Welding in Metalworking: Why Defects Are Costly
Welding is a critical joining process in metalworking. A welding defect may at best only mean rework — at worst, a safety risk. This makes it all the more important to know the most common sources of defects and to systematically avoid them.
The Most Common Welding Defects
1. Porosity (Gas Inclusions)
Porosity is caused by gas inclusions in the weld metal. Causes include contaminated material (oil, grease, rust), insufficient shielding gas coverage, draughts or damp electrodes. Porosity weakens the weld and can lead to leaks.
2. Cracks
The most dangerous type of defect. Hot cracks form during solidification, cold cracks after cooling — often due to hydrogen embrittlement or excessive residual stresses. Crack-prone materials require preheating and controlled cooling.
3. Lack of Fusion
With lack of fusion, the weld metal and base material do not fully merge. Causes: insufficient heat input, incorrect electrode angle or excessive welding speed. Lack of fusion defects are often not visible from outside.
4. Undercut
Grooves at the weld toe transition to the base material — caused by excessive current or incorrect torch handling. Undercuts act as stress raisers and can initiate fatigue cracks.
5. Distortion
Thermally induced component distortion is unavoidable in welded structures but can be minimised through correct welding sequence, counter-heating and fixtures.
Prevention Strategies
Careful Preparation
- Joint preparation: Clean, dry and grease-free surfaces
- Material verification: Base material and filler metal must be compatible
- Preheating: For thick-section or crack-prone materials
Correct Parameter Settings
Current, voltage, welding speed and gas flow must be matched to material, wall thickness and joint configuration. A WPS (Welding Procedure Specification) documents the correct parameters.
Regular Quality Inspection
- Visual testing (VT): First check of every weld
- Dye penetrant testing (PT): Detects near-surface cracks
- Radiographic testing (RT): Detects internal defects
- Ultrasonic testing (UT): For thick components and lack of fusion
Connection to CNC Machining
Welded structures are frequently machined after welding — mating surfaces, flat surfaces and bores are created by CNC milling or turning. The quality of the weld directly affects the achievable CNC precision.
Frequently Asked Questions (FAQ)
Can welding defects be repaired?
Yes — most welding defects can be repaired by grinding out and re-welding. However, this is time-consuming and expensive. Prevention is always cheaper than repair.
What welder qualification do I need?
For components per EN 1090 or the Pressure Equipment Directive, welders need a qualification per EN ISO 9606. For non-critical components, practical experience and in-house qualification suffice.
Conclusion: Quality Begins Before the First Weld
Most welding defects can be avoided through careful preparation, correct parameter settings and systematic quality control. Invest in prevention — it is always cheaper than rework.
Welded structures with CNC post-machining? Contact us — we offer welding and machining from a single source.