1. What is welding stress
Welding stress is the stress caused by the welding of welded components.
The internal stress generated in the weldment during welding and the shape and size changes of the weldment are caused by the welding thermal process.
The uneven temperature field in the welding process and the local plastic deformation and microstructure with different specific volumes caused by it are the root causes of welding stress and deformation.
When the uneven temperature field caused by welding has not disappeared, this kind of stress and deformation in weldment is called transient welding stress and deformation;
The stress and deformation after the disappearance of the welding temperature field are called residual welding stress and deformation.
Under the condition of no external force, the welding stress is balanced in the weldment.
Welding stress and deformation will affect the function and appearance of weldment under certain conditions.
2. Hazards of welding stress
Welding residual stress has six effects on weldments:
① Effect on strength:
If there are serious defects in the high residual tensile stress zone and the weldment works below the brittle transition temperature, the welding residual stress will reduce the static load strength.
Under the action of cyclic stress, if there is residual tensile stress at the stress concentration, the welding residual tensile stress will reduce the fatigue strength of the weldment.
In addition to the magnitude of residual stress, the fatigue strength of weldments is also related to the stress concentration factor, stress cycle characteristic coefficient and the maximum value of cyclic stress.
Its influence decreases with the decrease of stress concentration factor, intensifies with the decrease of stress cycle characteristic coefficient, and decreases with the increase of cyclic stress.
When the cyclic stress is close to the yield strength, the effect of residual stress disappears gradually.
② Effect on stiffness:
The superposition of welding residual stress and stress caused by external load may lead to local early yield of weldment and plastic deformation.
The stiffness of the weldment will be reduced.
③ Influence on the stability of pressure weldment:
When the welded rod is under pressure, the welding residual stress is superimposed with the stress caused by an external load, which may make the rod yield locally or make the rod unstable locally, and the overall stability of the rod will be reduced.
The influence of residual stress on stability depends on the geometry and internal stress distribution of the member.
The influence of residual stress on non closed section (such as I-section) is greater than that on closed section (such as box section).
④ Influence on machining accuracy:
The existence of welding residual stress has different degrees of influence on the machining accuracy of weldments.
The smaller the stiffness of the weldment, the greater the machining amount, and the greater the impact on the accuracy.
⑤ Influence on dimensional stability:
Welding residual stress changes with time and the size of weldment also changes. The dimensional stability of weldments is also affected by the stability of residual stress.
⑥ Effect on corrosion resistance:
Welding residual stress and load stress can also lead to stress corrosion cracking.
Influence of welding residual stress on structure and member:
Welding residual stress is the initial stress on the member section that has not borne the load. During the service of the member, it overlaps with the working stress caused by other loads, resulting in secondary deformation and redistribution of residual stress.
It will not only reduce the stiffness and stability of the structure but also seriously affect the fatigue strength, brittle fracture resistance, stress corrosion cracking and high temperature creep cracking of the structure under the joint action of temperature and medium.
3. Elimination method of welding stress
At present, the failure methods used to eliminate stress include vibration aging (eliminating 30% – 50% stress), thermal aging (eliminating 40% – 70% stress) and houckner PT aging (eliminating 80% – 100% stress).
Vibration aging treatment is a common method to eliminate the internal residual internal stress of engineering materials.
Through the vibration, when the vector sum of the residual internal stress and additional vibration stress in the workpiece exceeds the material yield strength, the material will have a small amount of plastic deformation, so as to relax and reduce the internal stress in the material.
Thermal aging is to heat the workpiece to the elastic-plastic transition temperature and keep it for a certain time to relax the residual stress of the workpiece, and then reduce the temperature very slowly to make the workpiece in a low-stress state after cooling.
The production time shows that if the process parameters are improperly selected in the process of heating, insulation and cooling, or the reasonable process specifications are not strictly observed during operation, the stress elimination result is often not obtained, and even the stress of the workpiece is increased.
Hawking energy aging
- At present, it is the most complete aging method to eliminate welding residual stress and produce ideal compressive stress. Hawke can eliminate 80% – 100% aging.
- It can increase the fatigue strength of welded joints by 50% – 120% and prolong the fatigue life by 5-100 times. The corrosion resistance of metal in a corrosive environment is increased by about 400%.
- It is used to eliminate welding stress. It can completely replace heat treatment, vibration aging and other aging methods. The treatment process is simple and the effect is stable and reliable (it can be treated wherever you want, and can be carried out at any time and in any process, so that you can do whatever you want, handy and easy to use).
- It is not limited by workpiece material, shape, structure, steel plate thickness, weight and site, especially in the construction site, welding process and welding repair, it is more flexible and convenient to eliminate welding stress.
- The weld reinforcement, pit and undercut at the weld toe can be directly processed into a smooth geometric transition, so as to greatly reduce the stress concentration factor.
- The micro cracks and slag defects at the weld toe can be removed to inhibit the early initiation of cracks.
- Because Hawking can eliminate stress and improve several factors affecting weld fatigue performance at the same time, such as residual stress, micro cracks and defects, weld toe geometry, surface strengthening, etc., it is the most effective method to improve weld fatigue performance at present, and has the effect of twice the result with half the effort.
- It is more suitable for stress relief treatment of site welds, ultra-high and low welds and welding repair welds of large structural parts.
- Environmental protection, energy saving, safety and pollution-free, and the use on the construction site is more flexible and convenient.