Welding is a process that joins two metals together, but there are good weld vs bad weld considerations that must be understood to create quality weld joints. A good welder can create seamless joints and the two materials may appear to be one. In this article, we will take a closer look at the differences between good and bad welds.
How to Determine a Good Weld vs Bad One
A bad weld may look untidy, but the primary concern is that it’s unsafe and yet you can learn about a weld with a visual inspection. If you notice cracks, splatters, and thin lines these are signs that the weld is bad.
Different welding processes can produce different results with varying degrees of difficulty to create good welds. If you cannot see any evidence of a weld or the join is seamless, you’re looking at a good weld.
What Are 5 Characteristics of A Good Weld?
When we consider good vs bad welds, there are five characteristics which indicate good weld quality.
- No evidence of cracks or holes.
- Uniform bead height, width, and waves.
- The design dimensions are satisfied with virtually no distortion.
- The weld is strong.
- Full and partial penetration welds have fused and joined the interface between weld joints or base materials appropriately to ensure rigidity.
5 Characteristics of A Bad Weld
When you look at a good weld vs bad weld, you will notice five key differences that indicate poor quality.
- The weld joint will look messy.
- There may be uneven joints.
- A physical test may indicate weakness and movement.
- There may be jagged edges.
- There could be cracks in the joint.
How Can You Tell the Quality of a Weld?
A wide range of weld joints can be created and there is no one-size-fits-all solution that could be applied across the board. Each welding process must be approached on an individual basis, but there are two desirable characteristics: quality and strength. These two characteristics are connected, if you have a low-quality weld, it will have low strength and vice versa.
Let’s look at four common weld processes and how you can determine the quality of the weld joints:
1. Stick Welding
A high-quality stick weld will have a straight and uniform appearance with no spatter drops. The thickness will be consistent, there will be no holes, cracks, or breaks in the bead. If the weld is poor there will be clear evidence of spatter, cracking, undercutting and the bead width will vary.
2. TIG Welding
A high quality TIG weld will be multiple tiny welds that are layered to create a pattern that can be aesthetically pleasing. There will be no evidence of burnout or slag. A bad TIG weld will have no pattern, the beads will be erratic and there may be signs of burnout.
3. MIG Welding
A good MIG weld will be straight, uniform, and have no cracks, holes, slag, or breaks. The weld joint should not be too thin, with no craters or dips along the length of the bead. There are no patterns, the surface should be smooth, and any signs of cracking indicate a poor MIG weld.
4. Oxy-Fuel Welding
A good oxyacetylene weld will have a uniform bead with no globules of melted metal and no holes.
Reasons Why You May Have Ended Up with A Bad Weld
To understand the good weld vs bad weld differences more completely, let’s look at five reasons why you may have a bad weld.
1. Insufficient Temperature for Welding
Welding requires high temperatures and welding machines have temperature settings that can range from 5,000ºF up to 36,000ºF. The correct welding temperature is essential to initiate the fusion process to create strong welds. If the temperature is too high or too low it may compromise the integrity of the weld.
At high temperatures, there is a risk that small pores are created along the weld joints and the metal may become brittle. At lower temperatures, the disparity between hot and cold metals and two surfaces that are much colder than the heat generated by the arc can be disastrous.
The weak weld joints created under these conditions may have instant cracks that form along the length of the joint. Preheating materials can help, this lowers the stresses and prevents metal shrinkage because hot materials expand and cooler materials contract.
2. Rapid Cooling Stress of a Weld
During rapid or inefficient cooling, thermal stress can buildup and this leads to cracking. The metal should be annealed to prevent this problem, and this is achieved by heating the weld again after fusion has occurred. The temperature should be lower than the melting temperature of the metal that you’re working with.
This is like preheating and this reheating can be done with a furnace or torch as required. The appropriate furnace range is usually 1450-1650 with incubation at a set time. Soak the metal for an hour for each inch of thickness and base your incubation temperature on the thickest part. The weld should cool naturally, and this allows the weld to purge the internal thermal stress.
3. Faulty Welding Equipment
Faulty equipment can have a huge effect on the quality of the weld joint. To illustrate this purpose let’s look at MIG welding which is widely regarded as the easiest welding process. The contact tip produces the arc where the heat is sufficiently high to fuse two metals together.
Burn back can occur if the gun is too close or if the wire is fed energy slowly. In both cases, the contact tip must be replaced to increase the feed speed. But if the feed speed is excessive it can cause the molten metal to spatter and ruin the bead.
Fusion may be incomplete; the weld may look bad and it’s probably weaker. Constant adjustments are necessary, and this cannot be achieved with faulty welding equipment.
4. Oxidation Issues of Metals Being Welded
Metals oxidize when they are exposed to oxygen and an electrolyte, and this is especially true when it comes to hot metals. An oxide layer is formed on the surface, and this diminishes the natural resistance to corrosion.
Weld purging is a technique which removes the oxygen and ion gases during and immediately after the welding process. An inert gas such as helium or argon has non-reactive properties which can prevent oxidation.
Welding may use gas or a flux rod that is embedded with carbon gas to achieve the same results. The gas is introduced slowly during the welding process and a purge monitor can be used to measure the gas reading.
5. Welding Dirty Surfaces
During the welding process, it’s important to have clean surfaces to prevent contamination from sources, such as dirt, rust, paint, and more. These contaminants may prevent uniform fusion, and this can lead to the formation of pores that weaken the weld.
How Can You Fix a Bad Weld?
Grinding away the voids and re-welding is the only option. If the bad weld was caused by a lack of penetration, it may be necessary to make multiple passes at higher temperatures.
Preventing A Bad Weld
When you are considering good vs bad welds there are three main ways to prevent a bad weld.
1. Careful Fit-up and Weld Joint Design
This will help to prevent hot cracking weld failures which usually occur when a welder attempts to compensate for bad design with a wider bead to fuse metals together. This can create thin and weak welds that create stress at the center which is known as bead-shape cracking. The joint should be designed with easy root access to ensure a proper bead depth (5:1) to width (2:1) ratio.
2. Pre and Post-Welding Considerations
Certain materials can be susceptible to weld failures, this can include high-strength steel with higher alloy and/or carbon levels. These materials are less ductile, they generate residual stress along the base metal and weld when cooling. So, these materials should be preheated for the recommended temperatures and times.
This will prevent the rapid cooling that can affect the ductile grain structure which causes cracking. Post-weld heat treatments (PWHT) should be applied to diffuse hydrogen and relieve residual stress as required to prevent cold cracking.
3. Match Base Material and Filler Metal Strengths Properly Before Welding
The risk of weld failures can be mitigated when the appropriate filler metal strength that matches the base material is considered. The filler metal strength should be closely matched to the base material to meet the requirements of the application.
If you need to weld a lower strength to a higher strength material, it’s important to match the filler metal to the lower strength material. Certain fillet welds or partial joint penetration (PJP) require under matched strength filler metal to be used with the base material. This can reduce the residual stress in the finished weld joint.
FAQ’s-Good Weld vs Bad Weld
Can you weld over a bad weld?
Yes, but this is rarely successful, and, in most cases, it makes the weld joint weaker.
Why are my welds not holding?
We covered the five possible causes earlier. They are low temperatures, inefficient cooling, faulty welding machines, oxidation, and poorly cleaned surfaces.
Why do You need to have a good weld?
An aesthetic appeal may indicate a good weld, but the primary purpose is to maintain strength across the entire weld joint.
Conclusion –Good Weld vs Bad Weld
Hopefully, we’ve shown the differences between a good weld vs bad weld. There are many factors to consider, a visual inspection can help, but at times it can be hard to identify a bad weld without experience. Adopting the techniques mentioned in this article should help you to improve your welds and mitigate the risks of creating a bad weld.