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Weldability of Steel
Steel is one of the most common materials in the world. It is widely used for its high tensile strength and unparalleled versatility. Used in everything from structural construction to detailed aesthetic designs, steel comes in a vast range of grades. Each grade has specific strengths and is optimized to a certain type of project.
The weldability of steel grades depends mostly on how hard it is. In turn, this depends on the material’s chemical composition, specifically its carbon content. Other alloying elements that have a lesser effect on the hardness of steel include manganese, molybdenum, chromium, vanadium, nickel and silicon. Therefore, in order to successfully weld this versatile material it is important to first familiarize yourself with the various types of steels and their properties.
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 Steel (Carbon Tool Steel)
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.
This category covers a wide range of metals. They are carbon steels that are then alloyed heavily with other elements, typically chromium, cobalt, manganese, molybdenum, nickel, tungsten, vanadium, and chromium-vanadium.
Alloy Steels often have superior hardness, higher corrosion resistance, but 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.
Nickel steel is a specific type of Alloy Steel that is unusual enough that it gets its own entry. Alloys contain 1-3% nickel may be carefully welded with low hydrogen welding processes. As the nickel content increases, the steel’s hardness increases. Like carbon, this means the weldability of these steels becomes 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.
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.
Common Weld Defects
Inadequate shield gas results in atmospheric contamination in your weld. Porosity is the typical result. Avoid this by checking all the setting on your equipment, and making sure your weld isn’t exposed to the wind.
Lack of Fusion
This defect occurs when there is not enough energy being absorbed by the steel and the weld. Double check your voltage and wire feed speeds to avoid this. Also, avoid rushing your welds to prevent this.