Introduction to Austenitic Stainless Steel
Date:2025-05-29View:19Tags:OCTG pipe,Seamless steel pipe manufacturer,Casing and tubing
Austenitic stainless steel is the most widely used and important type in the stainless steel family, accounting for more than 70% of the total stainless steel production. It has become the preferred material in many fields such as industry, construction, medical care, food, and chemical industry with its excellent comprehensive properties, especially its unparalleled corrosion resistance, good formability, weldability, and excellent toughness.
1. Core Features: Austenite Crystal Structure
Microstructure: Its name comes from its stable face-centered cubic (FCC) crystal structure at room temperature, called "austenite". This structure is achieved by adding sufficient amounts of nickel (Ni) and other austenite stabilizing elements (such as manganese Mn, nitrogen N, and carbon C) to the steel. Nickel is the most critical element, which inhibits the transformation of steel to ferrite or martensite during cooling.
Non-magnetic: The pure austenite structure makes it usually non-magnetic or weakly magnetic in the annealed state (a small amount of magnetism may be generated after cold working). This is an important feature that distinguishes it from ferrite and martensitic stainless steels.
Cannot be hardened by heat treatment: Austenitic stainless steel cannot be hardened by quenching like martensitic stainless steel. Its main strengthening method is cold working hardening.
2. Key alloying elements and their functions
Chromium (Cr): (16-26%) The cornerstone of stainless steel corrosion resistance. Forms a dense, stable, self-healing chromium oxide (Cr₂O₃) passivation film on the surface to isolate corrosive media.
Nickel (Ni): (6-22%) Core austenite forming element.
Stabilizes the austenite structure and ensures good low-temperature toughness (even to cryogenic temperatures).
Significantly improves corrosion resistance in reducing media, organic acids and alkaline media.
Improves formability and weldability.
Reduces the tendency to cold working hardening (relative to nickel-free or low-nickel austenitic-manganese steels).
Carbon (C): (<0.08%-0.15%) Improves strength, but too high will lead to:
Sensitization: In the temperature range of 425-815°C (such as the heat affected zone of welding), carbon combines with chromium to form chromium carbide (Cr₂₃C₆) that precipitates along the grain boundaries, causing chromium depletion near the grain boundaries and inducing intergranular corrosion. To solve this problem, the following were developed:
Low carbon grades (type L): such as 304L, 316L (C ≤ 0.03%), which significantly reduce the tendency of chromium carbide precipitation and improve the resistance to intergranular corrosion, especially for welded parts.
Stabilized grades: Add strong carbide-forming elements titanium (Ti) or niobium (Nb), such as 321 (Ti stabilized), 347 (Nb stabilized). They preferentially combine with carbon to form TiC or NbC, preventing the consumption of chromium and thus resisting sensitization.
Molybdenum (Mo): (2-7%) Key element that significantly enhances corrosion resistance.
Greatly improves pitting and crevice corrosion resistance in chloride-containing environments (such as seawater, salt spray, de-icing salt).
Improves corrosion resistance in reducing acids (such as sulfuric acid, phosphoric acid, organic acids).
Typical grades: 316 (2-3% Mo), 316L, 317 (3-4% Mo), 317L, super austenite (such as 904L, 254SMO - containing 6-7% Mo).
Nitrogen (N): An important alloying element.
A strong austenite-forming element that can partially replace expensive nickel (used in 200 series stainless steel).
Significantly improves strength and pitting resistance (increases PREN value - pitting equivalent).
Helps stabilize austenite and inhibits the precipitation of harmful intermetallic phases.
Commonly found in high alloy austenitic grades (such as 201, 202, 304LN, 316LN, 904L, super austenite).
Manganese (Mn): Austenite forming element.
Partially replaces nickel in 200 series stainless steel (such as 201, 202) to reduce costs.
Increase the solubility of nitrogen in steel.
Improve hot workability.
Copper (Cu): Sometimes added.
Improve corrosion resistance in reducing acids (especially sulfuric acid).
Improve cold formability.
Typical grade: 904L.
