Welded structures generally have deficiencies that are mainly reflected in problems with welded joints. These problems include the following aspects:
- The welding process involves uneven heating and cooling, which results in residual stress and distortion in the weld. These issues not only affect the overall dimensions and appearance quality of the welding structure, but also create difficulties for continuous processing after welding. In some cases, the strength of the welding structure can also be directly affected.
- Welded joints go through three stages: casting, solidification and heat treatment.
- Welding can change certain material properties.
Welding Stress and Distortion
Local high-temperature heating during welding results in uneven temperature distribution in the weldment, which in turn causes welding stress and distortion in the welding structure.
Welding stress is the main cause of brittle fracture, fatigue fracture, stress corrosion cracking and instability failure.
Welding distortion affects the shape and dimensional accuracy of the structure, making it difficult to meet technical requirements. This directly impacts the manufacturing quality and service performance of the structure.
Voltage generation and welding distortion
1. Basic knowledge of welding stress and distortion
1. Distortion
The shape and size of an object changes under the action of external force or temperature
2. Stress
The force of interaction within an object caused by external forces or other factors is called internal force. The internal force per unit cross-sectional area of an object is called stress.
3. Welding voltage and welding distortion
Welding stress is the internal stress that exists in welding during and after the welding process.
The change in weld size caused by welding is called welding distortion.
2. Causes of welding stress and distortion
1. Uneven heating of welds
(1) Stress and distortion caused by long strip central heating (similar to cladding)
Stress and distortion of the center of the steel strip during heating and cooling
(2) Stress and distortion caused by heating on one side of the long strip (equivalent to the plate edge surface)
Stress and distortion during heating and cooling on one side of the steel sheet edge
2. Shrinkage of welded metal
3. Change of metal structure
4. Welding rigidity and restraint
Welding Distortion
1. Types of welding distortion and their influencing factors
Welding distortion can be divided into five basic forms: contraction distortion, angular distortion, bending distortion, wave distortion and creep distortion.
1). Shrinkage distortion
The phenomenon that the weld size is smaller than before welding is called shrinkage distortion.
Longitudinal and transverse contraction distortion
(1) Longitudinal contraction distortion
(2) Transverse contraction distortion
two). Angular distortion
The root cause of angular distortion is the uneven distribution of transverse contraction throughout the thickness of the plate.
Angular distortion of multiple joints
T-Joint Angular Distortion
3). Doubling distortion
Bending distortion is caused by non-coincidence or asymmetry between the weld centerline and the neutral axis of the structural section, as well as the uneven distribution of weld shrinkage across the weld width.
(1) Bending distortion caused by longitudinal shrinkage
Bending distortion caused by longitudinal shrinkage of the weld
(2) Bending distortion caused by transverse contraction
Bending distortion caused by transverse weld contraction
4). Wave Distortion
Wave distortion often occurs in the welding process of thin plates with a thickness of less than 6 mm, also called instability distortion.
Wave distortion caused by weld fillet distortion
5). Distortion
The main cause of distortion is the uneven distribution of fillet weld distortion along the length of the weld.
I-beam distortion
2. Measures to control welding distortion
1). Project measurements
(1) Select a reasonable weld shape and size
1) Select the smallest weld size.
Cross joint with the same load capacity
2) Select a reasonable groove shape.
T-Joint Slot
(2) Reduce the number of welds
Profiles and stamped pieces are preferred options when possible. For structures with many and dense welds, cast weld joint structures can be used to reduce the number of welds. Additionally, increasing wall plate thickness to reduce the number of ribs, or using profiled structures instead of rib structures, can help prevent distortion of thin plate structures.
(3) Reasonable weld position arrangement
Beam, column and other welded components often exhibit bending distortion due to the eccentric configuration of the weld.
Box frame welding arrangement
Reasonably arrange the weld position to avoid distortion
two). Process measures
(1) Rebate method
(2) Inverse distortion method
Inverse Distortion Method for Flat Plate Butt Welding
(3) Rigid fixation method
1) Fix the welding on the rigid platform.
Rigid fixation when splicing thin plates
2) Welding is combined into a more rigid or symmetrical structure.
Rigid and anti-distortion T beam fixing
3) The welding accessory is used to increase the rigidity and containment of the structure.
Rigid fixation during butt splicing
4) Use temporary supports to increase the containment of the structure.
Temporary support during shield welding
(4) Select a reasonable assembly and welding sequence.
The assembly welding sequence has a great influence on the distortion of the welded structure.
(1) If conditions permit, large and complex welded structures should be divided into several parts with simple structures, welded separately and then assembled as a whole.
(2) The weld during welding should be as close as possible to the neutral axis of the structural section.
Assembly and welding of the main beam
3) For the structure with asymmetric weld arrangement, the side with few welds must be welded first during assembly welding.
Press upper die welding sequence
4) The structure with a symmetrical arrangement of welds must be welded symmetrically by paired welders.
Cylinder butt weld welding sequence
5) When welding long welds (more than 1m), the direction and sequence shown in the figure below can be used to reduce shrinkage distortion after welding.
(5) Reasonably select welding methods and welding process parameters
Asymmetric Section Structure Welding
(6) Thermal equilibrium method
Use heat balance method to avoid welding distortion
(7) Heat dissipation method
3. Method to correct welding distortion
1). Manual correction
two). Mechanical correction method
Correction of beam bending distortion by mechanical correction method
3). Flame heating correction method
Forms of flame heating include point heating, linear heating and triangular heating.
(1) Spot heating
(2) Linear heating
(3) Triangular heating
Flame correction of I-beam bending distortion
Correction of flame heating welding distortion depends on the following three factors:
(1) Heating mode
(2) Heating position
(3) Heating temperature and heating zone area
C residual voltage
1. Classification of welding residual stress
1). According to the causes of stress
(1) Thermal stress
(2) Corresponding stress
(3) Plastic deformation stress
two). According to the duration of the stress
(1) Instantaneous welding voltage
(2) Welding residual stress
2. Distribution of welding residual stress
1). Distribution of longitudinal residual stress σx
Butt joint distribution in weld cross section 0x
two). Distribution of transverse residual stress σy
(1) The transverse stress caused by the longitudinal contraction of the weldment and its adjacent plastic distortion zone is σ'y
(2) Mechanical stress caused by transverse retraction ano σ” y
Distribution of σ”Y during welding in different directions
3. Influence of welding residual stress on welding structure
1). Impact on structural strength
two). Influence on the dimensional accuracy of welding processing
Release of internal stress and distortion caused by machining
3). Influence on the stability of compression elements
4. Measures to control welding residual stress
1). Project measurements
1) Minimize the number and size of welds on the structure.
2) Avoid excessive concentration of welds and maintain sufficient distance between welds.
Welding the vessel nozzle
3) The joint shape with the lowest stiffness should be adopted.
Measures to reduce joint stiffness
two). Process measures
1) Adopt a reasonable assembly welding sequence and direction.
① When welding the weld in a plane, it must be ensured that the longitudinal and transverse contraction of the weld can be relatively free.
Reasonable assembly and welding sequence of seam welds
② The weld with the greatest shrinkage should be welded first.
Welding sequence of duplex beam structure with cover plate
③ The weld with the highest stress during operation should be welded first.
Top I beam welding sequence
④ When the flat cross weld is welded, it is easy to produce large welding stress at the weld intersection.
Welding sequence of flat cross welds
⑤ The structure where butt and fillet welds intersect.
2) Preheating method.
3) Cold welding.
4) Reduce the restriction of welds.
Reduce local stiffness and internal stress
Schematic diagram of the “stress relief zone” heating method
5. Method for eliminating or reducing welding residual stress
1). Heat treatment method
(1) Full heat treatment
(2) Local heat treatment
two). Mechanical stretching method
3). Temperature difference stretching method
Schematic diagram of residual stress elimination by “temperature difference tensile method”
4). hammer welding
5). Vibration method
6. Determination of welding residual stress
1). Mechanical method
(1) Cutting method
(2) Drilling method
two). Physical methods
(1) Magnetic method
(2) X-ray diffraction
(3) Ultrasonic method
