Intro4u2u

A perfect butt weld joint, when subjected to an external force, provides a distribution of stress throughout its volume which is not significantly greater than that within the parent metal.
This is achieved as long as the following features apply:
•Welds should consist of solid metal throughout a cross section at least equal to that of the parent metal
•All parts of a weld should be fully fused to the parent metal
•Welds should have smoothly blended surfaces
If any of these requirements are not fulfilled then the weld is imperfect and the stress distribution through the joint is disrupted.
A weld imperfection (or discontinuity) is therefore any object or shape which is capable of creating a stress concentration within a welded construction. Note that in practice, all welds contain small imperfections. However, the majority of these are so small that they do not significantly affect the performance of the joint. If an imperfection is considered of a sufficient size to be detrimental to the structure (as defined by an acceptance standard), it is classified as a defect.
43
5.1.2 Significance
Stress concentrations are detrimental to the performance of a structure as they can:
•lower the load bearing capacity of the joint
•initiate brittle fracture
•nucleate a fatigue crack
•initiate stress corrosion cracking
The greater the stress concentration produced by an imperfection, the more likely it is to cause failure of the weld in service. Therefore, imperfections need to be assessed according to the severity of their stress concentration. This is achieved mainly by the geometric descriptions volumetric or planar.
Volumetric imperfections (those which have three significant dimensions) are less effective stress raisers and are usually accepted in limited quantity and size.
Planar imperfections (those which are essentially two-dimensional) are more effective stress raisers and are almost invariably treated as unacceptable.
5.2 Types of Imperfection
Weld imperfections can be grouped into five distinct types according to their nature and shape. It is important that an imperfection is correctly identified to allow the welding procedure to be suitably modified to prevent their re-occurance. Details of these types of imperfections are contained in the appendix at the end of this document.
Cracks
•Solidification Cracking
•HAZ Hydrogen Cracking
•Weld Metal Hydrogen Cracking
•Lamellar Tearing
Cracks are more significant than other types of imperfection, as their geometry produces a very large stress concentration at the crack tip, making them more likely to cause fracture. Note that this section is concerned only with cracks produced at the time of welding, not subsequent service cracking, such as fatigue or stress corrosion cracking. Cracks can occur in the weld metal or heat affected zone. Due to their severity of stress concentration, crack-like imperfections are usually classed as defects.
44
Lack of Solid Metal
•Porosity
•Worm Holes
•Crater Pipe
•Root Concavity
•Underfill
•Slag Inclusions
•Inter-Run Imperfections
These imperfections are formed when there is insufficient weld metal to completely fill the cross-section between the parent metal plates. They are volumetric (blunt) in shape, and as such are usually only associated with a reduction in the load bearing capacity of a weld.
Lack of Fusion
•Incomplete Root Penetration
•Lack of Sidewall Fusion
These imperfections occur when there is incomplete fusion between the parent metal and weld metal or between weld runs. They are essentially two-dimensional in shape and so are effective stress raisers within the material. Therefore it is important to control them as they can lead to cracking within the weld.
Lack of Smoothly Blended Surfaces
•Surface Porosity
•Excess Weld Metal (Reinforcement)
•Excessive Penetration
•Undercut
•Overlap
It is not immediately obvious that irregularities on the surface of the weld are serious imperfections. However, any sudden change in the contours of the surface produce local stress concentrations. This can especially lead to the formation of fatigue cracks (most commonly at weld toes).
Miscellaneous
•Misalignment
•Arc Strikes
•Spatter
Several miscellaneous imperfections do not conform to any particular category.
45
5.3 Practical Aspects
5.3.1 Tolerance
Weld imperfections can seriously reduce the integrity of a welded structure. Therefore, prior to service of a welded joint, it is necessary to locate them using NDE techniques, assess their significance, and take action to avoid their reoccurrence.
The acceptance of a certain size and type of defect for a given structure is normally expressed as the defect acceptance standard. This is usually incorporated in application standards or specifications.
All normal defect acceptance standards totally reject cracks. However, in exceptional circumstances, and subject to the agreement of all parties, cracks may be allowed to remain if it can be demonstrated beyond doubt that they will not lead to failure. This can be difficult to establish and usually involves fracture mechanics measurements and calculations.
It is important to note that the levels of acceptability vary between different applications, and in most cases vary between different standards for the same application. Consequently, when inspecting different jobs it is important to use the applicable standard or specification quoted in the contract.
5.3.2 Repair
Once unacceptable defects have been found, they have to be removed. If the defect is at the surface, the first consideration is whether it is of a type which is normally shallow enough to be repaired by superficial dressing. Superficial implies that, after removal of the defect, the remaining material thickness is sufficient not to require the addition of further weld metal.
If the defect is too deep, it must be removed by some means and new weld metal added to make up to size.
Replacing removed metal or weld repair (as in filling an excavation or re-making a weld joint) has to be done in accordance with an approved procedure. The rigor with which this procedure is qualified will depend on the application standard for the job. In some cases it will be acceptable to use a procedure qualified for making new joints whether filling an excavation or making a complete joint. If the level of reassurance required is higher, the qualification will have to be made using an exact simulation of a welded joint which is excavated and then refilled using a specified method. In either case, qualification inspection and testing will be required in accordance with the application standard.
46
6. Conclusion
The weldability of a steel broadly refers to the ease with which it can be joined by fusion welding without introducing significant defects or deterioration in properties. Thus, in considering the welding of carbon-manganese steels, key factors which influence their weldability have been explored. In addition, the formal approach to welding quality controls the variables affecting properties, and ensures the welder has the skills required to produce sound joints.
It is hoped you will now have an appreciation of welding carbon-manganese steels which will enable you to recognise good practice. More importantly, when problems are encountered, you should be able to observe whether the causes have been identified and that appropriate corrective actions are taken.