Carbon is the most important of all alloying additions to steels because of the effects it produces on the microstructure as the welds cool from the very high temperatures associated with the deposition of weld metal. This applies as much to the heat-affected zone (HAZ) of the plate as it does to the weld metal. In addition, when carbon equivalents are of concern, they are generally related to the HAZ.
Two of the most troublesome problems associated with fabricating steels are hydrogen-induced cracking and poor toughness or ductility. Both are aggravated by a microstructure called martensite. Since martensite is very hard, its presence can be inferred by measuring the hardness of the HAZ, particularly in the coarsegrained regions which are close to the weld deposit. Carbon has a profound and direct effect on hardness. Other alloying elements also affect hardness, although not to the same degree. In total, they affect the facility with which a given hardness can be obtained in an alloy steel. This is called hardenability.
However, the most important use of this concept has not been in predicting hardness, but predicting the minimum preheat temperature needed to avoid the formation of the hard martensite. Since martensite is produced at higher cooling rates, anything that can be done to reduce cooling rates can be beneficial toward avoiding that microstructure or a high hardness.
Preheat is important because it has a very strong effect on the rate at which welds cool. Weldability, energy input and cooling rates are important variables. Investigators have measured the effects of alloy content on the preheat temperatures needed to prevent hydrogen-induced cracking or poor toughness, and have determined the relative importance of the alloying elements on that temperature. This is another measure of carbon equivalence (CE). One example follows:
The minimum preheat temperature needed to avoid hydrogen-induced cracking has been shown to have a value of:
PHmin = 200 CE – 20
where PH is preheat temperature in degrees Celsius.