Common alloying elements in weld metal and their functions
Alloying elements in weld metal play a crucial role in determining the mechanical properties, corrosion resistance, and overall performance of the weld. The composition of the weld metal is often adjusted through the addition of specific alloying elements to match or enhance the properties of the base material. Below are some common alloying elements in weld metal and their primary functions:
1.Manganese (Mn) and Silicon (Si): Manganese and silicon are the most common alloying elements in weld metals. Both elements act as deoxidizers, increasing the strength of the weld metal and altering the microstructure, which affects the toughness of the weld. When the Mn and Si content is within an appropriate range, fine-grained ferrite and acicular ferrite microstructures can be achieved, resulting in high toughness. However, using only manganese and silicon to enhance the toughness of the weld metal has its limitations. Particularly when high welding heat input is used, it is difficult to avoid the formation of coarse proeutectoid ferrite and side plate ferrite. In such cases, other grain-refining elements must be added to further improve the weld microstructure and enhance toughness.
2.Titanium (Ti) and Boron (B): Titanium can combine with boron, nitrogen, and oxygen to form small particles like TiB, TiN, and TiO during the crystallization of the weld pool. These particles act as non-spontaneous nucleation sites, refining the crystalline structure, inhibiting the growth of austenite grains, and promoting the nucleation of acicular ferrite, resulting in a fine and uniform acicular ferrite structure. This improves the toughness of the weld metal. When Ti is within 0.02% to 0.07% and B is within 0.0030% to 0.0060%, a microstructure dominated by acicular ferrite is achieved, maximizing impact energy absorption. However, excessive Ti and B content can decrease toughness.
3.Nickel (Ni): During the cooling of high-temperature welds, nickel significantly lowers the transformation temperature from austenite to ferrite. This suppresses the transformation of coarse proeutectoid ferrite and side plate ferrite while promoting the formation of acicular ferrite. The lower transformation temperature ensures that the acicular ferrite remains fine and uniform, thus improving weld toughness. Nickel’s ability to enhance toughness is effective only under low-carbon and low-sulfur conditions; otherwise, toughness may decline. Additionally, both Mn and Ni are elements that expand the austenite phase field, and the best toughness of weld metal is achieved when their contents are appropriately matched. The optimal Mn content is around 1.5%, and the optimal Ni content is around 1.3%.
4.Niobium (Nb), Vanadium (V), and Chromium (Cr): These elements form carbides or nitrides through metallurgical reactions in the weld pool, acting as nucleation sites for acicular ferrite under certain conditions. They also inhibit the precipitation of blocky ferrite and side plate ferrite at the austenite grain boundaries and within the grains during cooling, favoring the formation of acicular ferrite and enhancing the toughness of the weld metal. Additionally, the carbides of Nb, V, and Cr dissolve in the austenite, refining grains, strengthening the weld, pinning the austenite grain boundaries, and inhibiting grain growth. However, it should be noted that if the content of these elements exceeds a certain level, the toughness of the weld metal may decrease significantly.
5.Rare Earth Elements (REEs): Rare earth elements suppress the formation of proeutectoid ferrite and side plate ferrite while promoting the nucleation of acicular ferrite. They also inhibit or reduce the formation of M-A (martensite-austenite) constituents within the grains, thereby improving weld strength and toughness, reducing the impact of inclusions on weld cracking, decreasing the tendency for weld cracking, and enhancing the overall performance of the weld.