27th of June ,2022

Corrosion resistance of different grades of stainless steel

Corrosion of different grades of stainless steel will vary in different environments. The appropriate rating will depend on the service environment. Even trace amounts of certain elements can significantly alter corrosion resistance. Chlorides in particular can have a detrimental effect on the corrosion resistance of stainless steel. The products with the highest chromium, molybdenum and nickel content are the most corrosion resistant products.

Low temperature resistance is measured by sub-zero ductility or toughness. At low temperature, the tensile strength of austenitic stainless steel is much higher than ambient temperature. They also maintain excellent toughness. Ferritic, martensitic and precipitation hardening steels should not be used below zero. The toughness of these grades drops significantly at low temperatures. In some cases, this drop is close to room temperature.

Hardened grades of stainless steel have the advantage that the strength of the metal can be significantly increased by cold working alone. A combination of cold working and annealing stages can be employed to impart a specific strength to the manufactured part.

A typical example is the drawing of wires. The wire used as the spring will be machined to a specific tensile strength. If the same wire is used as a bendable tie wire, it will be annealed, resulting in a softer material.

Austenitic stainless steels maintain high strength at high temperatures. This is especially true for grades that contain high levels of chromium and/or silicon, nitrogen and rare earth elements such as grades 310 and S30815. High chromium ferritic grades like 446 can also exhibit high thermal strength. The high chromium content of stainless steel also helps resist scaling at high temperatures.

Ductility tends to be given by % elongation during tensile testing. The elongation of austenitic stainless steel is very high. High ductility and high work hardening rates enable austenitic stainless steels to be formed by rigorous processes such as deep drawing.

Austenitic stainless steels

Stainless steel tends to have higher tensile strength than mild steel. Duplex stainless steels have higher tensile strength than austenitic steels. The highest tensile strength is seen in martensite (431) and precipitation hardening grades (17-4PH). These grades can be twice as strong as TYPES 304 and 316 (the most commonly used stainless steels).

Magnetic response is the attractive force of steel to a magnet. Austenitic stainless steels are generally not magnetic, although a magnetic response can be induced in low austenitic grades by cold working. High nickel grades like 316 and 310 remain non-magnetic even when cold worked. All other grades are magnetic.

Although the corrosion resistance of stainless steel comes from the presence of chromium, other elements are added to enhance other properties. These elements change the microstructure of the steel. Stainless steels are divided into different families based on their metallurgical microstructure. The microstructure can consist of the stable phase austenite or ferrite, a "dual-phase" mixture of the two, martensite or a hardened structure containing precipitated trace elements.

Austenitic stainless steels contain a minimum of 16% chromium and 6% nickel. They range from basic grades such as 304 to super austenitic grades such as 904L and 6% molybdenum. By adding elements such as molybdenum, titanium or copper, the properties of the steel can be modified. These improvements can make the steel suitable for high temperature applications or improve corrosion resistance. Most steels become brittle at low temperatures, but the nickel in austenitic stainless steels makes them suitable for low or low temperature applications.

Austenitic stainless steels are generally non-magnetic. They cannot be hardened by heat treatment. Austenitic stainless steels work harden rapidly by cold working. Although they harden, they are the easiest stainless steel to form. The main alloying elements are sometimes reflected in the name of the steel. The common name for 304 stainless steel is 18/8, 18% chromium and 8% nickel.

Applications for austenitic stainless steel include: kitchen sinks, architectural applications such as roofing and cladding, roofing and gutters, doors and windows, railings, benches and food preparation areas, food processing equipment, heat exchangers, ovens, and more.

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