Stainless steel seamless pipe is an iron-based alloy wi […]
Stainless steel seamless pipe is an iron-based alloy with a mass fraction of 12% to more than 50% alloying elements. Alloying elements affect the stability of austenite, ferrite and martensite phases, thereby affecting the balance between the phases related to stability.
The elements added to the stainless steel tube can be divided into elements that form and stabilize ferrite and elements that form stable austenite. Martensite is a phase transformation product that is formed when austenite is cooled from high temperature to low temperature. If austenite is not formed at high temperatures, then martensite cannot be formed at low temperatures.
Austenitic stainless steel pipes contain high nickel content and other austenite-forming elements, which promote the formation of the austenite phase, making it stable at room temperature or even lower temperatures.
Ferritic stainless steel seamless pipes contain elements that weaken the formation of austenite, such as high chromium content, making ferrite the dominant phase component. The martensitic stainless steel tube is austenitic at high temperatures, but this austenite is suitably unstable and transforms when it is cooled. The microstructure of the stainless steel tube can be controlled by the balance between the austenite forming element and the ferrite forming element. The adjustment of the balance between the two elements plays an important role in the mechanical properties, corrosion resistance and weldability of stainless steel pipes.
The addition of aluminum, titanium, copper and molybdenum to the stainless steel seamless tube can promote the precipitation reaction and strengthen the steel. Austenitic stainless steel seamless tubes often contain titanium and aluminum to form nickel-titanium and nickel-aluminum precipitation phases, which are similar to precipitation strengthening phases in nickel-based superalloys. Aluminum is a ferrite-forming element in solid solution, while copper is a weak austenite-forming element. The precipitated phase with a composition close to pure copper can be used to strengthen martensitic steel.
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