Influence of austenitic interlayer on the properties of stellite sur- facing welds after impact-hardening

Stellites (Co-Cr-W-C) are the specific group of coating materials used for surface modification of the engineering materials and for remanufacturing too. The aim of the paper was to research the influence of austenitic (308LSi) interlayer present on hardening level of stellite 1 and 6 after impact treatment. The samples have been cladded by TIG welding method with interlayer and without. Before impact hardening the samples have been visually and penetrant non-destructive tested. The samples after impact hardening have been tested by metallographic and Vickers hardness methods. The highest impact hardening effect have been revealed for coatings deposited with interlayer. The highest impact hardening effect was achieved for the padding welds produced with the interlayer, i.e. for stellite 1 (increased by 29.8%) and stellite 6 (increased by 42.7%). The hardening of the coating samples deposited without interlayer was lower and amounted to stellite 1 (increased by 13.7%) and stellite 6 (increased by 29.8%) respectively. The highest hardness values were obtained for impact-hardened cladded welds without the use of an interlayer (stellite 1; 790 HV0.1 and stellite 6; 732 HV0.1). The use of an interlayer reduces the hardness of the stellite coating while increasing the susceptibility to hardening and plastic deformation of the produced coating.

Properties of different TIG coatings of Stellite on the Hardox 450 and St 52 steel

The aim of this study was to coat different types of Stellite (1, 6 and 12) whose carbon rates vary on Hardox 450 and St 52 steel by means of tungsten inert gas (TIG) welding. For the purpose of microstructure investigations after welding, SEM, EDX, X-Ray and microhardness tests were carried out. It was observed that different types of Stellite coating with different features have an effective influence on microstructure and in the presence of dendritic nets between the grains decreased when the carbon content in Stellite coating decreased and when the substrates varied. In addition, it was determined in microhardness examinations that hardness increased with increasing carbon content and that the highest hardness value of 762 HV in sample S1 was attained. Consequently, positive results have been given for coating Hardox 450 steel and St 52 steel with Stellite via the TIG method.

Microstructure Evolution and Failure Behavior of Stellite 6 Coating on Steel after Long-Time Service

The microstructure evolution, elements diffusion and fracture behavior of the Stellite 6 weld overlay, deposited on 10Cr9Mo1VNbN (F91) steel by the tungsten inert gas (TIG) cladding process, were investigated after long-time service. Obvious diffusion of Fe occurred from the steel and fusion zone to the Stellite overlay, resulting in the microstructure evolution and hardness increase in the coating, where hard Co–Fe phases, σ phases (Fe–Cr metallic compounds) and Cr-rich carbides (Cr18.93Fe4.07C6) were formed. Besides, the width of the light zone, combined with the fusion zone and diffusion zone, increased significantly to a maximum value of 2.5 mm. The fracture of the Stellite coating samples mainly occurred in the light zone, which was caused by the formation and growth of circumferential crack and radial crack under high temperature and pressure conditions. Moreover, the micro-hardness values in the light zone increased to the maximum (470–680 HV) due to the formation and growth of brittle Co–Fe phases. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service.

Fretting Fatigue and Wear of Ti-6Al-4V With HVOF Sprayed Stellite Coating on Contact Surfaces of Shrouds and Stubs of Turbine Blades

High velocity oxygen fuel (HVOF) sprayed stellite coating is often adopted on contact surfaces of the shrouds and stubs of titanium turbine blades, in order to increase fretting fatigue strength and reduce fretting wear. To confirm the effectiveness of the sprayed stellite coating, we conducted friction-type fretting tests originally developed to simulate the load condition of shrouds and stubs, vibratory force was carried only by friction force. We also investigated the fretting fatigue mechanism of coated materials by observing non-propagating cracks in the tested specimens and analyzed crack propagation behavior at the interface using fracture mechanics. The fretting fatigue tests confirmed that the sprayed stellite coating could double the fretting fatigue strength of Ti-6A1-4V. The fretting fatigue mechanism of Ti-6A1-4V with a sprayed stellite coating was revealed as follows: Final fracture was led by a newly initiated micro-crack at the interface as cracks initiated on the coating surface did not continuously propagate into the substrate but mainly propagated along the interface. The micro-crack propagation at the substrate depends on the length of interface crack. The longer interface crack delays micro-crack propagation to the substrate because the lines of force are less concentrated at the crack tip. Hence, the interface crack makes the fretting fatigue strength of coated Ti-6A1-4V higher than that of uncoated specimens. This finding was supported by the cross-sectional observations of non-propagating cracks in the tested specimens, and by FE analysis which revealed that ΔK of the substrate crack initiated at the interface decreases with an increase in delaminated crack length. Ti-6A1-4V with a sprayed stellite coating offers better wear resistance than uncoated Ti-6A1-4V because the coating’s friction coefficient is lower than that of Ti-6Al-4V, although the wear rates of coated and uncoated materials against the consumption energy per cycle were almost the same.

WEAR ASSESSMENT OF STELLITE COATING IN SEVERAL CORROSIVE SOLUTIONS

Stellite™ 6 coatings deposited by HVOF on a Super Duplex Stainless-Steel substrate and wear performance was subsequently assessed. Thus, reciprocating ball-on-plate wear tests were performed in several conditions, for both coating and substrate. Results showed better wear resistance for the coating in all test conditions.

Dilution and C Content Estimation of Stellite 6 Fabricated on a 1050 Steel Substrate by Laser Cladding

Stellite 6 was fabricated by laser cladding on a 1050 steel (MS) substrate with laser powers of 1 kW (MS-1) and 1.8 kW (MS-1.8). The chemical compositions and microstructures of the coatings were analysed by X-Ray Fluoroscense, optical microscopy and scanning electron microscopy. The microhardness of the coatings was examined and the wear mechanism of the coatings was evaluated using a ball-on-plate wear testing machine. The results indicated less cracking and pore development for Stellite 6 coatings applied to the 1050 steel substrate with the lower laser power (MS-1). Moreover, the Stellite coating for MS-1 was significantly harder than that obtained for MS-1.8. The wear test results showed that the weight loss for MS-1 was much lower than for MS-1.8. The evaluations of dilution and calculation of carbon content indicated that MS-1 has lower dilution and higher coating C content than MS-1.8. It is concluded that the lower hardness of the coating for MS-1.8, substantially reduced the wear resistance of the Stellite 6 coating and the lower hardness of the coating for MS-1.8 was due to higher level of dilution and lower coating C content. The coating-substrate couple must be considered in assessing the likely performance of the coating under service conditions.