Single Side Welding and Double Side Forming Basics

1. Definition and classification of single-sided welding and double-sided forming

1.1 Definition of single side welding and double side forming

The welding method of single side welding and double side forming is a necessary operation method and skill for welders engaged in welding of pressure vessels, important silos and boilers.

It is also used in the manufacturing and installation of some important welded steel structures, that is, the welding technology that requires full penetration but cannot be processed and re welded on the back of the components.

In the process of such welding, no other auxiliary measures need to be taken.

When positioning welding is only needed at the root of the groove, different gaps should be reserved according to different welding methods.

When welding is carried out on the front side of the groove, uniform, regular and qualified welds will be obtained on the front and back sides of the groove.

This unconventional welding operation is “single-sided welding and double-sided forming”.

1.2 Grading of single-sided welding and double-sided forming

1.2.1 Arc intermittent welding method

Place the small gap end of the test panel on the left side, position it on the left side of the test panel for arc striking, and use a long arc for a certain dwell time to preheat the welding components, and then press down the arc to swing the electrode laterally between the two blunt edges for welding.

When the molten metal of the blunt edge is connected with the molten metal of the electrode, and the “porphyrin” sound is heard, the arc light will be extinguished.

The feature of this operation is that the center of the welding electrode shall be aligned to 2/3 of the welding pool during each continuous arc, and the arc shall melt both sides of the blunt edge at the same time.

When the sound of “porphyrin” is heard, the arc is quickly extinguished so that each newly formed molten pool covers about 2/3 of the previously formed molten pool.

1.2.2 Continuous arc welding method

Continuous arc welding is a technology in which the arc burns continuously during welding, the arc light is not extinguished, small groove gap is adopted, and small welding current is used at the beginning, and short arc continuous welding is always maintained.

The basic elements of this technology: after striking the arc with the electrode, firstly press the arc to the minimum, and at the place where welding is to be started, use the sawtooth shaped electrode moving method with small pitch to swing the electrode laterally, at this time, the purpose is to preheat the welding components.

The welding rod shall be sent to the root as far as possible to conduct the electrode breakdown action.

When the “porphyrin” sound is heard, the weldment will form a dissolved hole.

At this time, the arc shall be transferred to any groove surface position immediately, and then a certain electrode angle shall be used between the groove surfaces.

The operation seems to be non-stop small action, and the time is about 2s.

When the arc melts the groove root on both sides for about 1.5mm, the welding rod shall be raised by 1~2mm.

Swing the electrode in a zigzag manner with small pitch so that the arc is in front of the melting hole while welding is carried out forward.

During welding, ensure that the center of the welding rod is aligned with the front edge of the weld pool and the junction of the base metal, so that each new weld pool overlaps with the previously generated weld pool.

During arc extinguishing, slowly bring the electrode to the right or left side behind the molten pool, and raise the electrode to extinguish the arc.

In joint welding, the arc is ignited at 10~15mm away from the crater at the beginning, and the electrode is moved to 1/2 of the crater at a normal speed.

Press the electrode downward. When a “porphyrin” sound is heard, make a slight swing for 1~2s, and then raise the electrode by 1~2mm, so that the electrode is moved forward to weld while melting the front of the melt hole.

Single Side Welding and Double Side Forming Basics 1

2. Process analysis of plate butt welding and double-sided forming

2.1 Preparation before welding

(1) Plate thickness: 12mm, specification: 300 × 200mm.

Cut with oxygen and acetylene gas or cut with plate shears and cutting machines, and then use mechanical methods to process V-shaped grooves, such as planers or angle grinders.

No chamfers are allowed on the back of the grooves.

Make sure that the grooves are straight, straight, smooth, free of burrs, bulges, etc.

(2) For the cleaning of the test panel, use a file to remove the sharp corners of the groove to keep the size of the blunt edge at 0.5~1.0mm, and remove rust, oil, oxide and other contaminants from the groove and within 20mm on both sides to make it appear metallic luster.

(3) Assembly and tack welding of test panel.

Tack welding shall be carried out on both ends of the back of the test plate, and shrinkage allowance shall be reserved.

Make the weld end gap 3.0~3.5mm, the final weld end 3.5~4.0mm, and the positioned weld length about 10~15mm.

The positioned weld should be firm, especially at the final weld end.

In order to avoid the influence on the continuous welding or the fracture in the welding process due to the reduction of the gap of the groove of the non-welded section due to the shrinkage of the weld, the reverse deformation allowance must be reserved during positioned welding, which is the reverse deformation of the angle of 3 °~4 °.

(4) Welding equipment: ZXG series DC arc welding machine.

Type and polarity of power supply: DC welding rod shall not be affected with damp and deterioration, the welding core shall be free of rust, and the coating shall not crack and fall off.

Bake to 350~400℃ before use, keep constant temperature for 2h, and weld layers are welded in four layers.

(5) Fixing the test plate: place the anti deformed test plate horizontally on the welding frame with proper height.

The welder shall not sit down during welding, and shall operate in a squatting position.

The groove angle of the test plate is 60 °.

2.2 Welding operation

2.2.1 Welding of bottom layer

The welding of the primer can be performed by either general welding or end arc welding. Continuous arc welding.

Ignition arc: during welding, the arc shall be started at the inner side of the groove of the test panel, and the bottom arc shall be pressed.

The welding rod shall swing slightly back and forth between the two blunt edges of the groove.

When the molten iron at the edge is combined with the molten droplet of the welding rod, and the sound of “porphyrin” is heard, the first molten pool will be formed.

At this time, there should be a dissolution hole in front of the dissolution tank, which is 0.5~1.0mm deep to both sides of the test plate groove.

Welding rod operation method: zigzag or crescent shaped transverse swing welding method is adopted.

The length of the swing arc is less than or equal to the electrode diameter.

A short arc is used to send the molten iron through the root of the groove to the back of the weldment.

When the welding rod is transported to both sides of the groove, a 1-2s pause is required, which is conducive to good fusion of the filler metal and the base metal, and prevents the formation of sharp corners at the junction of the weld and the groove, which is conducive to slag removal.

2.2.2 Electrode angle

The included angle of the welding forward direction is 70 °~80 °.

A proper electrode angle is conducive to separating the molten slag from the molten iron, keeping the molten pool clear and bright all the time, and avoiding clamping.

Main points of welding rod: look, listen, be accurate and short.

“Look” means to observe the shape of the molten pool and the size of the solution hole, which determines the height and reinforcement of the back weld.

The shape of the molten pool should be round or oval, and it should always be bright and clear to separate the slag from the molten iron.

The size of the melting hole shall be such that the arc melts all the blunt edges on both sides and penetrates 0.5~1.0mm into the base metal on each side.

When the size of the hole is too large, the weld on the back is too high, and in serious cases, weld beading or even burn through will occur.

At this time, the welding speed should be properly increased, the swing range of the electrode toward both sides of the groove should be increased, and the included angle between the electrode and the welding direction should be reduced.

When the weld hole is too small, the root of the groove is not fully welded, and the fusion of the weld root is not good.

At this time, the bottom arc should be pressed to increase the angle of the welding rod in the forward direction, slow down the welding speed and swing amplitude, reduce the angle between the welding rod and the forward direction of welding, and maintain the shape of the weld pool and the size of the solution hole is the key to the operation.

During welding, it is necessary to control the flow direction of molten iron and solution.

The electric arc should always burn in front of the molten iron.

The reverse blowing force of the gas generated by the electric arc and coating melting is used to blow the molten iron to the rear of the molten pool, so as to ensure the penetration of the weld, the separation of molten slag and molten iron, and reduce the possible defects of slag inclusion and porosity.

In addition, it is necessary to observe the fusion of the welded groove at any time. 

The rear edge of the molten pool shall be completely fused with the grooves on both sides.

“Listening” means that the welder should not have any distractions during welding, that is, listen to the “porphyrin” sound when the arc breaks through the test plate.

If this sound is not heard, then the groove root is not broken down by the arc. If the welding continues forward, the welding will be impermeable.

Generally, keep the end of the electrode 2 or 3 mm away from the groove root.

“Accurate” means that the arc contact position should be accurate, that is, each new molten pool should basically overlap the previous molten pool by 2/3 to ensure that 1/3 of the arc is directly in front of the molten pool.

When the welder hears the sound of “Por Por”, he should extinguish the arc quickly behind the molten pool.

In this way, the protective effect of the arc on the molten pool can be fully exerted to avoid the generation of pores.

“Short” refers to the time of arc extinguishing and butt welding, which should be completed in the shortest possible time, otherwise it is easy to produce cold shrinkage defects, and metal cracking is relatively easy due to insufficient fusion between molten pools.

While increasing the arc extinguishing frequency and reducing the arc contact time, when the previous molten pool has not yet solidified, the next molten pool has been formed, so that the molten pool is always in a high temperature state, which will avoid the chance of cold shrinkage holes.

Therefore, for the two-point breakdown method, the arc extinguishing frequency should be 50~60 times per minute, and for the single click breakdown method, the arc extinguishing frequency should be about 80 times per minute.

Single Side Welding and Double Side Forming Basics 2

2.2.3 Joints

The joints are divided into hot connection and cold connection, which are described separately below.

Hot connection:

When the arc crater is in the hot state, ignite the arc on the slope 10~15s behind the crater and weld to the front arc stop to gradually raise the temperature at the bottom of the crater, then press the electrode down along the pre-molten hole, and after hearing the sound of “porphyrin”, stop and lift the electrode for normal welding.

The sooner the electrode is replaced, the better.

Cold connection:

When the arc crater has cooled down, knock the arc crater and the first 10 mm of molten slag off the carbon steel, clean them, and then replace the welding rod.

At the lowest point of the slope, press the welding rod down quickly along the pre-molten hole. After hearing the sound of “porphyrin”, pause for about 1 s, and then raise the welding rod to start normal welding.

Arc break welding method: the arc break welding method has one click through the ink cartridge, two clicks through two methods.

The two click piercing technique is that the arc is ignited alternately on both sides of the groove, and a drop of molten iron is dropped on the left blunt edge first, and then a drop of molten iron is dropped on the right blunt edge, gradually reciprocating.

This welding operation mode is applicable to the work with large welding assembly clearance.

One-click penetration method is that the arc ignites the blunt edges on both sides of the groove and melts at the same time.

The basic mode of operation is as follows: ignite the arc.

During welding, first strike an arc at the inner side of the groove at the starting end of the test panel, preheat the starting part with a long arc, then press down the arc, and swing the electrode back and forth between the two blunt edges.

When the molten iron at the blunt edge of the groove is combined with the metal droplet of the electrode, and the sound of “porphyrin” is heard, the first molten pool will be formed and the arc will be extinguished.

At this time, the front end of the first molten pool becomes a molten hole, which should be made 0.5~1.0mm deep to both sides of the test plate.

Replace the welding rod joint.

The joint method for changing welding rod during arc break welding is basically the same as that for continuous arc welding.

Before changing the welding rod, in order to prevent cold shrinkage holes due to arc extinguishing, the arc shall not be extinguished too quickly, two or three drops of molten iron shall be sent to the edge or north of the weld pool quickly in advance, the back weld pool shall be filled, and the temperature of the weld pool shall be controlled to cool it slowly.

A weld hole shall be formed in front of the weld pool, the arc shall be pressed down to one side of the groove, and the arc shall be welded back about 10mm, and then the arc shall be extinguished.

This ensures that the molten iron in the rear molten pool is full and sufficient to prevent the defects of cold shrinkage.

After the rapid electrode change, the arc shall be started within 10~15mm of the front of the crater.

After the arc is ignited, the arc shall be stretched backward. The metal to be welded shall be preheated with a long arc.

Then, the arc shall be pressed down about 10mm behind the crater.

The welding rod shall be operated to the root of the crater by means of continuous arc welding.

After hearing the “porphyrin” sound, the arc shall be stopped for about 2s to extinguish the arc, and then the original intermittent arc welding method shall be used to continue welding.

Note:

(1) The weld thickness of the primer should be 1.5~2.0mm at the front and back of the groove.

(2) Welders shall rotate the bar flexibly by their wrists.

(3) Four 300mm long welds shall be welded.

2.2.4 Welding of filler layer

After the welding slag of the previous layer cools down, remove the slag and splashes thoroughly, especially the slag from the corners.

After the slag is removed, use a wire brush to clean it until the metal luster is exposed.

Start the arc at 10mm from the beginning of the weld, and then return the arc to the beginning for welding.

The crescent-shaped or sawtooth-shaped electrode shall be used. When the electrode swings to both sides of the groove, it shall pause slightly to stabilize the arc.

The middle movement shall be fast and the arc shall be kept as low as possible, so as to facilitate the floating of impurities at the sharp corners of the deep groove and prevent slag inclusion.

At the same time, the molten pool and both sides of the groove shall be balanced, so as to prevent the formation of slag inclusion at the junction between the filler metal and the base metal, which is not easy to clear.

The included angle between welding rod and welding progress is 75 °~85 °.

Short arc shall be adopted during welding. The welding pool shall be round or oval, and the shape and size of the pool shall be consistent.

The welding speed shall be uniform, and the thickness of the welding flesh shall be consistent.

When welding the joint, the thermal method shall be adopted.

Before changing the welding rod to stop the arc, the molten iron shall be added to the formed weld pool slightly.

After the rapid change of the welding rod, the arc shall be ignited about 10mm in front of the arc crater.

Then pull the arc to 2/3 of the crater, fill the crater first, and then conduct normal welding.

The connection joints of each layer of weld shall be staggered.

The filling height of the last layer should be 0.5~2.0mm lower than the surface of the base metal, and the shape should be high on both sides and concave in the middle.

The weld should be smooth and excessive in the groove to ensure that the groove can be seen clearly during the cover welding, so that the edge of the cover weld can be straight.

2.2.5 Welding of covering layer

The arc striking method of the capping layer is the same as that of the filling layer.

The crescent shaped or transverse sawtooth shaped electrode shall be used.

The swing amplitude of the electrode shall be slightly larger than that of the filler layer.

During the swing, the swing amplitude shall be consistent and the electrode speed shall be uniform.

The shape of the welding pool is oval. Try to keep the shape and size of the pool roughly the same, and use short arc welding.

When the welding rod swings to the edge of the groove, the arc shall be stabilized and the tip shall be stopped. The fusion at the edge of the groove shall be about 1.0-2.0mm.

The edges of both sides of the weld shall be well fused to prevent undercut.

The forward speed of welding shall be uniform to make the weld surface smooth and beautiful.

When welding rod joints are used, hot joint method shall be adopted.

The molten iron shall be slightly added to the weld pool before the welding rod is replaced for arc extinguishing.

After replacing the welding rod, ignite the arc about 10mm in front of the crater, then pull the arc back to 2/3 of the crater, fill the crater, and then carry out conventional welding.

If the joint part is deviated, the joint part will be higher;

If the joint is deflected, the defect of weld disjoint will occur.

3. Common defects of single side welding and double side forming

To sum up, the defects that are easy to occur in the process of plate to plate butt joint, single-sided welding and double-sided forming are: air hole, incomplete penetration, incomplete fusion, weld beading, etc.

The defects that are easy to occur are analyzed and anatomized below.

Single Side Welding and Double Side Forming Basics 3

3.1 Porosity

The formation of air holes refers to the holes formed in the weld metal when the gas in the weld pool does not escape from the metal before the cold solidification of the weld metal during welding.

This gas may be absorbed by the molten pool from the external environment. Most of the gas is hydrogen and carbon monoxide.

It may also be generated by the reaction of metal during welding.

For example, if the base metal contains too much sulfur, gas pictures will be produced during the fusion with the electrode metal.

(1) The main causes of porosity during welding are analyzed as follows:

The surface of the welded base metal or filler metal is not clean, and the sundries such as oil stain and rust are cleaned.

The welding flux or electrode for welding is not dried, which causes the moisture in the welding process to decompose into gas and cannot escape in time, thus increasing the number of pores.

The low linear energy during welding makes the cooling speed of the molten pool very fast, which makes it impossible for the gas to escape from the weld metal in time.

Insufficient deoxidation of weld metal will also increase the generation of oxygen pores.

The environmental impact of welding and excessive humidity, such as rainy days and foggy days, will cause the weld to absorb the outside moist air during welding, increasing the generation of porosity.

(2) Harm of stomata.

There are different regulations on the number of pores in different specifications.

However, the generation of air holes must reduce the effective sectional area of the weld, loosen the weld, reduce the strength of the joint, and cause production accidents such as leakage.

Porosity will also produce structural defects with concentrated stress, causing damage to components.

Reduce the strength, thus affecting the service life of the component.

Therefore, according to the causes of pores, analyze them one by one and avoid the external factors that cause pores one by one, so as to reduce the occurrence of pores.

3.2 Incomplete penetration and fusion

3.2.1 Causes of incomplete penetration

(1) The groove angle of the test plate is reduced, the blunt edge is too large or the assembly gap is too small, and the spot welding length is not enough and too thin.

Due to the tensile and contraction stress during welding, the gap at the spot welding point becomes smaller due to the violent spot welding point, or the diameter of the selected electrode is too large, so that the deposited metal cannot be sent to the root of the groove.

(2) The welding speed is too fast or the welding current is too small, which leads to the decrease of the arc penetration force, the shallowing of the molten pool, and the insufficient melting of the edges of the weldment;

Or the arc burning time of breakdown welding on both sides of the groove is too short to form a certain size of hole.

(3) The heat of the arc is lost or deviated to one side due to improper electrode angle or magnetic drift of the arc, resulting in incomplete penetration where the arc cannot be used.

(4) During the welding of the primer, incomplete penetration occurs at the joint.

The most common occurrence process in the plate weldment is: after changing the welding rod, the temperature of the striking arc decreases, the temperature difference between the welded part and the non welded part is large, the gap between the test plate is large, and the welding is carried out when the required preheating temperature is not reached after changing the welding rod and striking the arc.

As a result, the arc at the joint cannot quickly break through the blunt edge of the test piece, causing a section of incomplete penetration at the joint, causing defects.

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3.2.2 Measures to prevent incomplete penetration defects

(1) Select the appropriate electrode angle.

During backing welding, the welding speed shall be properly controlled to make the arc fully melt the root.

(2) Carefully clean the oil rust and other dirt on the groove weld.

(3) In the welding process, if the eccentric electrode causes arc deflection, the angle of the electrode shall be adjusted in time, and the electrode shall swing in the opposite direction of arc deflection to make the arc align with the molten pool or replace the electrode.

(4) During welding, observe the melting condition to ensure good fusion.

3.3 Overlap

(1) The bevel blunt edge is too large, and the assembly clearance is too large;

(2) When backing up flat welding, the welding current is too high and the welding is too slow.

If the temperature of the weld edge is high and the volume is large, the liquid metal will fall due to its own gravity;

(3) In the welding process, the angle of the welding rod is improper.

Prevention measures for welding overlap:

(1) Select the appropriate size of the blunt edge and assembly clearance, and control the size of the melting hole during the welding process;

(2) Strictly control the current and interlayer temperature;

(3) Select a proper electrode angle and swing the electrode faster in the middle and slower on both sides;

(4) Observe the state of the molten pool;

(5) When more small sparks are sprayed from the molten pool, the arc shall be extinguished immediately to make the temperature of the molten pool drop slightly before welding.

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