20.2 Welding Carbon Steels
David Colameco, M.Ed.
Basics of welding carbon steels
The majority of carbon steels will come in shapes you are familiar with or will become familiar with while welding and fabricating. Plates, pipes, H-beams, and C-channel for example are common shapes you will use to fabricate. As a welder and fabricator, the WPS and blueprints will provide you with the instructions you need to complete the fabrication to the required specifications. If the WPS or blueprint has something that isn’t clear to you, stop and ask a colleague or supervisor for clarification. Spending a few minutes to clarify something is better than spending an hour or more on rework.
Steels can be hot rolled, cold rolled, quenched and tempered, or processed in other ways to achieve the desired properties. When you weld, you are destroying those treatments and mechanical workings of the base material because the metal melts within the weld pool returning the metal to its original state prior to treatment. In areas where the base material does not melt, there is a heat affected zone (HAZ) where grain growth, allotropic changes, and absorption of elements into the crystal structure can occur. The heat applied to the base metal in the HAZ will change the mechanical properties of the base material and/or result in a weld with material properties that are less than desired. The end result could be failure of the weld joint due to cracking which is why following a WPS is so important. If a structural failure occurs, as welders we want that failure to occur in the base material away from the weld and not because of the weld.
The HAZ is a complex set of sub zones shown in the V-Groove Butt Joint of Figure 20.6 below. The weld metal is at the center, followed by a penetration zone directly next to the weld metal which represents the melted base material that fuses with the weld metal. Moving further outward we next reach the coarse-grained HAZ that experiences elevated temperatures from welding but not hot enough to melt the metal. In this zone the heat causes the metal grains to grow becoming more coarse. The further away we move from the weld bead itself the less heat there is in the base metal due to dissipation of heat. The Fine-grained HAZ has grains that are affected by the heat but grow to a lesser extent. This zone may have been tempered from the heat. Outside of the fine grained HAZ is the base metal which may have been heated up, but has not undergone any mechanical property changes. It is important to note that other sources may break up the HAZ into a more detailed structure. For the purposes of this chapter, this level of detail in the HAZ is adequate.
Steels from the mill are processed through cold and hot rolling. Rolling is a process where the steel is forced through rollers to flatten it out. Finer grained steel structures have a higher toughness. Figure 20.7 shows two metals being rolled together. The process for cold and hot rolling a single piece