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Valve and Actuator Technology for the Offshore Industry
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
There are some cavitation mitigation approaches, such as larger stem diameter, stronger connection of stem and plug as well as applying hard-faced materials such as stellite 6 or 21 to the seat and plug of the valve. Stellite is a type of cobalt and chromium alloy used for wear resistance on the valves’ internals, such as the seat and plug. From an operational point of view, it is not recommended to keep the valve <20% open, as this can cause severe wearing and erosion, which can intensify the cavitation effect. The other solution is to change the type of valve to either a Y-type globe or an axial control valve. A Y-type or Y-pattern globe valve, as illustrated in Figures 5.48 and 5.49, reduces the pressure drop inside the valve as the rotation of flow inside the valve can be reduced to a maximum of two times and the flow direction change occurs at a maximum of 45° rather than 90° in each rotation. Reduction of pressure drop value results in less cavitation.
On the performance of metallurgical behaviour of Stellite 6 cladding deposited on SS316L substrate with PTAW process
Published in Canadian Metallurgical Quarterly, 2022
Vivek Kalyankar, Avishkar Bhoskar, Dhiraj Deshmukh, Sanjay Patil
Ferozhkhan et al. [2] suggested that the different alloy composition of cladding materials has a significant effect on the cladded surface and microstructural studies allow users to modify the composition of alloy according to the applications. Various alloys such as cobalt-based, nickel-based and tungsten-based are used in industries as a cladding material using different surface cladding processes. Among these alloys, the cobalt-based Stellite 6 alloy is used extensively in applications such as valves, turbine blade and slurry carrying pipe [4]. Stellite 6 being a non-ferrous alloy, contains cobalt as a major element while remaining elements are chromium, carbon and tungsten which form different complex carbides in Co-Cr based alloyed matrix [5]. Ahmed et al. [6] observed that, cobalt-based Stellite 6 alloy shows an excellent characteristic against corrosion which makes it preferable alloy for the products that are used in corrosive environment.
Surface and bulk modification techniques to mitigate silt erosion in hydro turbines: a review of techniques and parameters
Published in Surface Engineering, 2022
Arji et al. flame sprayed stellite (Co–Cr–W–Ni–C) coating on mild steel and studied the erosion performance and the mechanisms [45]. The increased hardness improved the erosion performance. The pitting and indentations were observed erosion mechanisms. Ramachandran and associates hard-faced carbon steels by depositing nickel-based alloy (Colmonoy) [46] and cobalt-based alloy (Stellite 6) [47] by plasma transferred arc method. The applied hard-facing enhanced the impact toughness and hardness and thereby increased the erosion resistance. They found craters and indentations were responsible for the material erosion. Kennedy and Natarajan strengthened 304 austenitic stainless steel by applying Al2O3 + TiO2 coating using plasma, and detonation methods [48]. The performance against sand mixed with water was evaluated and found that the erosion took place by delamination of the coating through plastic deformation and ejection of grains. But the detonation gun spray showed better adhesion and hence reduced erosion rates. A similar kind of erosion mechanism was observed when nano-Al2O3 was deposited on the same material by plasma spray [49]. The erosion performance was improved owing to the increased hardness and high crack hindrance in the applied coating.
A study of laser cladding NiCrBSi/Mo composite coatings
Published in Surface Engineering, 2018
W. Kaiming, L. Yulong, F. Hanguang, L. Yongping, S. Zhenqing, M. Pengfei
Molybdenum (hereafter referred to as Mo) can refine grain, increase toughness, enhance plasticity and effectively reduce the crack sensitivity of the cladding layer [20–22]. Shin et al. [23] have investigated the effect of molybdenum on the microstructure and wear resistance of cobalt-base Stellite hardfacing alloys using a plasma transferred arc welding machine. The results show that with the increase of Mo content, the M23C6- and M6C-type carbides are formed instead of Cr-rich M7C3. The size of Cr-rich carbides in interdendritic region decreases, but M6C type carbide increases and Co-rich dendrites are refined. Wang et al. [24] have studied the effect of molybdenum on the microstructure and wear resistance of Fe-based hardfacing coatings by shield manual arc welding. The results indicate that the fraction of carbides in hardfacing layer increases with the increase of Mo content and the hardfacing layer has good cracking resistance. Wear resistance can be obtained when the amounts of Mo are controlled within a range of 3–4 wt-%.