Surface hardening of valves: materials and processes.(HY-industry technical centre)
Valve internals, such as valve seats and closures, are often at risk of erosion, wear, corrosion, and cavitation damage. Let’s take a look at what these hazards are and how surface hardening can help prevent damage.
Erosion is the loss of material weight caused by the process, especially when the fluid contains particles. Corrosion is the process by which an object is lost and destroyed by the surrounding medium. Wear is a form of wear caused by the adhesion between two sliding surfaces, typically occurring when parts of similar hardness slide against each other under dry or non-lubricating fluid conditions. When the pressure in the liquid suddenly drops and vapor bubbles form in the liquid, cavitation occurs. When the bubble bursts, a pressure wave is generated, which damages the inside of the valve. Cavitation can create irregular pits and corrosion inside the valve, as well as noise and vibration.
Hardening is a metalworking process that applies a harder material to a vulnerable surface to mitigate such damage. The entire vulnerable component can also be made of cemented carbide material, but this is usually a more expensive solution. Internal parts of many different types of valves may benefit from hard surface treatments such as gates, globe valves, check valves and butterfly valves.
Different cemented carbides are processed using different processes, including:
High speed oxygen spray
To ensure the quality of the hardening treatment, the hardness, corrosion, thickness, adhesion and other properties of the hardened layer are tested, and the base metal is subjected to metallographic examination.
The material most commonly used to harden trims is Stellite 6, a cobalt alloy containing chromium, tungsten, carbon and other elements.
The chemical composition and some physical properties of Stellite 6 are given in the table below. The cemented carbide in the CoCr alloy matrix gives the material high corrosion resistance and wear resistance.
22Cr duplex stainless steel is widely used in piping systems in the marine industry, including valves. However, the use of Stellite 6 on the 22Cr duplex is challenging due to the formation of the Sigma phase in the 22Cr duplex during the soldering process, which may result in embrittlement. Chromium and molybdenum usually increase the rate of precipitation of the Sigma phase. Due to the higher heat concentration during the surfacing process, the risk of Sigma formation in the smaller and thinner dual phase components using Stellite 6 surfacing is higher. Rather than using Stellite 6 hardening on smaller sized 22Cr dual phase components (eg, 4 inches). Another method is to manufacture the assembly with a solid Stellite 6 to avoid problems associated with surfacing.
Seawater applications are not suitable for Stellite6 because of the high risk of cracking in this environment. Ultimet or Tribaloy materials are more suitable for valves used in seawater applications.
Another cobalt alloy Ultimet (UNSR31233) also has excellent wear resistance. Ultimet contains chromium, nickel, molybdenum, iron and tungsten. Engineers found Ultimet to be more difficult to cover than Stellite 6. In some cases, achieving full adhesion of Ultimet to the parent metal can be a challenge, which can result in loss of the weld overlay after several years of operation. The marine industry typically uses Ultimet coverage on 25Cr dual valve assemblies used in seawater.
Cemented carbide surfaces with nickel- or cobalt-based friction alloys protect parts that are subject to extreme wear, high temperatures or corrosive materials. The alloy contains a large amount of molybdenum, which increases the resistance to seawater pitting. This cobalt-chromium-molybdenum alloy also has high corrosion resistance, corrosion resistance and wear resistance.
Tungsten carbide is a combination of tungsten and carbon that can withstand high temperatures and is extremely resistant to wear. Figure 2 shows a sphere using a tungsten carbide coating and a post-grinding ball valve.
Tungsten carbide coatings are typically produced using a high speed HVOF thermal spray process. This process pushes the coating particles towards the surface of the substrate at an extremely high speed and they melt to the surface of the substrate. The thickness of the tungsten carbide coating is usually kept at 150 micrometers or less because of the problem that pores or coatings may fall off when the coating is thick.
Stellite 6 laser surfacing case study
Laser cladding has many advantages over traditional solder overlays, including: less total heat input, less heat affected zone, less thermal deformation, better adhesion, more uniform coverage, and iron-dilute stellite in the matrix. The rate is low. The following figure shows a laser tool for applying Stellite 6 or other materials to the valve disc and seat of a metal seat butterfly valve.
The figure below shows the result: a high quality Stellite 6 covers the contact surface of the disk.
Valves and closures may present a high risk of erosion, wear, corrosion and cavitation damage inside the valve. The use of a harder carbide layer on the consumables mitigates such damage. Hard alloys such as cemented carbide, stellite, Ultimet, tungsten carbide, etc. can be sprayed by welding, spraying, high speed flame spraying (HVOF) or laser. Different cemented carbides use different processing techniques, and the process used to cover the cemented carbide must be compatible with the cemented carbide and the substrate. Care must be taken when selecting cemented carbide and application methods.
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