Specialized Terms of Stainless Steel
Specialized Terms of Stainless Steel
In layman’s terms, “stainless steel” refers to steels that are not prone to rusting. In fact, some stainless steels possess not only stainlessness but also corrosion (acid) resistance. The stainless and corrosion-resistant properties of stainless steel stem from a chromium-rich oxide film (passive film) formed on its surface. These properties are relative, not absolute. Experiments indicate that in weak media such as air or water, or in oxidizing media like nitric acid, the corrosion resistance of a steel increases as chromium content increases. When chromium reaches a certain threshold, the steel’s corrosion behavior undergoes a sudden transition—from easily rusting to relatively non-rusting, from non-corrosion-resistant to corrosion-resistant.
Austenitic Stainless Steel
At room temperature, these stainless steels possess austenitic microstructure. When the steel contains about 18 % Cr, 8–10 % Ni, and ~0.1 % C, the austenitic structure is stable. Austenitic chromium-nickel stainless steels include the well-known “18Cr-8Ni” steel and advanced high Cr–Ni steels with additions of Mo, Cu, Si, Nb, Ti, etc. Austenitic stainless steels are nonmagnetic and have high toughness and ductility, but relatively low strength; they cannot be hardened by phase transformation and must be strengthened by cold work. If elements such as S, Ca, Se, Te are added, good machinability (free-cutting) can be achieved. Besides oxidation resistance, if they contain Mo, Cu, etc., they also resist corrosion from sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. If the carbon content is kept below ~0.03 % or stabilized by Ti or Nb, their resistance to intergranular corrosion is significantly improved. High-silicon austenitic stainless steels may have excellent corrosion resistance to concentrated nitric acid. Because austenitic stainless steels offer a well-balanced set of properties, they are widely used across many industries.
Ferritic Stainless Steel
These are stainless steels that, in service, have a ferritic microstructure. Chromium content is usually in the range of 11 % to 30 %, and they have a body-centered cubic (BCC) crystal structure. These steels generally contain no nickel, and sometimes small amounts of Mo, Ti, Nb, etc. They have high thermal conductivity, low thermal expansion coefficient, good oxidation resistance, and good resistance to stress corrosion. However, their drawbacks include relatively poor ductility and a significant drop in ductility and corrosion resistance after welding. The advent of outside-furnace refining (such as AOD or VOD) allows significant reduction of interstitial elements like C and N, which helps to broaden their applications.
Austenitic–Ferritic (Duplex) Stainless Steel
This is stainless steel whose microstructure is roughly half austenite and half ferrite. Under low carbon content, Cr content is ~18 %–28 %, Ni ~3 %–10 %. Some grades also include Mo, Cu, Si, Nb, Ti, N, etc. This type combines advantages of both austenitic and ferritic steels: compared to ferritic types, it has better ductility, toughness, no brittle behavior at room temperature, and improved intergranular corrosion and weldability. Compared to austenitic types, it has higher strength and improved resistance to chloride stress corrosion and intergranular corrosion. Duplex stainless steels also typically have good pitting corrosion resistance, and may reduce nickel usage (“nickel-saving stainless steel”).
Martensitic Stainless Steel
These are stainless steels whose mechanical properties can be adjusted by heat treatment (hardening + tempering). Put simply, they are the “hardenable” stainless steels. Typical grades are “Cr13” types, such as 2Cr13, 3Cr13, 4Cr13, etc. After quenching, they may reach high hardness; different tempering temperatures yield different combinations of strength and toughness. They are used in turbine blades, cutlery, surgical instruments, etc. According to chemical composition, martensitic stainless steels can be divided into martensitic chromium steels and martensitic chromium-nickel steels. Depending on microstructure and strengthening mechanism, they can be further classified into martensitic, semi-austenitic (or semi-martensitic) precipitation hardening, or martensitic aging stainless steels.