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In actual production, there are mainly two ways to improve the mechanical properties of weld metal: one is the alloying of the weld metal, that is, by adding alloying elements such as Mn, Si, Ti, Nb, B, and rare earths to the weld； Second is to adjust the welding or post-weld heat treatment processes, such as adjusting the welding heat input, interpass temperature, and welding position. This article focuses primarily on the alloying of weld metal. Principle 01: Alloying elements are introduced into the weld metal through welding materials (electrodes, wires, or fluxes), and through complex metallurgical reactions, the weld structure is improved and its properties are optimized. 1. By adding alloying elements, the austenite grain boundaries are pinned, preventing the growth of austenite grains and refining the primary crystalline structure formed during weld solidification; this also leads to a refinement of the secondary crystalline structure that forms as the weld cools to room temperature ； 2. Prevent or inhibit the pro-eutectoid ferrite transformation, and increase the content of acicular ferrite as much as possible ； 3. Adding alloying elements results in the formation of high-melting-point inclusions distributed in a fine-grained or spherical pattern, which create ferrite nucleation sites within austenite; this promotes the transformation of acicular ferrite while preventing the formation of lateral lath ferrite and M-A components ； 4. Try to suppress the transformation of martensite and upper bainite. 02 Common weld alloying elements and their functions 1 Manganese and silicon: Manganese and silicon are the most common alloying elements in welds. Both can remove oxygen, thereby increasing the strength of the weld metal; they can also alter the structure of the weld metal, which in turn affects its toughness. When the manganese and silicon contents in the weld metal structure are within an appropriate range, a weld metal structure with fine-grained ferrite and acicular ferrite can be obtained, thereby granting higher toughness. However, relying solely on manganese and silicon to improve the toughness of the weld metal has limitations; especially when high welding heat input is used, it is difficult to avoid the formation of coarse pro-eutectoid ferrite and lateral lath ferrite in the weld metal. In such cases, other grain-refining elements need to be added to further improve the microstructure of the weld metal and enhance its toughness. 2 Titanium and boron: Titanium can form tiny particles such as TiB, TiN, and TiO with boron, nitrogen, and oxygen. During the crystallization process in the welding pool, these particles act as sites for non-spontaneous nucleation, thereby refining the crystal structure. They prevent the growth of austenite grains and promote the extensive nucleation of acicular ferrite, resulting in the formation of fine and uniform acicular ferrite structures; this in turn improves the toughness of the weld metal. A microstructure dominated by acicular ferrite can be obtained when Ti is at 0.02%~0.07% and B is at 0.0030%~0.0060%, resulting in a maximum impact absorption value; excessive levels of Ti and B lead to a decrease in toughness. During the cooling of nickel-containing high-temperature welds, the presence of nickel **lowers the transformation temperature of austenite to ferrite, thereby suppressing the transformation of massive pro-eutectoid ferrite and lateral lath ferrite and promoting the formation of acicular ferrite. Meanwhile, the lower transformation temperature ensures that the acicular ferrite remains uniform and fine, enhancing the toughness of the weld. Nickel improves the toughness of the weld metal, but its effect is only evident under conditions of low carbon and low sulfur; otherwise, toughness decreases. Additionally, both Mn and Ni are elements that expand the austenite region, and it is only when their contents are appropriately balanced that the weld metal achieves its best toughness. With a Mn content of 1.5% and a Ni content of around 1.3%, the weld metal exhibits the best toughness. Elements such as 4Nb, V, and Cr form carbides or nitrides in the weld pool through metallurgical reactions; under certain conditions, they can serve as nuclei for the formation of acicular ferrite. Additionally, they inhibit the precipitation of granular ferrite and lateral lath ferrite at the austenite grain boundaries and within the grains during cooling, which facilitates the formation of acicular ferrite and enhances the toughness of the weld metal. The carbides of Nb, V, and Cr, when dissolved in austenite, can also serve to refine grain sizes, strengthen the weld, pierce through the austenite grain boundaries, and inhibit the growth of austenite grains. It should be noted, however, that once the content of these elements in the weld metal exceeds a certain level, the toughness of the weld metal will **decrease**. The 5 rare earth elements inhibit the formation of pro-eutectoid ferrite and lateral lath ferrite, promote the nucleation of acicular ferrite, suppress or reduce the formation of intragranular M-A phases, thereby enhancing the strength and toughness of the weld, reducing the impact of inclusions on weld cracking, lowering the tendency for weld cracking, and improving the overall performance of the weld.