To create an initial protective or passive property against corrosion, a minimum of 11% chromium is added to steel, and to improve corrosion resistance, mechanical properties, and more, other elements are also added. The different effects of these elements on the microstructure give rise to six groups of stainless steel as described below:

1- Ferritic Group: with a composition range of C<0.1%, Ni=0-4%, Cr=11-30%

2- Martensitic Group: with a composition range of Ni=0-6%, Cr=11-18%

3- Austenitic Group: with a composition range of C<0.15%, Ni=8-20%, Cr=15-27%. Alloys with higher nickel content are considered super austenitic alloys and usually fall into this category.

4- Duplex Group: with a composition range of C<0.08%, Ni=4-7%, Cr=18-27%, and some additional elements like N and Cu.

5- Precipitation Hardening Group: with a composition range of C<0.15%, Ni=3-27%, Cr=12-28%, and some additional elements like N, Cu, B, Ti, Al.

6- Cast Group

The first four groups are classified based on microscopic structure, while the fifth group is identified based on heat treatment type.

The sixth group includes a collection of alloys from the five main groups produced via casting methods. In all of these groups, other elements such as phosphorus, silicon, sulfur, manganese, niobium, titanium, and molybdenum may be present as impurities or added to improve specific properties. Standards define the allowable limits for these elements.

• Ferritic Group

Ferrite is the primary phase of iron alloys and has a BCC crystal structure.

The mechanical strength of ferritic stainless steel is higher than that of plain low-carbon steels, but its ductility and formability are slightly lower, though still adequate. However, their mechanical strength at high temperatures and toughness at low temperatures, especially in thick sections, are much lower compared to the austenitic group.

In ferritic stainless steel (similar to martensitic and duplex stainless steel), temper embrittlement can occur. This phenomenon arises during tempering heat treatments in the 400 to 550°C range. Therefore, when high toughness is required, these steels should not be used or tempered in this temperature range. Additionally, the grain size of ferritic steels becomes coarse after welding, reducing their mechanical properties.

In general, in terms of general corrosion resistance, ferritic stainless steels rank below the austenitic and martensitic groups. Depending on the chromium content, corrosion resistance can range from poor to excellent. Unlike the austenitic group, however, ferritic steels are more resistant to stress corrosion cracking. Reducing carbon and nitrogen content significantly increases their corrosion resistance. Low-chromium ferritic alloys (about 11%), such as 409 and 405, have minimal corrosion and oxidation resistance but are widely used in applications requiring low corrosion resistance due to their low cost and ease of production. Medium-chromium ferritic alloys (16 to 18%), such as 430, are resistant to weak oxidizing acids and organic compounds and are used in automotive parts and kitchen utensils, though they have welding limitations.

The 430 alloy is the leader of this family, with 17% chromium offering good resistance to atmospheric conditions and some chemicals (especially oxidizing acids). This alloy is a suitable alternative to expensive austenitic alloys in certain applications. It has good formability and is used in nitric acid tanks, annealing baskets, and decorative parts.

Alloys 430F and 430FSe were developed for improved machinability of alloy 430 by adding sulfur or selenium. They are used in parts like screws, where cold heading and machining are required.

2- Martensitic Group

The alloying elements in this group are chromium and carbon. Martensite is another phase of iron. Due to the interstitial carbon in its atomic lattice, martensite is extremely hard. Martensitic alloys are ferromagnetic and heat-treatable, with high mechanical strength, hardness, fatigue resistance, and creep strength, but their usage temperature is limited to 650°C. High-carbon types are not recommended for welding.

The chromium content in martensitic alloys usually ranges from 11 to 18%. The lower limit of chromium determines corrosion resistance, while the upper limit defines the possibility of transforming the entire structure into austenite during heat treatment, as chromium promotes ferrite and narrows the stability zone for austenite.

The carbon content in martensitic alloys can reach up to 1.2%. Increasing carbon content up to 0.6% increases hardness, but beyond this limit, only wear resistance improves due to the formation of chromium carbides.

Other alloying elements, such as niobium, silicon, and vanadium, are added to refine the microstructure after quenching. A small amount of nickel is added to improve toughness and corrosion resistance. High chromium content improves corrosion resistance, and nickel reduces free ferrite, refining the structure.

The corrosion resistance of martensitic alloys is generally lower than that of many austenitic and ferritic alloys but is still moderate overall.

Alloy 410 is the most widely used alloy in the martensitic group. In the United States, this alloy is widely used in steam turbines, jet engines, gas turbines, general applications, machinery parts, and pump shafts.

3- Austenitic Group

Austenite is one of the phases of iron with an FCC crystal structure, stable in pure iron without alloying elements between 910°C and 1400°C. Adding austenite-stabilizing elements (such as Ni) can stabilize it at room temperature. The presence of 8 to 10% nickel with 18% chromium stabilizes the steel structure as austenitic. This phase transformation significantly changes mechanical properties, including increased ductility. These alloys are non-magnetic in the annealed state due to the presence of austenite, but they may develop slight magnetic properties during cold working due to phase changes.

Austenitic alloys contain chromium (16 to 26%) and nickel as the primary elements. In some alloys (2XX series), a portion of the nickel is replaced by manganese (4 to 15%) and nitrogen to reduce costs and increase strength.

Nitrogen is also used to improve mechanical strength at low temperatures, slow down chromium carbide formation, and increase pitting and crevice corrosion resistance (with the help of molybdenum). Other alloying elements, such as molybdenum, copper, silicon, aluminum, titanium, and niobium, are added to enhance specific properties like resistance to pitting and chloride ion corrosion or oxidation resistance.

Cold rolling gives this group good strength. Their low carbon and sulfur content also make them weldable. Adding new elements improves quality and meets the desired properties of the alloy. For example, adding titanium to alloy 304 transforms it into 321, another austenitic alloy. Another commonly used alloy in this group is 316, which contains 2% molybdenum, improving its resistance to chlorine corrosion. Increasing chromium content yields alloy 310, which has high resistance at elevated temperatures. The main weakness of this group is their susceptibility to chloride stress corrosion cracking.

Properties of Austenitic Group:

Non-magnetic, ductile, lower mechanical strength compared to other stainless steel groups, excellent formability, heat resistance up to 475°C, high corrosion resistance, hardenable

Overall, austenitic stainless steels have higher corrosion resistance than other stainless steel groups, making them more widely used. However, unlike ferritic steels, they are prone to stress corrosion cracking. High-nickel austenitic alloys containing molybdenum (about 6%) and nitrogen (0.15 to 0.25%) have excellent pitting corrosion resistance and good oxidation resistance at high temperatures.

Numerous austenitic alloys exist, classified under the 2XX and 3XX series and other trade names. The most well-known alloy is the (8-18) 302, and other alloys are derived from modifications of this base alloy.

2XX Series Alloys

In the 2XX series, such as 201, 202, 205, 216, 218, 219, etc., some of the expensive nickel is replaced by manganese and nitrogen, making these alloys more economical for applications requiring less corrosion resistance than the 3XX series. Their applications include kitchenware, piston rings, decorative parts, tanks, and material handling equipment.

3XX Series Alloys

The 3XX austenitic group is divided into four classes.

Class A: This group includes alloys 301, 302, 303, 303Se, 304, 304L, 304N, 321, 347, and 348, with slight differences in their compositions...