In the process of fabricating, stainless steel parts often encounter the deformation of parts due to welding.
Part deformation also is only the surface phenomenon, due to the effect of welding heat source, it is easy to make the weld metal and welding heat affected area overheating, often leads to coarse grains of weld metal and metal in heat affected zone which will produce defects worsen metal performance.
In addition to prevent and eliminate the negative influence of welding on the parts, to control the temperature of parts is necessary.
However, it takes a long time to wait for the parts to be cooled and then proceed the next seam welding process or some other process. The working efficiency will be seriously affected.
Therefore, several solutions are proposed for reference.
In specific welding, the volume of the parts is closely related to the density of the weld bead and the welding feet.
Especially in the process of stainless steel sheet parts processing, melt the base metal (parts) by the heat source (arc) during welding (sheet parts usually do not need to add solder wire), make the parts where need to welded melt into a molten pool, then after natural cooling crystallization form into weld seam.
Because the parts are too small, the welding heat cannot be distributed quickly, and the distortion of the parts will appear, which will have a great impact on the appearance and shape of the parts.
To solve this problem, we need to start with several aspects.
At present, two kinds of welding methods are used for stainless steel welding in general factory:
⑴ Welding electrode arc welding
This kind of welding method is a more traditional welding method, which has a high requirement for welders. It has a great influence on the heat of the parts. The processing time after welding is long and the welding quality is not well controlled.
However, the equipment is relatively simple and can be used to weld different materials with flexible welding rod.
⑵ Gas shielded welding
Gas shielded arc welding can be divided into several different method, here now we’re discussing argon arc welding for the stainless steel welding, which is taking argon or mixed gas (MAG welding) as protective gas.
The advantage of gas shielded welding is fast speed welding, small thermal influence area and simple processing after welding.
So as to reduce the thermal influence of the parts when welding stainless steel parts, try to use gas protection welding as much as you can.
In the process design of welding, it is necessary to adopt the right and left alternate welding method, symmetrical welding method and back-step welding method, and the specific principles are first inside and then outside, first less and then more, first short and then long.
Welding parameters such as welding current and arc voltage can also affect welding deformation.
When welding stainless steel components, along with the grow in size of the parts, the welding current should also be increased. At the same time, in order to make the weldment locally heated more evenly, the welding current should be strictly controlled. If the welding current is too small, can affect the welding quality; if the welding current is too large, the welding deformation is likely to be more serious.
Therefore, the welding parameters such as welding current and arc voltage should be adjusted according to the thickness and weld requirements of the material.
⑴ Small, simple parts
For example, if the welding lapping method is L-shape, T-shape or lap parts in plane, a copper plate can be added under parts (thickness > 8mm), as shown in Fig.1:
Fig.1 copper plate under parts
Because the heat transfer efficiency of copper plate is higher than the steel plate, it can quickly remove the welding heat and reduce the thermal deformation of the parts.
If the shape of parts is not very flat or swelling which is not easy to closely contact with copper plate, can also use the thick cotton with good water imbibition or soaked mat underneath the weld bead of the parts, and this can also effectively reduce the deformation of parts.
⑵ Large, complex parts
Because the shape is complex or without the space for the copper plate, the above solution can not be used to solve the problem, so the water cooling method should be adopted (Fig.2).
Fig.2 water cooling method
Water cooling is generally divided into two types:
① Spray cooling
On the back of the weld bead of parts adopt the method of the water spray cooling, this kind of method applies to the parts with larger area. At the same time must be T-shape or L-shape lap (need to adjust water current angle), to avoid the water flow into the weld position.
The advantage of this method is that the cooling effect is good, which is convenient for mass production. The disadvantage is that the welding conditions are high (requiring special equipment) and the types of processed parts are single;
② Wet sand cooling
The spray cooling method is not applicable to the welding path of planar joint form because it cannot guarantee that the water flow does not enter the welding pass position.
Can use wet sand cooling method: choose containers (greater than the welding parts) filled with sand, injection water to fully saturated sand, place the parts flat on the wet sand when welding, make the back side of parts weld bead fully contact with the wet sand, then welding can begin.
The advantages of this method are simple operation and suitable for all kinds of complex shapes.
The disadvantage is that it is not easy to fabricate large parts.
⑶ Welding of large plate parts
Generally refers to the welding of parts with thickness more than 6mm.
Due to the big size of parts, long weld length and high welding feet (large area of molten pool and hot influence area), the bending deformation caused by thermal deformation will occur during welding.
To solve this problem, we need to start with several aspects:
① The cooling measures are prepared ahead of welding (refer to the small parts cooling scheme);
②Weld allowance for deformation
Since most of parts is difficult to realize welding symmetrically or only in one side welding at the same time, and it will cause bending deformation during welding due to uneven heating.
Therefore, the deformation of the parts should be offset by the reverse direction of the deformation according to the length of the parts, the thickness of the material (the height of the foot) and the shape.
This operation need experienced engineering and technical personnel and skilled workers make judgments based on experience, using fixtures to fix parts, after finishing fabricate the first piece, then make fine adjustment according to the actual effect.
Fig.3 weld allowance for deformation
Stress Relief After Welding of Parts
Compared with the ordinary carbon steel parts welding, the coefficient of thermal conductivity of stainless steel is smaller than carbon steel. Besides, the stainless steel has bigger electric resistance and expansion coefficient, so stainless steel heat transfer is slow, and the thermal deformation is larger.
Even the parts surface deformation is not obvious after complete fabrication, it will also have changes during transportation, or changes during use because of vibration, stroke or temperature change. The appearance, size and using effect of parts will be affected directly.
Therefore, it is necessary to remove the stress of large parts after finish fabrication, especially for the thickness parts (high welding feet and large melting pool) and parts with many welding bead.
The stress elimination can be adopted by natural aging and artificial aging.
Natural aging is generally used in large castings, and it is not suitable for general welding parts. Besides, the aging period is long and not easy to control.
The artificial aging is divided into heat treatment aging and vibration aging.
Aging heat treatment is to heat the parts with temperature of 550 ~ 650 ℃ to carry out stress annealing. This method is time-saving than the natural aging and with high efficiency, but generally the factories do not have the processing condition, outsourcing processing will increase the transportation costs, so it is generally not been adopted.
Vibration aging is commonly used method to eliminate the residual internal stress for engineering materials, which is through the vibration to reduce the plastic deformation of components caused by internal residual stresses, so as to achieve the purpose of eliminating stress.
The working principle is to place a motor system with eccentric block (vibrator) on artifacts, and bearing components with elastic objects such as rubber pad, starting motor by controller and adjust the speed, to make the artifacts in a state of resonance.
The vibration treatment of 20 ~ 30min can achieve the aim of adjusting the internal stress, and the general accumulative vibration time should not exceed 40min.
This method has low requirement and simple operation for processing site, which is adopted by most factories.
The structure and appearance of the parts are not changed due to the influence of the external environment, so the stability of the parts is guaranteed.
In conclusion, welding deformation is unavoidable in stainless steel welding, and it has certain influence on the machining and actual use of stainless steel components.
In order to avoid these effects, more time should be spent on welding technology, including the welding method, technical parameters, sequence, component positioning and holding, welding post-processing, etc., to control the welding distortion to a minimum.
In the control of every fabrication process, engineering technical personnel and welding operators must work closely, combine theoretical data with actual situation, set up a reasonable construction scheme and fully control the welding deformation, only in this way, a more perfect product can be produced.