The austenite phase has a face-centered cubic (fcc) crystal structure, the ferrite phase has a body-centered cubic (bcc) crystal structure, and the martensite phase has a body-centered tetragonal (bct) crystal structure.
Stainless steels are divided into five primary subgroups or families depending on their crystal structure and strengthening mechanism. The difference between the families is fundamental on an atomic level. For example, the arrangement of atoms in the ferrite crystal is different from the one in the austenite crystal.
In ferritic stainless steels, the iron and chromium atoms are arranged on the corners of a cube and in the center of that cube. In austenitic stainless steels, the atoms, typically iron, chromium, and nickel are arranged on the corners of the cube and in the center of each of the faces of the cube. In molybdenum-bearing stainless steels, the molybdenum atoms would also occupy these locations.
This seemingly small difference in crystal structure profoundly affects the mechanical properties of these steels and, in general, the alloys in each stainless steel family have similar physical properties, which can vary greatly between the various families.
The austenitic family of stainless steels is characterized by having a microstructure that is either entirely or predominately austenite phase, which is a non-magnetic face-centered cubic phase. Some of the austenitic stainless steels also contain a small amount of ferrite phase, which is a ferro-magnetic body-centered cubic phase. The compositions of the austenitic stainless steels are adjusted to produce the desired microstructure or properties.
As a family, austenitic stainless steels tend to have a relatively low yield strength, high work hardening rate, high tensile strength, good ductility, and excellent low temperature toughness. They also tend to be more susceptible to chloride stress corrosion cracking compared to ferritic and duplex stainless steels. Austenitic stainless steels have good weldability and are readily fabricated into complex shapes. This family of stainless steels cannot be hardened or strengthened by heat treatment but can be strengthened by cold forming or work hardening (see ASTM A666).
The ferritic family is characterized by having a microstructure that consists of ferrite phase and possibly small amounts of carbides and nitrides. Because the ferrite phase is ferromagnetic this group of stainless steels has a magnetic attraction similar to carbon steels.
Ferritic stainless steels tend to have higher strength and much better resistance to chloride stress corrosion cracking than the austenitic grades; however, they do have reduced formability and weldability. The ferritic grades cannot be hardened or strengthened by heat treatment. They have limited toughness compared to other types of stainless steels. The toughness can be further reduced by a large grain size and thicker cross section. Because of the toughness limitations, the ferritic grades are only produced as thinner sheet and strip products. It is rare to find products thicker then 4 mm (0.150″), which restricts their use to tubing and other thin-gage applications.
The duplex family is called “duplex” because it has a two-phase microstructure consisting of ferrite and austenite. Wrought duplex stainless steel products typically have an austenite/ferrite phase balance of 50-55 volume-% austenite (45-50% ferrite). The duplex microstructure gives this stainless steel family a desirable combination of properties, including relatively high strength, good wear resistance, good toughness, and improved chloride stress corrosion cracking resistance. The duplex grades are use commonly for applications that can take advantage of their high yield strengths such as pressure vessels, tanks and structural members.
The martensitic family is characterized by having a microstructure that consists predominately of the martensite phase and possibly lesser amounts of secondary phases such as ferrite, austenite, and carbides. This family has the capability of being strengthened by heat treatment. At elevated temperatures 1040°C (~1900°F), this family of stainless steels has an austenitic structure that can be transformed to a highly strained body-centered tetragonal phase, martensite, when cooled to room temperature. In order to achieve a suitable combination of high strength (high hardness) and toughness, steels hardened by the martensite transformation must be tempered at lower temperatures of 100 to 700°C (200 to 1300°F).
Martensitic stainless steels have high strength, good wear resistance, low toughness, and a relatively high ductile-to-brittle transition temperature. They are very difficult to weld and typically require a post-weld heat treatment. Because of this, the martensitic grades are often restricted to non-welded applications such as shafts or fasteners.
The precipitation hardened family of stainless steels can be strengthened by heat treatment. The defining characteristic of this family is that they all rely on a precipitation mechanism for some or most of their strength. An age-hardening heat treatment is used to produce fine intermetallic precipitates that provide increased strength. Because of their higher chromium and molybdenum levels compared the martensitic grades, the precipitation hardened grades have better corrosion resistance than the martensitic grades and are used for high-strength applications that require better corrosion resistance.
Molybdenum is a ferrite former
The chemical composition of a stainless steel is adjusted to produce the desired microstructure and properties. This is done by maintaining a balance between elements that stabilize the austenite phase and elements that stabilize the ferrite phase. The role of the various alloying elements is summarized below.
Molybdenum is a ferrite former. That means that when molybdenum is added to improve the corrosion resistance of an austenitic stainless steel, there has to be an austenite former such as nickel or nitrogen added in order to keep the structure austenitic.
Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure. This is achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel.