8 Full Penetration First Level Weld Seam Welding Basics

1. General requirements

1. As shown in Fig. 1, the joints welded with full penetration first class welds can be divided into butt joint, corner joint and T-joint depending on the shape of components.

The stress transfer welding includes full penetration first level weld welding that can bear all kinds of stress as the base metal, fillet welding that mainly bears shear force, and partial penetration second level weld welding.

Among them, the full penetration first level weld welding under full management, except for low stress and high cycle fatigue such as crown block beam, is considered to have the same performance as the base metal for all loads including earthquake and other repeated loads in steel commonly used in buildings.

2. The full penetration level I weld shall be fully welded on the full section, and the strength of the welding position shall be equal to or above that of the base metal.

3. The throat thickness of the full penetration first level weld shall not be less than the thickness of the base metal (the thickness of the thinner base metal shall be taken when the thickness of the base metal is different, and the thickness of the base metal to be butt welded shall be taken when the corner joint and T-joint are used).

If the throat thickness is smaller than the base metal thickness, it will not only fail to ensure the design tolerance, but also become the cause of stress concentration.

On the contrary, when the welding crown is too high, it will also be the cause of stress concentration, and it is necessary to make the welding crown not too large.

8 Full Penetration First Level Weld Seam Welding Basics 1
Full penetration first level weld joint

4. For example, when the T-joint and corner joint have the force of pulling the steel plate in the direction of plate thickness, it should be noted that although there is no welding defect, the steel plate may crack due to its low strength.

As is known to all, the ratio of strength or elongation in the thickness direction to the rolling direction of the rolled steel plate is generally reduced.

In particular, when there are non-metallic mediators (Mn S is a component) whose layers are deflected in the rolling of steel, they will crack or peel at very low stress.

Therefore, the steel plates used for the parts with tensile force in the direction of plate thickness, such as the top column flange in the case of column penetration type, and the beam flange of outer column and corner column in the case of beam penetration type, should be reviewed and considered in particular.

In continuous casting, due to the difference of solidification temperature or the reduction of surface temperature, there is a deflection of MnS series composition in the center of the plate thickness, and there is a tendency of Al2O3 segregation on the surface layer, which has been improved in recent years.

Recently, with the large-scale construction, there are several reports on the problem of lamellar weld cracking caused by multi-layer welding of thick steel plates.

T-type full penetration first class weld joint is usually used in the column beam joint of steel structure with the maximum stress and the part bearing repeated plastic strain.

This part is the part most prone to layer welding cracking.

It is necessary to select a good steel plate and review the welding design and construction methods.

The cracking of other heat affected parts is almost low-temperature cracking, which can be prevented by proper preheating, so preheating management is particularly important.

2. Groove shape

The groove shape of the welded joint is the basic element for the soundness of the left and right joints.

If the root gap and groove angle are not appropriate, the quality of the welded joint will be damaged.

Therefore, it is necessary to determine the groove shape according to various welding methods and joint types.

For general welding methods, the standard groove shape of the joint type and its root surface size, if it is less than 2mm when using the backing plate, will be no problem.

If special welding methods or joint types are required, depending on their conditions, the data can be used to confirm that the groove shape is no problem in quality and construction.

3. Welding crown

In order to avoid stress concentration, the weld crown welded in the full penetration first level weld shall not be excessive.

The weld crown shall be smooth and continuous from the base metal surface, and the height of the weld crown shall be 0~3mm.

4. Crown height of T-joint

The welding crown of T-joint should mitigate the stress concentration near the welding position, which is very important to prevent the cracking or spalling of the butt plate.

The welding crown shall be as shown in Fig. 2. For the purpose mentioned above, it is important that the weld bead be smooth.

In addition, the height of the welding crown (h) is 1/4 of the thickness of the butt joint, and it is 10mm when the thickness exceeds 40mm.

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Fig. 2 Crown height of T-joint

5. Run-on plate

Generally speaking, poor penetration or air holes are easy to occur at the starting end of welding, and crater cracks and other defects are easy to occur at the end.

In order to prevent this defect from occurring in the effective welding of the structure, in principle, the steel run-on plate of appropriate shape shall be installed at the beginning and end of the welding for welding [see Fig. 3 for reference].

In general, the run-on plate does not need to be removed after welding, and there is no problem with the residue [refer to Fig. 4].

However, if the slab thickness is more than 50mm and the column and beam are the same width, it is expected to be cut.

For example, when the crown block beam is subject to low stress and high cycle fatigue, it needs to be cut off and refined with a grinder.

It is better not to directly assemble and weld the run-on plate at the column beam joint.

Because the assembly welding of the run-on tab is a short weld bead, which reduces the fracture toughness of the heat-affected part, this part may become the starting point of brittle failure of the whole flange plate.

Fig. 5 shows an example of arc strike plate assembly and welding at the column beam joint.

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The device of the run on plate is assembled and welded on the backing plate.

If it is unavoidable to do it in the groove, the construction shall be able to fully ensure the quality of the welding part after the positive welding.

However, in reality, the construction conditions are stricter than forward welding, which leads to insufficient quality of assembly welding.

Especially in the welding of such important joints as full penetration welding, by using any of the following methods, it is necessary to remove the weld bead of assembly welding or remelt it during forward welding.

① Remove the weld bead of assembly welding with gas arc before welding.

② Backhoe shall be applied to remove the residual assembly welding at the first welding end.

③ Use the welding method of re-melting the assembled weld bead without residual defects.

Assembly welding is conducted by covering electric welding or gas shielding welding.

When the steel of thicker plate is to be welded with shorter weld bead length, the welding part will be hardened by rapid heating and cooling, and welding cracks may occur due to the amount of diffusible hydrogen and the degree of restraint.

The higher the plate thickness or the higher the content of alloy composition, the more obvious this tendency is.

Therefore, when the covered electric welding is used for assembly welding, low hydrogen electrode shall be used depending on the plate thickness and steel type.

In fact, it is difficult to change the electrode depending on the thickness of the plate and the steel grade, so it is hoped that the low hydrogen electrode will be uniformly used when using the covered electric welding.

In addition, gas shielded welding is effective against cracking due to its very small amount of diffusible hydrogen gas, which should also be used in assembly welding.

In addition, preheating is effective for cracking, and it shall also be conducted according to the same conditions of normal welding during assembly and welding.

In addition to the steel run on plate method, there are other methods to replace the run on plate method with sintered fixed type run on plates such as flux or ceramics or stamped steel plate run on plates, or use the backing plate method, back welding method, end removal method and other methods to omit the run on plates.

The scope of application is that the welding posture is based on flat welding, and the applicable steels are Q235 and Q345.

However, beyond this scope, if the weldability test is used to confirm that the quality of the welding part is equal to or above that of the steel run on plate construction method, it can be applied.

The welding technician shall be a person who is qualified in the equivalent welding method and recognized by the project supervisor in accordance with the additional examination of welding technology of the equivalent run on plate method.

Among these construction methods of run on tab, the most common attention points of fixed type run on tab are shown below:

1. The material of the fixed type run on plate will not invade the welding metal due to the welding position, which will lead to the degradation of the joint performance.

2. The backing plate is designed to prevent the weld metal from melting down at the end of the welding process. It needs to grow about 10mm from the base metal end.

3. The fixed type run on plate is tightly connected with the base metal by using steel wire, magnet fixture, etc.

As for the welding method, as shown in Fig. 6, the arc starts towards the end from the point where the base metal end enters 15~20mm, and then turns back for welding to ensure that the welding of the initial layer can always be fully penetrated at the end.

6. Joints with different plate thicknesses

When the thickness of butt joint materials is different, the welding shall be carried out from the thinner plate to the thicker plate so that the stress can move smoothly.

As shown in Fig. 7 (a), when the segment difference exceeds 10mm due to the difference in plate thickness or when the crown block beam is subjected to low pressure and high cyclic fatigue, the thicker sheet shall be processed into an inclination of less than 1/2.5, and the groove part shall be made thinner.

The sheets are at the same height.

However, as shown in Fig. 7 (b), when the wing plate joint of SRC beam forming through arch head is welded on both sides (backhoe), if the plate thickness difference exceeds 10mm, it can only be used for reinforcement fillet welding.

In addition, the single side welding of the backing plate shall be recognized by the project supervisor.

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7. Air gouging

When welding from both sides, air gouging shall be carried out in principle before welding the initial layer on the back.

Generally, welding at the initial layer of the surface is prone to crack, poor penetration, slag inclusion and other defects due to fast cooling speed.

For this reason, it is necessary to remove the defects of the initial layer of the surface by air gouging.

However, in submerged arc welding, when construction test or non-destructive inspection is used to confirm that the initial layer on the back has good penetration, air gouging can be omitted.

8. Backing plate

When using the backing plate, in order to obtain sound root penetration, sufficient root clearance shall be taken to make the backing plate closely connected.

If the tightness of the backing plate is not complete and the root gap is too narrow, it is easy to produce welding defects such as poor penetration or slag inclusion at the root.

In principle, Q345B material shall be used for backing plate.

For other materials, it is necessary to investigate the chemical composition of P, S, Cu, C, etc., and confirm that there is no problem with the weldability.

In principle, the backing plate of the column beam joint of the backing plate shall be installed inside the wing plate.

However, the lower flange plate can also be installed on the outside if it is necessary to weld it on-site.

The device position of the backing plate is shown in Fig. 8.

The assembly welding of the backing plate is subject to tensile resistance and slight bending stress, that is, the fillet welding is subject to the maximum stress, which is relatively simple and will break.

In addition, there are many cases of direct welding without preheating for this fillet welding.

Although the original weld bead length of the assembly welding is quite enough, the fracture toughness of the welding metal or HAZ is very low, so the fracture of the fillet welding is connected with the brittle failure of the whole joint as it is.

In this case, the assembly and welding of the backing plate at the column beam joint of the backing plate shall not be carried out within 10mm of both ends of the beam flange or within 10mm of the R toe or fillet weld toe of the web fillet [Fig. 9 (1)].

The assembly and welding of the backing plate shall be carried out at the position of the run on plate [Fig. 9 (2)] or 1/4 of the width of the beam wing plate [Fig. 9 (3)].

When the run on plate is to be installed on the outside of the backing plate or beam flange, it shall not be assembled and welded on the outside of the beam flange and column flange as shown in Fig. 9 (2).

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