Types of Steel
According to the American Iron & Steel Institute (AISI), Steel can be categorized into four basic groups based on the chemical compositions. Each group varies in carbon content and therefore possess various weldabilities:
As stated earlier, the ease of welding carbon steels is largely dependent of the amount of carbon present. As the carbon content increases, the weldability tends to decrease. This is because the resulting increase in hardness makes the steel more prone to cracking. However, the majority of carbon steels are still weldable. Let’s take a look at the weldability of the varying levels of carbon steel:
Low Carbon Steel (Mild Steel)
Low carbon steels typically contain less than 0.3% carbon content, up to 0.4% manganese. They are one of the largest groups of carbon steel. Low carbon steels with 0.15-0.3% carbon and up to 0.9% manganese posses good weldability. Those with carbon content below 0.2% are particularly preferable.
As long as their impurity levels 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 increased 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 hardenable by heat treatment, which causes them to be prone to cracking. Medium carbon steels should always be welded using a low hydrogen welding process or controlled hydrogen fillers.
High Carbon Steel (“Carbon Tool Steel”)
High carbon steels contain 0.60-1.0% carbon and 0.30-0.90% manganese. They possess an excellent hardness and strength, but also have poor weldability and are difficult to weld without cracking.
Once heat treated it becomes 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 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.
Nickel steels containing 1.0-3.0% nickel may be carefully welded with low hydrogen welding processes. As the nickel content increases, the steel’s hardenability increases. This improved hardenability also means the weldability of these steels becomes progressively worse. Steels containing 5-9% nickel have poor weldability. They are too hard to be welded without the risk of cracking.
When welding nickel steel, it is essential to use a low hydrogen process or controlled hydrogen fillers.
Molybdenum, Chromium-Molybdenum, and Chromium-Molybdenum-Vanadium Steels
This category, beyond having a mouthful of a name, also has poor weldability. They are prone to cracking when welded unless attention is paid to preheat, interpass temperature, cooling rate and post-weld treatment. Like the other hardenable steels, low hydrogen processes or hydrogen controlled filler are recommended to reduce the risk of cracking.
Stainless steels are a group of high alloy steels that contain at least 10.5% chromium. They are widely-preferred because of their performance in even the most aggressive environments. Stainless steels are usually alloyed with a number of other elements to improve heat resisting properties, enhance mechanical properties and/or fabricating characteristics, and to improve corrosion resistance. These alloying elements also influences their weldability. Learn more about how to weld this versatile material in our Weldability of Stainless Steel post.
As with carbon steels, the weldability of steels with more than 0.2% carbon is considered to be poor. This is due to their hardness and risk of cracking when welded. Tool steels, which contain 0.3–2.5% carbon, are therefore difficult to weld and many steel suppliers will actually recommend against it. However, with advancements in welding equipment, techniques, procedures, tool steel and fillers, it is possible though best left to people with good welding skills.