Stainless steel is the name given to high-alloy steels used primarily for their corrosion resistance. The defining feature of the stainless steel family is that every grade contains at least 10.5% chromium. This allows the steel to withstand corrosive atmospheres and aggressive chemical environments. There are around 60 grades of stainless steel divided into five categories. Although stainless steel is more expensive than carbon and alloy steel and is used in smaller volumes worldwide, it holds a very important place in global markets.
The History of Stainless Steel
Stainless steel was discovered in 1913 by the English metallurgist Harry Brearley. During an experiment he observed that raising the chromium content of steel to 12% or more increased its resistance to corrosion. Brearley is regarded as the founder of stainless steel and contributed to the development of many grades. Development stalled during the First World War, but by the late 1920s martensitic and austenitic stainless steels had been recognized as the most useful types. Martensitic stainless steels contain 13% to 18% chromium, while the austenitic group contains 18% chromium and 8% nickel. Today the term stainless steel refers to steel containing a minimum of 10.5% chromium with high corrosion resistance. Chromium forms a self-repairing passive layer on the surface of the steel, which is the key factor preventing rust. Stainless steel continues to evolve and is divided into five main groups:
- Austenitic
- Ferritic
- Martensitic
- Duplex
- Precipitation-hardening alloys
Main Categories of Stainless Steel
300 series, austenitic alloys: grades 301, 302, 303, 304, 305, 308, 309, 310, 314, 316 and 321 contain chromium and nickel. They cannot be hardened by heat treatment and are non-magnetic. They have excellent formability. The 300 series, or austenitic stainless steels, account for roughly 70% of world stainless steel production. The austenitic structure is obtained with an addition of about 8–10% nickel; nickel alone, however, does not create it, and other elements such as manganese, nitrogen, carbon and copper also help stabilize the austenitic phase. Corrosion resistance is further improved by adding molybdenum. These grades are used in interior design, machinery manufacturing, home appliances, the chemical industry and sink production.
400 series, ferritic alloys: grades 405, 409, 429, 430, 434, 436, 442 and others contain chromium. They are magnetic but cannot be hardened by heat treatment. Ferritic stainless steels generally contain no nickel and a high proportion of chromium (12% to 30%), together with carbide-forming and ferrite-stabilizing elements such as molybdenum, titanium and vanadium. Their high chromium content gives ferritic grades very good corrosion resistance. Their mechanical and physical properties are closer to those of their relatives, the carbon steels; unlike austenitic grades they are magnetic, they cannot be hardened by heat treatment because of their low carbon content, and they roll easily. Annealing is the only heat treatment applicable to these steels. They are less suitable for forming than austenitic grades.
| Grade | Symbol | Chemical Composition (%) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| C | Si | Mn | P | S | Ni | Cr | Others | ||
| 301 | STS301 | ≤0.15 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 6.00~8.00 | 16.00~18.00 | - |
| 301L | STS301L | ≤0.030 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 6.00~8.00 | 16.00~18.00 | N less than 0.20 |
| 304 | STS304 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 8.00~10.50 | 18.00~20.00 | - |
| 304L | STS304L | ≤0.030 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 9.00~13.00 | 18.00~20.00 | - |
| 304J1 | STS304J1 | ≤0.08 | ≤1.70 | ≤3.00 | ≤0.045 | ≤0.030 | 6.00~9.00 | 15.00~18.00 | Cu 1.00~3.00 |
| 305 | STS305 | ≤0.12 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 10.50~13.00 | 17.00~19.00 | - |
| 316 | STS316 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 10.00~14.00 | 16.00~18.00 | Mo 2.00~3.00 |
| 316L | STS316L | ≤0.030 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 12.00~15.00 | 16.00~18.00 | Mo 2.00~3.00 |
| 321 | STS321 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.030 | 9.00~13.00 | 17.00~19.00 | Ti more than 5×C% |