Since stainless steel is aesthetically pleasing and corrosion resistant, it’s often used in manufacturing items such as cookware, kitchen sinks, and vehicles. The term stainless steel refers to any of several iron-based alloys that include the element chromium as at least 10.5% of their composition.
Although all stainless steel is defined by its crystalline structure, the metal comes in more than 100 grades. The American Iron and Steel Institute first developed a structure of grades for steel in the 1930s and 40s. Later, this system combined with one from the Society of Automotive Engineers (SAE) International to form a more standardized numbering system. Today, steel manufacturers may use more than one numbering system, however, which can complicate matters.
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The general idea behind grading the metal is that stainless steel can be divided into three broad categories — austenitic, ferritic, and martensitic — based on the crystalline structure of its iron atoms. Each category is further divided into series and grades. The grades reflect the specific alloy’s durability, quality, and temperature resistance. Numbers listed after the grade refer to the item’s chemical composition, specifically to it’s chromium and nickel percentages.
The SAE developed a four-digit grading system for carbon steel and a three-digit one for stainless steel. The first digit refers to the primary alloy element, the second and third digits to the alloy’s carbon percentage. Carbon steel needs four digits because there are more carbon steel alloys than stainless steel alloys.
The ASTM developed a six-digit system for categorizing stainless steel. Composed of both letters and numbers, this classification system begins with the letter A, which stands for any ferrous material, followed by a sequential series of numbers unrelated to the metal’s properties. The letter M, standing for “metric,” follows this number series. The last two digits refer to the year the standard for the grade was adopted or last revised.
Other organizations have also published grades and series of steel, including the British Standards (BS), German Standard (DIN), Chinese Standard (GB), European Standard (EN), Japanese Industrial Standards (JIS), and the International Organization for Standardization (IOS).
Each system functions entirely differently. For example, the SAE’s grade 304 steel would be graded as follows:
Regardless of its classification number, the specific alloy must meet the composition standards set for it by the system’s governing agency. To keep things as simple as possible, let’s use the SAE’s system to look at some of the most common grades of stainless steel.
Each grade of stainless steel has a slightly different chemical composition and consequently a distinct physical appearance.
The 300 series of stainless steel is the most common with 304 stainless steel and 316 stainless steel being the most prevalent grades. Two other common grades are 409 stainless steel and 430 stainless steel.
Stainless steel grades in the 300 series are classified as austenitic with 18-30% chromium and 6-20% nickel as their major alloying additions. It is solid iron containing face-centered cubic crystals.
The composition of 300-series stainless steel grades makes it especially useful in the construction, automotive, and aerospace industries. Since it can withstand extremely high temperatures, series 300 stainless steel is hardenable only by cold working methods.
Attractive and malleable, grade 301 features high strength and corrosion-resistant composition. Since it is highly malleable and abrasion resistant, grade 301 stainless steel often gets used for decorative structural applications. Kloeckner Metals supplies 301 stainless steel sheet.
Grade 304 represents 50% of the steel produced around the world. Grade 304 contains 18% chromium and 8% nickel, which makes it aesthetically pleasing as well as durable. It’s one weakness is saline, so grade 304 can pit or crevice when used near salt water. We stock 304 stainless steel sheet, 304 stainless steel plate, 304 stainless steel bar, and 304 stainless steel tube.
The second most commonly produced steel, grade 316 contains 2-3% molybdenum. Consequently, it’s more resistant to saline and thus better for coastal environments. Many surgical supplies also get made from grade 316 stainless steel. Kloeckner Metals supplies the ultra low carbon variant of this grade stainless steel. It is available in 316L stainless steel sheet, 316L stainless steel plate, 316L stainless steel bar, and 316L stainless steel tube.
This grade provides high tensile strength at elevated temperatures. It is more resistant to corrosion and pitting than either grade 304 or grade 316 stainless steel. Due to its composition, grade 317 stainless steel is a more expensive alloy than most other 300-level grades.
This series of stainless steel contains 11% chromium and 1% more manganese than 300-series grades, making it slightly magnetic. This steel also features a higher carbon content. As a result, 400-series stainless steel offers higher strength and more wear resistance, but it is also more easily corroded than 300-series grades. Heat-treating hardens 400-series stainless steel.
A ferritic steel, grade 430 forms easily but does not resist extreme temperatures or corrosion well. It’s often used for decorative purposes such as automotive trim. Grade 430 stainless steel is also commonly used in backyard barbecue grills. It is less expensive than grade 304 since it does not contain nickel. 430 stainless steel sheet is routinely stocked.
Among the most widely used ferritic steels, grade 434 is highly corrosion and oxidation resistant. It can also resist temperatures of up to 1500°F. Grade 434 stainless steel cannot be hardened by heat treatment and is generally cold formed like low carbon steel. It is typically used for automotive trim.
Grade 420 stainless steel is a very bright steel and one of the earliest carbon/iron alloys. Containing 12% chromium, this grade of steel is highly corrosion resistant. It is often called “blade grade steel” since manufacturers frequently use it to make surgical instruments.
A non-stabilized austenitic stainless steel, 904L grade products contain copper as part of the alloy. As a result, this grade of steel can resist corrosion from many acids. The high cost of this grade of stainless steel, however, has caused many manufacturers to turn to the lower-cost duplex stainless steel 2205 as an alternative.
Stainless steel comes in many series and grades. It’s important to choose the right kind of stainless steel for your product in your environment. Learn more about stainless steel on our website.
Kloeckner Metals is a full-line stainless steel supplier and service center. Kloeckner Metals brings a national footprint with the latest fabrication and processing technologies and most innovative customer service solutions.
The name stainless steel covers a variety of corrosion resistant steels that contain a minimum of 11% Chromium. Changing the Chromium content and adding other elements like Nickel, Molybdenum, Titanium and Niobium changes the mechanical and physical properties of the steel.
Stainless Steel Grades Explained
This results in hundreds of different grades of stainless steel that are covered by a range of national and international standards.
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Each one of the grades is grouped into one of five stainless steel families. These families are named after their metallurgical microstructure. The five groups are austenitic, ferritic, duplex, martensitic and precipitation hardening.
The lack of other alloying elements means ferritic stainless steel are known as plain Chromium steels. They have a Chromium content between 12 and 18%. The carbon content in ferritic stainless steel is very low.
Ferritic stainless steel:
Common ferritic grades include the proprietary grade 430 stainless steel and the cheapest stainless steel, grade 409 stainless steel. 409 stainless steel is the material of choice for automotive exhausts due to its combination of low price, corrosion resistance and excellent formability.
Adding nickel to stainless steel in sufficient amounts, changes the microstructure to “austenite”.
70% of commercially produced stainless steel are austenitic. The most common grade of austenitic stainless steel is 304 (1.4301). Globally, 304 accounts for more than 50% of stainless steel consumed. A common name for 304 stainless is 18/8. This name refers to the average composition, 18% chromium and 8% nickel. It is sometimes used generically for austenitic stainless steel other than 304, even if the actual composition is vastly different.
Some of the features of austenitic stainless steel include:
Other than 304 stainless steel, other common austenitic grades include the popular marine grade, 316 stainless steel and the machining bar grade, 303 stainless steel.
The first stainless steel to be developed for commercial applications were martensitic stainless steel. These steels were used for cutlery. When compared with other stainless steel, the martensitic stainless group have a relatively high carbon content (0.1 - 1.2%). Like ferritic stainless steel, they are plain chromium steels containing between 12 and 18% chromium.
Features of martensitic stainless steel include:
Martensitic grades include 420 stainless steel, which is used in engineering applications like shafts and 440C stainless steel – the hardest and most abrasion resistant of all the stainless steel.
Duplex stainless steel get their name from the fact that they contain both a ferritic and austenitic microstructure. They have a relatively high chromium content of between 18 and 28%. Nickel content is moderate at 4.5 to 8%.
At this level, the nickel content is too low to generate a fully austenitic structure. This results in a duplex microstructure containing both ferritic and austenitic phases.
Duplex stainless steel also tend to contain 2.5-4% molybdenum.
The prime advantage of duplex stainless is the combination of properties derived from both austenitic and ferritic stainless steel.
Duplex stainless steel have:
With excellent corrosion resistance the common duplex grade, 2205 stainless steel, is used in heat exchangers, chemical tanks and refineries.
Precipitation hardening stainless steel can be martensitic, semi-austenitic or austenitic. They offer the combined properties of corrosion resistance from austenitic grades with the heat treatability of martensitic grades.
Precipitation hardening grades, like 17-4 PH (also known as 630 stainless steel), are supplied as solution treated bars. They can then be machined before hardening.
The hardening process is a single, low temperature, ageing step.
Properties of precipitation hardening grades include:
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Grade compositions, mechanical properties and production specifications are governed by a range of international and national standards for stainless steel. While the old AISI three digit stainless steel numbering system (e.g. 304 and 316) is still commonly used for the classification of stainless steel grades, new classification systems have been developed.
These systems include a 1-letter + 5-digit UNS number, like S30400, as defined by SAE and ASTM. European countries are adopting unified Euronorm standards. These countries are either replacing or adapting their own country specific standards to mirror the Euronorm standards. Other designations being replaced include old BS and EN numbers like 304S31 and 58E.
Some grades are not covered by standard numbers and could be proprietary grades or be named using standards for specialist products like welding wire.
Stainless steel standards are explained in detail in the British Stainless Steel Association “Guide to Stainless Steel Specifications”, also known as the BSSA “Blue Guide”.
The table 1 lists a range of stainless steel grades, their old BS designation, new UNS number and new EN designation.
Table 1. Stainless steel grades and their international equivalents
Grade
UNS No
BS
Euronorm No.
301
S30100
301S21
1.4310
302
S30200
302S25
1.4319
303
S30300
303S31
1.4305
304
S30400
304S31
1.4301
304L
S30403
304S11
1.4306
304H
S30409
-
1.4948
(302HQ)
S30430
394S17
1.4567
305
S30500
305S19
1.4303
309S
S30908
309S24
1.4833
310
S31000
310S24
1.4840
310S
S31008
310S16
1.4845
314
S31400
314S25
1.4841
316
S31600
316S31
1.4401
316L
S31603
316S11
1.4404
316H
S31609
316S51
-
316Ti
S31635
320S31
1.4571
321
S32100
321S31
1.4541
347
S34700
347S31
1.4550
403
S40300
403S17
1.4000
405
S40500
405S17
1.4002
409
S40900
409S19
1.4512
410
S41000
410S21
1.4006
416
S41600
416S21
1.4005
420
S42000
420S37
1.4021
430
S43000
430S17
1.4016
440C
S44004
-
1.4125
444
S44400
-
1.4521
630
S17400
-
1.4542
(904L)
N08904
904S13
1.4539
(253MA)
S30815
-
1.4835
(2205)
S31803
318S13
1.4462
(3CR12)
S41003
-
1.4003
(4565S)
S34565
-
1.4565
(Zeron100)
S32760
-
1.4501
(UR52N+)
S32520
-
1.4507
ASTM does not recognise the designations in brackets. Many other grades and specifications are available.
Material supplied by Aalco has been manufactured to comply with a number of standards depending upon the product. Standards also cover the finish of the material.
of Stainless Steel
Required mechanical properties are normally given in purchase specifications for stainless steel. Minimum mechanical properties are also given by the various standards relevant to the material and product form. Meeting these standard mechanical properties indicates that the material has been properly manufactured to an appropriate quality system. Engineers can then confidently utilise the material in structures that meet safe working loads and pressures.
Mechanical properties specified for flat rolled products are normally tensile strength, yield stress (or proof stress), elongation and Brinell or Rockwell hardness. Property requirements for bar, tube, pipe and fittings typically state tensile strength and yield stress.
of Stainless Steel
Unlike mild steels, the yield strength of annealed austenitic stainless steel is a very low proportion of the tensile strength. Mild steel yield strength is typically 65-70% of the tensile strength. This figure tends to only be 40-45% in the austenitic stainless family.
Cold working rapidly and greatly increases the yield strength. Some forms of stainless steel, like spring tempered wire, can be cold worked to lift the yield strength to 80-95% of the tensile strength.
of Stainless Steel
The combination of high work hardening rates and high elongation / ductility makes stainless steel very easy to fabricate. With this property combination, stainless steel can be severely deformed in operations like deep drawing.
Ductility is normally measured as the % elongation before fracture during tensile testing. Annealed austenitic stainless steels have exceptionally high elongations. Typical figures are 60-70%.
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Hardness is the resistance to penetration of the material surface. Hardness testers measure the depth that a very hard indenter can be pushed into the surface of a material. Brinell, Rockwell and Vickers machines are used. Each of these has a different shaped indenter and method of applying the known force. Conversions between the different scales are therefore only approximate.
Martensitic and precipitation hardening grades can be hardened by heat treatment. Other grades can be hardened through cold working.
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Tensile strength is generally the only mechanical property required to define bar and wire products. Identical material grades may be used at various tensile strengths for completely different applications. The supplied tensile strength of bar and wire products directly relates to the final use after fabrication.
Spring wire tends to have the highest tensile strength after fabrication. The high strength is imparted by cold working into coiled springs. Without this high strength the wire would not function properly as a spring.
Such high tensile strengths are not required for wire to be used in forming or weaving processes. Wire or bar used as raw material for fasteners, like bolts and screws, needs to be soft enough for a head and thread to be formed but still strong enough to perform adequately in service.
The different families of stainless steel tend to have different tensile and yield strengths. These typical strengths for annealed material are outlined in Table 2.
Table 2. Typical strength for annealed stainless steel from different families
Tensile Strength
Yield Strength
Austenitic
600
250
Duplex
700
450
Ferritic
500
280
Martensitic
650
350
Precipitation Hardening
1100
1000
of Stainless Steel
The reason for choosing stainless steel is normally due to advantages given by physical properties such as corrosion resistance.
In addition to corrosion resistance, the advantageous physical properties of stainless steel include:
of Stainless Steel
Good corrosion resistance is a feature of all stainless steels. Low alloy grades can resist corrosion in normal conditions. Higher alloys resist corrosion by most acids, alkaline solutions and chloride environments.
The corrosion resistance of stainless steel is due to their chromium content. In general, stainless steel contain a minimum of around 10.5% chromium. The chromium in the alloy forms a self-healing protective clear oxide layer that forms spontaneously in air. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained.
Some stainless steel grades can resist scaling and retain high strength at very high temperatures. Other grades maintain high mechanical properties at cryogenic temperatures.
of Stainless Steel
Most grades of stainless steel can be cut, welded, formed, machined and fabricated using standard methods and equipment used for other types of steel.
Component designs and fabrication methods can be altered to take advantage of the work hardening of stainless steels that occurs when they are cold worked. The resultant high strengths can allow the use of thinner material, leading to lower weights and costs.
Other stainless steel can be heat treated to increase strength.
Stainless steel can be supplied with a range of surface finishes. It can also be polished after fabrication to give the desired finish. The finish of stainless steel is easy to clean and maintain.
o
f CleaningStainless steel is non-toxic and readily cleaned. This makes stainless steel the material of choice for use in hospitals, kitchens, food, drink and pharmaceutical processing plants.
The durability and corrosion resistance of stainless steel means it will outlast many competing products. The low maintenance characteristics also add to stainless steel often being the lowest cost choice in a life cycle cost comparison.
Stainless steel is fully recyclable. New stainless steel normally contains between 50 and 80% recycled material. Scrap stainless steel can be stored without degradation to its value as a raw material.
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Magnetic permeability is the ability of a material to attract a magnet. The austenitic grades are the only stainless steel grades that are not magnetic. Cold working can induce a limited degree of magnetism in austenitic grades other than 310 and 316 stainless steels.
DISCLAIMER
This Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn. In particular, the mechanical property requirements vary widely with temper, product and product dimensions. The information is based on our present knowledge and is given in good faith. However, no liability will be accepted by the Company is respect of any action taken by any third party in reliance thereon.
As the products detailed may be used for a wide variety of purposes and as the Company has no control over their use; the Company specifically excludes all conditions or warranties expressed or implied by statute or otherwise as to dimensions, properties and/or fitness for any particular purpose.
Any advice given by the Company to any third party is given for that party’s assistance only and without liability on the part of the Company. Any contract between the Company and a customer will be subject to the company’s Conditions of Sale. The extent of the Company’s liabilities to any customer is clearly set out in those Conditions; a copy of which is available on request.
This information has been sourced, reviewed and adapted from materials provided by Aalco - Ferrous and Non-Ferrous Metals Stockist.
For more information on this source, please visit Aalco - Ferrous and Non-Ferrous Metals Stockist.