Carbon Equivalent (CE) is an empirical value in weight percent, relating the combined effects of different alloying elements used in the making of carbon steels to an equivalent amount of carbon. This value can be calculated using a mathematical equation. By varying the amount of carbon and other alloying elements in the steel, the desired strength levels can be achieved by proper heat treatment. A better weldability and low temperature notch toughness can also be obtained.
In terms of welding, the Carbon Equivalent governs the hardenability of the parent metal. It is a rating of weldability related to carbon, manganese, chromium, molybdenum, vanadium, nickel and copper content. There are several commonly used equations for expressing Carbon Equivalent. One example of such mathematical formula is:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
The table below shows the preferred and maximum weight percent content of some elements and the diagram that follows shows the influence of certain element content on the hardness.
|
Element |
Composition |
|
|
Preferred
(%) |
High
(%) |
|
|
Carbon |
0.06 to 0.25 |
0.35 |
|
Manganese |
0.35 to 0.80 |
1.40 |
|
Silicon |
0.10 or less |
0.30 |
|
Sulphur |
0.035 or less |
0.05 |
|
Phosphorus |
0.030 or less |
0.04 |
Base
composition: 0.25% C, 0.30% Si, 0.70% Mn
The ability to form hard metallurgical constituents such as martensites or any other hard phases is dependent on the carbon equivalent and the cooling rate of the steel involved in cooling from the transformation temperature. The higher the carbon equivalent value, the faster the cooling rate, the higher the tendency for hard, brittle phases to form during cooling.
The metallurgical characteristics of steels are mainly determined by its chemical composition. As such, any small changes in its chemical composition of the base and filler metals can substantially increase cracking tendency. The risk of cracking also increases with increasing hardness of the Heat Affected Zone (HAZ) in welding for a particular hydrogen level and joint restraint. The diagram below shows the influence of carbon content and the transformation temperature on the HAZ microstructure and toughness.
As such, the value of the Carbon Equivalent is a useful guide to the possibility of cracking in alloy steels by comparison with an equivalent plain carbon steel. The two main problems faced in the cracking of the welded metals are hot cracking and cold cracking.
Hot cracking occurs immediately after solidification in a weld, caused by the segregation of certain alloying elements during the solidification process. Sulphur, boron and other elements that tend to segregate excessively are reduced in order to prevent hot cracking. Cold cracking, also known as delayed or hydrogen-induced cracking, develops after solidification of the fusion zone as the result of residual stress. It generally occurs below 200°C, sometimes several hours, or even days after welding.
Although a carbon equivalent is sometimes useful in planning welding procedures, its value is limited because only the chemical composition of the steel is considered. The section size being welded and joint restraint is of equal or greater importance, because of their relations to heat input and cooling rate.
References
1. http://www.howellpipe.com/howgloss.htm
2. N. Bailey, Welding Steels without Hydrogen Cracking, Cambridge: Abington Pub, 1993, p. 11-12, 35-36
3. http://www.weldind.com/wl4.html
4. http://www.key-to-steel.com/articles/art50.htm
5. http://www.bethsteel.com/customers/pdfs/welding.pdf
6. http://www.us.cbmm.com.br/english/sources/techlib/info/weldabil/fig8.htm
7. http://www.welding.org/newsletters/winter1999/metallurgy.html
8. http://www.weldind.com/wl3.html