Types of Steel
According to the American Iron & Steel Institute (AISI), Steel comes in four main groups based on the chemical compositions. Each group varies in carbon content and therefore possess different weldabilities. The four groups are Carbon Steel, Alloy Steel, Stainless Steel, and Tool Steel.
The ease of welding carbon steel largely depends of the amount of carbon present. As the carbon content increases, the weldability tends to decrease. This is because the increase in hardness makes the steel more prone to cracking. However, the majority of carbon steels are still weldable.
Low Carbon Steel (Mild Steel)
These steels typically contain less than 0.3% carbon content and up to 0.4% manganese. Low carbon steels with 0.15-0.3% carbon and up to 0.9% manganese posses good weldability. Those with less than 0.2% carbon are ideal.
As long as the impurities are kept low, these metals rarely present problems during the welding process. Steels with carbon over 0.25% are be prone to cracking in certain applications. Conversely, steels with less than 0.12% carbon can be susceptible to porosity. All low carbon steel can be welded using any of the common welding processes. But the steels with more carbon content are best welded with a low-hydrogen process or with low-hydrogen fillers.
Medium Carbon Steel
Medium carbon steels contain 0.30-0.60% carbon and 0.60-1.65% manganese. They are stronger than low carbon steel, but are more difficult to weld. This is because they are more prone to cracking. Medium carbon steels should always be welded using a low-hydrogen welding process or controlled hydrogen fillers.
High carbon steels contain 0.60-1.0% carbon and 0.30-0.90% manganese. They are are very hard and strong, but also have poor weldability and are difficult to weld without cracking.
Once heat treated, these are extremely hard and brittle. If welded, high carbon steels require preheating, careful interpass temperature control, and post weld stress relief. Low-hydrogen processes to low-hydrogen fillers are necessary when welding these steels.
Carbon-Manganese steels have 0.15-0.5% carbon and 1.0-1.7% manganese. Generally these steels are weldable, although some steels will require controls on preheat and heat input. When welding carbon-manganese steels with higher amounts of carbon, it is recommended to use low-hydrogen welding processes or controlled hydrogen fillers.
Similar to other carbon steels, many low alloy steels are weldable. But their weldabilty again varies with its carbon content. Specifically, the weldabilty of alloy steels depend on the carbon equivalent to its its alloying additions: manganese, chromium, molybdenum, vanadium and nickel.