Stainless steel is called “stainless” because of its resistance to rust. Steel contains iron and carbon in certain percentages: adding the element chromium transforms it from steel to stainless steel. Chromium oxidizes much faster than the iron in the alloy. The resulting chromium oxide seals and protects the rest of the metal in a process called passivation. Starting with these three basic elements, metallurgists have created a huge range of stainless steels. Types of metals in the alloy, alloying instructions, heat treatments, and post-production work go into the description of each grade. These specifications sort into four major sub-types: austenitic, ferric, martensitic, and duplex. All are useful, but austenitic steels stand out for their superior utility. 70% of stainless steel items are made from austenitic stainless steel.
Properties of austenitic stainless steel
Austenitic stainless steel has many useful properties:
- Highly corrosion resistant
- Does not become brittle at cryogenic temperatures
- Larger hot-strength range than other stainless types
- Greater thermal expansion than martensitic and ferritic stainless steels
- Lower thermal conductivity than martensitic grades
- Usually non-magnetic; mildly magnetic if cold-worked
These properties make austenitic stainless versatile and ideal for many applications, including kitchens and food processing equipment, labs and hospitals, exterior site furnishings and cladding, ovens and furnaces, heat exchangers, and more. The commercially-common 300 series, including 304 and 316 grades, are austenitic steels.
Austenitic steel has these useful properties because of its molecular structure. However, it is expensive to create and maintain the austenite molecules within the steel. These steels are therefore only used where their enhanced properties are necessary.
Metal’s crystalline microstructure
When metals freeze out of the molten state, they crystallize and form grains. This crystal structure determines many of the metal’s mechanical properties.
There are many factors that influence this microstructure. The types of atoms in an alloy change the way the structure forms by making different molecules possible. Percentages of each material, too, can change what arrangements the atoms form.
Temperature also has a profound effect on the shape of a metal’s crystal lattice. Different structures begin to form at specific temperatures. Alloys have phase tables that demonstrate what types of grains are common given a percentage of an alloyed element and range of temperatures.
The iron-carbon phase diagram shows the way temperature and carbon affect the formation of grains in steel. It shows there are three phases of iron formation:
- Ferrite, or alpha iron, (α) is the standard grain formed at temperatures below 912°C.
- Austenite, or gamma iron, (γ), has more densely-packed grain crystals and appears between 912–1394°C.
- Delta iron (δ) forms at heats above 1395°C, before iron turns to liquid at 1538°C. The delta iron phase more closely resembles α-iron, or ferrite.
However, the addition of carbon to the steel or iron influences the steel and how these basic grains crystallize, stabilize, and interact with one another. At different temperatures, the same percentage of carbon can either saturate the metal grains, or remain separate to produce other types of molecules. For example, at many temperatures, an iron-carbon alloy will also contain Fe3C cementite molecules. In pure form, cementite is classified as a ceramic, but it becomes part of the structure of the metal. Graphite can also form at some temperatures at a molecular level. Ledeburite, sometimes shown as µ-iron, is a eutectic composition of austenite and cementite together that allows some of the structure of austenite to maintain at lower temperatures.
How quickly a metal is cooled, and whether it is heat treated or worked after being made, also affect grain size and shape, and in turn, the properties of the metal.
Austenitic steels are those that have an austenite lattice with γ-iron. On the iron-carbon phase diagram this lattice is normally found at high temperatures. However, adding nickel and/or manganese allows austenite to remain as the steel cools. The austenite microstructure is known as “face-centered cubic.”
Body-centered vs. face-centered cubic microstructures
Each cell of a lattice is made up of atoms. The number of atoms in each lattice cell, and how they connect with each other, influence the properties of the crystalline material they create.
- Each atom in this primitive cube sits at a corner of the cell. Each atom is a connection point in the lattice.
- Each corner atom is shared equally with the cells around it. Therefore, each atom is part of eight adjacent cubes.
- The unit cell contains 1 atom in total. Because each of the corner atoms is shared with eight adjacent cubes, only 1/8 of each atom is inside the primitive cell.
8 x 1/8th piece of each corner atom = 1 atom total.
Body-Centered Cubic (BCC)
- Like the primitive cubic form, there are atoms at each corner of the cell.
- Additionally, an atom sits in the middle of the cell. This atom is shared by no other cells: there are 8 cells joined to the lattice and one only to the atom.
- The unit cell contains 2 atoms in total:
8 atoms x 1/8 share per atom, as in the primitive cubic structure, plus the atom in the center.
- Alpha-iron (ferrite) and delta-iron are both BCC metals.
Face-Centered Cubic (FCC)
- The face-centered cubic structure has atoms at each corner of the cell and additionally an atom in the center of each face of the cube.
- The “face” centered atoms are shared only with two cells, and so each contribute 1/2 an atom’s worth.
- The unit cell contains 4 atoms total:
8 atoms x 1/8 share for the corner atoms, and 6 atoms x 1/2 share for the face-centered atoms.
- Gamma-iron (austenite) is an FCC metal.
Steel, without nickel or manganese, achieves a stable face-centered cubic structure between 1,674—2,541°F. At these temperatures, carbon in the steel permeates each cell. This steel, cooled in a regular (unquenched) fashion, will become ferritic and body-centered cubic (BCC). Body-centered lattices are more vulnerable to some types of mechanical strain than more densely-packed face-centered cubic (FCC) structures.
Ferritic steel is magnetic. It becomes brittle at cryogenic temperatures and loses strength quickly in elevated temperatures. These properties are due to the body-centered cubic form. The cells of the BCC crystal are less densely packed than cells with FCC structure, carrying only two atoms to FCC’s four. Lack of cell density forms many of the observable properties of the material. Within each “loosely” packed BCC cell, not all electrons are able to find and pair with electrons of the opposite spin. It is these unattached electrons that create the magnetism of the ferritic material.
These same loosely-packed cells also have a harder time sliding past one another while maintaining internal shape and structure. All lattices have “slip systems.” These are lines of shear along which the lattice can slide without deforming. Cubic lattices have lots of symmetry and therefore more slip planes. BCC and FCC structures both have symmetry-based slip planes, yet the close-packed FCC cells slide more easily past one another without shattering. This means that face-centered cubic systems have more ability to slide without creating strain.
Plastic deformation at the micro level supports the material’s ductility at the macro level. There’s a wider range of resilience in face-centered cubic structures. Ferritic structures are more likely to shatter on impact, or fracture when stretched, in challenging environments.
Martensitic steels do not have a simple cubic form. They are formed by quenching, which means rapid cooling of the surface. This causes displacements within the lattice. Martensitic microstructures are under strain, in a body-centered tetragonal shape, and do not line up evenly. This allows martensitic surfaces to be harder, but they are also more brittle, even at room temperature.
Duplex steels are a relatively new addition to the varieties of stainless steels. These steels have a blend of microstructures. Interleaved layers of ferrite and austenite give the final material properties of both. The lower percentages of nickel and/or manganese needed for duplex stainless lowers the cost compared to austenitic stainless.
Heat-tolerant and cryogenic stainless steels
Face-centered cubic structures are hardier at extreme temperatures. Austenitic stainless steels are the only stainless types that do not become brittle and easily fractured in cryogenic applications. They also maintain hot-strength at higher range than the other types.
Austenitic steel keeps most of its toughness and elongation even below -292°F. In comparison, when duplex and ferritic steels are tested in cold environments, they evidence a “transition” temperature, below which they shatter easily under some or all mechanical strains. Low-temperature embrittlement is characteristic of all types of metals with body-centered cubic microstructures: unalloyed iron, chromium, molybdenum, tungsten, and others. All evidence a transition temperature after which they’re embrittled.
Nickle, copper, aluminum, lead, and silver, each have face-centered cubic microstructures. They maintain some ductility at low temperatures.
Duplex steels are made of both. The austenite offers some of its resilience at lower temperatures. Yet even duplex steels start failing below -58°F with the embrittlement of their ferritic components.
Heat tolerance, or hot-strength, is also an important feature in austenitic stainless steels. When metals are heated, they often soften. Austenitic stainless steel softens as heat increases. Like other steels, it begins to lose strength between 900–1000°F, yet it does not lose strength as quickly as its martensitic and ferritic counterparts. Maximum continuous service temperatures in martensitic and ferritic stainless steels are in the 1300–1500°F range. In comparison, the maximum continuous service temperature for austenitic 310 stainless steel is 2100°F.
The complexity of metals
Metals get their unique material properties from their atomic crystal lattice formation. These grains are influenced by many different aspects of the metal’s production.
Steel is created when iron is alloyed to carbon, producing a strong, ductile, but rust-vulnerable alloy. Chromium is added to help create a passive oxide layer and prevent rust. When heat treated between 1,674–2,541°F, carbon permeates through the lattice and the stainless now has greater ductility and strength. The only way to maintain this structure at room temperature is having nickel and/or manganese in the alloy. These additions provide chemical scaffolding for the face-centered cubic cells. With all these elements, austenitic stainless steel is created: non-magnetic, heat and cold tolerant, ductile, and weldable.
Resilient austenitic stainless steels keep working in many industrial environments. Their mechanical properties make them by far the most popular choice in stainless grades. However, added nickel and manganese make austenitic steels more expensive: new duplex steels, which interleave austenite and ferrite, tend to have some of the properties of both. They’re a cheaper way to have some austenitic steel benefits in non-extreme environments. However, for cryogenic applications, and heat-intensive ones such as boilers, heat-exchangers, and steam lines, austenitic stainless steel will remain the most popular choice.