In the study of control valve failure analysis, it is necessary to have a detailed knowledge of the wear mechanisms since the detachment of material from a mechanical component can vary remarkably, from very intense to insignificant.
It means a total loss of efficiency or complete utility of the item.
The wear mechanisms that affect the valves and in general mechanical elements, can be classified into abrasion, erosion, corrosion, and cavitation.
It is a type of superficial wear, caused by movement relative to hard particles on the surface of some mechanical part.
Pure or low-stress abrasion
It has the following characteristics:
- Final/medium granulation abrasive. Free impact
- Small angles of incidence
- Low pressures (abrasives on metal)
The following relationship should be taken into consideration, the greater the. The hardness of the abrasive, with respect to that of the metal surface, lowers the rate of wear.
To protect metal exposed to pure abrasion, use a coating of high hardness and high deposit density.
In this case, the abrasive element.
It is made up of small particles that do not impact the surfaces of wear but are trapped and exposed to high pressure, in which there is relative motion. The stresses generated on the surfaces are large, many times higher than the elastic limit of the metal, and even to the point of rupture, which leads to the detachment of the material. To increase stamina to wear, metallurgical structures such as carbides + hard matrix must be used
Also known as “Penetration abrasion” or “Cutting abrasion”. This abrasion differs from the previous one in which the abrasive element is larger and many times there is an impact associated with the wear mechanism.
The pressures exerted on the wear surface are generally very high, and surface deterioration manifests with the detachment of volumes of microscopic metal. The cutting and penetrating simply imply deformation plasticity of the surface, this means that the plastic deformation produces a hardening, which in turn reduces the point toughness in the metal, and against new efforts, this area is detached.
For this type of abrasion, tough and resistant materials must be used, such as even austenitic manganese steel.
This type of wear is related to abrasion, is manifested with the impact of solid or liquid particle flow on the surface, and is generally transported by a fluid at high speed.
Erosion wear is a function of the kinetic energy, of the particles and how this energy dissipates when impact occurs. A part of the energy is consumed by the part, causing elastic or plastic deformation depending on the degree of toughness of the metal. Another part is exhausted by the abrasive particle itself, either as speed rebound or energy that leads the particle to fracture.
The degree of surface deterioration will depend on the toughness and hardness of the base metal, hardness, and granulometry of the abrasive, in addition to the angle of incidence and of the erosive beam concerning the surface. It is extremely important to know the angled impact because it is the most critical when you want to select a material.
At low-impact angles wear occurs as a result of the micromachining and its speed depends fundamentally on the surface hardness. With a greater impact angle, erosive particles hitting the surface
The use of warping can lead to chipping. That is why the material must have a great capacity to absorb energy without plastic deformation.
This type of wear occurs particularly where the parts are in contact with fresh or saline water, in basic or acid environments, and also when they are exposed to gases at normal or elevated temperatures.
Corrosion can be defined as a chemical attack on a material of the surrounding environment. This reaction can be initiated by various phenomena, there are two basic types of corrosion phenomena, which can be defined as follows:
In dry environments or oxidation corrosion
It manifests itself through a chemical reaction between the metal and the surrounding environment, without initiating an electric current, oxidation is a good example.
This type of corrosion occurs in the form of localized, making it more difficult to detect. It manifests itself through a reaction.