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Understanding the Components and Operations in Valve Hardfacing

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Are you familiar with the process called hardfacing? It is a metalworking operation sometimes called cladding or mainly hard surfacing. It is done by applying a wear-resistant substance to the component’s surface by thermal spraying, welding, or any similar process that attains the purpose of wear reduction.

Like closures and seats, the internal valve parts are usually at risk of abrasion, erosion, corrosion, galling, and harm from cavitation. The hardfacing process can be applied to required parts and provide a value-added feature or repair previously deteriorated segments. Its whole materials could be made from hard elements though it may be costly.

Types of Wear

The hardfacing materials to be used rely much on the type of wear it needs to protect against. Check out the list of different threats that can harm internal valve parts.

  • Erosion – It is the deficit of component weight caused by flow processes, mainly if the fluid contains bits of solid particles. 
  • Abrasion This type of wear happens when minute, hard mineral particles rub through a metal surface and cut away particles from it. 
  • Corrosion This type of wear occurs in machines used for hot working applications. The tools are bare to cyclic thermal loads that eventually result in fatigue failures. Corrosion can also hasten the occurrence of erosion.
  • Galling is caused by adhesion among two sliding surfaces and typically happens when materials with the same hardness slide over one another in a non-lubricated fluid condition.
  • Cavitation This wear type occurs whenever the pressure in a fluid suddenly drops, and vapor bubbles form from it. Once the bubbles breakdown, they create pressure waves that may harm the valve internals. Irregular pits, valve erosion, and even noise and vibrations can be produced from cavitation activities.

 

Surface Treatments

Hard surfacing for improving the performance of castings are typically aimed at improving wear. It could be beneficial to several various types of valves, including ball and butterfly valves that are available at www.tubomart.com

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One type of surface treatment is welding. It is a construction or an assembly process that links components using high heat. This heat is used to melt down the parts that need to be joined and cool down, causing fusion welding. 

Here are some welding methods showcasing what hardfacing can do to avoid such damages.

  • Flame Sprayingthis treatment uses heat produced from fuel gas and oxygen mixture combustion.
  • Plasma Sprayingthis treatment uses an electric arc from direct current to form a high-temperature plasma gas.
  • LaserIt is also known as laser surface alloying (LSA). It is an operation wherein a high-powered laser beam is used to deposit one or many layers of a particular clad component onto a substrate to make both pore-free and crack-free coating.

 

Components for Hardfacing

To ascertain the quality of a hardfacing operation, a metallographic test on the base material is done to check:

  • hardness
  • corrosion
  • thickness
  • adhesion and
  • other characteristics of overlay

 

Stellite 6

 

The Stellite 6 is the most often utilized hardfacing valve internals. It is a cobalt alloy composed of carbon, chromium, tungsten, and other elements. This component coated on stainless steel is used for valve application in a high-pressure and high-temperature environment, where such materials might melt or degrade.

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Ultimet 

An ultimet is another cobalt-base alloy that offers excellent resistance to galling, cavitation, erosion, and different forms of corrosive attack. It contains chromium, iron, molybdenum, and tungsten. As a result, it presents higher weldability and can be utilized to weld overlay critical surfaces in order to increase their resistance to corrosion wear. This alloy is available in different forms, such as castings and powders.

Several valve manufacturers find Ultimet harder to overlay than Stellite 6. In some cases, attaining enough adhesion of Ultimet together with base material can be a great challenge. The offshore enterprise utilizes this cobalt-base alloy on the features of duplex valves used in seawater. 

 

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Tribaloy

A tribaloy, or also called as T-800, can be either a nickel-base or cobalt-base alloy. In the same manner that was mentioned above, it can also protect components from high-temperature, extreme wear, or corrosive materials. The T-800 alloy has a high content of molybdenum, which intensifies its resistance to pitting corrosion through water service. It also inhibits galling among dry sliding surfaces.

 

Tungsten Carbide

This surface treatment is a combination of tungsten and carbon. It is strong, durable, provides the most abrasion resistance. In the wear-resistant buildup, the tungsten carbide is applied at the tool surface to reshape and ultimately restore the part or tool.

The coatings for tungsten carbide are often applied by a high-velocity oxygen fuel (HVOF) thermal process. This operation boosts coating particles toward the substrate surface at extremely high velocity. They combine to the surface because of their kinetic energy, instead of using elevated temperature. The thickness of tungsten carbide is generally held to a hundred and fifty microns or anything lesser since it will have a porosity issue with thicker coatings.

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Conclusion

To sum it up, internal valve parts are at high risk of wear. There are several types of wear, namely erosion, abrasion, corrosion, galling, and cavitation. Fortunately, a process called hardfacing, a type of surface treatment, exists that provides damage reduction.

 

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