Stainless Steel Types or 'Families'
There are five main types or 'families' of Stainless Steel, they are primarily classified by their crystalline structure. Below is a detailed explanation of each type.
1. Austenitic stainless steel
Austenitic stainless steel is the largest family of stainless steels, comprising of about two-thirds of all stainless steel production worldwide. Austenitic alloys possess an austenitic microstructure, which is a face-centered cubic crystal structure. This microstructure is achieved by alloying with sufficient nickel and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures from the cryogenic region to the melting point. Due to this, austenitic stainless steel alloys are not hardenable by heat treatment as they possess the same microstructure at all temperatures.
Thinner sheets and small bars can be strengthened by cold working methods. Their austenitic microstructure provides excellent formability and weldability and they are essentially non-magnetic and maintain their ductility at cryogenic temperatures.
They can be further subdivided into two sub-groups, 200 series and 300 series:
- 200 Series alloys are chromium/manganese/nickel varieties, which aim to maximise the use of manganese and nitrogen in order to minimise the use of nickel. Due to the addition of nitrogen, they possess approximately 50% higher yield strength than 300 series stainless steel alloys. Type 201 stainless steel is ‘hardenable’ through cold working while Type 202 is a general purpose stainless steel. It must be noted that decreasing nickel content and increasing the manganese content results in a weak resistance to corrosion.
- 300 Series alloys are chromium/nickel alloys, which achieve their austenitic microstructure almost exclusively by alloying with nickel. Some grades with very high nickel content include some nitrogen to reduce the nickel requirements. The 300 series of stainless steel is the largest group and the most widely used. The best known grade is Type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8%/10% nickel, respectively. The second most common austenitic stainless steel is Type 316. The addition of 2% molybdenum provides greater resistance to acids and localised corrosion caused by chloride ions.
Low-carbon versions such as 316L or 304L are used to avoid corrosion problems caused by welding. The “L” means that the carbon content is “Low” classified as being below 0.03%.
2. Ferritic Stainless Steel
Ferritic stainless steel alloys possess a ferrite microstructure just like carbon steel. A ferrite microstructure is a body-centered cubic crystal structure and contains between 10.5% and 27% chromium with very little to no nickel present. Due to the chromium addition, this microstructure stays present at all temperatures which just like austenitic stainless steels described above, are not hardenable by heat treatment. However, ferritic stainless steels cannot be strengthened by cold work to the same degree as austenitic stainless steels. Ferritic stainless steels are magnetic like carbon steel.
It must be noted that electrical resistant ferritic grades (Fr-Cr-Al) are not included in these groups as they are designed for oxidation resistances at elevated temperatures.
3. Martensitic Stainless Steel
Martensitic stainless steel alloys provide a wide range of properties and are used mainly as stainless engineering steels, stainless tool steels and creep resisting steels.
They fall into four categories with some overlapping:
- Fe – Cr – C grades: They were the first grades used in engineering and wear-resistant applications. They are still widely used today.
- Fe-Cr-Ni-C grades: In these grades, some of the Carbon is replaced by Nickel. They offer increased toughness and corrosion resistance.
- Precipitation Hardening grades: Grade EN 1.4542 (a.k.a. 17/4 Precipitation Hardening), the most well known grade, combines both martensitic hardening and precipitation hardening. It achieves high strength and good toughness and is used in various applications including aerospace.
- Creep-resisting grades: small additions of Nb, V, B, Co increase the strength and creep resistance to about 650 °C.
Heat Treatment of Martensitic Stainless Steels
Martensitic stainless steels are a family of stainless steels that can be heat-treated to provide the required level of mechanical properties.
The heat treatment process typically involves three steps:
- Austenitising – A process in which the steel is heated to a temperature in the range of 980 – 1050 °C. The temperature used depends on the grades of the steel. The austenite is a face centered cubic phase.
- Quenching – The quenching process involves the rapid cooling of the steel in air, oil or water. In this process, the austenite is transformed into martenisite, a hard a body-centered crystal structure tetragonal in shape. The as-quenched martensite is extremely hard and far too brittle for use in most applications and in some cases some residual austenite may remain.
- Tempering – Tempering involves the heating of the steel to around 500 °C then holding the steel at this temperature before air cooling. It is important to note that increasing the tempering temperature will decrease the yield and resulting tensile strength of the steel but also increases the elongation and the impact resistance of the material.
Nitrogen-alloyed Martensitic Stainless Steels
Replacing some of the Carbon in martensitic stainless steels by Nitrogen is still regarded as a fairly recent development in stainless steel manufacturing. The limited solubility of Nitrogen has been increased by the Pressure Electroslag Refining process (PESR) whereby melting is carried out under a high nitrogen pressure. Up to 0.4% Nitrogen contents has been achieved resulting in increased hardness/strength and higher corrosion resistance properties. Due to the PESR process being more expensive, lower yet significant Nitrogen contents have still been achieved using the standard Argon Oxygen Decarburisation (AOD) process.
Nitrogen-alloyed martensitic stainless steels are magnetic and due to their lower chromium content, are not as resistant to corrosion as the common ferritic and austenitic stainless steels.
4. Duplex Stainless Steel
Duplex stainless steels are unique in that they have a mixed microstructure consisting of austenite and ferrite with the aim usually being to produce a equal 50%/50% mix. However, in most commercial alloys the ratio may be closer to 40%/60%. Duplex stainless steels are characterised by high chromium (19–32%) and molybdenum (up to 5%) content and a lower nickel content than standard austenitic stainless steels. Duplex has approximately double the strength when compared to austenitic stainless steel varieties and their mixed microstructure provides improved resistance to chloride stress corrosion cracking when compared to austenitic stainless steels such as Types 304 and 316, making them a good candidate for water piping applications.
The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades of stainless steel. This makes their use far more cost-effective for many different applications, particularly in water pipes and fittings. Duplex stainless steel grades are grouped and characterised based on their corrosion resistance properties and alloy content.
5. Precipitation Hardening Stainless Steel
Precipitation hardening stainless steels have corrosion resistance properties that are comparable to austenitic alloys but can be hardened by precipitation to even higher strengths than the other martensitic grades.
PH alloys are based around the following three types:
– 17-4 PH – this alloy contains about 17% Chromium, 4% Nickel, 4% Copper and 0.3% Niobium.
Type 17-4 PH stainless steel is the most widely used alloy of all the PH stainless steels. Its unique and valuable combination of properties provides designers with opportunities to add reliability to their products while often reducing costs. The alloy is a martensitic stainless steel that provides an superb combination of high strength, good corrosion resistance, and good mechanical properties even when used at temperatures up to 316°C. This unique combination of properties make this alloy a very effective solution to many design and production issues.
– 17-7 PH – this alloy contains approximately 17% Chromium, 7.2% Nickel and 1.2%Aluminium.
Type 17-7 PH stainless steel in sheet form provides a valuable combination of properties that are particularly well suited for aerospace applications. Additionally, this unique alloy provides benefits for other applications that require high strength with good corrosion resistance. The alloy also has excellent properties for flat springs up to 316°C. The 17-7PH alloy provides high strength and hardness with superb fatigue properties, great corrosion resistance and minimal distortion on heat treatment.
– A286 – this alloy has a typical analysis of 15% Chromium, 25% Nickel, 2.1% Titanium, 1.2% Molybdenum, 1.3% Vanadium and 0.005% Boron.
Type A286 alloy is an iron-based ‘superalloy’ useful for applications requiring high strength and corrosion resistance at temperatures up to 704°C. Applications may include jet engines, gas turbines, turbo parts, etc. Type A286 alloy is a heat and corrosion resistant austenitic material that can be age hardened to a rather high strength level. Additionally, the alloy is also used for low temperature applications requiring a ductile, non-magnetic high strength material at temperatures ranging from above room temperature down to at least -196°C. The alloy may also be used for moderate corrosion applications in aqueous solutions.
The designation “CRES” is used in various industries to refer to corrosion-resistant steel. Most mentions of CRES refer to stainless steel, although the correspondence is not absolute, because there are other materials other than stainless steel that are also corrosion-resistant.