Types of Welding Discontinuity And What is Discontinuity
Welding Discontinuities – Do you know that no matter how you try to keep things in order, but in the end, a mistake will still happen? For example when you casually drive to your campus or workplace, no matter how often you check your car’s condition, in one of those days your car will experience a malfunction. Either your tire blew out or transmission malfunction or even external disturbance like crashes and getting a ticket from patrolling police officer. It seems like there’s a force of nature that intervenes to turn things into disorder or chaos. If you think so, know that you’re not wrong. This nature’s behavior is called entropy, a behavior that requires us to put extra effort to negate the tendency of things from its disorder state and keeping it in order. That’s why you have to regularly check your car and bring that extra tire everywhere you go.
The same force of nature also applies in welding. As a welder, you really want to weld a sound weld. A sound weld means that there’s uniformity (with a very little variation tolerance) everywhere you see, either on weld surface or in sub-surface. A sound weld is the best form of the weld, it will have the ability to withstand its designed stress, which means that the quality of fusion between the weld metal and the parent metal is top notch. However, entropy also plays its role to spread chaos in welding. If the welder is unaware and somehow make mistake, it will result in a non-uniform weld metal. Now, this non-uniform phenomenon is what we call as discontinuity. So what’s discontinuity and how to avert the forming of discontinuities during welding? This article will give you a clear definition of discontinuity itself that will surely help you as a welder.
What is Discontinuity?
Now we know that the existence of discontinuity means that the weldment is not sound. Basically, there is two main category of discontinuity based on its inflicted damage on the material.
- Reparable Discontinuity
As the name implies, discontinuity under this category is considered harmless to the material that a simple repair action would solve the problem. Most of the mechanical damage such as; scratch, dent, and even surface corrosion falls under this category with the exception a huge dent because you somehow find a way to drop a huge concrete block on it. While welding discontinuities such as; porosity, slag inclusion, and undercut can either called discontinuity or defect.
Even when this type of discontinuity is highly tolerated. It is wise for you to always avoid it to train your mind to give your best when doing something. So when the time comes for you to handle a job with a stricter regulation on discontinuity, you’ll be ready.
- Irreparable Discontinuity
In some cases, a discontinuity can greatly affect the performance of the weld. Discontinuities like lack of fusion, laminar tearing, and most forms of cracks are one of them. No matter how hard you try to repair it, you’ll only waste your time and resources, making it not worth the fuss. So it’s best for you to avoid it from the very first place by following the correct welding procedure.
How to Deal With Discontinuities?
Because there is just so many forms of discontinuity, there are also many ways to treat them. To make things simpler, let’s talk about it one by one.
Porosity is one of that classic discontinuity that happens in welding. It happens when there are just so many gas molecules trapped inside during welding. Those gasses might come from a wet electrode because the water molecules on the electrode will dissolve due to the heat of welding and result into trapped oxygen that never comes out until the weld pool turns solid. A windy environment can also promote porosity because the wind will disrupt the shielding environment around the welding arc and allow gasses to enter the molten metal. This porosity can easily be avoided by avoiding contact with the wet environment during welding and making sure that the weld pool is properly shielded with either flux, gas shielding, or both. Several examination methods can detect porosity such as; visual examination, liquid penetrant, radiography examination, etc.
Slag inclusion is also one of the classic discontinuity that occurs in welding. It’s a situation when the slag – a protective layer that supposed to resurface and protect the molten weld pool – is trapped under the geometry of the weld itself. The trapped slag replaces the supposed filler weld metal with a chunk of Manganese Oxide (may vary) that when you chipped it off will leave the hole gaped, making it good for nothing. The various thing can cause slag inclusion, dirty weld surface and a relatively low welding parameter being used is supposed to be the primary cause. Detectable visually, but for sub-surface slag inclusion can only be detected by radiography examination or ultrasonic examination.
The undercut is one of the classics, along with slag inclusion and porosity, which occur most in welding. It primarily happens because bad maneuver is done by the welder that the arc melts the side of the pool without properly filling it. While it looks minuscule, undercut could be one of the sources of initial crack that later on will propagate and break the material. To avoid this, try to avoid using an overly high welding parameter while moving along the weld bead evenly to avoid traveling too fast. Undercuts are usually surface defect, thus can easily be recognized visually. Liquid penetrant and magnetic particle examination can also help to detect it.
While there are many causes on which a crack can occur, the crack can also occur during welding, making it worth mentioning. Crack is highly dangerous because it will greatly reduce the material’s strength and will propagate along the way when the material is loaded. Therefore a complete repair procedure should be done at once when the crack is detected by any examination method (radiography, liquid penetrant, and magnetic particle). Some crack can happen during a non-welding process such as lamellar tearing. While cracks like hot and cold crack happen mostly because of welding.
In hot crack, cracks happen because of the pre-existing contaminant like; sulfur and phosphorus inside the base metal. Those contaminant form a carbide that has a lower melting temperature than the base metal itself. When the heat from welding highly increase the temperature, those carbides will melt and decrease the material’s strength. Combined with the stress applied to the material, it will break and propagate fast without even waiting for the material to cool down. Hence the name hot crack. Ordering a killed material can greatly reduce the chance of hot crack to happen, or simply regulating the inter-pass temperature can also work.
A different thing happens in cold crack, high hydrogen concentration around the weld metal is the primary cause. When the weld metal is hot, the hydrogen in the environment might enter the metal by the mean of diffusion. This hydrogen atom will mostly reside in the HAZ (Heat-Affected Zone) where the crystalline is most susceptible due to its coarseness. The existence of hydrogen will turn the material into its brittle state that continuously builds internal stress up until its breaking point. This process takes so much time that its breaking point will happen even when everything is already cooled down. Hence the name cold crack. To avoid this, using low-hydrogen electrode will greatly help, and also doing PWHT (Post Weld Heat Treatment) can reduce the residual stress and also allowing the hydrogen to escape.
After we learn that there are just too many variables that may cause a discontinuity. It’s just impossible to avoid each and every one of them, so making mistake every now and then is pretty normal as long as we know how to deal with it. However, it doesn’t mean that we have to take things for granted. We can surely do our best to avoid making mistakes so frequently if we strive. That’s what we should do to fight the entropic force of nature, by striving for excellence.