Carbon equivalent formulae in relation to hydrogen cracking

Carbon equivalent formulae were originally developed to give a numerical value for a steel composition which would give an indication of a carbon content which would contribute to an equivalent level of hardenability for that steel. These formulae were later extended to represent the contribution of the composition to the hydrogen cracking susceptibility of steel. They are also used as compositional characterising parameters for other properties that may be linked to hardness, such as toughness and strength.

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These kind of relationships originate from about 1940 when Dearden and O'Neill proposed a carbon equivalent formula to predict steel strength, hardenability and HAZ hardness ^[1]. In 1967, the International Institute for Welding (IIW) adopted a somewhat simplified form of Dearden and O'Neill's formula for hardenability which became a generally accepted measure of steel weldability - CE_IIW

Since its adoption by IIW, the equation has been incorporated into a number of material standards and codes, including EN 1011-2:2001 ^[2] (replaces BS 5135-1984 ^[3] ) and in a modified form in AWS D1.1 ^[4] , with a "+Si/6" term added to the equation.

Further development of Carbon equivalent formulae has taken place and several can be found in technical literature today. Three of the more common ones are P_cm, CEq and CEN. Ito and Bessyo ^[5] developed P_cm in Japan based on a wider range of steels than the IIW formula:

The CEq formula devised by Düren ^[6] has a similar appearance:

Both the P_cm and the CEq formulae were developed for low carbon steels for which the CE_IIW is less suitable. P_cm is generally used for modern steels typically used for pipeline manufacture, where carbon contents are no more than ~0.11 wt% ^[7] .

However, it should be noted that the P_cm formula was derived largely from lower C low alloy steels.

The CEN formula was proposed to evaluate the weldability of a wide variety of steels. For the higher C range, the values of CEN correlate well with carbon equivalents such as CE_IIW, whereas for lower carbon steels the values are close to those of the CEq formula. CEN is given by:

Yurioka ^[8] illustrated a good correlation between P_cm and CEN for structural steels, low-alloy steels (Ni-Cr-Mo type) and carbon steels, provided the carbon content was less than 0.17 wt%. From this comparison the following relationship was derived:

CEN = 2P _cm - 0.092 (C ≤ 0.17%)

For carbon steels, the values deviate from this relationship as the P_cm overestimates the cold cracking susceptibility. Where the carbon content exceeds 0.17 wt%, there is a better correlation between CEN and CE_IIW :

CEN = CE_IIW + 0.012 (C ≥ 0.17%)

Yurioka ^[8] grouped a number of carbon equivalents for the assessment of weldability, as follows:

Group A are characterised by 1/6 as the coefficient of Manganese; Group B has Carbon as more important than the other alloying elements and is more applicable to modern steels; Group C includes interactions between Carbon and other elements.

Other existing formulae that could be classified as Group B are the CEw developed by Cottrell ^[9] , and the CET included in the standard EN 1011-2:2001 ^[3] . Cottrell claimed that the CEw formula could improve the predictions of cracking by a factor of three, compared to CE_IIW. The data on which the formula was based was collected from results published in open literature, and covered a wide range of composition and welding conditions. The resulting formula is as follows:

The CET formula is based on similar elements to the CE_IIW formula with the exception of Vanadium, although carbon is considered to have more significance than the other elements:

References

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