Carbon steel is just as described: a steel alloy containing added carbon, with the carbon making up 0.05% to 2.0% of its weight. This percentage range means carbon steel is a sprawling category, covering many different types of steels with varying levels of carbon. Steel with any other combination of alloying elements can be included in this category, so long as they contain carbon.
Clearly the many possible chemical compositions of carbon steel make it difficult to make blanket statements about this metal. Carbon increases the hardness and strength of steel, but the percentage of carbon added creates different results with the steel’s mechanical properties. Those properties then affect how the metal reacts to further processing such as welding. It is very important to understand the exact qualities of a selected carbon steel to produce a successful weld; doing otherwise risks metal damage or a weld that may fail later during use.
Carbon Content
The most important decision before beginning a weld will be the type of steel to use. Carbon steels are organized as three basic types:
• Low carbon steels: containing less than 0.30% carbon by weight
• Medium carbon steels: containing 0.30% to 0.60% carbon by weight
• High carbon steels: containing 0.61% to 2% carbon by weight
Each of these groups will weld differently due to their carbon content. The higher the steel’s carbon level, the more prone it will be to weld cracking. Low carbon steels can be much more easily welded, which is why they are the typical choice for this process.. However, high carbon steels can be welded, provided extra care is taken with the more brittle material. Heating high carbon steel prior to the weld, as well as afterwards, will help prevent cracking. Additional filler metals may also be used with high carbon steel to create a better weld.
Carbon Equivalency
Once the type of carbon steel is selected, further determination of its weldability is done using the carbon equivalency formula. This formula examines what effect the other alloying elements besides carbon may have on a weld. The percentages of alloying elements are calculated as an additional degree of carbon – and that boosted carbon level translates into a steel considered more difficult to weld. As an example: using the formula means a medium carbon steel with no added manganese will be judged as easier to weld than the same carbon metal including manganese alloy.
Overall Chemical Composition
Even after selecting a lower carbon steel, and calculating an acceptable carbon equivalency, the overall chemical composition should still be reviewed. Some elements will just not take well to welding, even with a great deal of effort. Any amount of lead in carbon steel, for instance, will significantly raise the probability of weld cracks. Other common alloying elements such as sulfur or phosphorus can have the same effect, resulting in a weak or failed weld. While carbon steel with trace amounts of either sulfur or phosphorus can still be welded, the percentages must remain extremely low (approximately 0.05% or less) to avoid weld cracking.
Cooling Rate
The job is still not complete after creating a weld, because as it cools, cracking may develop. Carbon is again the main culprit here, due to it making the steel more brittle. With higher levels of carbon and equivalent alloys, the metal must be cooled at a slower rate to prevent cracks from occurring.
In addition to the steel’s carbon percentage, other factors must be accounted for during weld cooling. The thickness of the steel being welded will affect the cooling rate, and thinner material has a higher risk of distortion. The temperature where the weld is being performed is also important. In cold temperatures, preheating may be necessary even for lower carbon steels, while it can be performed without prior operations in warmer environments.