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Electric Resistance Welding

Date:2021-01-08    keywords:electric resistance welding
Electric resistance welding  (ERW) refers to a method in which electric current passes through the weldment and the resistance heat generated at the contact point as a heat source to locally heat the weldment while applying pressure to weld. When welding, no filler metal is required, the productivity is high, the deformation of the weldment is small, and it is easy to realize automation. [1] Electric resistance welding is widely used, for example, in the manufacture of steel pipes and the assembly of automobile bodies.

Electric resistance welding uses the resistance heating effect generated by the current flowing through the contact surface of the workpiece and the adjacent area to heat it to a molten or plastic state to form a metal bond. There are four main resistance welding methods, namely spot welding, seam welding, projection welding, and butt welding.



Introduction:
Electric resistance welding is a type of welding method that uses resistance heat as the energy source, including electroslag welding that uses slag resistance heat as the energy source and resistance welding that uses solid resistance heat as the energy source.

Electric resistance welding is generally a welding method in which the workpiece is placed under a certain electrode pressure and the contact surface between the two workpieces is melted by the resistance heat generated when the current passes through the workpiece to realize the connection. Usually use a larger current. In order to prevent arcing on the contact surfaces and to forge the weld metal, pressure must always be applied during the welding process. When performing this type of resistance welding, the surface quality of the welded workpiece is of paramount importance for obtaining stable welding quality. Therefore, the contact surface between the electrode and the workpiece and between the workpiece and the workpiece must be cleaned before welding.

Classification of electric resistance welding methods:
1. Spot welding
Spot welding is a resistance welding method in which weldments are assembled into lap joints and pressed between two cylindrical electrodes, and the base metal is melted by resistance heat to form a welding spot. Spot welding is mainly used for thin plate welding.

The process of spot welding:
(1) Preload to ensure good contact of the workpiece.
(2) Energize to form nugget and plastic ring at the weld.
(3) Power-off forging, so that the nugget cools and crystallizes under the continued pressure to form a densely structured solder joint without shrinkage or cracks.

2. Seam welding
The seam welding process is similar to spot welding, except that the cylindrical electrode is replaced by a rotating disc-shaped roller electrode, and the weldment is assembled into a lap or butt joint, and placed between the two roller electrodes. The roller pressure welding It is a resistance welding method in which the parts are rotated, continuously or intermittently, to form a continuous weld. Seam welding is mainly used for welding structures with regular weld seams and sealing requirements. The thickness of the plate is generally below 3mm.

3. Butt welding
Butt Welding is a resistance welding method that welds the weldment along the entire contact surface.
(1) Resistance Butt Welding
Resistance butt welding is a method of assembling the weldment into a butt joint so that its end faces are in close contact, using resistance heat to heat it to a plastic state, then the power is cut off and the upsetting force is quickly applied to complete the welding. Resistance butt welding is mainly used for simple cross-section and diameter Or weldments with side length less than 20mm and low strength requirements.

(2) Flash Butt Welding
Flash butt welding is to assemble the weldment into a butt joint, turn on the power, and make the end surface gradually move closer to achieve local contact. The contact points are heated by resistance heat. Under the action of large current, flash is generated to melt the end metal until the end When the part reaches a predetermined temperature within a certain depth range, the power is turned off and the upsetting force is quickly applied to complete the welding.
The joint quality of flash welding is better than that of resistance welding, and the mechanical properties of the weld are equivalent to the base metal, and there is no need to clean the pre-welded surface of the joint before welding. Flash butt welding is often used for welding important weldments. It can weld the same kind of metal, but also dissimilar metals; it can weld 0.01mm metal wire, and also can weld 20,000mm metal rod and profile.

4. Projection welding
Projection Welding is a variant of spot welding; there are prefabricated bumps on a workpiece, and during projection welding, one or more nuggets can be formed at the joint at a time.
The quality of resistance welding is determined by the following 4 factors:
1) Current; 2) Power-on time; 3) Pressure; 4) Diameter of the top end of the resistor.

Principle of electric resistance welding:
The generation of welding heat and the factors affecting heat generation The heat generated during spot welding is determined by the following formula: Q = I2Rt
In the formula Q-the heat generated (J)
I2——The square of welding current (A)
R——Resistance between electrodes (Ω)
t——welding time (s)
Resistance R and the factors affecting R. 

When the workpiece and the electrode are timed, the resistance of the workpiece depends on its resistivity. Therefore, electrical resistivity is an important property of the material being welded. Metals with high resistivity have poor thermal conductivity (such as stainless steel), and metals with low resistivity have good thermal conductivity (such as aluminum alloy). Therefore, spot welding of stainless steel is easy to generate heat but difficult to dissipate, and spot welding of aluminum alloy is difficult to generate heat but easy to dissipate. When spot welding, the former can use a small current (several thousand amperes), and the latter must use a large current (tens of thousands of amperes).


Advantages and disadvantages of electric resistance welding:

1. Advantages

(1) When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.
(2) The heating time is short and the heat is concentrated, so the heat-affected zone is small, and the deformation and stress are also small. Usually, it is not necessary to arrange correction and heat treatment procedures after welding.
(3) It does not require filler metals such as welding wires and electrodes, and welding materials such as oxygen, acetylene, and hydrogen, and the welding cost is low.
(4) Simple operation, easy to realize mechanization and automation, and improve working conditions.
(5) The productivity is high, and there is no noise and harmful gas. In mass production, it can be compiled on the assembly line together with other manufacturing processes. However, flash butt welding needs to be isolated due to sparks and splashes.

2. Disadvantages
(1) There is still a lack of reliable non-destructive testing methods. Welding quality can only be checked by destructive tests on process samples and workpieces, as well as by various monitoring techniques.
(2) The lap joint of spot welding and seam welding not only increases the weight of the component, but also forms an angle around the welding nugget of the two plates, resulting in lower tensile strength and fatigue strength of the joint.
(3) The equipment has high power, high degree of mechanization and automation, which makes the equipment more expensive and difficult to maintain, and the commonly used high-power single-phase AC welding machine is not conducive to the balanced operation of the power grid.

Tips: In case ASTM A53 Grade B in ERW (electric resistance welded) pipe, the weld seam of A53 erw pipe shall be done the heat treatment with a minimum 1000°F [540°C]. In this way the no untempered martensite remains.
In case ASTM A53 B pipe in cold expanded, then expansion should not exceed 1.5% of the required OD.


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