Stainless steel is a steel alloy that contains chromium. In addition, it may also contain other elements such as nickel, carbon, manganese, molybdenum, nitrogen, copper, titanium, phosphorus, sulfur, selenium, niobium, silicon, cobalt and calcium. As a manufacturing metal, stainless steel provides a number of desirable qualities, including corrosion resistance, strength and relatively high-temperature resistance. It is usually shaped into castings, forgings or extrusions.
Stainless steel alloys represent the largest single first use of nickel. The addition of this silvery-white, hard and ductile metal gives stainless steel high corrosion resistance, ease of production, high formability and good weldability.
Applications of stainless steel and nickel alloys include various industries, such as construction, transport, architecture, food and beverage handling (cookware), medical equipment and pharmaceutical industry. Both kinds of alloys are also used to produce military parts, marine products, jewelry as well as industrial and petrochemical parts.
Stainless Steel Alloys
Stainless steel is a generic term used to describe a variety of steel types and grades that provide high corrosion and oxidation resistance. In general, stainless steels contain iron and a minimum of 10.5% chromium. Of other alloying elements, the most important one is nickel.
In terms of mechanical properties, stainless steel can be divided into four grades:
1. Austenitic grades
This is the most common type of stainless steel. In addition to 16-25% of chromium, it contains nickel and sometimes nitrogen in solution. Austenitic steel is is nonmagnetic, extremely formable and weldable. It is also characterized by low proof strength, strong work hardening and very high ductility.
2. Ferritic grades
Ferritic grades demonstrate high corrosion and oxidation resistance, in addition to being highly resistant to stress corrosion cracking. These magnetic steels contain no nickel and cannot be strengthened by heat treatment. They also exhibit low proof strength, limited work hardening and decreased ductility at high chromium levels. Ferritic grades are widely used in homes (sinks, decorative trims) as well as in transport (car exhaust system).
3. DUPLEX GRADES
Duplex grades are essentially a ferritic-austentic stainless steel combination. They contain more than 20% chromium in addition to small amounts of nickel and molybdenum. This composition gives them good resistance to corrosion and stress corrosion cracking, good abrasion and corrosion resistance, high mechanical strength, high energy absorption as well as low thermal expansion and good weldability. The many applications of duplex stainless steel include paper industry, pipe systems in chemical tankers, seawater systems, firewalls, bridges, storage tanks, heat exchangers, water heaters, structural design components and more.
4. MARTENSITIC GRADES
Martensitic grades of stainless steel contain a relatively high amount of carbon in addition to 12-18% chromium. They are hardenable by tampering and quenching and can be subject to heat treatment in order to improve their properties. Depending on the type of heat treatment, these grades can also exhibit high ultimate tensile strength in addition to low elongation to fracture.
So-called supermartensitic stainless steels are a modern type that displays high strength, low-temperature toughness and acceptable corrosion resistance.
The choice of a particular stainless steel alloy and grade relies largely upon corrosion resistance requirements. Various mechanical and physical properties of stainless steels should also be considered to achieve particular service performance requirements.
In general, nickel alloys can be divided into the following two categories: nickel-copper alloys and nickel-chromium base alloys.
1. Nickel-copper alloys
Nickel-copper alloys are also known as Monel alloys. In addition to nickel, they contain 29-33% copper as well as small amounts of iron and manganese. The main properties of nickel-copper alloys include high corrosion resistance, great mechanical strength, good ductility and a low coefficient of thermal expansion. However, they do tend to quickly work-harden.
In general, nickel-copper alloys are used where a higher strength is required compared to pure nickel. Because of their superb resistance to acids and alkalis, they are frequently used in chemical plant equipment. With small additions of aluminum and titanium, Monel alloys can also be used in the aerospace industry. Other common applications include valve stems, heat exchangers, screw machine products, piping systems, fuel and water tanks, kitchen sinks and more.
2. Nickel-cHROMIUM BASE ALLOYS
Nickel-chromium alloys provide resistance to oxidation and metal failure through a loss of strength that often occurs at high temperatures (with the exception of high-temperature sulfur-rich environments). Thanks to their excellent resistance to acid, alkalis, water and seawater, oxidation, flammability, and organic solvents, nickel-chromium alloys are usually applied where particular heat and corrosion resistance is required.
Nickel-chromium alloys can be divided into two categories:
1. The first group includes cast alloys that contain 40-50% nickel and 50-60% chrome. These alloys are heat and corrosion resistant. Their applications include structural parts, containers, supports, hangers, spacers etc. that are exposed to temperatures up to 1994 °F (1090 °C).
2. The other group of nickel-chromium alloys contains significantly more nickel (70-80%). Their applications include resistance heating elements exposed to temperatures up to 2102°F (1150 °C).
The more recent nickel-chromium base alloys contain a variety of ancillary elements, the addition of which creates new and improved alloy properties.
Low expansion and magnetic alloys
Two particular nickel group alloys that exhibit distinctive properties are low expansion alloys and magnetic alloys.
Low expansion alloys are nickel-iron alloys exhibit low and constant expansion coefficient over a particular temperature range. They are used in medical, scientific and automotive industries to manufacture precision instruments, scientific instruments, laboratory devices, motor valves, aircraft industry parts and electronic parts.
Magnetic alloys are complex alloys. The high-nickel magnetic alloys exhibit high initial permeability and fairly low saturation induction while soft magnetic nickel-iron alloys are used in switchgear and for direct current motors and generators.