Lack of root fusion
Incomplete fusion at the weld root: The lack of metallurgical bonding in the weld root area, usually due to insufficient heat input at the underside. This leads to a significant reduction in cross-section.
Physical-Chemical Mechanism
Incomplete fusion occurs when the weld is not heated sufficiently, especially at the underside. This prevents a complete metallurgical bond between the pieces being welded. The lack of fusion results in a weak zone at the weld root.
Metallurgical Relevance & Risks in Welding
Incomplete fusion can lead to cross-section weakness and cracks in the weld. These weaknesses make the parts prone to mechanical stress and can cause premature failure. Additionally, the lack of fusion can lead to hot cracking and other metal deformation issues in the weld root area.
Shop Practice & Guidelines
To avoid incomplete fusion, it is crucial to ensure sufficient heat input at the underside of the weld. This can be achieved by using a suitable backing bar and a correct heating strategy. The heating time should be at least 30 seconds to ensure complete fusion. For heavy steels or high stresses, a longer heating time is required.