Several common methods to prevent welding deformation I welding steel structure

1 anti-deformation：

When we do steel plate fabrication work or weld steel structure, when assembling before welding, purposely preset reverse direction deformation to offset (compensation) welding deformation, this method is called anti-deformation method. For example, 8-12 thick steel plates butt-weld with single V groove, the welding deformation nearly can be prevented by anti-deformation method.

2 Control deformation by assembly and welding sequence:

Take reasonable assembly and welding procedures to reduce deformation when welding steel structure , this is a good way and workable way in production practice. Many steel structural sections are symmetrical and the weld seams are symmetrical too, but after assembly and welding it still has deformation, it is not cancelled out by each other, this deformation is caused mainly by un-correct assemble and welding sequence, means that the deformations cause by each welding seam cannot be cancelled each other. Welding sequence is one of the major reasons to cause the deformation, when arranging welding sequence, we should pay attention to following principles;

 (1) Try to use symmetry welding. For the works which having symmetry welding, it is better to do welding symmetrically by paired welders, by this way, the deformation caused by each welding seam can be cancelled out partially.

 (2) For the steel structure with unsymmetrical welding seams, welding should start from less welding seam side.

 (3) According to the characteristics of different welding sequence, to control the welding deformation by means of welding procedures, there are 5 commonly used methods, which are:

  a. Back-step welding:  This way is suitable for any position welding except vertical welding, for thick steel plate and long welding seam, arc baffle plate can be used for more welders work  together. The advantage is that the heat affected zone can be reduced, and avoid deformation. Each section welding seams should be 0.5-1m. 

  b. Back-step welding at middle. This way is suitable for medium thick steel plate and thinner steel plate, its advantage is that heat fast at middle and temperature difference is smaller at ends. By this way, the temperature at welding heat affected soon does not increase sharply, so that it can reduce or avoid the thermal expansion deformation. The welding way is suitable for flat welding and overhead weld, normally not for horizontal position welding,not suitable for vertical welding in welding steel structure at all. 

  c. Skip welding: It suitable for flat welding, horizontal position welding, overhead weld except vertical welding, normally for 6-12mm thick steel plate in long welding seams, and for casting steel, stainless steel, copper materials welding too, it can disperse welding heat and avoid or reduce deformation. Welding seam should be between 200-400mm long for steel material; Cast iron welding parts according to the specification of cast iron welding; Due to stainless steel and copper have fast heat conduction, so the welding seam should not be longer than 200mm (for thinner plate , welding seam even shorter). 

  d. Alternating welding method: This welding method is basically the same as that of skip welding, but the length of each welding line is elongated, especially for thin plates and long welding seam.

 e. Middle symmetry: it is suitable for welding with shorter welding seam, in order to reduce deformation, welding is divided into 2 section, and start from at middle and finish once.

3. Rigid fixed method:

Rigid fixed method is workable method, and it does not to pay attention to the welding sequence. The advantage is that some big size steel structure or welding components is not easy to do rigid fixing, and after removing the fixing, small deformation and large residual stress still exist. This way is suitable for thinner materials w below 6mm thickness with good toughness. If it combines with anti-deformation method together, the result is even better. For complex shape, big size steel structure, and mass production, it is wise to design a rotatable welding jig, by this way, it can avoid deformation and increase production efficiency.  When having big size steel structure, and small quantity, some supporting or pulling rods can be temporarily welded at the place where deformation is easy to happen, so that the rigidity of the workpiece can be increased, and the welding deformation can be effectively reduced

4. Heat dissipation method:

Heat dissipation method is also called forced cooling method, means quickly dissipate the heat at the welding place when welding steel structure, and greatly reduce the metal heating surface near the weld seam, so that reduce the welding deformation. But heat dissipation is complicated, and it is not suitable for steel material with high quenching tendency, otherwise steel will crack.

5. Hammering weld method:

Hammering weld method means use small hammer to knock the welding seam, to reduce the welding deformation. Therefore, if the weld seam is properly forged, and let welding seam extended to compensate for the shortening, the deformation and stress can be reduced. Hammering force should be uniform, normally use 0.5kg-1.0kg hand hammer with round reduce (R=3-5mm) at hammer head. Welding seam at bottom and surface cannot be hammered, to avoid steel surface cold-work hardening. The other welding seam can be hammered as soon as finishing, until the welding surface have evenly and denser knocking point.