Consumables Corner: Choosing the correct carbon equivalency formula

Q: We're a midsize fabrication shop that builds and welds machines and structural components from varying types of carbon steel. Some of the material is mild carbon steel, while some is high-strength, low-alloy (HSLA) or stronger grades of structural steel. If we're welding on thicker sections of material, we typically determine preheat temperatures based on just the carbon content.

Someone mentioned that we should be using a carbon equivalency formula to determine preheat temperatures. Can we use the one provided by AWS D1.1 or do we need to consider others?

A: This question comes up rather frequently, so let's take a look at some background information on carbon equivalency (CE) formulas.

The first CE formula developed by Dearden and O'Neill dates back several decades and was initially used to give a numerical value that related to the hardenability of carbon steel. In the mid-1960s the International Institute of Welding (IIW) derived a simplified formula for determining this property.

Various other formulas have been released over the years; some are for low-carbon steel, while others take into consideration grades that have small amounts of alloying elements. The reason is small amounts of alloying elements such as chromium (Cr), nickel (Ni), molybdenum (Mo), and manganese (Mn) increase the potential for martensite formation in the heat-affected zone (HAZ), which is the area of base material adjacent to a weld or high-temp cutting process.

From a welding standpoint, a couple of common formulas based on the parent material chemistry serve as a guideline for welding considerations such as hardenability, weldability, and HAZ hydrogen-induced cracking potential.

The equation that follows is the early version from IIW that is commonly found today in various publications, and the elements are calculated by weight percentages. This formula is suitable for carbon and carbon-manganese type steels:

CE(IIW) = %C + (%Mn)/6 + (%Cr+%Mo+%V)/5 + (%Ni+%Cu)/15

The American Welding Society (AWS D1.1 code) adopted this formula with an additional term to include silicon (Si); it is mostly used in applications involving various structural steel grades. The next formula is used mostly for steel with moderate levels of carbon to determine minimum preheat temperatures:

CE(AWS)=%C + (%Mn)/6 + (%Cr+%Mo+%V)/5 + (%Ni+%Cu)/15 + (%Si)/6

Neither of these formulas should be used with HSLA or low-alloy CrMo steels.

The next two formulas are better suited for low-carbon grades of steel and place an emphasis on carbon being the primary element that produces the greatest potential for microstructural changes. The first one is called the critical metal parameter equation or P_cm.

P_cm = %C + (%Si)/30 + (%Mn+%Cu+%Cr)/20 + (%Ni)/60 + (%Mo)/15 + (%V)/10 + 5%B

The last equation was derived by Düren and is similar to P_cm :

CEq = %C + (%Si)/25 + (%Mn+%Cu)/16 + (%Ni)/40 + (%Cr)/10 + (%Mo)/15 + (%V)/10

These two formulas are more applicable to a wide array of modern-day alloyed steels in which carbon content typically is less than 0.11 percent by weight.

Depending on which formula you use and the chemistry of the base material, the calculated result will typically fall between 0.30 and 0.70. The lower value corresponds with material that is readily weldable with a low tendency for increased hardness or potential for hydrogen-induced cracking. The middle to high range corresponds to poor weldability or high potential for increased hardness and cracking.

The best practice is to follow the code you are working to, keep customer requirements (if any) in mind, and consult the steel manufacturer for welding guidelines.