The weldability of steels is influenced primarily by the carbon content. At higher carbon levels, steels may need either pre- or post- weld heat treatment in order to prevent stress build up and weld cracking.
Generally speaking, if the Carbon Equivalent (CE) is 0.35 or below, no pre- or post- weld thermal treatment is needed. In our experience with maintenance welding, we have found that preheating was beneficial between 0.35 and 0.55 CE. Above 0.55 CE we usually both pre- and post- weld heated to relieve stress and prevent cracking.
So CE= .35 max.
However the other elements that are contained in the steel also have an effect on the steel’s “carbon equivalence.” These additional elements can really add up in scrap fed electric arc furnace steels that now predominate in our market.
Here are two formulas for calculating Carbon Equivalents.
CE=%C+(%Mn/6)+(%Cr+%Mo+%Va)/5 + (%Si+%Ni+%Cu)/15
This is the first formula I learned when I took over metallurgical support for maintenance ‘back in the day.’
In this formula you can see that 6 points of Manganese are approximately equal to one point of Carbon. 5 points of Chrome, Moly or Vanadium are roughly equal to a point of Carbon, while it takes about 15 points of Silicon, Nickel or Copper to get about the same effect as one point of Carbon.
The GE formula for Carbon Equivalency is CE= C+(Mn/6)+(Ni/20)+(Cr/10)+(Cu/40)+(Mo/50)+(Va/10). If this is less than .35 max, you should have no need to pre or post weld thermal treat in most cases.
As long as CE is no more than .35, you probably won’t need to preheat or post weld stress relieve your welded parts. above .35 CE, you may need either or both depending on section thickness and CE.
* (I) added (extra parentheses) to keep (the terms) clear in (this post); no (scathing rebukes) from (math teachers) please!