Lack of Penetration

Lack of fusion defects occur when the base material in the root is not properly melted, usually because the heat is not applied long enough or strongly enough. The result is incomplete penetration, which weakens the weld and can lead to cracks or fractures in service. Make sure the heat really reaches the root and the weld pool connection is clean, then you'll avoid the problem. For critical root passes, it helps to inspect the weld from the back side or weld a backup pass.