Steel is an alloy of iron. Since the primary element used in the alloying process, other than iron itself, is carbon, steel is defined by its carbon content. Steel can have anywhere between 0.002% and 2.1% carbon by weight, but always contains smaller amounts of the elements manganese, phosphorus, sulphur, silicon, and traces of oxygen, nitrogen and aluminium in addition to carbon and iron. While these elements are required for an alloy to be accurately referred to as ‘steel’, others such as manganese, nickel, chromium, molybdenum, boron, titanium, vanadium and niobium can be added to modify the characteristics of the alloy. This article will focus on stainless steel, which is formed by creating a steel alloy containing a minimum of 10.5% chromium by mass.
How does stainless steel differ from regular steel?
Under conditions of high moisture and high salinity, the iron found in steel will oxidise to form iron oxide. A layer of iron oxide on the surface of the steel is able to increase the rate of its own formation, thus encouraging more corrosion to occur, and causing the structure of the steel to be compromised. Stainless steel contains a sufficient proportion of chromium to facilitate the formation of a layer of chromium oxide on the surface of the material. This layer of chromium oxide does not catalyse the formation of more chromium oxide, and acts to protect the iron within the structure from oxidation, a property referred to as ‘passivation’. This property of stainless steel makes it incredibly corrosion resistant, although the lustre of the material can be lost under certain conditions, so it is not truly ‘stainless’.
Grades of stainless steel
Since steel can be alloyed with an infinite number of combinations of alloying elements, a classification system has been developed to facilitate easy identification of the compounds used. Stainless steel has its own list of classifications, each of which will be explained in outline below. Note that some of the types of stainless steel are described as ‘austenitic’, meaning that they are composed of the gamma allotrope of iron:
- Type 102 – an austenitic, general purpose stainless steel widely used in furniture.
- 200 Series – austenitic chromium-nickel-manganese alloy series used for general purpose metalwork.
- 300 Series – austenitic chromium-nickel alloy series. Contains the most popular grade of stainless steel, grade 304. This series of stainless steel is used for food utensils and surgical scrub sinks and instruments.
- 400 Series – ferritic and martensitic chromium alloy series. This series contains some of the cheapest stainless steel, used for car exhausts, some higher grade cutlery steel, and some of the hardest stainless steel, used for replica swords and knives.
- 500 Series – heat-resistant chromium alloy series.
- 600 Series – martensitic alloy series formed by precipitation hardening.
- Type 2205 – a duplex (both ferric and austenitic) alloy with excellent strength and corrosion resistance.
- Type 2304 – an alloy similar to type 2205 in all respects except for a lower molybdenum content, leading to reduced pitted corrosion resistance.
300 Series Stainless Steel Grades
Austenitic Chromium-Nickel Alloys| Grade | Description |
|---|---|
| Type 301 | Highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304. |
| Type 302 | Same corrosion resistance as 304, with slightly higher strength due to additional carbon. |
| Type 303 | Easier machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with International Organization for Standardization ISO 3506 |
| Type 304 | The most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with International Organization for Standardization ISO 3506. |
| Type 309 | Better temperature resistance than 304 |
| Type 316 | The second most common grade (after 304); for food and surgical stainless steel uses; Alloy addition of molybdenum prevents specific forms of corrosion. Also known as "marine grade" stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of this grade. Rolex is an exception in that they use Type 904L. 18/10 stainless often corresponds to this grade. Also referred to as "A4" in accordance with International Organization for Standardization ISO 3506. |
| Type 321 | Similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding. |
400 Series Stainless Steel Grades
Ferritic and Martensitic Chromium Alloys| Grade | Description |
|---|---|
| Type 408 | heat-resistant; poor corrosion resistance; 11% Chromium, 8% Nickel. |
| Type 409 | cheapest type; used for automobile exhausts; ferritic (iron/chromium only). |
| Type 410 | martensitic (high-strength iron/chromium). Wear resistant, but less corrosion resistant. |
| Type 416 | easy to machine due to additional sulfur |
| Type 420 | "Cutlery Grade" martensitic; similar to the Brearley's original "rustless steel". Also known as "surgical steel". Excellent polishability. |
| Type 430 | decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance. |
| Type 440 | a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat treated properly. It can be hardened to Rockwell 58 hardness, making it one of the hardest stainless steels. Also known as "razor blade steel". Available in three grades 440A, 440B, 440C (more common) and 440F (free machinable). |
304 Grade stainless steel
The most common type of stainless steel in use is grade 304. This grade has an 18-20% chromium content and a 8-10.5% nickel content by mass, although 304L exists as a low-carbon alternative to 304, and 304Cu and 304N versions, enriched in copper and nitrogen respectively, also exist. This type of steel is also often referred to as A2 stainless steel. Grade 304 offers an impressive strength, as well as incredible corrosion resistance, lending it to applications in which contact with food, chemicals or fresh water must be tolerated. Type 304 stainless steel also has properties which make it well suited to welding and machining applications, thereby making it an excellent material for use in making machined parts.
316 Grade stainless steel
The second most common type of stainless steel is 316. The physical and mechanical properties are very similar to 304 stainless steel, but the key difference is that 316 stainless steel incorporates about 2-3 percent molybdenum. Molybdenum has the ability to protect against chlorides and other industrial solvents, which gives the steel a stronger corrosion resistance.
Duplex Stainless Steel
Duplex stainless steels come with numerous benefits compared to other stainless steel alloys. For instance, they provide high levels of yield and impact strength, along with excellent corrosion resistance, the ability to hold up in temperatures ranging from -50° C to 250° C, and optimal weldability and machinability. They’re also more cost-effective than other more expensive alloys.
Duplex stainless steels feature an even combination of ferritic and austenitic microstructures. The alloy’s microstructure defines the atoms’ crystal structure, which is crucial in determining the alloy’s specific properties. While 3xx and 4xx stainless steels contain solely austenitic or ferritic structures respectively, duplex stainless steels feature both, combining the best features of each structure.
Duplex and super duplex stainless steels
Based on their specific needs, different applications can use either duplex or super duplex stainless steel alloys.
Duplex stainless steels
Pulp and paper manufacturing applications first used duplex stainless steels in the 1930s, replacing 3xx grades to provide more corrosion resistance and strength. Originally, the designation for duplex stainless steels was UNS S31803 (F51), but later enhancements made to increase corrosion resistance led to a re-designation of UNS S32205 (F60).
Super duplex stainless steels
Off-shore oil and gas exploration and other developments requiring improved alloys led to the creation of super duplex stainless steels. Specifically, Ferralium came first in 1967 as the original super duplex stainless steel. Following the launch of this alloy, Alloy 32760 and Alloy 32750 came along with varying compositions.
Applications for duplex and super duplex stainless steels
Many applications use duplex and super duplex stainless steel alloys. The following are some of the most common applications.
Oil & Gas
Downhole tooling, subsea and well equipment, pumps, and valves use super duplex stainless steels. H2S-containing environments such as sour service wells also frequently use these materials.
Bolts and Fasteners
Many bolts and fasteners feature super duplex stainless steel construction. This is due to the material’s high strength and ease of hardening for added strength.
Pollution Control Scrubbers
Pumps, fans, and precipitators often use super duplex stainless steels. They offer superior corrosion resistance in systems using seawater and acids.
Marine Applications
Shafts, seals, rudders, and propellers frequently use super duplex stainless steel. Specifically, they will use this metal if austenitic stainless steels such as XM-19 are incompatible with an application.
Chemical Process Industry
Chemical processes often use phosphoric, sulphuric, and nitric acid during production. Super duplex stainless steels offer the wear, corrosion, and abrasion resistance needed in these applications.
Vegetable Processing
Equipment used for processing vegetables and grains often features duplex stainless steels. Ferralium 255 offers reliable resistance to wear and corrosion in sugar cane processing, centrifuges, and mixers.
Water Treatment
Some water treatment applications for super duplex stainless steel include swimming pools, desalination, and sewage treatment. Many of these environments use seawater, brackish, or contaminated solutions that require sufficient corrosion resistance.
Paper and Pulp
Components used to produce pulp and paper are often made with super duplex stainless steels. While duplex and lean duplex alloys are sometimes used, areas at a greater risk of failure typically use super duplex alloys.
Pump Shafts
Super duplex stainless steels often appear in pump shaft components. The popularity of these alloys in these applications comes down to the material’s combined strength and resistance to wear and corrosion.