How to Weld Extra Thick Plates?

What kind of process do you want to use to solve the problem of welding base metal with a thickness of 270mm, is it robot welding or narrow gap welding?

Then let’s see how the genius makes a 4810mm x 4810mm x 270mm sheet by butt welding 270mm Q235D steel plate.

1. Problems of welding large thick plates and measures to solve them

Requirements: flatness requirements are from 8 to 10 mm to ensure the material properties of the steel plate after welding.

(1) Number and size of welded steel plates 

It is made of 3 steel plates, with widths of 1,900mm, 1,900mm and 1,050mm and lengths of 4,830mm, which are butted together.

Considering the problem of welding shrinkage, a margin of 9 mm was reserved.

After the completion of production, it is found that the shrinkage is 10 ~ 12.

Because the machining allowance is 25-30mm with a maximum 3mm error, which does not affect the use of processing.

(2) Welding method and groove type

General thick plate welding methods are electro-slag welding, submerged arc welding, gas-shielded welding and electrode arc welding.

Considering the existing conditions of the enterprise as well as the efficiency of various welding methods, it uses CO2 gas to protect backing welding with submerged arc welding and cover surface.

The groove form of the thick plate is mostly Ⅰ, X-shape and U-shape, etc., but after comprehensive comparison, the U-shaped groove is chosen.

Considering the degree of difficulty of root clearing, the asymmetric U-shaped groove is chosen.

In order to ensure welding quality, the production of the groove must be completed by machining means.

It must ensure that the size and surface roughness value is 12.5μm.

How To Weld Extra Thick Plates

(3) Pre-test welding

To ensure welding quality, it uses a steel plate with a length of 1m and thickness of 200mm to do a welding test, which is not only to exercise welders but also to find the shortage of the actual operation process.

During the bottom welding test, it was found that during the process of moving the welding end to the other end, the opening at the unwelded end did not change significantly.

(4) Welding materials and parameters

First, a specific analysis of the main causes of cracking:

① Hardening tendency

The material is the steel plate Q345D steel, and the upper limit of carbon content is 0.18%; wP, S ≤ 0.03%.

With a low hardening tendency and good weldability, so it is not the main reason for cold cracks.

② The function of hydrogen

The welding materials used are strictly dried and the workshop environment is dry.

Even if a small amount of hydrogen remains in the weld during welding, the content is low, which is not the main reason for cold cracks.

③ Uneven temperature distribution in the thickness direction during welding will cause large lateral compression plastic deformation;

Uneven shrinkage in the thickness direction when cooling after welding can easily cause angular deformation between the two connectors.

Secondly, the selection principle of welding materials is that the alloy composition and strength performance of the weld metal basically meet the lower limit specified by the base metal standard or reach the minimum performance index specified by the product technical conditions.

Therefore, it is determined to choose welding wire THQ-50C, φ1.2mm; submerged arc welding wire H10Mn2, φ4mm; flux SJ101 (preheating at 100°C for more than 4h before welding), welding parameters are as follows.

Welding parameters

welding bead arc voltage/V welding current/A welding wire grades and diameter/mm welding speed flux notes
backing welding 26-32 140-180 1.2/THQ-50C 300-400 CO2 gas shielded arc welding
filling welding 32-34 550 4/H10Mn2 200 SJ101 filling welding/AC
cover welding 40-42 650 4/H10Mn2 334 SJ101 filling welding/AC

Note: The interlayer temperature of the welding area is 120~180℃

Finally, reverse deformation is used to control the deformation during the welding process.

In the welding construction process, due to the reverse deformation caused by welding, it is necessary to turn the workpiece in time and weld the other side, so that the cyclic operation can realize the control deformation.

(5) Heat treatment

Preheating of the workpiece must be required for welding, and it is an important issue to ensure even heating of the workpiece.

After many trials, it was decided to drill several holes evenly on one side of a pipe about 4.8 m long.

The gas-cutting nozzle is then welded to the pipe to ensure a tight seal, and heated by gas ignition.

A total of two pieces are made so that both sides of the weld can be heated simultaneously.

At the end of welding, a large amount of welding residual stresses are generated inside the workpiece.

To prevent delayed cracking and deformation during processing, it is necessary to perform in-furnace de-stressing annealing after welding.

2. Implementation

Place the sheet material 1~1.2m above the ground, the anti-deformation angle is 1°~1.5°, and the butt gap is about 2mm.

Before welding, the 200mm area on each side of the reverse side of the weld will be heated at multiple points simultaneously to ensure uniform preheating temperature and the preheating temperature of the forward side: 90~120°C.

The side with the large groove is welded first, and the CO2 gas-shielded arc welding is used for the base.

At this time, the deformation of the farthest part of the control plate needs to be measured (the measuring points are not less than 4 points).

When the deformation of the workpiece is between 1° and 1.2° (calculate the number, set to A), i.e. the measurement point is above the plane value ≤ A, the workpiece is turned over.

It should note that the workpiece needs to be flipped on a large thick plate and welded on both sides of the weld concave rib plate, to prevent lifting when the weld cracks.

The width of the control submerged arc filler weld should be <18mm to reduce defects. The weld width is the same.

After the overturned workpiece, carbon arc gouging is required to remove the bottom weld, expose the weld metal and polish the smooth surface, and then start welding with submerged arc welding.

During the welding process, the deformation of the farthest side plate is measured continuously.

When the reverse deformation is 0°, the concave ribs at the weld are removed, leaving only three weld ribs (evenly spread), and when the reverse deformation reaches (A-5) mm, the workpiece is turned over again.

After the workpiece has been turned and placed firmly, the weld ribs are removed and the deformation of the plate is observed (our observation values are very small, about 2mm).

Then starting submerged arc welding, when the reverse deformation ≤ 10 mm (measuring point as above), it should turn over the workpiece again.

This process is done with a high-temperature UT flaw detection process, if available, to reduce the amount of work required to rework the final defect.

After the workpiece is turned over, submerged arc welding is conducted and the inverse deformation is controlled within 5mm.

Until the welding is completed as a whole, the workpiece is turned over and welded on the other side.

After welding, the workpiece is kept warm for 6h.

After natural cooling, the surface of the weld is polished, UT inspection is done, and then the whole is annealed at 620℃×10h for stress relief in the furnace.

When annealing, it should pay attention to the deformation of the large plate due to its own weight and large plate can also be flattened by methods such as self-weight and external gravity.

After the welded parts are subjected to stress relief annealing and cooled to room temperature, the welding defects and flatness are tested, and the next step is carried out after meeting the requirements.

The other steel plate is welded with the welded steel plate, and the welding method and operation steps are the same as above.

After the overall welding, the weld is kept warm for 6h, naturally cooled, UT detected, and then the whole is stress relieved and annealed again.

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