How much do you know on weld indications, the discontinuities, imperfections, flaw and weld defects? Well, we will make this easy so that there is enough clarity on what defects are, and how to prevent these defects or imperfections.
Table of contents
- What is a Defect?
- What’s a Discontinuity?
- Defining Indication
- What’s Imperfection?
- What is Flaw defect in welds?
- How to prevent weld defects in materials?
- What are the types of discontinuities in welds?
- 1. Porosity in welds
- 2. Inclusions
- 3. Lack of Penetration
- 4. Lack of Fusion
- 5. Undercuts
- 6. Burn-Through
- 7. Overlap
- 8. Spatter
- 9. Craters
- 10. Weld crack
- Closing Opinion
Before we go right into the main topic, it is very important to have a general understanding on the various terms related to weld defects as stated below.
What is a Defect?
In general, a weld defect is a discontinuity that is unable to meet a minimum acceptable specification or standard, designates rejectability of a material.
In other words, a lack of homogeneity in a material’s mechanical, metallurgical, or physical characteristics is a Defect.
What’s a Discontinuity?
Discontinuity, on the other hand, is the lack of homogeneity in mechanical, metallurgical, or physical characteristics of material acting as interruption of the typical structure of material.
Moreover, a discontinuity is not necessarily a defect. But, certainly, have the potential to become defects.
Before testing to relevant standards, an indication is basically a signal of discontinuity in the material. After an NDE, the indication is referred to as being imperfect because it deviated from the required standards.
An unintentional discontinuity is generally, an imperfection. A crack, inclusions, and porosity are examples of unintentional discontinuities that are also imperfections.
What is Flaw defect in welds?
Once evaluated to the NDE standards, an indication becomes a Flaw, an imperfect or unintentional discontinuity. Therefore, a flaw becomes a defect and deemed rejectable, if the size, shape, orientation, location or property does not comply to the relevant acceptable standards.
How to prevent weld defects in materials?
The right techniques practiced in welding, for instance, the equipment used, the qualification, the approved procedures, and the right skill are very important in preventing the below intentional, and unintentional discontinuities that may lead to flaws in welds.
What are the types of discontinuities in welds?
Briefly, the most widely found discontinuities in welds are as below:
- Lack of Penetration
- Lack of Fusion
1. Porosity in welds
A porosity is a cavity type discontinuity due to gas entrapment as small bubbles or large blowholes, or during shielding gas entrapment as elongated wormholes, during solidification process of weld metal while welding.
Moreover, a leading cause of excess porosity is contamination. Using insufficient electrode deoxidant in electrodes may also be a possible contributor. In fact, porosity can be caused by open doors, wind, weather conditions or air emitted from machinery. Porosity mainly observed in the weld metal, but is also noted in base metal and HAZ zones for casting materials.
Porosity discontinuities, therefore, may be found as uniformly scattered, clustered, aligned or elongated.
How to avoid porosity in welds?
Welding steel materials either as bars or plates can cause porosity if you weld a straight line across the grain. Consequently, this can be reduced by following some standard practices as below:
- Limit long arc strikes.
- Avoid welding at a bad angle.
- Proper cleaning of base metals and tools (gun liners and nozzles – GMAW) before welding.
- Follow the approved welding procedure specifications parameters.
- Monitor required distance of the Gun From the Work piece during welding. Neither too close or too from from weld.
- Presence of moisture.
- Maintaining uninterrupted and required gas flow rate
- Use high quality tools, materials and equipment’s.
When nonmetallic materials or slag (clear and glasslike material) or tungsten (GTAW Process) are trapped in the molten metal without rising to surface of the weld pool, it may lead to the failure of the weld. It is a type material that is formed due to flux materials melting inside or on the weld area, and doesn’t rise to the surface of the fused metal. Moreover, as it cools form a deposit on the weld bead. Occurs mostly in arc welding processes.
For that reason, these non metallic materials like slag and oxides, are called inclusions. They may weaken the joint to such extent to cause failure.
How to avoid inclusions in welds?
Some of the best way to avoid inclusions are as below:
- To keep the base metals clean from impurities like slags from previous welds, rust, corrosion and mill scales.
- Proper electrode size.
- Proper angle.
- Smooth type of weld bead.
3. Lack of Penetration
Lack of joint penetration (LOP) is a defect that occurs when molten weld cannot completely fill the root part of the required thickness. Mostly, the failure is imminent as it acts as a stress raiser.
How to avoid LOP in welds?
Poor penetration defects can be avoided by considering appropriate weld preparation (root gap), the joint design, proper current, equipment selection and right techniques like heat, travel speed for welding.
4. Lack of Fusion
The failure of a welding process to provide a complete fusion is called a lack of fusion (LOF) or an incomplete fusion. They are basically of two types interpass (between weld layers) and side wall fusion (face of weld).
How to avoid LOF defects in welding?
Lack of fusion issues in weld can be avoided, of course, by ensuring proper heat input, welding current, and travel speed is applied.
Undercutting in welding involves the occurrence at weld interface of a groove, at the toe of a weld or weld root in the base metal.
Undercut acts as a stress raiser during fatigue loading. Moreover, it runs parallel to the weld metal. Consequently, visible as a small pit, which decreases the cross-sectional thickness of the base metal and may cause failure.
How to avoid undercuts in welds?
Especially, avoid undercuts by controlling the inappropriate travel speeds, arc length, weld angle, electrode distance, high current, and large diameter rods are some considerations to avoid causing undercuts.
When we weld materials of lesser thickness, the weld metal/filler material completely penetrates through the weld metal. For that reason, is called the burn through of welds.
How to avoid burn-through in welds?
Specifically, the burn-through in welds can be prevented by controlling heat exposure, voltage and wire speed in the weld.
When the face of the weld extends past the toe, it overlaps. Moreover, the base material can’t be fused to the filler, as it flows over the weld joint and cools down.
How to avoid overlap defect in welds?
Most importantly, ensure proper welding technique, weld angle, and travel speeds are followed to avoid overlap defects in welds.
Spatter is the expulsion of droplets of molten material from the welding process. It’s external visible defect, but in excess may cause cracks in the welded components or compromising the integrity of the weld.
How to avoid spatter in during welding?
Spatter, consequently, can be avoided by controlling the wire speed, high voltage, right shielding gases, and correct polarity selection.
Craters are welding defects that occurs when the welding process is stopped before a pass is completed. They form stress points that eventually cause cracking in welding.
Star cracks or crater cracks happen when due to the pulling effect when the center of the weld pool is solidified before its surroundings.
How to avoid crater defects in welds?
To avoid craters, ensure proper terminations, especially at stops by reducing the rate of extinguishing the welding arc, and the other is by reducing the welding current to shorten the weld puddle size. Also ensuring proper filling in the areas between tack welds, and the weld beads.
10. Weld crack
Obviously, any weld crack are one of the most unacceptable welding defects is it in any size or shape, microscopic or macroscopic. When the ultimate Tensile stress of the material is greater than the localized stress, the crack occurs. Weld cracks, for instance, can be internal, external, or in an area affected by strong heat.
Hot cracks occur during welding or soon after the completion of welding on weld metal as solidification cracks. When it occurs at base metal (HAZ) it is termed Liquation Crack. Star cracks (crater cracks) happen when due to the pulling effect when the center of the weld pool is solidified before its surroundings.
While Cold cracks occur after solidification of weld metal. Probability is high on the HAZ than weld metal and they develop slowly after several days.
How to avoid cracks in welds?
Most of the cracks can be avoided by practicing proper pre-heating and post heating procedures and right filler metals for the job.
We have listed most of the weld defects, and how to prevent the defects before or during a welding process. An ideal weld is one that has good penetration with sufficient fusion between the metal and the edge.
Proper monitoring of welds helps us to detect problems that can occur during a welding operation that could lead to defects, and therefore, we can apply the recommended practice, before it is too late.
This article is for a general guidance only. The acceptance criteria should be as stated in the engineering design and should at least meet the limits stated in Site document scope.
Have I missed any, of your specific interest, please comment.