Carbon Equivalent formula for steel – An overview

The carbon equivalent calculation for steel is carried out on the basis of the chemical composition (preferably the product analysis). The value is used to estimate hardening and susceptibility to cold cracking during welding.

In addition to carbon, other alloying elements also favour cold cracking. Carbon equivalents are often determined to estimate the susceptibility to cracking. There are numerous formulae for describing the carbon equivalent, in which the individual alloying elements are weighted differently.

CE_IIW carbon equivalent

Avoidance of hydrogen cracking (also known as cold cracking). The formula based on the International Institute of Welding (IIW). This carbon equivalent CE_IIW (also called CEV) is included in various standards and codes for pressurised containers e.g. as method A of ISO/TR 17671-2 and method A of the harmonised standard EN 1011-2 Annex C par. 2.2.
Applicable steels: C-Mn steels, fine grain steels and low alloy steels.
Limits: CE_IIW = 0,30 to 0,70 %wt;

Carbon equivalent calculation is performed according to the formula below:

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

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

• 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 to avoid hydrogen cracks and relates to arc welding of ISO 15608 material groups 1 to 4, specially high strength steels. This carbon equivalent is included in various standards and codes for pressurised containers e.g. as method B of ISO/TR 17671-2 and method B of the harmonised standard EN 1011-2 Annex C par. 3.2. The carbon equivalent CET is preferably used to calculate the preheating temperature with this determined value. There is a linear relationship between the carbon equivalent CET and the preheating temperature. An increase in the carbon equivalent CET of around 0.01% leads to an increase in the preheating temperature of around 7.5 °C.
Applicable steels: C-Mn steels, fine grain steels, low alloy steels

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

Read More

CET – Equivalent Calculator

CEN ISO/TR 17844: Welding — Comparison of standardised methods for the avoidance of cold cracks