Weldability Of Steels
The carbon equivalent of steel is converting the content of alloying elements in steel, including the carbon element, which has an effect on hardening, cold cracking and lubrication. The steel can be pre-measured and the welding process conditions such as preheat, post-weld heat treatment and line energy can be reasonably determined by estimating the carbon equivalent and cold crack susceptibility index of the steel.
First of all, the concepts of carbon equivalent and carbon content are not the same. Carbon content is the main factor determining strength and weldability in carbon steels. As the carbon content increases, the quenching tendency increases and the plasticity decreases, and this tends to produce weld cracks. In other words, the higher the element carbon, the worse the weldability. For alloy steels (especially low alloy steels), various alloying elements other than carbon will affect the strength and weldability of the steel, and the carbon content cannot be used as a mere gauge index. The concept of carbon equivalent is used through a large number of test data to facilitate the expression of strength properties and weldability of these materials. The main difference between carbon equivalent and carbon content is that the carbon equivalent can be used to evaluate the effect of all alloying elements in the steel on the weldability of the steel.
According to IX Numbered Commission (Welding Capability Commission) of the International Institute of Welding (IIW) Commission, the formula for carbon equivalent is as follows:
According to the carbon equivalent result obtained, the weldability is expressed as follows.
| Karbon eşdeğeri (CE) | Kaynaklanabilirlik | |
| ≤0,35 | Excellent | |
| 0.36–0.40 | Very Good | |
| 0.41–0.45 | Good | |
| 0.46–0.50 | Average | |
| ≥ 0,50 | Poor |
The carbon equivalent is usually abbreviated as CEV, sometimes CE, CET, CEQ, and etc. It can be seen from the table above that the greater the CE value, the worse the weldability. There are some precautions to be taken when welding metals with high carbon equivalent. If, we list them:
Karbon eşdeğeri yüksek olan metallerde kaynak yapılırken alınması gereken bazı önlemler vardır. Bunları listeleyecek olursak:
• As the carbon equivalent increases in the welded metals, the cooling rate should be slowed down.
• The hardness of the heat affected zone should be less than 350 HV.
• Before welding, the pre-annealing temperature suitable for the carbon equivalent of the part should be selected and pre-annealing should be applied to the part when necessary.
Pre-annealing temperatures corresponding to carbon equivalents are given in the table below.
• For parts with a carbon equivalent greater than 0.60%, stress relief annealing should be applied to the part after welding. This process should be done before the weld cools down. Once made, the part should be cooled as slowly as possible.
