Stainless steels can be produced with five different crystal structures: ferritic, austenitic, martensite, duplex, and precipitation hardening.
pheasic
Ferritic stainless steel contains 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 cubic-centered body crystal structure (BCC) – the same as pure iron at room temperature.
Due to its crystalline structure, ferritic stainless steel is magnetic. Its relatively low carbon content results in low strength. Other disadvantages of the ferritic type include poor weldability and reduced corrosion resistance. However, they are desirable for engineering applications due to their superior toughness. Ferritic stainless steel is often used for vehicle exhaust, fuel lines, and architectural trim.
Austenitik
Austenitic stainless steels have a face-centered cube crystal structure (FCC); They consist 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 steel, austenitic stainless steel 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 grade; The main driver behind the popularity of austenitic stainless steel is its ease of shaping and welding, which makes it ideal for use in high-efficiency manufacturing.
There are many sub-groups of austenitic stainless steels, with a wide variation in carbon content. These properties are further tuned by the addition of alloying elements such as molybdenum, titanium, and copper.
Austenitic stainless steel is often used to manufacture sinks, window frames, food processing equipment, ovens, chemical tanks, and outdoor furniture such as benches and sidewalks.
Martensitik
Martensitic stainless steels have a tetragonal body-centered structure (BCT). They contain 12 – 18 percent chromium, and have a higher carbon content (0.1 – 1.2 percent) than austenitic or ferritic stainless steels. Like ferritic BCC structures, BCTs are magnetic. The main difference is that martensitic stainless steel can be hardened by heat treatment due to its high carbon content. This makes it useful for a number of applications where ferritic stainless steel is not suitable, including aerospace parts, cutlery, and blades.
Martensitic stainless steels are especially useful in situations where the strength of the steel is more important than its weldability or corrosion resistance. Hardened martensitic steel cannot be cold formed.
Rangkap
Duplex stainless steel is the latest type of stainless steel. 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 crystal structure. The roughly half- and half-phase mixtures of austenitic and ferritic in duplex stainless steels provide several unique advantages.
Duplex grades are a compromise: they are more resistant to stress corrosion cracking than austenitic grades, harder than ferritic grades, and approximately twice as strong as pure forms of both. The main advantage of duplex stainless steel is corrosion resistance equal to – and, in the case of chloride exposure, exceeding – austenitic values.
Another significant advantage of duplex stainless steels is cost efficiency – the strength and corrosion resistance of duplex stainless steels is achieved with a lower alloy content than equivalent austenitic grades.
Duplex stainless steels are regularly used to manufacture parts for chloride-exposed applications such as desalination, food picking, and petrochemicals.
Precipitation hardening
Precipitation-hardening stainless steels can have a variety of crystal structures, but they all contain chromium and nickel. Their general characteristics are corrosion resistance, ease of fabrication, and very high tensile strength with low-temperature heat treatment.
Austenitic precipitation-hardenable alloys have largely been replaced by higher-strength superalloys, but semi-austenitic hardenable stainless steels continue to be used in aerospace applications, and even applied to new forms.
Martensitic precipitation stainless steel that can be hardened is stronger than ordinary martensite grades, often used to produce bars, bars, and wire.
