spraying (HVOF), atmospheric plasma spraying (APS), and direct laser cladding were employed to obtain novel NiCr-Re and NiCr-Re-Al2O3 coatings intended for application in combustion boilers. The main objective was to assess the suitability of these three techniques to deposit NiCr-Re and NiCr-Re-Al2O3 composite coatings on a 16Mo3 steel substrate. For this purpose, a comparative analysis of the coatings behavior in selected tests was conducted. Of the three processing techniques, thermal spraying by HVOF turned out to be the optimum choice for the studied coatings. From among several variants of the HVOF-sprayed coatings, namely NiCr+1%Re, NiCr+2%Re, NiCr+1%Re+5%Al2O3, and NiCr+1%Re+10%Al2O3 (all vol.%), the NiCr+1%Re material exhibited the lowest extent of cracking in the disk bend test, the highest tensile strength (405 MPa) in the in-plane tension test, the highest Vickers hardness (379 HV2), the lowest specific wear rate (2.23·10-4 mm3/N m), and the lowest level of average residual tensile stress (120 MPa).

alumina, HVOF, mechanical behavior, nickel-chromium coatings, rhenium, residual stress, wear resistance

The high-velocity oxy-fuel technique (HVOF) was used to produce dense NiCrRe coating on boiler steel substrate with a minimal amount of oxide impurities and low porosity. Microstructure analysis, tribology and nano-hardness measurement have been realized with the aim to characterize the systems. The microstructure was studied using scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Tribological characteristics have been studied under the dry sliding condition at applied loads of 5, 10 and 20 N using the ball-on-flat technique with SiC ball at room temperature. Nano-hardness was investigated in continuous stiffness measurement (CSM) mode, the indentation depth limit was 1500 nm. Microstructure analyses proved that the HVOF sprayed layer, with a thickness approximately 870 µm, contains a relatively low volume fraction of porosity with a chemical composition based on Nickel, Chromium, with white areas of Rhenium. The wear rate of the coating is significantly lower than the wear rate of 16Mo3 steel. The average values of indentation modulus and hardness were EIT = 237.6 GPa and HIT = 6.3 GPa, respectively.

HVOF, mechanical properties of NiCrRe, NiCrRe coating

This paper presents the results of the investigation of NiCrRe coating deposited by plasma spray process at the atmospheric pressure on boiler steel substrate. These coatings were characterized by means of a scanning electron microscopy, and Energy-dispersive X-ray spectroscopy. The wear resistance of plasma sprayed NiCrRe coatings has been investigated under dry sliding conditions at applied load of 10 N in air. The continuous stiffness measurement (CSM) method was used for the investigation of nanohardness using Agilent G200 Nano-indenter in order to determine the mechanical properties of the coatings. Microstructural observations pointed out that the NiCr layer with white isles of rhenium possessed porosity, oxidized, un-melted and semimelted particles, and inclusions. According to the results the thickness of the layer is 450 μm, the indentation modulus 158 ± 24.4 GPa, hardness 3.74 ± 0.76 GPa and the coefficient of friction is 0.45.

coating, plasma, tribology

The development of technologies for joining advanced materials is connected with an introduction of new materials and new applications of their bonds, to work in ever more difficult conditions. One of possibilities of obtaining this type of joints is using the friction welding technique. This paper presents the results of joining intermetallics (Fe-Al and Ni-Al type) with steel (S235JR) and ceramics (Al2O3) using friction welding technique. The focus of the investigations was selecting: appropriate rotational speed (10000-25000rpm) of joining elements, welding pressure and time (1500-4500ms) of its application and swelling time (1000-7000ms). The paper presents the results of microstructure investigations, investigations of microhardness (perpendicularly to joint surface) and mechanical properties (tensile strength).

friction welding, intermetallics, microstructure of joints, microhardness, tensile strength