Stainless steel pipes come in numerous grades, categorized based on their crystalline structure (microstructure) and chemical composition. These variations result in different mechanical properties and corrosion resistance levels. The main families of stainless steel grades used for pipes and tubes include:
- Austenitic Stainless Steels (300 Series and 200 Series): This is the largest and most common group. They are non-magnetic (typically), have excellent corrosion resistance, good formability, and weldability.
- 300 Series: Based on chromium and nickel additions. Common grades include 304 (general purpose), 316 (enhanced corrosion resistance due to molybdenum, often used in stainless steel pipe applications), 316L (low carbon version of 316 for better weldability), 321 (stabilized with titanium for high-temperature use), and 310 (high temperature resistance).
- 200 Series: Substitute manganese and nitrogen for some of the nickel content, making them a lower-cost alternative. Common grades include 201 and 202. They offer moderate corrosion resistance (less than 304) and good strength but can be more susceptible to certain types of corrosion.
- Ferritic Stainless Steels (400 Series): These contain chromium but very little or no nickel. They are magnetic, have moderate corrosion resistance (generally less than austenitic but better than carbon steel), good ductility, and are resistant to stress corrosion cracking. Common grades include 409 (often used in automotive exhausts) and 430 (used in decorative applications and appliances).
- Martensitic Stainless Steels (400 Series): Also primarily chromium-based but with higher carbon content than ferritic grades. They can be hardened by heat treatment, resulting in high strength and hardness. They are magnetic but have lower corrosion resistance compared to austenitic and ferritic grades. Common grades include 410 and 420, used for cutlery, surgical instruments, and valves.
- Duplex Stainless Steels (e.g., 2205): These have a mixed microstructure of austenite and ferrite (roughly 50/50). They combine some of the best properties of both: high strength (often twice that of austenitic grades), good resistance to pitting, crevice corrosion, and stress corrosion cracking, especially in chloride environments. They contain chromium, nickel, and often molybdenum and nitrogen. Used in demanding applications like chemical processing, pipe systems in oil and gas, and desalination plants.
- Precipitation-Hardening (PH) Stainless Steels: Offer a unique combination of high strength (achieved by heat treatment) and good corrosion resistance (comparable to austenitic grades). An example is 17-4 PH. Used in aerospace, nuclear, and other high-technology industries.
The choice of grade depends heavily on the specific application requirements, including the corrosive environment, temperature, pressure, required mechanical strength, weldability, formability, and budget.