Stainless steels can be produced with five distinct crystalline structures: ferritic, austenitic, martensitic, duplex, and precipitation hardening.

Ferritic

Ferritic stainless steels contain iron, carbon, and between 10.5 and 18 percent chromium. They may contain other alloying elements such as molybdenum or aluminum, but usually in very small amounts. They have a body-centered-cubic (BCC) crystal structure – the same as pure iron at ambient temperature.

Due to their crystal structure, ferritic stainless steels are magnetic. Their relatively low carbon content produces correspondingly low strength. Other weaknesses of the ferritic type include poor weldability and reduced corrosion resistance. They are, however, desirable for engineering applications because of their superior toughness. Ferritic stainless steels are often used for vehicle exhaust, fuel lines, and architectural trim.

Austenitic

Austenitic stainless steels have a face-centered cubic (FCC) crystal structure; they are composed of iron, carbon, chromium, and at least 8 percent nickel. Due to their high chromium and nickel content, they are highly corrosion resistant. They are non-magnetic. Like ferritic stainless steels, austenitic stainless steels cannot be hardened by heat treatment. However, they can be hardened by cold working. The high nickel content in austenitic stainless steels makes them capable of functioning well in low-temperature applications.

The two most common stainless steels-304 and 316-are both austenitic grades; the primary driver behind the popularity of austenitic stainless steels is the ease with which they can be formed and welded, which makes them ideal for use in high-efficiency manufacturing.

There are many sub-groups of austenitic stainless steel, with wide variations in carbon content. The properties are further tuned by the addition of alloying elements such as molybdenum, titanium, and copper.

Austenitic stainless steels are frequently used to produce kitchen sinks, window frames, food processing equipment, ovens, chemical tanks, and outdoor site furnishings such as benches and bollards.

Martensitic

Martensitic stainless steels have a body-centered tetragonal (BCT) structure. They contain 12 – 18 percent chromium, and have a higher carbon content (0.1 – 1.2 percent) than austenitic or ferritic stainless steels. Like the ferritic BCC structure, BCT is magnetic. The major distinction is that martensitic stainless steel can be hardened by heat treatment because of their high carbon content. This makes it useful for a number of applications for which ferritic stainless steel would be unsuitable, including aerospace parts, cutlery, and blades.

Martensitic stainless steels are highly useful in situations where the strength of the steel is more important than its weldability or corrosion resistance. Hardened martensitic steels can’t be cold formed.

Duplex

Duplex stainless steels are the newest stainless steel type. They contain more chromium (19 – 32 percent) and molybdenum (up to 5 percent) than austenitic stainless steels, but significantly less nickel. Duplex stainless steels are sometimes referred to as austenitic-ferritic because they have a hybrid ferritic and austenitic crystalline structure. The roughly half and half mix of austenitic and ferritic phases in duplex stainless steels gives it some unique advantages.

Duplex grades are a compromise: they are more resistant to stress corrosion cracking than austenitic grades, tougher than ferritic grades, and roughly two times stronger than a pure form of either. The key advantage of duplex stainless steels is a corrosion resistance equal to – and, in the case of chloride exposure, exceeding – austenitic grades.

Another significant advantage of duplex stainless steels is cost efficiency – the strength and corrosion resistance of duplex stainless steel are achieved with a lower alloy content than equivalent austenitic grades.

Duplex stainless steels are regularly used to produce parts for chloride-exposed applications like desalination, food picking, and petrochemical.

Precipitation Hardening

Precipitation hardening stainless steels can have a range of crystalline structures, however they all contain both chromium and nickel. Their common characteristics are corrosion resistance, ease of fabrication, and extremely high tensile strength with low-temperature heat treatment.

Austenitic precipitation hardenable alloys have mostly been replaced by higher strength superalloys, however semi-austenitic precipitation hardenable stainless steels continue to be used in aerospace applications, and even applied to new forms.

Martensitic precipitation hardenable stainless steels are stronger than regular martensitic grades, are frequently used to produce bars, rods, and wire.