Introduction to Steel Grades
Steels are impure iron-carbon alloys of low carbon content, usually 0.1–1.5% carbon by weight. The amount of carbon and level of impurities and additional elements, both metallic and non-metallic, determine the properties of each steel grade [1].
Various types of steel are manufactured in relation to the needed properties for their application, and different grading systems are utilised to differentiate steels based on these properties. According to the World Steel Association, there are more than 3,500 steel grades encompassing distinct chemical, environmental, and physical properties [2].
Here, you will learn about:
- the chemical composition of steel grades,
- the chemical composition’s influence on the material’s mechanical properties,
- the different grading systems currently in use by various industries
Chemical composition
Listed below are some of the chemical elements used to influence the mechanical properties of steel grades [3]:
- Carbon
Carbon is one of the most important chemical elements in steel. An increase in carbon content yields a material with lower ductility and higher strength. - Manganese
Manganese is used as a neutraliser in hot rolling production of steel together with oxygen and sulphur, and it produces effects on the steel grades’ material properties similar to those of carbon. - Chromium
Chromium is present in small amounts and is used in combination with copper and nickel to increase the material’s resistance to corrosion. - Aluminium
Aluminium is one of the most important deoxidisers, and aids in forming a more fine-grained crystalline microstructure. - Copper
Copper is also used to increase resistance to corrosion. It is the main anti-corrosion component of steel grades A242 and A441 (withdrawn, replaced by A572). - Molybdenum
Molybdenum improves the steel’s strength at high temperatures and also increases its resistance to corrosion. For steel grade A514, a common amount of molybdenum ranges between 0.15–0.65%. - Sulphur and phosphorus
Sulphur and phosphorus generally constitute a restricted amount in steel alloys for they have undesirable effects on steel’s durability and strength.
Other alloying elements such as titanium, nitrogen, and boron are also used in small amounts by some steel grades. These chemical elements are combined with the major components to further improve the performance of the material [3].
Steels can be broadly categorised according to their chemical composition – alloy steel, carbon steel, and stainless steel.
Mechanical properties
Each steel grade, following international standards, reflects the measured mechanical properties of the material [4]:
- Strength
Strength refers to the force needed to deform a material. The metal strength of steel can be improved through normalisation, which creates a uniform microstructure throughout the material. - Hardness
Hardness is the material’s ability to resist abrasion. Increasing the carbon content and quenching the material results in improved hardness. - Ductility
Ductility refers to the ability of a metal to plastically deform under tensile stress. By annealing the cold-formed steel, its low ductility can be improved, as annealing enables the reforming of crystals, thus eliminating the dislocations in the microstructure. - Machinability
Machinability refers to how easy it is for steel to be ground, cut, or drilled. It is greatly affected by hardness. As hardness increases, machining becomes more difficult. - Toughness
Toughness is the ability of a material to resist stress without fracturing. Toughness can be improved by adding spheroids in the microstructure as in tempering. - Weldability
Weldability refers to the ease with which a material can be welded without defects. Heat conductivity, along with melting point and electrical conductivity can influence the weldability of a material. It is mainly dependent, however, on the heat treatment used and the chemical composition of the material.
Steel grades numbering system
The steel grade communicates the chemical composition, properties, fabrication processes, heat treatments and forms of steel. Grading is very important to fabricators, engineers and consumers as it gives a standard language for effectively noting the properties of steel [4].
Listed below are some of the most common international standard organisations, each with their steel grades numbering system.
American Iron and Steel Institute (AISI)
AISI is the most popular and the oldest numbering system for all steels in the US. It states the chemical composition of an alloy based on ladle analysis but does not indicate other properties. AISI utilises a four-digit numbering system for carbon steels and a three-digit numbering system for stainless steels, having a “type” prefix for identification. Some steel grades contain suffixes that indicate the modifications in the composition, such as type 303Se, showing the addition of selenium into the composition. The AISI compositions and designations act as the primary standards for a wide range of industries [5].
Society of Automotive Engineers International (SAE)
Similarly for SAE, alloy and carbon steels are assigned with a four-digit number, where the first digit specifies the main alloying element. The second number shows the top grade element while the last two digits indicate the carbon composition of the steel (in hundredths of a percent by weight) [6].
The table below shows the different classifications of steel and its corresponding SAE designation [7]:
|
SAE designation |
Type |
|
1xxx |
|
|
2xxx |
|
|
3xxx |
|
|
4xxx |
|
|
5xxx |
|
|
6xxx |
|
|
7xxx |
|
|
8xxx |
|
|
9xxx |
For stainless steel, SAE utilises a five-digit numbering system, the last three numbers of which are in compliance with the designations of AISI alloy standards [5]. It mainly describes standards and practices that can be at the base of designing, constructing, and characterising automotive components.
Unified Numbering System (UNS)
UNS uses a prefix letter and a five-digit numbering system designed to correlate the different numbering systems for alloys and metals that are commercially used by various nations and standards organisations [5].
Shown below is the table of different UNS categories [8]:
|
UNS Series |
Type |
|
A00001 to A99999 |
|
|
C00001 to C99999 |
|
|
D00001 to D99999 |
Specified mechanical property steels |
|
E00001 to E99999 |
|
|
F00001 to F99999 |
|
|
G00001 to G99999 |
AISI and SAE carbon and alloy steels (except tool steels) |
|
H00001 to H99999 |
AISI and SAE H-steels |
|
J00001 to J99999 |
|
|
K00001 to K99999 |
Miscellaneous steels and ferrous alloys |
|
L00001 to L99999 |
Low-melting metals and alloys |
|
M00001 to M99999 |
Miscellaneous nonferrous metals and alloys |
|
N00001 to N99999 |
|
|
P00001 to P99999 |
Precious metals and alloys |
|
R00001 to R99999 |
Reactive and refractory metals and alloys |
|
S00001 to S99999 |
Heat and corrosion resistant (stainless) steels |
|
T00001 to T99999 |
|
|
W00001 to W99999 |
Welding filler metals |
|
Z00001 to Z99999 |
American Society for Testing and Materials (ASTM)
ASTM steel grade system provides the chemical composition and performance requirements of the material. It also bears the test method standards along with the minimum and common values for a variety of physical and mechanical properties [5]. Examples include ASTM 36 and ASTM A53.
Other organisations utilising their own numbering systems include the American National Standards Institute (ANSI), American Society of Mechanical Engineers (ASME), Steel Founders Society of America, and American Welding Society (AWS) [9].
Sources
[1] W. Hume-Rothery, The Structure of Alloys of Iron: An Elementary Introduction,H.M. Finniston, D.W. Hopkins, W.S. Owen (Ed.s), Elsevier, 2016.
[2] “Most Common types of Steel in Process Piping Industry,” n.d. [Online]. Available: https://www.theprocesspiping.com/common-types-steel-process-piping-industry/
[3] “Chemical Composition of Structural Steels,” n.d. [Online]. Available: http://web.mit.edu/1.51/www/pdf/chemical.pdf
[4] “Steel Grading: Chemistry and Properties”, 2018, from: https://www.reliance-foundry.com/blog/steel-grades
[5] E. Klar, P.K. Samal, Powder Metallurgy Stainless Steels: Processing, Microstructures, and Properties, OH: ASM International, 2007.
[6] E.P. Degarmo, J.T. Black, R.A. Kohser, Materials and Processes in Manufacturing (9th ed.).Wiley, 2003.
[7] L.F. Jeffus, Welding: Principles and Applications. Cengage Learning, 2016.
[8] E. Oberg, H.L. Horton, F.D. Jones, H.H. Ryfell, and C.J. McCauley, Machinery's Handbook (29th ed.).Industrial Press Inc., 2012.
[9] “Engineering Handbook Technical Information”, n.d. [Online]. Available: https://www.isibang.ac.in/~library/onlinerz/resources/Enghandbook.pdf