Top Techniques for Correcting Welding Deformation

During the fabrication of welded structural components, although a series of necessary measures are taken, residual deformation often inevitably occurs; therefore, when the residual deformation exceeds the technical requirements, measures must be taken to correct it.

Common correction methods include manual correction, mechanical correction, flame correction, and electromagnetic correction, etc.

1. Manual correction method

The manual correction method involves using tools such as hammers to strike the deformed parts of the weldment. This method is mainly used for bending deformation of small and simple weldments and wave deformation of thin plates.

2. Mechanical correction method

Hydraulic presses, jacks, specialized straightening machines, and hammers are commonly used. External forces are applied to cause plastic deformation in the component opposite to the direction of welding deformation, thereby neutralizing each other. Figure 9-80 shows a schematic of mechanical straightening of a bent I-beam after welding using a press or jack. The angle deformation of the I-beam flange can be corrected using the roller machine shown in Figure 9-81.

a) Press correction b) Jack correction

Deformation after welding is mainly caused by the shrinkage of the weld and its nearby area. If forging or rolling is performed along the weld area to achieve plastic elongation, it can compensate for the plastic deformation that occurs during welding, thus eliminating the deformation. Small welded parts with few quantities are generally forged with a hand hammer. For thin plate structures with regular welds, rolling equipment can be used to roll the weld and its nearby areas, achieving good technical and economic effects.

Figure 9-82 shows a schematic of correcting the bending deformation of an aluminum cylinder after welding using a rolling machine, where the longitudinal seam is rolled. Changing the direction of the pressure roller can also roll the circumferential weld. Rolling forging of the weld not only eliminates residual welding deformation but also eliminates residual welding stress.

Mechanical correction methods are only suitable for simple structures of medium and small welded parts.

3. Flame correction method

The flame correction method, also known as the heating correction method, uses a flame as a heat source to locally heat the metal, causing it to undergo compressive plastic deformation. As the metal cools, it contracts, and the deformation caused by this contraction is used to counteract the residual deformation caused by welding.

This method generally uses a gas torch and does not require specialized equipment. It is simple and convenient to operate, flexible, and can be used to correct large and complex structures.

(1) The three essentials of flame correction

There are three main factors that determine the effect of flame correction: heating position, heating temperature, and the shape of the heated area.

1) Heating position

It is the key factor for success or failure. An incorrect heating position will not only fail to correct the deformation but may even worsen the existing deformation. Therefore, the selected heating position must cause the deformation in the opposite direction to the residual deformation from welding, to counteract it.

The main reasons for bending or angular deformation are that the welds are concentrated on one side of the neutral axis of the workpiece. To correct these deformations, the heating position must be chosen on the opposite side of the neutral axis, as shown in Figure 9-83. The farther the heating position is from the neutral axis, the better the correction effect.

a) Angular deformation from build-up welding b) Finned tube bending deformation

2) Heating temperature

The temperature of the heated area must be higher than that of the adjacent unheated area, causing the heated metal to thermally expand and be obstructed, resulting in compressive plastic deformation. For thick carbon steel plates or welded components with high rigidity, local heating temperatures above 100°C can produce compressive plastic deformation. In production, the temperature for flame correction heating of structural steel is generally controlled between 600~800°C.

On-site temperature measurement is inconvenient, generally, the color of the heated part is observed with the eyes to estimate the approximate temperature. Table 9-13 lists the colors of the steel plate surface during the heating process and their corresponding temperatures.

Table 9-13 Colors of steel plate surface and their corresponding temperatures

3) The shape of the heating area.

The shapes of the heating area include dot, stripe, and triangle, as shown in Figure 9-84, with dot heating shown in Figure 9-85, line heating shown in Figure 9-86, and triangle heating shown in Figure 9-87.

a) Dot b) Stripe c) Triangle

a) Straight-through heating b) Chain heating c) Band heating

(2) Common flame correction methods

Common methods for correcting welding deformation by flame are shown in Table 9-14.

Table 9-14 Methods for correcting welding deformation by heating

Examples of flame correction of welding deformation are shown in Figure 9-88.

a) Lateral bending of asymmetric arch steel

b) Upward deflection of asymmetric I-beam

c) Angular deformation of butt arch joint

d) Wavy deformation of medium thin plate

(3) Test results after flame correction

Results of flame correction tests for some commonly used low-alloy steels are shown in Table 9-15.

Table 9-15 Partial test results of commonly used low-alloy structural steels after flame correction

The water-fire bending plate is a process name in shipbuilding. The process is exactly the same as flame straightening; it involves using flame to locally heat the steel plate to achieve the desired deformation, with rapid cooling by water during the heating process.