The selection of materials for control valve components is a very complex undertaking. Control valves are required to function with precision in some very extreme environments. A number of factors must be considered to ensure that a material will perform properly in service. These factors fall primarily into two categories:

1. The material's suitability to function mechanically, and
2. The material's compatibility with the environment.

To make matters difficult, these categories conflict in many instances, making it difficult or impossible to satisfy all considerations with a single material. In these cases, the best compromise must be identified.

Material Properties

Mechanical and Physical Properties

When selecting materials, the mechanical and physical properties that must be considered vary depending upon the component. Obviously, the properties that are important in selecting a body material are different from those used in selecting trim material. Some of the properties that must be considered when selecting valve materials are the following:

Elastic modulus: In metallic materials, stress (S = load divided by area) is proportional to strain (e = change in length divided by initial length) provided that the stress is below a threshold stress, called the yield stress, where permanent (plastic) deformation begins to occur. The elastic modulus (E) relates stress and strain by the equation:

The elastic modulus is basically a measure of the stiffness or spring rate of the material and is only dependent upon composition and temperature.

Tensile strength: The tensile strength is the stress required to cause rupture. Tensile strength is not generally used directly in design since it is seldom desirable to utilize a component in a situation where it is on the verge of failure. However, the tensile strength value is utilized in the computation of allowable stresses in most codes.

Yield strength: The yield strength of a material is the stress required to cause a permanent deformation of 0.2%. This parameter is also utilized in the computation of allowable stresses in most codes. It is generally a critical factor that is considered when selecting materials for parts that carry loads, such as valve stems, cages, seat rings, bolting, and the like.

Hardness: Hardness is defined as a material's resistance to penetration, indentation, or scratching and is one of the most difficult material properties to fully understand. In metals it is usually measured by loading an indenter into the material and measuring either the depth of penetration or the surface area of the indentation. The deeper the penetration or the greater the surface area of the indentation, the lower the hardness. Thus, the hardness as measured in this way is a function of a number of other properties, such as yield strength, work hardening rate, elastic modulus, and so on.

There is a general impression that hardness is directly related to the service life of a trim component and that the hardness levels of two materials can be used to compare their "value" (hardness/dollar). However, the use of hardness as a gauge of wear resistance, erosion resistance, cavitation resistance, or galling resistance is merely a first-order approximation. There are a number of other material characteristics that contribute to resistance to these types of wear. The composition and crystal structure of a material, which are strongly related, can have a much greater effect than the actual hardness. This is the reason that cobalt-based hard-surfacing materials are superior in most wear situations, even though -their hardness is relatively the same as that of hardened stainless steels. It has been shown that the reason for the excellent performance of cobalt-based alloy 6 in wear applications is the crystal structure of its soft matrix phase, not its average hardness or its very hard carbide phase.