Answer: In addition to reducing the strength of the weld cross section, the main harm of undercuts to welds is that they cause stress concentration. When the stress concentration is severe, weld toe cracks may also occur.
For welding-sensitive materials, the main reasons for paying attention to undercut inspection are:
(1) Welding-sensitive materials generally have poor weldability, high carbon equivalent, high hardening tendency, complex welding process, and are prone to cold cracks.
(2) The weld toe is the junction between the parent material and the weld. The metallographic structure is discontinuous and the chemical composition is different, which is prone to cracks.
(3) The geometric shape of the undercut is discontinuous, which is prone to stress concentration. The area with severe stress concentration is prone to cracks.
(4) The undercut area is mostly in the heat-affected zone, where the hardness is high and has the conditions for cracks.
(5) The welding process requirements for welding-sensitive materials are strict. If there is a slight loss of control, cracks will occur.
(6) Corrosive substances are easily accumulated in the undercut area during use, which accelerates the corrosion process. Changes in working conditions are prone to cracks.
(7) Statistical rules show that there are more cracks in the undercut area.
