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NON-FERROUS METALS
STAINLESS STEELS
The term stainless is a general term for stainless steel that requires a minimum of 10.5% Chromium. The chromium content in steel is very important as it forms a thin layer of chromium oxide on the metal surface to prevent corrosion of the metal.
What is Stainless Steel?
Stainless steels are a group of steels with a minimum chromium content of 10.5% and resistant to corrosion. This is considered in the stainless category for other steels, including some tool steels that contain chromium levels. The major difference between these steels and stainless steels comes from the addition of a certain amount of chromium in stainless steels mainly to improve corrosion resistance.
The European standard for stainless steels EN 10088 (published in the UK as BS EN 10088) defines stainless steels as steels with a minimum (Chromium value) of 10.5% for use in applications where corrosion resistance is required.
Many commercially available stainless steels contain more than this minimum level of chromium, and also nickel, molybdenum, titanium, niobium, etc. They contain varying amounts of other alloying additions, including These additions are used not only to increase the corrosion resistance of the steel, but also to improve mechanical strength, hardness, toughness, magnetic properties, etc. It is added to improve other properties or to modify working properties, including formability, machinability, and weldability.
Stainless steels can also be used in heat resistant and cryogenic applications as they have excellent oxidation resistance. These special steels often have enhanced chromium levels to improve their heat resistance properties. They are covered by the BS EN 10095 standard.
The behavior of various stainless steels during manufacture and use depends not only on their chemical composition, but also on the design and surface quality of the manufactured parts. Being careful in this regard, selection of the most suitable steel grade, design and surface finish is vital for the successful application of stainless steels.
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What are the types of Stainless Steels?
Stainless steels have “family” names that reflect their metallurgical (atomic) nature.
These groupings, often referred to as families, are:
- Ferritic
- Austenitic
- Duplex (ferritic-ausenitic)
- Martensitic and precipitation hardening
The main characteristics of each family are:
Ferritik (Ferritic)
- Magnetic
- low carbon
- Chromium major alloying element, typically in the range of 10.5-17%
- Cannot be hardened by heat treatment
- Typical grades 409 (1.4512), 430 (1.4016)
Austenitic
- Non-magnetic (when fully softened)
- Low carbon Chromium content typically 16-20%
- Nickel content is typically 7-13%
- Cannot be hardened by heat treatment, but becomes significantly stronger during cold working
- Typical grades 304 (1.4301), 316 (1.4401)
Duplex
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Magnetic
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low carbon
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Chromium content typically 21-26%
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Nickel content is typically 3.5-6.5%
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Nitrogen content, typically 0.05-0.40%
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Cannot be hardened by heat treatment but exhibits higher annealed strength levels than ferritic or austenitic stainless steels
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Typical grade 2205 (1.4462)
Martensitic and precipitation hardening
- Magnetic or non-magnetic
- Carbon can be up to 0.95%
- Chromium content typically 12.0-17%
- Nickel content is typically 2-7%
- Precipitation hardening types also contain aluminum or copper Can be hardened by heat treatment
- Typical grades 420 (1.4021), 17/4PH (1.4542)
What are the Physical Properties of Stainless Steels?
In addition to corrosion resistance, certain stainless steel grades have other properties that can be useful in service or during fabrication. These characteristics can vary greatly between families and various degrees and include:
Mechanical Strength
The mechanical strength of ferritic (about 500 MPa) and austenitic (about 600 MPa) stainless steels is similar to that of low alloy steels. Austenitic steels retain their strength with temperature increases better than ferritic stainless steels or non-stainless structural steels. This makes austenitic stainless steels good choices for fire and heat resistant applications.
Duplex steels have about twice the strength of these other steels, with a yield strength of about 450 MPa and a tensile strength of 700 MPa. Their mixed structure of ferritic and austenitic mixture together with their nitrogen content enhances these high levels of strength.
The strength of martensitic and precipitation hardening steels can be varied over a wide range by heat treatment. The precipitation hardening types in particular have very high strength levels up to 1100 MPa.
Hardness
Hardness, resistance to indentation or scratching is similar for all “softened” steels such as ferritic, austenitic and duplex family grades. Only martensitic grades with enhanced carbon levels can be hardened. Grades such as 440C (1.4125) can be hardened to approximately 60 HRC (Rockwell C). In contrast, precipitation hardening grades, despite their name, are used primarily for their heat treatable strength rather than hardness.
Corrosion Resisdance
All stainless steels can withstand some degree of corrosion. There is a wide variety of service environments, different formats and locations where a corrosion attack can take place. This is the main reason why there are four main types of stainless steel and each has its own grade range. All stainless steels have a corrosion resistant, tightly adherent, chromium-rich passive layer.
Hardening
Hardening is the gradual increase in strength and hardness of a material (as it is “cold” worked). Most metals harden to some degree during processes such as cold forming, bending or machining.
The hardening rate of austenitic stainless steels is higher than most other metals. This is because some of the austenite turns into the stronger martensite component. These are structural changes in steel.
Hardening can be both a disadvantage and an advantage. It is undesirable during machining, but advantageous during some cold drawing operations where it can delay premature failure during operation.
Interstage annealing operations are often needed, but when producing severely drawn or complex shaped parts. The effects of cold working can be reversed by annealing heat treatments at about 1050°C.
Magnetic Permeability
Magnetic permeability is a measure of the attractiveness of a permanent magnet. Most materials, including many metals, are not attracted to a magnet. The common exception comes from iron and most steels, which can also be converted into magnets, i.e. magnetized and demagnetized by electric fields.
Softened austenitic steels do not behave this way. They have very low magnetic permeability with relative permeability values of around 1.005. Ferritic and martensitic stainless steels, by contrast, have relative permeability values of around 15. With this very low relative permeability, austenitic stainless steels can be used in equipment that is very sensitive to magnetic field interference but also needs to be corrosion resistant.
These applications include medical body scanner bodies and marine minesweeper equipment. With its additional nitrogen content, grade 316LN (1.4406) is well suited for such applications. It should be noted that hardened austenitics can be somewhat magnetic.
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