Have you ever wondered why welded structures sometimes fail despite their robust appearance? This article dives into the hidden challenges of welding, exploring how uneven heating and cooling can lead to stress, distortion, and even structural failure. By the end, you’ll understand the key factors affecting welding quality and how to mitigate these issues.
Welded structures, while widely used in industry, often present inherent challenges primarily associated with the welded joints. These challenges can be categorized into several key areas:
1. Residual Stress and Distortion:
The welding process induces localized, intense heating followed by rapid cooling, resulting in non-uniform thermal expansion and contraction. This thermal cycling leads to residual stresses and distortion in the welded structure. These issues not only compromise dimensional accuracy and aesthetic quality but also complicate post-welding operations. In severe cases, the overall structural integrity and load-bearing capacity may be significantly impaired.
2. Microstructural Changes:
Welded joints undergo three distinct phases during formation: melting, solidification, and heat-affected zone (HAZ) development. Each phase alters the material’s microstructure, potentially modifying its mechanical properties, corrosion resistance, and fatigue behavior. The HAZ, in particular, can exhibit markedly different characteristics from the base metal and weld metal.
3. Material Property Alterations:
Welding can significantly alter the properties of the base material. These changes may include modifications to strength, ductility, toughness, and hardness. In some alloys, precipitation hardening or grain growth can occur, while in others, softening due to over-aging might be observed. Understanding and mitigating these metallurgical changes is crucial for maintaining the desired material properties in the welded structure.
4. Stress Concentration and Failure Modes:
Welding stress, particularly residual stress, acts as a significant contributor to various failure mechanisms:
5. Dimensional Inaccuracies:
Welding distortion manifests as changes in the shape and dimensions of the welded structure. This can lead to:
These distortions can significantly impact the manufacturing quality, functional performance, and service life of the welded structure. Mitigation strategies such as proper joint design, controlled heat input, sequencing of welds, and post-weld treatments are often necessary to minimize these adverse effects.
1. Distortion
Distortion in welding refers to the unintended alteration in the shape and dimensions of a workpiece, resulting from the non-uniform heating and cooling cycles inherent in the welding process. This phenomenon occurs due to the localized thermal expansion and contraction of the material, as well as phase transformations in the heat-affected zone (HAZ).
2. Stress
Stress is defined as the internal force per unit area acting within a material in response to external loads or other factors such as thermal gradients. In welding, stress manifests as:
3. Welding stress and welding distortion
Welding stress refers to the complex system of internal stresses that develop within a weldment during and after the welding process. These stresses arise from:
Welding distortion is the measurable change in the geometry and dimensions of a weldment caused by the cumulative effects of welding stresses. Common types of welding distortion include:
Understanding the interrelationship between welding stress and distortion is crucial for implementing effective mitigation strategies in welding design and fabrication processes.
1. Uneven heating of weldments
(1) Stress and distortion caused by central heating of long strip (similar to surfacing)
Stress and distortion of steel strip center during heating and cooling
(2) Stress and distortion caused by heating on one side of the long strip (equivalent to plate edge surfacing)
Stress and distortion during heating and cooling on one side of steel plate edge
2. Shrinkage of welded metal
3. Change of metal structure
4. Rigidity and restraint of weldment
Welding distortion can be divided into five basic forms: shrinkage distortion, angular distortion, bending distortion, wave distortion and deformation distortion.
Basic forms of welding distortion
1). Shrinkage distortion
The phenomenon that the size of weldment is shorter than that before welding is called shrinkage distortion.
Longitudinal and transverse shrinkage distortion
(1) Longitudinal shrinkage distortion
(2) Transverse shrinkage distortion
2). Angular distortion
The root cause of angular distortion is the uneven distribution of transverse shrinkage along the plate thickness.
Angular distortion of several joints
Angular distortion of T-joint
3). Bending distortion
Bending distortion is caused by the non-coincidence or asymmetry between the centerline of the weld and the neutral axis of the structural section, as well as the uneven distribution of the shrinkage of the weld along the width of the weldment.
(1) Bending distortion caused by longitudinal shrinkage
Bending distortion caused by longitudinal shrinkage of weld
(2) Bending distortion caused by transverse shrinkage
Bending distortion caused by transverse shrinkage of the weld
4). Wave distortion
Wave distortion often occurs in the welding process of thin plates with a thickness of less than 6mm, which is also called instability distortion.
Wave distortion caused by weld fillet distortion
5). Distortion
The main cause of distortion is the uneven distribution of weld fillet distortion along the weld length.
Distortion of I-beam
1). Design measures
(1) Select a reasonable weld shape and size
1) Select the smallest weld size.
Cross joint with the same bearing capacity
2) Select a reasonable groove form.
Groove of T-joint
(2) Reduce the number of welds
Profiles and stamping parts are the 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 the thickness of the wall plate to reduce the number of ribs, or using profiled structures instead of rib structures, can help prevent distortion of thin plate structures.
(3) Reasonable arrangement of weld position
Beam, column and other welded components often have bending distortion due to the eccentric configuration of the weld.
Weld arrangement of the box structure
Reasonably arrange the weld position to prevent distortion
2). Process measures
(1) Allowance method
(2) Inverse distortion method
Inverse distortion method for flat plate butt welding
(3) Rigid fixation method
1) Fix the weldment on the rigid platform.
Rigid fixation during thin plate splicing
2) The weldment is combined into a more rigid or symmetrical structure.
Rigid fixation and anti-distortion of T-beam
3) The welding fixture is used to increase the rigidity and restraint of the structure.
Rigid fixation during butt splicing
4) Use temporary supports to increase the restraint 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 when welding should be close to the neutral axis of the structural section as much as possible.
Assembly and welding of the main beam
3) For the structure with the asymmetric arrangement of welds, the side with few welds shall be welded first during assembly welding.
Welding sequence of upper die of press
4) The structure with a symmetrical arrangement of welds shall be welded symmetrically by even welders.
Welding sequence of cylinder butt weld
5) When welding long welds (more than 1m), the direction and sequence shown in the figure below can be used to reduce the shrinkage distortion after welding.
(5) Reasonably select welding methods and welding process parameters
Welding of asymmetric section structure
(6) Heat balance method
Use the heat balance method to prevent welding distortion
(7) Heat dissipation method
1). Manual correction
2). Mechanical correction method
Correction of bending distortion of the beam by the mechanical correction method
3). Flame heating correction method
The ways of flame heating include point heating, linear heating and triangular heating.
(1) Spot heating
(2) Linear heating
(3) Triangular heating
Flame correction of bending distortion of I-beam
The correction of welding distortion by flame heating depends on the following three factors:
(1) Heating mode
(2) Heating position
(3) Heating temperature and area of the heating zone
1). According to the causes of stress
(1) Thermal stress
Thermal stress arises from non-uniform temperature distribution during the welding process. As the weld metal and surrounding base material heat up and cool down at different rates, localized expansion and contraction occur, leading to stress development.
(2) Transformation stress
Transformation stress, also known as phase transformation stress, results from volume changes associated with microstructural alterations in the material during heating and cooling cycles. This is particularly significant in steels undergoing martensitic or bainitic transformations.
(3) Plastic strain stress
Plastic strain stress develops when the material experiences localized yielding due to thermal gradients and constraints during welding. This non-uniform plastic deformation contributes to the residual stress state after cooling.
2). According to the time of stress existence
(1) Welding transient stress
Welding transient stress, also referred to as instantaneous stress, occurs during the welding process itself. It is a dynamic stress state that evolves rapidly as the heat source moves along the weld path, causing continuous changes in temperature distribution and material properties.
(2) Welding residual stress
Welding residual stress is the static stress that remains in the welded structure after it has completely cooled to ambient temperature and all external loads have been removed. This stress can significantly impact the mechanical behavior and service life of welded components.
1). Distribution of longitudinal residual stress σx
Distribution of butt joint on weld 0x cross section
2). Distribution of transverse residual stress σy
(1) The transverse stress caused by longitudinal shrinkage of welding and its adjacent plastic distortion zone is σ’y
(2) Mechanical stress caused by transverse shrinkage year σ” y
Distribution of σ” Y during welding in different directions
1). Impact on structural strength
2). Influence on the dimensional accuracy of weldment processing
Internal stress release and distortion caused by machining
3). Influence on the stability of compression members
1). Design measures
1) Minimize the number and size of welds on the structure.
2) Avoid excessive concentration of welds, and keep sufficient distance between welds.
Welding of vessel nozzle
3) The joint form with less rigidity shall be adopted.
Measures to reduce the rigidity of joints
2). Process measures
1) Adopt reasonable assembly welding sequence and direction.
① When welding the weld on a plane, it shall be ensured that the longitudinal and transverse shrinkage of the weld can be relatively free.
Reasonable assembly and welding sequence of splicing welds
② The weld with the largest shrinkage shall be welded first.
Welding sequence of duplex beam structure with cover plate
③ The weld with the largest stress during operation shall be welded first.
Welding sequence of butt I-beam
④ When the plane cross weld is welded, it is easy to produce large welding stress at the intersection of the weld.
Welding sequence of plane cross welds
⑤ The structure where butt welds and fillet welds intersect.
2) Preheating method.
3) Cold welding.
4) Reduce the restraint of welds.
Reduce local stiffness and internal stress
Schematic diagram of heating “stress relief zone” method
(1) Integral heat treatment
(2) Local heat treatment
2). Mechanical stretching method
3). Temperature difference stretching method
Schematic diagram of eliminating residual stress by “temperature difference tensile method”
4). Hammer weld
5). Vibration method
1). Mechanical Methods
(1) Sectioning Method
The sectioning method involves carefully cutting the welded component into smaller pieces, allowing for the release and measurement of residual stresses. This destructive technique provides a comprehensive stress profile across the weld and heat-affected zone (HAZ).
(2) Hole-Drilling Method
This semi-destructive method involves drilling a small hole in the welded area and measuring the resulting strain relaxation. It’s particularly effective for near-surface residual stress measurement and can provide stress distribution data at various depths.
2). Non-Destructive Physical Methods
(1) Magnetic Methods
Techniques such as Barkhausen Noise Analysis (BNA) and Magnetostriction exploit the relationship between a material’s magnetic properties and its stress state. These methods are particularly suitable for ferromagnetic materials and can provide rapid, on-site measurements.
(2) X-ray Diffraction (XRD)
XRD utilizes the principle of Bragg’s Law to measure lattice deformations caused by residual stresses. This highly accurate method is non-destructive and can measure surface stresses with excellent spatial resolution, making it ideal for complex geometries and multi-pass welds.
(3) Ultrasonic Methods
These techniques leverage the acoustoelastic effect, where the velocity of ultrasonic waves changes with the stress state of the material. Time-of-flight diffraction (TOFD) and critically refracted longitudinal (LCR) waves are commonly used for through-thickness residual stress measurement in thick welded components.
(4) Neutron Diffraction
While not mentioned in the original list, neutron diffraction is a powerful non-destructive method for measuring residual stresses deep within thick welded components. It offers excellent penetration depth and is particularly useful for complex geometries and multi-phase materials.
As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.
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