What makes steel 'Stainless'?
Stainless steel, also known as corrosion resistant steel, Inox steel or Inox (from French word “inoxydable”), is defined as a steel alloy with a minimum of 11% chromium content.
Stainless steel does not corrode, or rust as easily as ordinary steel. Stainless Steel performs better than ordinary steel in high temperature and cryogenic applications. Stainless steel is used where both the properties of steel and resistance to corrosion are required.
What makes stainless steel so resistant to corrosion?
Stainless steel differs from carbon steel by the amount of chromium present. Stainless steel contain sufficient chromium to form a passive film of chromium oxide, which prevents surface corrosion and blocks corrosion from spreading into the metal’s internal structure. The sizes of chromium atoms and their oxides are similar, they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. The passive film requires oxygen to self-repair, so stainless steel has poor corrosion resistance in low-oxygen and poor circulation environments.
Types of Stainless steel
Stainless steel comes in a variety of types as well as many grades within those types of stainless steel.
The four main types are Austenitic, Ferritic, Martensitic and Duplex. These types are identified by their microstructure or predominant crystal phase.
Austenitic steels have austenite as their primary phase, face centred cubic crystal. These are alloys containing chromium and nickel (sometimes manganese and nitrogen). Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304 and 316. These grades are non magnetic.
Typical application for the various grades of Austenitic include:
304/304L Tanks, storage vessels and pipe work. Process equipment in the mining, chemical, cryogenic, food, beverage and pharmaceutical industries. These stainless steels are also used for manufacturing holloware, cutlery, architectural products and sinks.
309/310 These grades have a higher chrome and nickel content that the 304 grade. As a result of their high oxidation resistance, these steels are used for high temperature applications such as furnace, kiln and catalytic converter components.
318/316L Tanks, pressure vessels, pipe work and components for more aggressive or corrosive conditions and specialised applications, such as the manufacture of tank containers for bulk transportation of chemicals and corrosive liquids. The molydenum content enhances the corrosion resistance.
321/316Ti These are the "stabilised" grades. They are resistant to sensitisation and thus the possibility of Intergranular Corrosion. In addition they are used in components, which require elevated temperature strength and corrosion resistance, such as afterburners, super heaters, compensators and expansion bellows.
Ferritic Stainless Steel are plain chromium stainless steels, usually with a low carbon content. They are magnetic and have good ductility and resistance to corrosion and oxidation. They are generally resistant to stress corrosion cracking. However, there are weldability limitations which restrict their use to thinner gauges. 3CR12 is a special grade, developed and patented by Columbus Stainless which largely overcome this problem.
Typical Ferritic grades and applications for the conventional different grades include :
409 Automotive exhaust tubing and catalytic convertor casings
430 Kitchen sinks, washtroughs, cutlery, kitchen and catering equipment and utensils.
441 This grade is specially produced by Columbus Stainless for used in automotive components. Its superior mechanical strength at elevated temperatures (up to 850 deg C) makes it the ideal material for the front end (close to the engine) of an exhaust system. It can also be used for fabrication of heat exchanger tubes.
AISI 444 AISI Grade 444 has a very similar PRE (Pitting Resistance Equivalent) to Grade 316, meaning that its corrosion resistance is similar in aggressive outdoor environments, e.g. at the coast. Grade 444 tubing can be polished, bent and welded by conventional methods and tubing will be marked clearly on the inside of the tube. PRE is defined as %Cr + (3.3%Mo) + (16%N) which gives an indication of the steels resistance to pitting corrosion.
3CR12 This is a price competitive, corrosion resisting, weldable, utility ferritic stainless steel with particular advantages in wet abrasive applications. Unlike other ferritic stainless steels it can be welded in thicknesses of up to 30mm. It is used in mining, materials handling and sugar industries due to its resistance to atmospheric corrosion and wet abrasive corrosion.
Martensitic steel is a type of stainless steel that, because of its chemical composition, can be hardened and strengthened through heat and aging treatments. These methods make Martensitic steel stronger than other types which makes it a good choice for the fabrication of medical instruments, mechanical valves, turbine parts, mechanical instruments, and other various applications.
410 Applications include fasteners, springs, pins, cutlery, hardware, gun clips, micrometer parts, turbine blades, coal screens, pump rods, nuts, bolts, fittings, ball bearings, shafts, impellers, pistons, and valves. Hardness levels can be changed with slight variations in hardening and tempering heat treatments.
420 Some applications of 420 stainless steel include fasteners, cutlery, machine parts, bushings, surgical tools, firearms, and valve trim.
440 Type 440A stainless steel has a greater hardening capability than Type 410 or Type 420, but limited formability in the annealed condition. This grade of stainless steel is hardenable to over RC50 which makes it very attractive for blanking into blade applications. The high hardness of this grade means that blades remain sharper longer. Typical uses include: Cutlery Surgical Instruments Bearings
Halfway between the ferritic and austenitic stainless steel is a family called the duplex stainless steels, which are about 50% ferrite and 50% austenite. Because of this duplex structure, they are resistant to stress corrosion cracking, which can affect the austenitic stainless steel in hot waters containing chlorides. Duplex stainless steels are inherently stronger, for example, a grade such as 2205, which contains about 0.15% nitrogen, has over twice the yield strength of Type 316L.
2205 Includes S31803 and S32205. Often used in industrial environments such as petrochemical, chemical, oil, gas and paper plants and military tanker use. The grade is generally not suitable for use at temperatures above 300oC or below -50oC because of reduced toughness outside this range.