Excessive root penetration / root sag
Root sagging and drop-through are uncontrolled sagging of molten weld metal at the root side caused by excessive welding heat or a too-wide gap. This leads to unacceptable cross-section distortion.
Physical-Chemical Mechanism of Action
During the welding process, the molten weld metal can sag through the gap under high temperature, especially if the gap is too wide or the heat input is too high. This leads to improper filling of the metal at the root side and uncontrolled sagging.
Metallurgical Relevance & Risks in Welding
Root sag and droop lead to unacceptable cross-sectional deformations that impair the weld microstructure. This can cause cracks, fractures, or reduced mechanical strength of the joint. Additionally, it can lead to uneven cooling and thus further metallurgical issues such as martensite formation.
Workshop Practice & Action Guidelines
To avoid root sag and droop, it is important to ensure that the gap is not too wide (maximum 2 mm) and that heat input is controlled. The use of shielding layers or barriers can help reduce the temperature at the root side. Proper cooling of the weld metal is also crucial to prevent uneven microstructure development.