Carbon equivalent CE(IIW), Pcm and CET

Hydrogen-assisted cold cracking needs three things at once: a susceptible microstructure, dissolved hydrogen and tensile stress. Composition controls the first of those. Every alloying element that delays the austenite-to-ferrite transformation on cooling pushes the steel toward untempered martensite in the heat-affected zone, and martensite is where hydrogen collects and cracks initiate. A carbon equivalent collapses the whole chemistry into a single number expressing that hardenability relative to carbon.

The IIW carbon equivalent, CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, is the oldest and most widely specified. It was derived for plain carbon and carbon-manganese steels in the 0.15 to 0.25 %C range and it weights carbon heavily. It is the number quoted on most European and ASTM mill certificates and the one AWS D1.1 and API 5L limit.

The Pcm parameter, Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B, was developed in Japan for modern low-carbon micro-alloyed steels where carbon is below about 0.12 %. For those steels CE(IIW) over-predicts risk because it gives too much weight to manganese; Pcm splits the alloying contributions more finely and is the parameter behind most pipeline and offshore preheat rules. Note the boron term — 5B — which is enormous, because a few tens of parts per million of boron transforms hardenability.

The CET, CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40, is the carbon equivalent used by EN 1011-2 Method B, which is the calculation route for preheat temperature in European practice. If you intend to compute a preheat, CET is the number you need, not CE(IIW).

A carbon equivalent on its own decides nothing. It ranks a steel's sensitivity; the actual cracking risk depends equally on joint thickness (which sets cooling rate and restraint), diffusible hydrogen from the consumable, and heat input. Use the value as the input to a preheat calculation, not as a pass/fail.

CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Pcm     = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
CET     = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40
All element contents in weight per cent