Equivalent carbon content
Encyclopedia
The equivalent carbon content concept is used on ferrous materials, typically steel
and cast iron
, to determine various properties of the alloy when more than just carbon
is used as an alloyant, which is typical. The idea is to convert the percentage of alloying elements other than carbon to the equivalent carbon percentage, because the iron-carbon phases are better understood than other iron-alloy phases. Most commonly this concept is used in welding
, but it is also used when heat treating and casting cast iron.
. Higher concentrations of carbon and other alloying elements such as manganese
, chromium
, silicon
, molybdenum
, vanadium
, copper
, and nickel
tend to increase hardness and decrease weldability. Each of these materials tends to influence the hardness and weldability of the steel to different magnitudes, however, making a method of comparison necessary to judge the difference in hardness between two alloys made of different alloying elements. There are two commonly used formula for calculating the equivalent carbon content. One is from the American Welding Society
(AWS) and recommended for structural steels and the other is the formula based on the International Institute of Welding
(IIW).
The AWS states that for an equivalent carbon content above 0.40% there is a potential for cracking in the heat-affected zone
(HAZ) on flame cut edges and welds. However, structural engineering standards rarely use CE, but rather limit the maximum percentage of certain alloying elements. This practice started before the CE concept existed, so just continues to be used. This has led to issues because certain high strength steels are now being used that have a CE higher than 0.50% that have brittle failures.
The other and most popular formula is the Dearden and O'Neill formula, which was adopted by IIW in 1967. This formula has been found suitable for predicting hardenability in a large range of commonly used plain carbon and carbon-manganese steels, but not to microalloyed high-strength low-alloy steels or low-alloy Cr-Mo steels. The formula is defined as follows:
For this equation the weldability based on a range of CE values can be defined as follows:
The Japanese Welding Engineering Society adopted the critical metal parameter (Pcm) for weld cracking, which was based on the work from Ito and Bessyo, is:
If some of the values are not available, the following formula is sometimes used:
The carbon equivalent is a measure of the tendency of the weld to form martensite
on cooling and to suffer brittle fracture. When the carbon equivalent is between 0.40 and 0.60 weld preheat may be necessary. When the carbon equivalent is above 0.60, preheat is necessary, postheat may be necessary.
The following carbon equivalent formula is used to determine if a spot weld will fail in high-strength low-alloy steel due to excessive hardenability:
Where UTS is the ultimate tensile strength in ksi
and h is the strip thickness in inches. A CE value of 0.3 or less is considered safe.
A special carbon equivalent was developed by Yorioka, which could determine the critical time in seconds Δt8-5 for the formation of martensitic in the Heat Effected Zone (HAZ) in low-carbon alloy steels. The equation is given as:
where:
Then the critical time length in seconds Δt8-5 can be determined as follows:
This CE is then used use to determine if the alloy is hypoeutectic, eutectic, or hypereutectic; for cast irons the eutectic is 4.3% carbon. When casting cast iron this is useful for determining the final grain
structure; for example, a hypereutectic cast iron usually has a coarse grain structure and large kish graphite flakes are formed. Also, there is less shrinkage as the CE increases. When heat treating cast iron, various CE samples are tested to empirically determine the correlation between CE and hardness. The following is an example for induction hardened gray irons:
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
and cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
, to determine various properties of the alloy when more than just carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
is used as an alloyant, which is typical. The idea is to convert the percentage of alloying elements other than carbon to the equivalent carbon percentage, because the iron-carbon phases are better understood than other iron-alloy phases. Most commonly this concept is used in welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
, but it is also used when heat treating and casting cast iron.
Steel
In welding, equivalent carbon content (CE) is used to understand how the different alloying elements affect hardness of the steel being welded. This is then directly related to hydrogen-induced cold cracking, which is the most common weld defect for steel, thus it is most commonly used to determine weldabilityWeldability
The weldability, also known as joinability, of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others...
. Higher concentrations of carbon and other alloying elements such as manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
tend to increase hardness and decrease weldability. Each of these materials tends to influence the hardness and weldability of the steel to different magnitudes, however, making a method of comparison necessary to judge the difference in hardness between two alloys made of different alloying elements. There are two commonly used formula for calculating the equivalent carbon content. One is from the American Welding Society
American Welding Society
The American Welding Society is a nonprofit organization dedicated to advancing the science, technology, and application of welding and allied joining and cutting processes, including brazing, soldering, and thermal spraying...
(AWS) and recommended for structural steels and the other is the formula based on the International Institute of Welding
International Institute of Welding
The International Institute of Welding is an international scientific and engineering body for welding, brazing and related technologies. Its membership consists of the national welding societies from around the world. The Institute was founded in 1948 by 13 national societies...
(IIW).
The AWS states that for an equivalent carbon content above 0.40% there is a potential for cracking in the heat-affected zone
Heat-affected zone
The heat-affected zone is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding...
(HAZ) on flame cut edges and welds. However, structural engineering standards rarely use CE, but rather limit the maximum percentage of certain alloying elements. This practice started before the CE concept existed, so just continues to be used. This has led to issues because certain high strength steels are now being used that have a CE higher than 0.50% that have brittle failures.
The other and most popular formula is the Dearden and O'Neill formula, which was adopted by IIW in 1967. This formula has been found suitable for predicting hardenability in a large range of commonly used plain carbon and carbon-manganese steels, but not to microalloyed high-strength low-alloy steels or low-alloy Cr-Mo steels. The formula is defined as follows:
For this equation the weldability based on a range of CE values can be defined as follows:
| Carbon equivalent (CE) | Weldability |
|---|---|
| Up to 0.35 | Excellent |
| 0.36–0.40 | Very good |
| 0.41–0.45 | Good |
| 0.46–0.50 | Fair |
| Over 0.50 | Poor |
The Japanese Welding Engineering Society adopted the critical metal parameter (Pcm) for weld cracking, which was based on the work from Ito and Bessyo, is:
If some of the values are not available, the following formula is sometimes used:
The carbon equivalent is a measure of the tendency of the weld to form martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
on cooling and to suffer brittle fracture. When the carbon equivalent is between 0.40 and 0.60 weld preheat may be necessary. When the carbon equivalent is above 0.60, preheat is necessary, postheat may be necessary.
The following carbon equivalent formula is used to determine if a spot weld will fail in high-strength low-alloy steel due to excessive hardenability:
Where UTS is the ultimate tensile strength in ksi
Ksi
Ksi is a letter of the early Cyrillic alphabet, derived from the Greek letter Xi . It was mainly used in Greek loanwords, especially words relating to the Church....
and h is the strip thickness in inches. A CE value of 0.3 or less is considered safe.
A special carbon equivalent was developed by Yorioka, which could determine the critical time in seconds Δt8-5 for the formation of martensitic in the Heat Effected Zone (HAZ) in low-carbon alloy steels. The equation is given as:
where:
Then the critical time length in seconds Δt8-5 can be determined as follows:
Cast iron
For cast iron the equivalent carbon content (CE) concept is used to understand how alloying elements will affect the heat treatment and casting behavior. It is used as a predictor of strength in cast irons because it gives an approximate balance of austenite and graphite in final structure. The following formulas are used to determine the CE in cast irons:This CE is then used use to determine if the alloy is hypoeutectic, eutectic, or hypereutectic; for cast irons the eutectic is 4.3% carbon. When casting cast iron this is useful for determining the final grain
Crystallite
Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...
structure; for example, a hypereutectic cast iron usually has a coarse grain structure and large kish graphite flakes are formed. Also, there is less shrinkage as the CE increases. When heat treating cast iron, various CE samples are tested to empirically determine the correlation between CE and hardness. The following is an example for induction hardened gray irons:
| Composition [%]† | | Carbon equivalent‡ | | Hardness [HRC] (convert from hardness test) | |||
|---|---|---|---|---|---|
| C | Si | HRC | HR 30 N | Microhardness | |
| 3.13 | 1.50 | 3.63 | 50 | 50 | 61 |
| 3.14 | 1.68 | 3.70 | 49 | 50 | 57 |
| 3.19 | 1.64 | 3.74 | 48 | 50 | 61 |
| 3.34 | 1.59 | 3.87 | 47 | 49 | 58 |
| 3.42 | 1.80 | 4.02 | 46 | 47 | 61 |
| 3.46 | 2.00 | 4.13 | 43 | 45 | 59 |
| 3.52 | 2.14 | 4.23 | 36 | 38 | 61 |
| †Each sample also contained 0.5–0.9 Mn, 0.35–0.55 Ni, 0.08–0.15 Cr, and 0.15–0.30 Mo. ‡Using the CE second equation. |
|||||
Further reading
- Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 99949-25-82-2. (Page 3.3-3)
- Weman, Klas (2003). Welding processes handbook. New YorkNew YorkNew York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
: CRC Press LLC. ISBN 0-8493-1773-8. - American Welding Society (2004). Structural Welding Code, AWS D1.1. ISBN 0-87171-726-3.