Nickel-Induced Microstructural Evolution and Its Impact on the Mechanical Properties of Welded Joints

<title>Abstract</title> <p>Nickel is widely used as an alloying element to improve the performance of welded joints, particularly in structural steels. In the present work, the influence of controlled nickel additions on the mechanical and microstructural properties of mild steel welded joints fabricated using manual metal arc welding (MMA) was investigated. Nickel concentrations of 1.3 wt.% and 1.5 wt.% were introduced through nickel-based filler metals, and their effects were compared with nickel-free welds. Mechanical characterization was carried out using Charpy V-notch impact testing and Vickers hardness measurements, while microstructural features of the weld metal and heat-affected zone (HAZ) were examined using field emission scanning electron microscopy (FESEM). The results showed a notable improvement in impact toughness and hardness with nickel addition. Compared to nickel-free welds, impact energy increased from 160 J to 187 J, while hardness improved by approximately 15–31%. Microstructural analysis revealed refined grain structures and reduced porosity in nickel-containing welds, which contributed directly to the observed mechanical enhancement. The novelty of this study lies in the systematic evaluation of narrow nickel additions (1.3 &amp;1.5 wt.%) and their correlation with impact energy partitioning and microstructural refinement under controlled welding conditions. The findings indicate that moderate nickel additions significantly enhance weld integrity and can be effectively employed to improve the reliability of mild steel welded structures.</p>