Welding defect
[3] According to the American Society of Mechanical Engineers (ASME), the causes of welding defects can be classified as follows: 41% poor process conditions, 32% operator error, 12% using the wrong technique, 10% incorrect consumables, and 5% bad weld grooves.
[4] The magnitude of residual stress caused by the heating, and subsequent cooling, from welding can be roughly calculated using:[5] Where
[6] Arc strikes result in localized base metal heating and very rapid cooling.
When located outside the intended weld area, they may result in hardening or localized cracking and may serve as potential sites subsequent fracturing.
This type of crack is usually observed in the heat affected zone (HAZ), especially with carbon steel, which has limited hardenability.
For other alloy steels, with a high degree of hardenability, cold cracking could occur in both the weld metal and the HAZ.
With high cooling rates, the risk of forming a hard, brittle structure in the weld metal and HAZ is more likely.
They are usually caused by high shrinkage stresses, especially on final passes, or by a hot cracking mechanism.
Conditions that help prevent reheat cracking include preliminary heat treating with a low-temperature soak and then with rapid heating to high temperatures, grinding or peening the weld toes, and using a two-layer welding technique to refine the HAZ grain structure.
These defects can be eliminated using a high current at the starting and proper filler material.
These are generally the result of longitudinal shrinkage stresses acting on weld metal of low ductility.
To alleviate certain types of distortion, the workpieces can be offset so that after welding, the product is the correct shape.
[19] The following pictures describe various types of welding distortion:[20] Gas inclusion—gas entrapment within the solidified weld—manifests itself in a wide variety of defects, including porosity, blow holes, and pipes (or wormholes).
[21] Isolated inclusions occur when rust or mill scale is present on the base metal.
Deeper defects can be detected using Radiographic (X-rays) and/or Ultrasound (sound waves) testing techniques.
[24] Since the 1970s, changes in manufacturing practices, limiting the amount of sulfur used, have greatly reduced the incidence of this problem.
[25] Adding rare earth elements, zirconium, or calcium to the alloy, to control the configuration of sulfur inclusions throughout the metal lattice, can also mitigate the problem.
Another reason is poor technique that doesn't deposit enough filler metal along the edges of the weld.
A third reason is use of an incorrect filler metal, which will create greater temperature gradients between the center of the weld and the edges.
