There are numerous misconceptions about welding. This article sparks the opportunity to summarize some of the more common misconceptions or misunderstandings about welding-related topics…
“Preheat steel to get the moisture out.”
This is the most misunderstood concept in the welding industry. The reason we sometimes preheat is to slow the cooling rate of the weld deposit and the adjacent heat-affected zone. The purpose is to prevent the formation of the martensite microstructure that can occur in the HAZ on thick base material sections or high-strength, low-alloy steels. Martensite is a brittle microstructure that can lead to brittle failure, reduced cold-weather toughness values, and greater susceptibility to hydrogen-induced cracking.
This misconception stems from the preheating method. Most shops use some form of torch or open flame preheat. When a fuel such as propane is mixed with oxygen and undergoes a chemical reaction, a flame is created; the main byproducts are carbon dioxide and water. Because the flame temperature is many times greater than the boiling point of water, water vapor is produced in very high concentrations. When the gases from the flame travel far enough away from the center, the air temperature cools drastically. When this happens, the air temperature is not high enough to support the amount of water dissolved (vapor) in the air, called the dew point, and it condenses out and deposits on the surface of the weld plate.
This mechanism is the reason for the incorrect statement and the illusion that water is trapped inside the steel, which is physically impossible. It’s very similar to how hot, humid days lead to morning dew on grass or automobiles.
The additional reasons we preheat include reducing distortion, minimizing weld cracking in highly constrained weld joints, and burning off surface impurities such as paint or grease. If the welding procedure requires preheat, never skip this critical step!
“Welds should be blue, straw, rainbow, or some other color.”
This generalization comes from the process pipe and reactive or refractory metals industry. The color of a weld may be an indication of a poor weld; it is not proof of a good weld. The surface color of a weld can be correlated to the amount of oxygen exposure, the metals temperature during exposure, and the type of metal or its alloying elements.
Weld color is commonly talked about with titanium and stainless steel welds. With titanium, a blue color indicates oxygen in the welding chamber, but the acceptability of the weld may depend on the class of the weld being performed. With stainless steels, the color can have a direct impact on corrosion resistance.
“Push your welds for better penetration.”
This comment frequently arises with the gas metal arc welding process and wire feeding. Sometimes it’s stated the other way, suggesting you should drag your welds for better penetration.
The most important thing here is to stay on the leading edge of your weld puddle to ensure good base-metal penetration. This can be accomplished with either a push or a drag technique. However, dragging mostly guarantees you will stay on the leading edge, whereas pushing can allow the weld puddle to roll under the welding arc. If this happens, the weld puddle acts like an insulating layer between the arc and the base material, thereby reducing penetration.
Typical results of dragging a weld bead include a narrower profile, higher weld reinforcement, and deep penetration. Conversely, when using the push technique, the weld profile is flatter, wider, and slightly shallower, but that doesn’t mean inadequate penetration.
The misconception stems from hand-me-down knowledge and the welding backgrounds of the individuals involved. If welders spent most of their time welding in the structural steel or pressure vessel industries, they likely will advocate for a dragging technique. If welders have experience working with thinner materials, such as metal furniture, lightweight frames, or sheet metal work, they will advocate for a pushing technique. Welders who work across a variety of steel industries will understand the differences and know how to leverage them to achieve the desired outcome. If they want deep penetration, they’ll most likely drag the weld. If they want to minimize burning through their workpiece or there are gaps in the joint fit-up, they will typically push.
Rule of thumb: When the welding process produces slag, you should drag. This will prevent slag from getting trapped at the root of the weld, which can lead to weld failure in service.
“Stick electrodes and flux-cored wire are stronger than solid wire.”
This misconception stems from welders who have seen welds fail when using solid wire and the GMAW process. Two main reasons exist for this incorrect belief.
First, often a stick electrode in shielded metal arc welding or flux-cored wire is much larger in diameter than solid wire, requiring a higher amperage to maintain an arc when compared to using GMAW wires, which are mostly used in 0.035- or 0.045-in. diameters. Amperage is the welding variable that determines penetration depth. Second, the shielding gas used for GMAW and the transfer mode will significantly affect the depth of penetration.
The SMAW process produces its own carbon dioxide shielding gas during welding. Gas-shielded FCAW uses either carbon dioxide or a blend of carbon dioxide and argon, both of which produce a deep-penetrating weld. GMAW often uses blends with higher argon content, which reduces total penetration. Additionally, GMAW can use weld parameters low enough to achieve short-circuiting transfer, a low-voltage, low-amperage transfer mode. This transfer mode can produce welds that appear to have good fusion but may, in fact, have very little, which can lead to weld failure.
If weld penetration is adequate, all welding processes produce welds of similar strength, as defined by the AWS electrode classification. For example, an ER70S-6 solid electrode for GMAW, an E70T-1 electrode for FCAW, and an E7018 electrode for SMAW will all have a minimum ultimate tensile strength of 70,000 PSI in weld metal.
So, regardless of the welding process used, if penetration is adequate for partial-joint-penetration welds or for complete-joint-penetration welds, the electrode classification determines the weld’s strength.
“Good welds look like stacked dimes.”
This misunderstanding arises from the visually appealing pattern formed during some aluminum gas tungsten arc welds. Though pleasant to look at, this weld profile can lead to stress risers and incorrect weld size.
Because of the lower solidification temperature and techniques used in manual GTAW, the ripples are more pronounced with aluminum alloys than other metals. Prominent ripples can cause excessive weld size due to the throat needing to be measured in the low points. These valleys can also lead to stress concentrations, which can cause premature failure in cyclically loaded weldments. GMAW aluminum welds should not have this appearance; if they do, it’s likely caused by lack of penetration.