Author: Bill Lenling, TST Coatings
Metal seated valves attain sealing by metal to metal contact. A variety of coatings are used to protect valves and actuators against wear and corrosion. In the case of metal to metal sealing surfaces coatings are designed to protect against wear due to galling of the sealing surfaces. Coatings also protect from corrosive, erosive and abrasive wear caused by the product that is flowing through them.
There are a variety of coating technologies used to deposit protective coatings. Plating is used to deposit materials such as hard chrome and electroless nickel. Boronizing is a high temperature diffusion process where boron atoms defuse into a surface to form hard metal borides for protection. This paper concentrates on the coating processes using thermal spray technologies for protection. Specifically, the thermal spray processes of HVOF and spray and fuse are described.
The use of thermal spray coatings can greatly improve the performance of a variety of valve types. Coating materials such as metal alloys, carbides, and oxide ceramics are used to create wear and corrosion resistant surfaces, which can provide substantial improvement in the life of the valve. Two thermal spray processes are commonly used to create the coatings. HVOF technology, (High Velocity Oxyfuel,) produces very well adhered as-sprayed coatings. Spray and fuse technology combines thermal spraying followed by a post coating heat treatment to create tenacious metallurgically bonded coatings. Both coating technologies can be used on gate, ball, and plug valve components to improve their performance when the valves are used in challenging applications.
High-Velocity Oxyfuel Process and Coatings (HVOF)
The HVOF process uses high-volumes of combustible gases or liquids fed into a combustion chamber. The high volume of gas or liquid flow, coupled with the high temperature of combustion can create gas velocities able to exceed Mach 5 and combustion temperatures near 6000°F. Powder metal, ceramic, or cermet (metal and ceramic composite) is injected into the device where the heat from the combustion fuel enable melting of the material. The high velocity of the gas jet accelerates the powder. This provides the powder with substantial kinetic energy resulting in high density coatings with excellent as-sprayed adhesion. Figure 1 above is a schematic of an HVOF device.
Typical coatings produced for valve components with the HVOF process are cermets (ceramic and metal composites). The ceramic phase in the coating is a carbide material, with tungsten carbide and chromium carbide being the most common. The metal phase in the cermet coating can vary and should be selected based on the operating conditions of the valve. Cobalt is a common metal used to cement the carbides together in the coating. If the environment is corrosive and can attack the cobalt, chrome may be added to increase the corrosion resistance or other metal binders can be used in place of the cobalt. Table 1 is a list of some carbide based coatings that can be used for valve components.
Table 1: Composition of HVOF deposited coatings for valve components
The exact properties of the coatings will vary depending on the composition of the coating material and the process spray parameter that is used to produce the coating. A properly deposited HVOF carbide coating will have low porosity content, high hardness, and excellent adhesion. Figure 2 is a metallurgical cross section of an HVOF deposited WC-10%Co,4%Cr coating deposited on a gate valve gate. Figure 3 shows a gate being HVOF coated.
Figure 2: Cross section of HVOF deposited WC-10%Co-4%Cr coating
Coating Hardness: 1284 Vickers, 72 HRC Coating Adhesion: >12,000 psi, measured by ASTM C633 test Coating Porosity: 0.11%
Figure 3: HVOF coating of gates
Thermal Spray and Fuse Technology
The spray and fuse process uses a thermal spray process to deposit an as-sprayed coating that is then heat treated to achieve a metallurgical bond to the coated component. Usually flame spraying is used to deposit the as-sprayed coating, but other thermal spray processes can also be used including HVOF. The coating is then fused by heating the coating up to a temperature within the solidus and liquidous range of the coating. This heat treating can be done by different methods including torch, induction, or furnace heating. Significant changes take place in the coating microstructure because of the fusing process. Diffusion will occur at the coating/substrate interface creating a strong metallurgical bond of the coating to the component. The metallurgical bond allows the coating to withstand impact without the coating chipping. Diffusion will also occur within the coating at the splat boundaries between the individual particles. Splat boundaries are eliminated and because of this, the corrosion resistance of the coating is improved. Corrosive attack of thermal spray coatings can happen by the corrosive chemicals attacking the splat boundaries of a coating. A fused coating can eliminate splat boundaries and reduce this attack. The porosity in the coating will also be reduced, which also improves the coating corrosion and wear resistance. Figure 3 shows as-sprayed and fused coating microstructures.
Figure 4: Metallurgical cross section of as-sprayed and fused coating
Most of the spray and fuse coatings are nickel based alloys that have boron added to lower the coatings melting point. Fusing temperatures will differ depending on the exact composition of the coating, but the typical fusing temperatures range is 1000°C-1150°C. Non-fusible materials can be blended into the alloy as seen in Figure 4 which has WC-12% Co added. The addition of the carbide phase can increase the coatings’ hardness and improve wear resistance.
There are several coating materials that can be fused. Table 2 is a list of the general chemistries of different groups of coatings that can be fused. All the groups have boron added to lower the alloys melting point.
NiCrBSiC + WC Co
NiCrBSiC + WC Ni
Table 2: General chemistries of coatings designed to be spray and fused
The hardness of the coatings will be dependent on the coating chemistry and how the coating is applied and heat treated. With pure metal alloys, the hardness of the fused coatings can be in the lower to middle 60’s on the Rockwell C scale. Higher hardness can be achieved by blending in additional hard phases like WC-Co with the alloy. With well controlled fusing methods the coating porosity will be very low, less than 0.5%.
Finishing of the coatings can be done in different ways, including grinding, lapping, honing and hard turning. Very smooth finishes can be achieved to produce excellent metal on metal sealing surfaces. Figure 5 shows a ball valve ball and seat that has been coated with a spray and fuse coating that has been ground and lapped to a smooth finish. These parts are commonly lapped together to achieve a tight metal to metal, gas tight seal.
Figure 5: Ball valve ball and seat coated with nickel based thermal sprayed and fused coating that was ground and lapped to achieve a metal on metal seal.
The are several valve materials that can be coated with the spray and fuse technology. Many stainless steels, alloyed steels, and nickel based alloys can be coated and subsequently fused. With alloys that have a high degree of hardenability, such as alloy steels, fusing of the coating can be difficult due to the formation of coating cracks while the part is cooled. With these alloys, the cracking occurs as the part is being cooled while a sufficient amount of martensite is being formed. When martensite is formed, there is an expansion of the steel, which can crack the coating.
To prevent the coating from cracking, the formation of martensite needs to be minimized. In the example of using 4140 as the base metal, Figure 6 shows the coated part should be cooled from the fusing temperature to approximately 1200°F and held at that temperature until the high temperature austenitic phase is transformed to ferrite and carbide, (F + C). In practice, the holding time should be twice the time shown on the TTT diagram due to variations in the alloy’s chemistry which can shift the cooling curves. For 4140 the part should be held at 1200°F for a minimum of 30 minutes to ensure most of the austenite has transformed to ferrite and carbide.
Figure 6: Time temperature transformation (TTT) diagram for 4140 steels
HVOF and thermal spray and fuse technologies are versatile engineering tools that when used correctly can greatly improve the performance of valve components. With a good understanding of which coating material to use for a particular component and environment, along with a good understanding of how to properly apply the coating, can result in a successfully engineered coating solution. The coatings produced with these techniques allow the use of coated valves in the most challenging applications in mining, power generation, chemical processing, oil & gas exploration, production and refining among others.
- Crawmer, Thermal Spray Processes, ASM Handbook, Volume 5A Thermal Spray Technology, p 39-41
- ASM, Atlas of Isothermal Transformation and Cooling Transformation Diagrams, p 153