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Stainless Steel Grades
Stainless Steel is the name given to a group of steel alloys which contain more than 12% Chromium, and is divided into the following stainless steel grades:
Chromium is known for its high affinity to oxygen and forms a stable oxide film (known as a passive oxide layer) on the surface of the stainless steel. This layer forms instantaneously in normal atmospheres, is self healing and rebuilds itself once it has been removed. This film is the reason why stainless steel is such a high resistance to corrosion.
Note: Also see Maximum Operating Temperatures of Stainless Steel Grades.
This group of stainless steels contain:
- 17% - 25% Chromium
- 8% - 20% Nickel
- As well as a number of additional elements that help manufacturers achieve the required properties.
Fully annealed conditions of this grade deliver a range of physical and mechanical properties. Mechanical properties can further be enhanced through cold working techniques.
Welding of these stainless steel grades can be done, with the low carbon content resulting in fewer problems during welding than seen with the Martensitic and Ferritic grades. These stainless steels are also normally non-magnetic, however they will become slightly magnetic when cold working techniques have been used.
The Basis Austenitic Grades:
The original general purpose 18/8 (18% Chromium/ 8% Nickel) stainless steel grade. The majority of other stainless steel forms have been developed from this grade and it exhibits the following physical properties:
- Excellent ductility.
- Excellent welding characteristics
This grade was specially developed for machining during which production involves extensive machining in automatic screw machines.
Here, Sulphur or Selenium is added in order to deliver excellent free machining properties as well as non-seizing properties. However, the adding of Sulphur or Selenium results in a reduction in the steel’s resistance to corrosion – slightly below that of T304. Not a recommended grade for welding, this grade is non-hardenable.
Delivering the best all-round performance, this stainless steel grade is one of the most versatile and most widely used. With a lower carbon Carbon content, this grade shows a slightly higher resistance to corrosion that T302.
Following welding, it has a lesser susceptibility to inter-granular corrosion.
A low carbon stainless steel, this grade has a resistance to corrosion that is similar to that of T304. It does however have a superior inter-granular corrosion resistance after welding stress-relieving processes. This grade is recommended for use in parts which have been fabricated through welding processes and, as a result, cannot be annealed.
Products made using this grade are limited to a service temperature of a maximum of 426°C.
Specifically developed for high temperature services (a maximum of ± 1 100°C) in which high creep strengths are a requirements, this grade is however not recommended in long service situations as brittleness may occur.
This grade is also non-magnetic when annealed as well as when cold worked.
A marine alloy, this grade has between 2% and 3% Molybdenum – this element helps to improve resistance to corrosion. This grade has a superior resistance to corrosion when exposed to chemical corroding agents s as well as a higher resistance to corrosion in marine applications when compared to other grades in this category.
T316 has a better creep resistance and strength at high temperatures when compared to T304, and greater properties when it comes to work hardening.
Other applications of this alloy includes the textile, chemical and paper industries.
A low carbon grade, it has a resistance to corrosion that is similar to that of T316. It does however exhibit a better inter-granular corrosion resistance after stress relieving and cold working processes. Parts that are manufactured from this grade are limited to service temperatures of no higher than 426°C.
This grade will be recommended in cases where manufacturer parts cannot be subsequently annealed.
This grade is T304 which has been stabilized by the addition of Titanium. Titanium is added to five times the carbon content. This addition of Titanium helps to prevent inter-granular corrosion and offers a scale resistance at high temperatures (of up to 850°).
T321 has a slightly lower resistance to corrosion than that of T304 and will not be recommended where bright or mirror polishing is required.
T347 is a modified T304 alloy which has been stabilized by the addition of Tantalum and Columbium. Recommended for fabricated parts produced through welding techniques, which cannot be annealed, the Columbium content delivers the following beneficial properties:
- Aids in the resistance to inter-granular corrosion after stress relieving, treatments or welding.
- Curbing the precipitation of harmful carbides into the boundary of the grain.
This alloy is similar in analysis to alloy T321.
These stainless steel grades contain:
- 12% - 14% Chromium
- 0.08% - 2.00% Carbon
Because of Martensitic Grades’ high carbon content, these stainless steels respond well to heat treatment, used for various mechanical strengths, such as hardness.
The high carbon content also means that extreme care is required during welding processes. Note that, when Martensitic Grades are heat treated, the results are a useful combination of mechanical properties and resistance to corrosion.
A construction stainless steel, T409 is intended for use in structural applications where mechanical properties and resistance o corrosion is more important than appearance, for example automotive exhaust systems.
A heat resistant and general corrosion resistant steel, it is easily machined and forged. Delivering good cold working properties, T410 is not recommended in situations where severe corrosion occurs. One of the most affordable corrosion resistant stainless steels (for general uses), it is a magnetic and commonly used for the production of cutlery.
With a higher carbon content than T410, it has a higher hardness (± 500 Brinell). When hardened or tempered, T420 has optimum resistance to corrosion and is, like T410, magnetic in all conditions.
A Nickel bearing stainless steel, T431 is designed for heat treatment processes to achieve the highest possible mechanical properties. Also magnetic in all conditions, this type has a better resistance to corrosion when compared to T410 and T430.
Nearest Relevant Specifications for Martensitic Grades:
Ferritic grades contain:
- A minimum of 17% Chromium
- 0.08% - 2.0% Carbon
An increased Chromium content leads to an increased resistance to corrosion at higher temperatures, but has a lack of mechanical properties as these grades cannot undergo heat treatment.
Magnetic, welding this group of stainless steels should be done with the utmost care.
A heat resisting stainless steel, this alloy exhibits a better resistance to corrosion and heat when compared to T410.
Non-hardenable with an excellent weldability, this alloy has only mild cold working properties as a result of the high Chromium content. T430 also does not require the use of annealing processes.
Nearest Relevant Specifications for Ferritic Grades:
Maximum Operating Temperatures of Stainless Steel Grades
Back to Main Page: Custom Stainless Steel Fabrication.
Further Suggested Reading:
- Stainless Steel Fabrication Process
- Stainless Steel Tube Fabrication
- Stainless Steel Finishes
- TIG Welding Stainless Steel
- Tolerances in Stainless Steel Fabrication