Calculation method Carbon Equivalent of Steel

The carbon equivalent for steel is calculated based on its chemical composition (preferably the product analysis). This value is used to estimate hardening and susceptibility to cold cracks during welding.

In addition to carbon, other alloying elements also promote the formation of cold cracks. Carbon equivalents are often determined to estimate susceptibility to crack formation. There are numerous formulas for describing the carbon equivalent, in which the individual alloying elements are weighted differently.

CE_IIW carbon equivalent

Prevention of hydrogen cracking (also known as cold cracking). The formula is based on the International Institute of Welding (IIW). This carbon equivalent CEIIW (also known as CEV) is included in various standards and regulations for pressure vessels, for example as Method A of ISO/TR 17671-2 and Method A of the harmonized standard EN 1011-2, Annex C, Section 2.2.

Applicable steel grades: C-Mn steel, fine-grained steel, and low-alloy steel.
Limit values: CEIIW = 0.30 to 0.70 percent by weight.

The carbon equivalent is calculated using the following formula:

CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 [wt%]

• EN 13445-4 clause 8.9: recommendation for pre-heating;
• Many material standards (EN…, ASTM…) contain maximum CE_IIW limit values for certain steels suitable for welding or provide for an optional ordering option.
• ASME BPVC Sect. VIII par. UCS-56-1: recommendation for pre-heating for partial materials;
• ASME BPVC Sect. IX table QW-290.4: WPS essential variables for temper bead procedure qualification
• API Specification 5L clause 9.2.4 and 9.2.5: max. values only PSL 2 pipe with carbon content C > 0,12 %;
• IGC Doc 120/14 clause D8: CE_IIW limit is max. 0,43% for syngas pipelines with carbon content C > 0,12 %;
• IGC Doc 121/04 clause D8: CE_IIW limit is max. 0,43% for hydrogen pipelines with carbon content C > 0,12 %;

Limits of chemical composition for the main alloying elements [wt%]: C = 0,05 to 0,25; Si = max. 0,8; Mn = max. 1,7; Cr = max.0,9; Cu = max. 1,0; Ni = max. 2,5; Mo = max. 0,75; V = max. 0,20.

CET carbon equivalent

This method is intended to prevent hydrogen cracking and applies to arc welding of material groups 1 through 4 according to ISO 15608, particularly high-strength steels. This carbon equivalent is included in various standards and regulations for pressure vessels, for example as Method B of ISO/TR 17671-2 and Method B of the harmonized standard EN 1011-2, Annex C, Section 3.2. The carbon equivalent (CET) is preferably used to calculate the preheating temperature based on this established value. There is a linear relationship between the carbon equivalent (CET) and the preheating temperature. An increase in the carbon equivalent (CET) of approximately 0.01% results in an increase in the preheating temperature of approximately 7.5 °C.

Applicable steel grades: C-Mn steel, fine-grained steel, low-alloy steel.

CET = C + (Mn+Mo)/10 + (Cr + Cu)/20 + Ni/40 [wt%]

Limits of chemical composition for the main alloying elements [wt%]: C = 0,05 to 0,32; Si = max. 0,8; Mn = 0,5 to 1,9; Cr = max. 1,5; Cu = max. 0,7; Ni = max. 2,5; Mo = max. 0,75; V = max. 0,18; Nb = max. 0,06; Ti = max. 0,12.

Pcm carbon equivalent

The carbon equivalent Pcm (also called CE_Pcm) is based on Japanese results from Ito and Bessyo in 1969. Pcm is designed for newer microalloyed steels with low carbon contents. For steels with low carbon contents or carbon equivalents Pcm ≤ 0.12% has been found be to a better indicator of crack susceptibility. The effect of carbon becomes critical to an HAZ containing large amounts of martensite. Thus, Pcm is a good indicator of hydrogen-assisted cracking in the HAZ because carbon is a heavily weighted factor in this formula:

Carbon equivalent calculation is performed according to the formula below:

Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B [wt%]

Standards and pressure vessel codes where this carbon equivalent Pcm is used:

• ISO 3138: max. Pcm values (0,25%) for only PSL 2 pipe material with carbon content C ≤ 0,12 %.
• API Specification 5L clause 9.2.4 and 9.2.5: max. values for only PSL 2 pipes with carbon content C ≤ 0,12 %.
• IGC Doc 120/14 clause D8: Pcm limit is max. 0,20 % for syngas pipelines with carbon content C ≤ 0,12 %.
• IGC Doc 121/04 clause D8: Pcm limit is max. 0,20 % for hydrogen pipelines with carbon content C ≤ 0,12 %.
• AWS D1.1 clause B6.2.2

Reference.. ped-online.com

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