CRYSTAL STRUCTURE OF VARIOUS CLASSES OF STAINLESS STEEL

CRYSTAL STRUCTURE OF VARIOUS CLASSES OF STAINLESS STEELS

There are five classes of stainless steel (Ferritic, austenitic, martensitic, duplex, and precipitate hardened) and only one is nonmagnetic (austenitic). However, the austenitic class just happens to include the most widely and universally used types of stainless steels in the market. The first four classes are defined based on the microstructure of the metal with the last class, PH, based on its heat treatment. Microstructure is important because this is what gives the stainless steel its Magnetic properties.

Crystal Structure of some metals (at room temperature)

CRYSTAL STRUCTURE OF VARIOUS CLASSES OF STAINLESS STEEL

Austenitic hasFCC type grain structure. Austenitic is the most widely used type of stainless steel. It has excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. Austenitic steel is used in housewares, industrial piping and vessels, construction, and architectural facades. 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.

Ferritic has BCC type grain structure. Ferritic stainless steel has similar properties to mild steel (the most common steel), but better corrosion, heat, and cracking resistance. Ferritic steel is commonly used in washing machines, boilers and indoor architecture. Ferritic steel is a common BCC steel. It becomes brittle at cryogenic temperatures, loses strength quickly in elevated temperatures, and is magnetic. These properties are due to the body-centered cubic (BCC) form. 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 weld ability 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 pipes, fuel lines, and architectural trim.

  Martensitic has BCT type grain structure. Martensitic stainless steel is very hard and strong, though it is not as resistant to corrosion as austenitic and Ferritic grades. It contains approximately 13% chromium and is used to make knives and turbine blades. Martensite is formed in carbon steels by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Martensitic stainless steels are highly useful in situations where the strength of the steel is more important than its weld ability or corrosion resistance. The major distinction is that martensitic stainless steel can be hardened by heat treatment because of their high carbon content. This makes them useful for a number of applications including aerospace parts, cutlery, and blades. Martensitic steels are another type of steel with a very different type of grain at the surface. These steels do not have a simple cubic microstructure. Martensite is formed by quenching: a rapid cooling of the surface. The environmental shock causes the lattice to heave as it freezes. 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 has combined type grain structure. Duplex stainless steel is a composite of austenitic and Ferritic steels, making it both strong and flexible. Duplex steels are used in the paper, pulp, shipbuilding, and petrochemical industries. Newer duplex grades are being developed for a broader range of applications.

Duplex has a structure of approximately equal amounts of ferrite and Austenite. 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 unique advantages. 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 corrosion resistance equal to, or exceeding, austenitic grades in the case of chloride exposure. 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.

 Martensitic or semi-austenitic steels can also be classified as precipitation hardening stainless steels. These steels are made to be extremely strong with the addition of elements such as aluminium, copper and niobium.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 hardening alloys have mostly been replaced by higher strength super alloys. However, semi-austenitic precipitation hardening stainless steels continue to be used in aerospace applications, and even applied to new forms. Martensitic precipitation hardening stainless steels are stronger than regular martensitic grades and frequently used to produce bars, rods, and wires.

 FCC CRYSTAL STRUCTURE FOR AUSTENITIC STAINLESS STEEL

BCC- CRYSTAL STRUCTURE FOR FERRITIC STAINLESS STEEL

BCT type grain structure

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