The amount of carbon in iron or steel can affect its strength and brittleness, as well as the way it is processed and welded. However, carbon is not the only alloying element; other elements also contribute to the material properties. The challenge is to consider the impact of each element and how it individually interacts with all the others. Therefore, the carbon equivalent concept is used to "convert" all alloying elements to carbon equivalent percentages. The carbon equivalent of steel is the conversion of the content of alloying elements in steel, with carbon included, which has an effect on hardening, cold cracking and embrittlement. By estimating the carbon equivalent and cold crack sensitivity index of steel, the cold crack sensitivity of low alloy and high strength steel can be preliminarily measured, and the welding process conditions such as preheating, post-welding heat treatment and line energy can be reasonably determined.
Firstly we should be clear about the difference between carbon equivalent and carbon content. Carbon content is the main factor that determines strength and weldability in carbon steels. With the increase of carbon content, the quenching tendency increases, and the plasticity decreases, which tends to produce welding cracks. In other words, the higher the carbon content, the worse the weldability. For alloy steels (mainly low alloy steels), a variety of alloying elements other than carbon will affect the strength and weldability of the steel, and the carbon content cannot be simply used as a measure index. To facilitate the expression of the strength properties and weldability of these materials, the concept of carbon equivalent is used through a large number of test data. The fundamental difference between carbon equivalent and carbon content is that carbon equivalent can be used to assess the effect of all alloying elements in steel on the weldability of the steel.
Carbon equivalent is often abbreviated as CEV, sometimes CE, and in practice includes CET, CEQ, and so on. It can be seen from the above table that the larger the CE value, the worse the weldability. How do we get this value? The calculation formula is as follows:
International Institute of Welding(IIW) :
It’s suitable for medium and high strength non-quenched and tempered low alloy steels (σb=500 ~ 900MPa). When the thickness of the steel plate is less than 20mm and the Ce (IIW) is less than 0.40%, the steel has little tendency to harden, good weldability, and does not need preheating. CE(IIW)=0.40% ~ 0.60%, especially when greater than 0.5%, the steel is easy to harden and need preheating before welding
JIS and WES：
It’s suitable for quenching and tempering low carbon low alloy high strength steel (σb=500 ~ 1000MPa). Japanese scientists have put the forward formula to calculate the cold crack sensitivity index after a lot of tests, which is suitable for low alloy high strength steels with C=0.07% ~ 0.22%, σb=400 ~ 1000MPa. Applicable to the following material chemical composition range:
Cold crack sensitivity index: Pcm = C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B
For the steel with a chemical composition with C:0.07%～0.22%；Si:0～0.60%；Mn:0.40%～1.40%；Cu:0～0.50%；Ni:0～1.20%；Cr:0～1.20%；Mo:0～0.70%；V:0～0.12%；Nb:0～0.04%；Ti:0～0.05%；B:0～0.005%。
German Thyssenkrupp steel：