Medium carbon steel becomes difficult to weld when its carbon content exceeds:

Medium carbon steel typically contains carbon concentrations of approximately 0.3% to 0.6%. When the carbon content exceeds about 0.35%, the steel begins to exhibit increased hardness and brittleness, making the welding process more challenging. Higher carbon levels promote the formation of hard microstructures such as martensite during welding, which can lead to cracking and poor mechanical properties in the heat-affected zone (HAZ) after welding. This gradual increase in carbon content results in difficulties with maintaining the integrity and ductility of the weld. Therefore, 0.35% is recognized as a critical threshold in the welding of medium carbon steel, beyond which the risks associated with welding increase significantly.