Stainless steel
4-20 usd/kgStainless steels have very good corrosion resistance compared to carbon steels, coupled with impressive mechanical properties. This combination is the result of the addition of a relatively high proportion of alloys, in particular chromium (Cr). They tend to be more expensive, and so reserved for applications that demand their superior properties.
Based on their crystalline structure, they are grouped in five families: ferritic, austenitic, martensitic, precipitation-hardening (PH) and duplex. Ferritic stainless steel is not seen much outside industrial applications, automotive and white goods, for which it is well suited. The lower alloy content makes it less expensive than the more common austenitic types.
Austenitic stainless steel, which includes the very popular 300 series, is a good all-rounder with high resistance to corrosion. It is non-magnetic (which makes it easy to distinguish from other steels), with excellent formability, machining properties and weldability.
Martensitic stainless steel has high hardness and strength, good for industrial applications, household appliances and some knife blades, with moderate resistance to corrosion. Its high hardness makes it relatively more difficult to form, cut and weld.
Duplex types are a modification on austenitic grades, designed to provide higher strength with comparable or improved corrosion resistance. PH stainless steels are a family of corrosion resistant alloys, which can be heat treated to tensile strength several times greater than austenitic types. They are used in very demanding applications in aerospace, off-shore and nuclear energy, for example.
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Precipitation hardening (PH) stainless steels are a family of high performance alloys, some of which can be heat-treated to yield very high tensile strength (1,700 MPa) and hardness – far superior to 300 series austenitic alloys. However, due to their high hardness, they have relatively low toughness. Corrosion resistance is moderate to good. Precipitation hardening is achieved by the addition of copper, molybdenum, aluminium and titanium either singly or in combination. The full potential is achieved through a process of solution heat treatment, or annealing, followed by a single low temperature age hardening process. As it is at low temperature, the risk of distortion is reduced. After the initially annealing phase, they can be formed – by pressing, bending and hydroforming, for example – and are readily machinable and can be welded with conventional techniques.
PH stainless steels are divided into three main types: martensitic, semi-austenitic and austenitic.
Alloy 693 (A693 Tp630, FV 520, UNS S17400, DIN 1.4542) is a chromium-nickel-copper, martensitic precipitation hardening stainless steel and is also known as 17-4PH, because it contains 17% chromium (Cr) and 4% nickel (Ni). The martensitic structure is formed as it cools following annealing at over 1040 degC. It is one of the most used PH alloys.
Alloy 631 (A693 Tp631, UNS S17700) is a chromium-nickel-aluminium, semi-austenitic precipitation hardening stainless steel and is also known as 17-7PH, because it contains 17% chromium (Cr) and 7% nickel (Ni). It is more formable than 17-4 PH, because it retains a predominantly austenitic structure.
Alloy 286 (A286, JBK 75) is a nickel-chromium, austenitic precipitation hardening stainless steel and also known as 17-10PH, because it contains 17% chromium (Cr) and 10% nickel (Ni). They retain their austenitic structure after annealing and are softer than martensitic and semi-austenitic types. They are non-magnetic. It is suited to applications that require high strength and corrosion resistance at higher temperatures, up to 700 degC.
These alloys are used in the oil and gas, nuclear and aerospace industries where a combination of high strength and corrosion resistance is required, such as for gears, valves, jet engine parts, turbine blades and tanks.