Why are Higher Carbon Steels More Difficult to Weld?

Why are higher carbon steels more difficult to weld? In the first instance, we know that as the carbon content of a steel increases then so does the tensile strength. This is illustrated graphically in Figure 1. and, as the strength goes up, then the joining of these steels will become more problematical. 

However, the question “why are higher carbon steels more difficult to weld” is not as straight forward as just looking at the carbon content of the steel in isolation.

Figure 1. Tensile Strength Increasing with Increasing Carbon Content

Modern steels do not necessarily gain all their strength from simple carbon additions but also from the added effect of small amounts of other alloying elements. The effect of these elements, combined with other parameters such as grain size control, is part of the design to improve the overall properties of a particular steel. 

To address this fact, we need to consider the other strengthening elements and should, therefore, be talking about the concept known as Carbon Equivalent, or CE.

The simple carbon equivalent formula, shown below, considers the weighting of six additional elements to that of carbon. These elements are Manganese, Chromium, Molybdenum, Vanadium, Nickel and Copper. 

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

So why are steels with higher carbon and higher CE’s more difficult to weld? 

• As the CE goes up then the need to preheat kicks in as the thickness of the steel increases. This is because the higher CE steels have an increased probability of being subject to the cracking mechanism known as Hydrogen Induced Cold Cracking (HICC). Preheat slows down the cooling rate of the weld and gives more time for any hydrogen present in the weld pool to diffuse out beyond the susceptible weld zone. Dependant on the steel, preheating can also soften the weld zone microstructure.

• As the CE of the steel increases, the need to use low hydrogen consumables or low hydrogen processes such as GMAW or GTAW comes into play. 

• Using SMAW coated electrodes, designated as low hydrogen, means the necessity of controlling their time in the atmosphere to prevent hydrogen ingress.

     Low hydrogen electrodes, designated E XX18-H8 for example, must be kept in holding ovens when taken from their containers. Depending on the strength level, these electrodes can only be out of the holding ovens for a defined period of time which is critical. If the electrodes exceed this critical time in the atmosphere they are no longer “low hydrogen” and they cannot they be reclaimed by putting them back in the holding oven.

• The other fluxed welding process that could be used is FCAW where, once again, as the CE and thickness goes up it becomes necessary to employ low hydrogen wires. Low hydrogen wires with an H designator, such as E491T-12MJ-H4 must be used. 

• In using low hydrogen consumables, we have the added difficulty that the flux used is defined as “basic”. Consumables with basic fluxes are considered to be more difficult to use than non-basic types, thus demanding greater skill on the part of the welder.

Since with higher CE steels, the probability of HICC is increased we must also take care of the parent metal prior to initiating any welding. This means the removal of anything that contains moisture in the immediate area of welding such as rust, oxides and grease. Most fabrication standards contain a clause as per:

“Areas to be welded shall be free, within x mm of any weld locations, from loose or thick scale, slag, loose rust, paint, grease, moisture, and other foreign material” 

In conclusion then, the fabrication of higher Carbon /CE steels demands a much more detailed approach at the welding procedural stage and a much greater awareness and level of control during the application of the weld itself. These extra controls, using preheat and low hydrogen approaches, are absolutely necessary to prevent hydrogen induced cold cracking in the weld zone.