Impact of Alloy Elements on Welding Performance

Carbon (C)

The influence of carbon on the welding properties and the metallographic performance of the weld metal mainly manifests in the improvement of strength and hardness. However, as the strength and hardness increase, the plasticity and toughness of the weld metal decrease.

Manganese (Mn)

Manganese comes from pig iron and deoxidizers. Mn has excellent deoxidation ability, can remove FeO from the steel, and can also form MnS with S to eliminate the harmful effects of S. Most of these reaction products enter the slag and are removed, while a small portion remains in the steel as non-metallic inclusions.

Therefore, Mn can improve the quality of the steel, reduce its brittleness, and enhance its hot working performance. In addition to forming MnO and MnS as impurities in the steel, Mn can dissolve in the ferrite at room temperature, exerting a certain strengthening effect on the steel.

Silicon (Si)

Silicon comes from pig iron and deoxidizers. Si has a stronger deoxidation ability than Mn and is the main deoxidizer, capable of eliminating the adverse effects of FeO inclusions on the steel. Si can react with FeO to form SiO2, which is then removed as slag. Apart from forming SiO2 as an impurity in the steel, most of the Si can dissolve in the ferrite at room temperature, thus exerting a strengthening effect on the steel.

Chromium (Cr)

Chromium is the main alloying element in stainless steel. Cr forms a protective film of Cr2O3 with oxygen to prevent oxidation. However, when Cr reacts with C, it forms Cr23C6, which is the main cause of intergranular corrosion in stainless steel. In low-alloy steel with Cr content <1.6%, it improves the hardenability of the steel without reducing its impact toughness.

Nickel (Ni)

Adding nickel to steel can increase its strength and impact toughness, with Ni working even better when combined with Cr. Generally, increasing the Ni content in low-alloy steel improves its yield strength, but when the Ni content in the steel is relatively high, the tendency for hot cracking (mainly liquation cracking) significantly increases.

Titanium (Ti)

Ti has a strong affinity for O and disperses in the form of tiny oxide particles in the weld metal, promoting refinement of the weld metal grain. TiC particles formed by the combination of Ti and C contribute to the dispersion strengthening of the weld metal. The combined addition of Ti and B has the best effect on the weld metal properties. The optimal range of Ti and B content in low-alloy steel weld metal is Ti=0.01%~0.02% and B=0.002%~0.006%.

Molybdenum (Mo)

Adding a small amount of Mo to low-alloy steel weld metal not only increases its strength but also improves its toughness. Further addition of trace amounts of Ti to the weld metal enhances the beneficial effects of Mo, resulting in a more uniform weld metal structure and a significant improvement in impact toughness.

For Mo-Ti series weld metal, when the Mo content is 0.20%~0.35% and the Ti content is 0.03%~0.05%, a uniform fine-grained ferrite structure can be obtained, leading to good toughness in the weld metal.

Niobium (Nb) and Vanadium (V)

A proper amount of Nb and V can improve the impact toughness of the weld metal. When the Nb content is 0.03%~0.04% and the V content is 0.05%~0.1%, the weld metal exhibits good toughness.

If Nb and V are used for toughening the weld metal and no normalizing treatment is performed after welding, the nitrides of Nb and V exist as fine coherent precipitates, significantly increasing the strength of the weld metal but causing a decrease in its toughness.

Alloying elements mainly exist in two forms in steel or weld metal: solid solution and compounds. The effects of some alloying elements in steel and their influence on welding performance are shown in Table 1-8. The effects listed in the table are general; in practical applications, the interaction between alloying elements should also be considered.

The interaction effects of various alloying elements are very complex.

To obtain weld metal with excellent comprehensive performance, attention should be paid to the existence form of alloying elements in the weld metal, their strengthening effects, and their effects on microstructural transformation during the process of welding material development, by adjusting the alloy composition of the weld metal through calculation, comprehensive investigation, and experimentation.

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