Electric Resistance Welding
- September 16, 2020
- Controlled Thermal Processing
- Cryogenic Processing
Welding is an integral component of many manufacturing processes, from the assembly of car and truck parts to the production of steel pipes. It’s one of the primary ways to join multiple pieces of metal or thermoplastics together to form a single unit.
Most forms of welding rely on heat, but the heat source can vary widely depending on the method. These energy sources can range from gas flames and lasers, to electron beams and ultrasound equipment.
Welding can be done almost anywhere, including under water or in space, but is usually performed in industrial or open air environments. Protective equipment must be used to avoid burns, and damage to the eyes or lungs due to laser or vapors.
In this article, we’re going to take a look at one subset of welding – known as electric resistance welding – as well as how cryogenic cooling fits in.
What is electric resistance welding?
First, how does electric resistance welding differ from other types of welding? In short, electric resistance welding (or ERW) uses an electric current as a heat source.
Like other forms of welding, it works by heating up the metal and melting it so that one or more pieces of metal can be attached together. In this case, the heat is supplied by an electric current, which produces a patch of molten metal at the joint. Pressure is applied to the seam to create a forged weld.
This is distinct from arc welding, in which an electric arc is used to create a transfer of energy from an electrode to a metal piece. Arc welding doesn’t require pressure; the metal bonds together as it cools.
Arc welding can be used on thicker metals, while resistance welding works best on thin materials, primarily steel or stainless steel. It’s known for being a fairly efficient process that creates less pollution than other forms of welding.
The amount of current and pressure required depends on several factors, including how conductive the metal is, whether or not it’s coated, and more.
What is it used on?
Electric resistance welding can be used on a variety of metal parts, from sheet metal to pipes. For example, ERW pipes are produced by rolling a sheet of steel into pipe form, and then applying heat and pressure to seal the seam together.
Due to the high heat and precision required, most electric resistance welding takes the form of an automated process rather than a manual one. The types of electrodes used vary depending on the technique, but are usually made from alloys of copper. Water is used to keep the welding machine from overheating.
You can find resistance welding technology in the aerospace and automotive industries, as well in as other manufacturing industries. It’s also common in the production of steel pipes used to transport oil, water, and other low- to medium-pressure liquids.
Types of electric resistance welding
Electric resistance welding is a broad category that can be broken down into several subcategories based on the type of weld and what kind of pressure is applied. All of them have specific applications that they’re best suited for.
Let’s take a closer look at the five main types of resistance welding and how they go about creating the heat required to form a joint.
Resistance spot welding
Spot welding is a common technique that can be used to join metal sheets, fins, tubing, and other components that measure 3mm in thickness or less.
In this method, the electrodes are embedded in the clamping mechanism that’s used to fuse the metal ends together. Because the electrodes are held in the jaws of the device itself, electricity flows directly between them and the metal being processed.
This combination of heat and pressure causes the molten metal to combine, forming a seam. Once the electricity is stopped, the pressure must be maintained in order for the parts to successfully fuse into a single unit.
The precise amount of pressure and current is programmed into the machine, usually in a timeframe of milliseconds. The electricity may be applied as an alternating current, a direct current, or in some cases, a half-wave direct current.
The timing is important, because if any of these factors are off, it can result in an escape of molten metal called “expulsion,” which weakens the finished product. This can occur when the current is applied too strongly or too quickly, or if there are problems with the thickness or conductivity of the metal’s coating.
Spot welding is more cost-effective than riveting, and can be performed at high speed using modern automation technology. However, it produces multiple weld spots rather than a single seam, which may not be strong enough for some applications.
Resistance projection welding
Projection welding is a technique that relies on a concentrated point of contact with the metal, rather than a clamping mechanism, to apply the necessary pressure.
There are four different types of projection welds: coined projection, corner projection, stamped project, and machined projection. Depending on the shape of the mechanism and the type of metal, it may produce visible projections on the metal surface, such as ridges or dimples, that remain on the finished project.
The benefit to this approach is that it’s less dependent on electrodes and can carry a stronger current. As a result, it can be used to weld thicker pieces of metal.
It can also be used to remove plating to ensure that the primary metal is able to bond. For example, when working with copper that’s plated with tin, the lower melting point of the tin plating means that it’s pushed out of the way and the copper can bond directly.
One common example of a projection weld is cross-wire welding. In this case, two wires are welded together at the point where they meet, such as in welded wire fencing.
Resistance butt welding
Butt welding is a technique that’s often used in structural elements, including pipes and flanges. Its name refers to the fact that the end, or “butt,” of each component is welded together, without any overlapping elements.
Depending on the part, it can be performed by hand or by machine. As with other types of resistance welding, it uses a combination of pressure and an electric current to force the ends of the material to fuse together.
The strength of a butt weld joint depends on whether it has been welded from one side or two, and how far the metal has been penetrated.
This results in four main types of joints:
- Single sided full penetration butt weld
- Single sided partial penetration butt weld
- Double sided full penetration butt weld
- Double sided partial penetration butt weld
In addition, butt welds can be categorized by their shape and configuration, including square-groove, bevel-groove, and V-, J-, and U-groove butt welds. Because there are so many ways to apply this technique, butt weld joints are some of the most common joints out there, and can be created using either resistance or flash welding.
Flash butt welding
Flash butt welding is another variation on the technique, but it uses two stages to weld the material together, rather than the single stage used in resistance welding.
In this approach, a gap is left between the materials being fused together. This creates resistance when the current is applied, resulting in an arc that melts the metal.
These two stages of flash welding are defined by flash time and upset time. Flash time refers to the stage when the current is applied to heat the metal, and upset time refers to the period when the metal is pressed together.
The distance between the metal and the timing of each stage have to be just right in order to ensure a sufficient bond and to avoid any impurities.
Flash welding is common in the railroad industry, where it’s used to create smoother joints on the rails of high-speed rail lines.
Resistance seam welding
Finally, there’s resistance seam welding, which is used to create a single seam instead of a series of spot welds or projection welds. This is especially important for fuel tanks and other components that require a continuous, unbroken connection.
While spot welding relies on a clamping mechanism, seam welding machines use two copper wheels instead. As the material passes between them, they receive pulses of electricity that create the heat needed to melt the metal.
Seam welds may be either intermittent or continuous, which refers to whether or not the wheels run continuously, or stop at each desired welding point.
What is cryogenic weld cooling?
So far, we’ve been talking a lot about heat, which is the key component in the welding process. But one side effect of all that heat is it can generate distortion in the finished product, resulting in weaker materials overall.
Cryogenic weld cooling uses cryogenic gases, such as CO2, to cool the metal as soon as it’s been joined together. Because cryogenic gases are so cold (−238 °F or below), they’re a more effective coolant than jets of air or sprays of mist.
They can be blown directly onto the material in the form of gas or “snow,” resulting in a faster cooling process with less distortion or expulsion.
To learn more about how you can use cryogenic cooling in your welding process, reach out to the team at CTP Cryogenics. With over 30 years of experience in cryogenics, we can help you choose the right treatment for your materials today.
Contact us now for a free quote that’s custom-made for your industry!