Due to uneven heat input during welding, the internal temperature field, stress field, and microstructure of the component undergo rapid changes, which can easily lead to uneven elastic-plastic deformation. Therefore, workpieces processed using welding technology are more affected by residual stress compared to other processing methods.

01.Types of welding stress

The stress present in welded structures can be roughly divided into two categories based on its causes and properties

Thermal stress:The stress caused by uneven heating and cooling during the welding process. It is the instantaneous stress that changes during the welding process.

Phase transition stress:The stress caused by uneven tissue transformation in the joint area during the welding process is often generated when the carbon equivalent is high or the process is improper.

Constrained stress:Stress caused by the structure itself or external constraints during the welding process.

Hydrogen induced stress:Local stress in the welded joint area caused by diffusion hydrogen accumulation at micro defects after welding. When the hydrogen content is high, it is easy to produce.

Welding residual stress

The stress that exists within the structure after welding, sometimes referred to as welding residual stress, is a self balancing internal stress at any cross-section within the structure.

The magnitude and distribution of various welding stresses are related to the characteristics of the welding material and steel (such as strength and expansion coefficient), welding process method, heat input, process parameters, welding assembly sequence and operation method, as well as the structure itself or external constraints, welding environmental conditions, etc. They often appear in combination and superposition.

02 Hazards of Welding Stress

Causing welding cracks

Under the interaction of temperature, organization, and structural rigidity constraint, when the welding stress reaches a certain value, it will become the main cause of various hot cracks, cold cracks, etc., affecting the quality of the structure, causing potential danger, and leading to repair or scrapping of the welded parts.

Reduce the load-bearing capacity of the structure

(1) The superposition of residual stresses in welded components and working stresses increases the stress level that the component can withstand, but in reality reduces the load-bearing capacity of the structure or lowers the strength safety margin of the structure.

(2) When the stress level exceeds the yield limit of the material, it will cause tensile plastic deformation in the joint area, consuming a portion of the material's plasticity.

(3) In the welding area of thick walled structures, three-dimensional intersecting welds or defects in welds may cause triaxial tensile stress, reduce the ability of materials to undergo plastic deformation, and may become the origin point of low stress brittle fracture.

(4) Under low cycle fatigue loads, higher tensile residual stresses can cause a certain degree of deformation in structures that have been used for a long time.

Causing stress corrosion

The presence of residual tensile stress causes stress corrosion cracking in the structure of the workpiece in corrosive media, leading to stress corrosion and low stress brittle fracture.

Affects the stability of structural dimensions

Especially for structures that need to be processed after welding, the balance of internal stress will be disrupted after processing, causing structural deformation or unstable processing dimensions.

03 Factors affecting welding stress

The influence of structural form

(1) Tablet docking. The distribution of residual stresses in longitudinal and transverse welding is shown in Figure 1.

(2) Pressure vessel cylinder circumferential seam. The magnitude and distribution of residual stresses in longitudinal welding are related to the diameter of the cylinder, the thickness of the cylinder wall, and the width of the compressive plastic deformation zone (as shown in Figure 2), and increase with the increase of the cylinder diameter, while decreasing with the expansion of the plastic deformation zone.

Figure 1 Distribution of residual stress in the butt joint of a flat plate

Figure 2 Distribution of residual stress in longitudinal welding of cylindrical circumferential seam

The impact of rigid constraints

(1) Tablet docking. Two steel plates are rigidly restrained in the transverse direction before welding, and there is no significant impact on longitudinal stress after welding. The two transverse sides have a single tensile stress (as shown in Figure 3). Narrow plates result in high constraint stress, while wide plates result in reduced constraint stress. For long welds, the stress at the first welding end is relatively small. After removing the external constraint, the constraint stress is eliminated and the residual stress will be redistributed.

(2) The closed weld seam of the embedded block connection. There are pipe sockets or inserts in the shell structure, which provide strong rigid constraints. The greater the rigidity, the greater the internal stress. The longitudinal stress (i.e. tangential stress σ t) in the embedded block (as shown in Figure 4) is tensile stress near the weld seam, which can reach up to σ s; Lateral stress (i.e. radial stress σ r) is also tensile stress near the weld seam. At the center of the inlay, where σ t=σ r, there is a bidirectional stress field. The smaller the diameter of the inlay, the higher the bidirectional stress value. The stress of the takeover weld is related to the joint form, and the stress of the outer seat type is smaller; The plug-in type has high stiffness and high stress.

Figure 3: The Effect of Rigid Constraints on Welding Stress

Figure 4 Welding stress in the closed weld seam of the disc insert block

The influence of plate thickness and groove form

The distribution of residual stress varies with the thickness of the plate, and the transverse stress σ y perpendicular to the weld axis cannot be ignored. The measured values of residual stress in multi-layer submerged arc welding of 2.25Cr-1Mo ultra thick plate. It is worth noting that the residual stress near the surface reaches its peak, and this bidirectional or triaxial stress is an important reason for the occurrence of transverse cracks in this type of steel weld. If the V-shaped groove is changed to a double V-shaped groove, compressive stress will be generated at the root of the double V-shaped groove, which is beneficial for avoiding welding cracks, as shown in Figure 6. a) Test piece; b) 55mm thick plate: c) 100mm thick plate

Figure 6 Residual stress distribution during symmetrical welding of double V-shaped groove

The influence of welding process parameters

With the increase of welding heat input, the heating width and residual stress increase, and the width of tensile residual stress also increases.

Influence of welding direction

Lateral residual stress is the stress synthesis induced by the longitudinal and transverse shrinkage of the weld seam and its adjacent plastic deformation zone. Its size and distribution are related to the plate length and welding direction. When welding from the center to both ends, the center is the compressive stress; When welding from both ends to the center, there is compressive stress at both ends, as shown in Figure 7. a) Welding from the center to both ends; b) Weld both ends towards the center

Figure 7: The Influence of Welding Direction on the Distribution of Lateral Residual Stress

The impact of phase transition

When welding high-strength steel with high carbon equivalent, the HAZ and weld microstructure will undergo a transformation from austenite to martensite, resulting in an increase in specific volume. At this transition temperature, the material has regained its elasticity, resulting in phase transition stress. It is superimposed with welding stress caused by uneven plastic deformation, which may be compressive stress in the phase transition zone, and volume (triaxial) expansion may also cause significant transverse tensile phase transition stress in certain areas, which is one of the main factors leading to cold cracking.

04 Methods to prevent and reduce welding stress

Adopting a reasonable welding sequence and direction

The basic principle is that when welding seams on a flat surface, both longitudinal and transverse shrinkage should be relatively free. The weld with the largest shrinkage in the structure should be welded first, such as welding the butt weld first and then the fillet weld. When welding cross welds, the welding sequence should ensure that the intersection points are not prone to defects and have low rigidity. As shown in Figure 8 ABC is reasonable and D is unreasonable.

Figure 8 Welding sequence of cross welds

Try to use small welding heat input as much as possible

A small welding heat input can reduce the range of uneven heating zones and the amount of weld shrinkage. In operation, high-energy welding methods such as small-diameter welding rods, multi-layer and multi pass welding, low current fast non swinging welding, and concentrated welding heat sources are used to control interlayer temperature for segmented welding and segmented reverse welding, in order to reduce heat input.

Adopting overall preheating

Overall preheating can reduce the temperature difference between the welding joint area and the overall structure, in order to reduce the uneven plastic deformation caused by uneven expansion and contraction in the welding thermal cycle and reduce welding stress. For example, in the hot welding method of cast iron, the casting is heated to 600 ℃.

Hammer

After welding, hammering the weld bead quickly and evenly can cause plastic deformation of the weld metal, which can reduce welding deformation and welding stress. Generally, a flat, long, and round headed hammer (which can be modified with a chisel, but not with a pointed tip) should be used to strike in sequence, with moderate force to affect the 2mm range. The length of the weld bead and the hammering temperature should be selected based on the material properties. Generally, the root weld bead should not be hammered to avoid cracking, and the cover weld bead should not be hammered to avoid affecting the appearance.

Reduce the impact of hydrogen

Especially for high-strength alloy steels with a tendency towards cold cracking, attention should be paid to reducing the influence of hydrogen. If low hydrogen welding rods and alkaline fluxes are used, and dried and stored in a drying cylinder according to regulations, they can be taken as needed to remove moisture, oil, rust, etc. from the groove surface, control the welding environment temperature, and if necessary, perform dehydrogenation treatment, that is, immediately heat up to 250 ℃ for 2-3 hours or 350 ℃ for 1-2 hours after welding, depending on the situation.

Stress relief treatment

Eliminating welding residual stress is achieved by causing tensile plastic deformation near the welding area and reducing the degree of residual plastic deformation.
(1) Stress relief heat treatment (stress relief annealing). Heat the welded structure as a whole or locally to 20-30 ℃ below the phase transition point of the steel for insulation, in order to generate the plastic deformation required for stress relief through creep during insulation. Holding at a certain temperature for about 1 hour can effectively eliminate stress, and most of the time required for heat treatment of thick walled structures is used for heating and cooling. This method can generally relax 70% to 90% of residual stress. At the same time, it has also improved the material of most steel welding areas. For materials with temper brittleness or a tendency towards reheat cracking, it is important to carefully choose the heating speed and insulation temperature.

(2) Loading method. Using mechanical principles to load and induce plastic deformation in the residual stress zone of the welded joint, and relaxing the tensile force in the joint zone after load reduction. This method is only applicable to plastic materials with relatively low yield strength, and attention should be paid to the water temperature being above the brittle transition temperature of the material. The hammering method should also follow this principle. In recent years, explosion stress relief method and vibration stress relief method have also been developed.

(3) Temperature difference stretching method (or low-temperature stress relief method). Heat each side of the weld seam with an appropriate oxyacetylene torch, spray water with the drainage pipe at a certain distance behind (see Figure 9), and maintain equal distance to create a temperature field with high temperature on both sides (about 200 ℃) and low temperature on the weld seam (about 100 ℃). This causes thermal expansion on both sides to cause tensile plastic deformation of the weld seam, offsetting the original shrinkage plastic deformation and relaxing residual stress. This method is not a big problem for low carbon steel, but special attention should be paid to the influence of temperature on the material for alloy steel.

Figure 9 Temperature difference stretching method