Carbon steel is a large category of steel that encompasses many varying chemical compositions. Although composed mostly of iron, there are several other elements that can be added to carbon steel (amount of carbon, for instance) that can have a large effect on its weldability. It is incredibly important to understand what type of carbon steel is being selected for a project that involves welding. Not knowing the important variables, such as the added elements or carbon content range in each grade, can result in weld failure. To weld carbon steel, you need to know:
- The carbon content
- The carbon equivalency
- The cooling rate
- Other chemical composition concerns
One of the most important items to consider when thinking about welding carbon steel is the carbon content, which typically ranges from nearly 0% by weight to around 2.1%.
- Low carbon steels contain less than approximately 0.30% carbon
- Medium carbon steels contain approximately 0.30% – 0.60% carbon
- High carbon steels contain approximately 0.61% – 2.1% carbon
Each type of carbon steel requires different considerations when welding. Low carbon steel is typically the most readily welded steel in a room temperature environment. Examples of low carbon steel suitable for welding include C1008, C1018, A36, A1011 and A500. Medium carbon steel such as C1045 typically requires preheat and post-heat treatment to avoid weld cracking. High carbon steel is even more prone to weld cracking than the other two groups of carbon steel. Welding high carbon steel will most likely require very thorough preheating and post-heating processes to avoid this. It could also require special welding filler metals.
The carbon equivalency formula can also be used to help determine the weldability of carbon steels. The carbon equivalency formula takes into account other elements in the steel that can have an effect on weldability besides carbon. Generally, the higher carbon equivalent the carbon steel has, the less weldable it is. If the carbon steel has a relatively high carbon equivalent, preheat and post-heat processes may have to be implemented to avoid weld cracking. Although two separate plates of C1045 steel should have the same amount of carbon, they could have varying amounts of a different element, such as manganese. This could create a difference in the carbon equivalencies of two materials with the same designation. It should be noted that carbon equivalencies are typically more of a concern with alloy steels.
The weld cooling rate of the steel is also important when selecting the right carbon steel to weld. High cooling rates can increase the likelihood of weld cracking. Generally, a carbon steel with higher amounts of carbon and other elements in the carbon equivalency formula will require slower cooling rates to prevent weld cracking from occurring.
Many factors can contribute to the weld cooling rate. One factor is the thickness of the carbon steel being welded. The thicker the material, the more volume the heat can dissipate into. This larger volume allows the weld to cool faster than it would on a thinner section. However, thicker carbon steel will also be less likely to distort which can leave residual stresses which could lead to weld cracking. Another factor affecting the cooling rate of the weld is the ambient temperature. Preheating may be more necessary when welding steel during Canada’s winter months versus welding steel in Texas in July.
Other Chemical Composition Concerns
Some elements that can be found in carbon steel are just not conducive to welding no matter what preheat or filler metal is used during the welding process. Grades such as C12L14 contain Lead that promotes solidification cracking of welds. Because of this, Leaded steels should be avoided when welding is going to be performed. Other materials that can cause weld cracking to occur are sulfur and phosphorus. While carbon steels with small amounts of sulfur and phosphorus are readily weldable, carbon steels with amounts over 0.05% of either could be prone to solidification cracking. Free machining steels such as C1141 and C1144 should be avoided when welding has to be performed because of their high amounts of sulfur and phosphorus.
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