The performance change of 304 stainless steel pipe fittings in high temperature environment is closely related to its material characteristics and the physical and chemical changes caused by high temperature. The following systematically explains the performance of 304 stainless steel pipe fittings at high temperature from the aspects of strength, corrosion resistance, dimensional stability, etc.
The main components of 304 stainless steel pipe fittings are iron, chromium, and nickel, which have good strength and toughness at room temperature. However, when the temperature rises, the thermal motion of atoms in its crystal structure intensifies, making the dislocation motion inside the metal easier. As the temperature continues to rise, the tensile strength and yield strength of the pipe fittings will gradually decrease. Generally speaking, below 400℃, the strength decreases relatively slowly and can still maintain a good load-bearing capacity; but after exceeding 500℃, the strength decreases significantly faster. When the temperature reaches 600℃ - 700℃, the strength of 304 stainless steel pipe fittings may drop to about half of that at room temperature. At this time, the ability to carry heavy objects or withstand pressure is greatly weakened, and it is easy to deform or even break during use.
High temperature environment will also affect the corrosion resistance of 304 stainless steel pipe fittings. The reason why 304 stainless steel is corrosion-resistant is that there is a dense chromium oxide passivation film on its surface. At high temperatures, the stability of this passivation film will be challenged. On the one hand, high temperature accelerates the reaction between oxygen and metal, which may thicken the passivation film, but the thickened film structure may become loose, reducing the protective effect on the metal; on the other hand, when there are corrosive media such as chloride ions and sulfides in the environment, high temperature will enhance the activity of these media, and they are more likely to penetrate the passivation film and react with the internal metal, thereby accelerating the corrosion of the pipe fittings. For example, in a high-temperature chemical environment, steam containing chloride ions may cause pitting or stress corrosion cracking of 304 stainless steel pipe fittings.
High temperature will also cause thermal expansion of 304 stainless steel pipe fittings. Since the thermal expansion coefficient of 304 stainless steel is relatively stable, the pipe fittings will expand evenly within a certain temperature range. However, if the temperature changes frequently and the amplitude is large, the pipe fittings will expand and contract repeatedly, which will generate thermal stress inside the pipe fittings. When thermal stress exceeds the fatigue limit of the material, fatigue cracks may occur in the pipe fittings, especially in stress concentration areas such as welding parts and bends of the pipe fittings. In addition, thermal expansion may also cause changes in the matching accuracy between the pipe fittings and the connectors, and the parts that were originally tightly connected may become loose, affecting the sealing of the pipeline system.
High temperature also has a significant effect on the oxidation resistance of 304 stainless steel pipe fittings. In normal temperature air, the oxide film formed on the surface of 304 stainless steel can effectively prevent further oxidation. However, when the temperature exceeds 570℃, the composition and structure of the oxide film will change, from the original protective chromium oxide film to iron oxide, and the iron oxide is loose and porous, which cannot prevent oxygen from continuing to diffuse inward, resulting in accelerated oxidation of the pipe fittings, peeling and flaking on the surface, and long-term use will cause the wall thickness of the pipe fittings to continue to thin, shortening the service life.
In high temperature environments, 304 stainless steel pipe fittings may also undergo sensitization. When the pipe fittings stay in the temperature range of 450℃ - 850℃ for too long, carbon will combine with chromium to form chromium carbide, which will precipitate at the grain boundaries. This will lead to a decrease in the chromium content near the grain boundary, forming a chromium-poor area, which greatly reduces the corrosion resistance of the pipe fittings, especially in a strong oxidizing environment, where intergranular corrosion is prone to occur. Once intergranular corrosion occurs, the surface of the pipe fittings looks intact, but the bonding force between the internal grains has been destroyed, and it may suddenly rupture under pressure or external force.
Although 304 stainless steel pipe fittings have the above-mentioned performance changes at high temperatures, their high-temperature performance can be improved through some treatment methods. For example, solid solution treatment of pipe fittings can fully dissolve carbon and alloy elements in austenite, reduce the precipitation of chromium carbide, and reduce the risk of sensitization; coating the surface of pipe fittings with high-temperature resistant anti-corrosion coatings, such as ceramic coatings and high-temperature resistant paints, can enhance the oxidation resistance and corrosion resistance of pipe fittings; reasonable design of the pipeline system and reserved expansion compensation devices can alleviate the impact of thermal stress on pipe fittings.
Although 304 stainless steel pipe fittings have a certain adaptability to use within a certain high temperature range, it is not a special high-temperature resistant material. If it is necessary to use it in a higher temperature or harsher environment, a more suitable high-temperature resistant stainless steel material should be selected, such as 310S stainless steel, whose higher chromium and nickel content gives it better oxidation resistance and strength at high temperatures; or pipe fittings made of special materials such as nickel-based alloys should be used to meet the needs of specific high-temperature working conditions.
