Selection of surfacing material for valve sealing surfaces

Thread Content

The sealing surfaces of steel valves are generally manufactured using surfacing welding. Materials used for valve surfacing welding are classified into 4 major categories based on alloy type, namely cobalt-based alloys, nickel-based alloys, iron-based alloys, and copper-based alloys. These alloy materials are used to produce welding electrodes, welding wires (including flux-cored wires), fluxes (including transition alloy-type fluxes), and alloy powders, etc. They are used in surfacing welding methods such as manual arc welding, oxy-acetylene welding, tungsten inert gas welding, submerged arc automatic welding, and plasma arc welding. The material chosen for welding on the valve sealing surface is generally determined based on the valve’s operating temperature, working pressure, and the corrosiveness of the medium; or it may be determined by the type of valve, the structure of the sealing surface, the required sealing pressure and allowable sealing pressure; or it could depend on the manufacturer’s production capabilities, its equipment for processing, its skills in welding techniques, as well as the requirements of the customer. Optimized design should also be employed; under the condition of meeting the performance requirements of the valve, sealing surface materials that are inexpensive, have simple production processes, and high production efficiency should be selected. The materials used for surfacing valve sealing surfaces come in only one form, either welding electrodes, welding wires, or alloy powders; therefore, only one surfacing method can be employed. Some are available in various forms such as welding electrodes, welding wires, or alloy powders. For example, the Stellite L6 alloy is available in the form of a welding electrode (D802), a welding wire (HS111), and an alloy powder (PT2102); it can therefore be used for surfacing welding using various methods such as manual arc welding, oxygen-acetylene welding, tungsten inert gas welding, flux-cored plasma welding, and powder plasma welding. When selecting the surfacing material for valve sealing surfaces, it is necessary to consider adopting a surfacing method that is technically mature, simple in process, and high in production efficiency within the company, so as to ensure the desired performance of these sealing surfaces. The sealing surface is a critical part of a valve, and its quality directly affects the valve’s service life. Choosing the material for the valve sealing surface appropriately is one of the important ways to improve the service life of the valve. Mistakes should be avoided when selecting materials for valve sealing surfaces. ● Myth 1: A higher hardness of the valve sealing surface material implies better wear resistance. Experiments show that the wear resistance of a valve sealing surface material is determined by the structural organization of its metal material. Some metal materials with an austenitic matrix and a small amount of hard phases do not have very high hardness, but they exhibit excellent wear resistance. The valve sealing surface has a certain degree of high hardness to prevent damage and scratches caused by hard particles in the medium. Taking everything into account, a hardness value of HRC 35 to 45 is appropriate. ● Myth 2: A higher price for the material used in a valve’s sealing surface means better performance. The price of a material is related to its inherent commercial properties, while its performance in actual use is determined by its physical properties; there is no inevitable connection between the two. In cobalt-based alloys, the metal cobalt is imported and comes at a high price; as a result, the cost of these alloys is also high. Cobalt-based alloys are characterized by good wear resistance at high temperatures; when used in normal or moderate temperature conditions, they offer a favorable price/performance ratio. When selecting the material for valve sealing surfaces, a material with a relatively low cost/performance ratio should be chosen. ● Myth 3: Just because a valve’s sealing surface material exhibits good corrosion resistance in a certain highly corrosive medium, it necessarily will also be suitable for use in other corrosive media. The corrosion resistance of metal materials follows complex mechanisms; a material may have good resistance in one highly corrosive medium, but even slight changes in conditions such as temperature or medium concentration can alter its corrosion resistance. For another corrosive medium, the corrosion resistance changes even more significantly. The corrosion resistance of metal materials can only be determined through experiments; when drawing on relevant information, it is necessary to understand the associated conditions, and one must not rely on such information blindly.