What is stainless steel?
Stainless steel is iron alloys with a minimum of 10.5% chromium content, Other alloying elements are added to enhance their structure and properties such as formability, strength and cryogenic toughness, used for corrosion or oxidation resistant applications, main elements added such as:
In the process of smelting stainless steel, various addition of alloy elements in stainless steel are enable to improve performance in strength and corrosion resistance.
Stainless steel classifications and grades
Stainless steels are classified several families, Austenitic stainless steel, Ferritic stainless steel, Martensitic Stainless Steel, Duplex stainless steel, Precipitation-hardening stainless steel, there are various kinds of grades, as well as chemical composition.
Stainless steel alloys are usually designated by 200 series, 300 series, 400 series, 500 series, 600 series and Nickel grades
Nickel and Chromium content are the most elements as base material, the difference between them create specific grades and properties.
Seeing left chart, there are different Nickel and Chromium chemical composition percentage in Austenitic, Ferritic, Martensitic, Duplex and Precipitation-hardening steels.
Austenitic stainless steels (Fe-Cr-Mn and Fe-Cr-Ni)
Austenitic stainless steels are the largest type and wide used alloys, they contain more than 16% chromium and 6% nickel content, including 200 series grades and 300 series grades.
- 301, 302, 303
- 304, 304L, 304H
- 316, 316L, 316H, 316Ti, 316LN
- 317, 317L, 317LN
- 321, 321H
- 347, 347H
Austenitic grades have good comprehensive performance, good weldability and formability and excellent corrosion resistance, changing and improve the performance after adding different alloy elements such as molybdenum, titanium, copper, they are non-magnetic in the solution-annealed condition.
304 is a typical 18-8 Chromium Nickel grade, as well as basic grade of Austenitic stainless steels, 304L is low carbon content 304, 316L is low carbon and addition of molybdenum element grade.Increasing and decreasing chemical element are enable to improve certain stainless steel properties, for example, reducing carbon content improve weldability, adding Molybdenum content improve corrosion performance. Various chemical elements in stainless steel affect their properties.
Ferritic stainless steels (Fe-Cr)
Ferritic stainless steels contain chromium content of 10.5-18%, adding chromium element improve the corrosion resistance, toughness and weldability, ferritic grades are magnetic due to their ferritic microstructure, bad mechanical properties and process performance compared to Austenitic stainless steels.
Common grades in Ferritic stainless steels are 430, 409, 439, 436, 441, 444 by adding different elements to the corresponding changes in its performance.
Martensitic Stainless Steels (Fe-Cr-C)
Martensitic stainless steels are improved mechanical properties by heat treatment, chromium alloy is basis element, chromium content 12–14%.
High carbon content, 0.1–1%, molybdenum, 0.2–1%, nickel, less than 2%.
The chromium content is lower than that of Ferritic stainless steels, it is also magnetic.
Common Martensitic Stainless Steel grades have 403, 410, 410S, 414, 414L, 416, 418, 420, 422, 431, 440 and 500 series
Duplex stainless steels (Fe-Cr-Ni-(Mo)-N)
Duplex stainless steels contain Ferrite and Austenite structure in its microstructure, separately containing 23% chromium, 4% nickel and 22% chromium, 5% of the nickel in addition of other alloy elements.
Duplex grades have Ferrite and Austenite stainless steels benefits, high strength and toughness, excellent corrosion resistance, good weldability and magnetic.
The common duplex grades contain 2101, 2304, 2205 and 2507, with high strength and excellent corrosion resistance.
precipitation-hardening stainless steels (Fe-Cr-Ni (Mo-Cu-Al-Nb)- N)
This group grades contain both chromium and nickel, with a high tensile strength. Matrix for the Austenitic or Martensitic crystal structure
Commonly used grade is PH 17-4, also known as 630 grade, with 17% of chromium, 4% of nickel, 4% of copper and 0.3% of niobium.
Precipitation hardening stainless steel adding strengthening elements of different composition and percentage, the process of precipitation in different composition and percentage of carbide, nitride, carbon nitride and intermetallic compounds, a kind of high strength stainless steel can increase the strength of the steel and maintain sufficient toughness.
Stainless Steel features
Stainless steel is known as corrosion resistance feature, that is most greatest difference with carbon steel, thus stainless steels are used in corrosion environment that are required.
Chromium element existed and Molybdenum additions increase corrosion resistance, to many Chromium base and Molybdenum base grades, when exposed in air, that will form protective passive film at the surface prevent rust and corrosion.
Here are main corrosion attacks:
- Pitting corrosion
- Intergranular corrosion
- Stress corrosion cracking
- Crevice corrosion
- General corrosion
Chromium content and Molybdenum content increase to improve corrosion resistance, in general, Austenitic Stainless Steels is better than Ferritic Stainless Steels, Duplex Stainless Steels perform best, in the different environment, such as air, sea water, acid, high temperature condition, stainless steel selection should meet corrosion resistance requirement.
Thought stainless steels have excellent corrosion resistance, and not easy to rust, unfortunately, the corrosion may be occur under certain condition, that will be rust and stain, internal elements and external environments are key reason for rust.
In the making process of stainless steels, due to equipment and technology reason, defective steels many be smelted, the base material itself is inferior.
When stainless steels exposed under harsh environment, it also may rust, such as strong acid, strong alkali, strong oxidizing solution condition.
Although stainless steels are low maintenance material, rust or stain should still be avoided.
Magnetic or non Magnetic
Austenitic Stainless Steels are non magnetic under heat solution condition, Ferritic stainless steels, Martensitic Stainless Steels and Duplex stainless steels are magnetic.
When it is good or bad stainless steel via judge weather magnetic or not, that is a false understanding, magnetic not means rust.
Stainless Steel finishes
Stainless steels are alloy material in good properties, like high strength and excellent corrosion resistance. There is a oxide film on at the surface of stainless steel because of Chromium content existed, and different finishes to protect surface to avoid damaging.
Mill finishes and mechanical polish are common surface types, also others, here list typical finishes for flat products complying to ASTM A480.
|NO.1||Hot rolled, annealed, pickled, dull non reflective finish|
|NO.2D||Cold rolled, annealed, pickled, smooth non reflective finish|
|NO.2B||Cold rolled, annealed, pickled, smooth reflective finish, the most wide used finish|
|BA||Smooth, bright, reflective finish by annealing in the controlled atmosphere furnace|
|NO.3||non reflective finish, light polishing using grinding belt, 100 – 120 grit|
|NO.4||Reflective dull polishing, brushed finish, using 120 – 320 grit belt after light grinding|
|NO.6||Cold rolled, dull satin finish by tampico brushing a No.4 finish|
|NO.7||High reflective finish, by a finely ground surface, 320 grit is buffed|
|NO.8||High reflective, bright polishing, smooth surface, mirror finish|
|TR||Cold worked to obtain a specified properties|
Forms and applications
Stainless steel have high strength and excellent corrosion resistance, and easy to weld and clean, safe and durable material, with various surface finished for option, it can be recycled and reused.
Stainless steel is a ideal material for various applications, mainly designed and fabricated in flat and long forms, like plate, sheet, strip, bar, wire, pipe, tube.
Chemical Processing, Pharmceutical and Petrochemical Industries
Stainless steels are used for these corrosive and high temperature environments, including stainless steel equipment and parts.
Architecture, building and construction
Stainless steels are great corrosion resistant material for transport water, contain drinking water and waste water.
Food and Beverage
Stainless steel are used in food and beverage industries because of safe and clean features, not only in the process, also in storage, they not affect food taste.
For example, fork, kitchenware, tanks are made from stainless steel, usually it requires polishing and sanitary surface.
it is used for car body structure, exhaust system and other parts, high strength and perfect appearance.
Stainless steel have bright surface, even mirror surface, after concern processing, it can be finished for decorative use, like Jewelry, ring, watch etc.
Stainless steels are ideal material to resist corrosion, and keep good mechanical properties, 316L type is a common grade to use in 3D print industries.