In the production process, a part of stainless steel frequently encounters deformity because of welding.
Deformity of parts appears just on the surface, which depends on the effects of weld heat sources. The welding metal and the zone exposed to the heat get overheated easily. This brings about a large-grain effect for welding metals and metals in the overhead area. This, in turn, affects the performance of metals.
Controlling the part temperature is essential to avoid the undesired weld effect on parts.
Yet, the cooling of parts requires a great deal of time, but only after it can the next seaming weld and other processes follow. Due to this effectiveness of the process can seriously suffer.
Thus, a few ways out are delivered for reference purposes.
In certain welds, part volumes closely relate to the density of welding beads and weld foot.
Particularly while machining parts of stainless steel it is recommended to melt base metals through the heat fount (arch) in the welding process (parts generally require no solder wires). Welding seams should be formed only after the cooling process.
Extremely little sizes of parts prevent the quick distribution of the weld heat causing part distortions and deteriorating the part shapes.
To avoid the worst effects we are delivering the following guidelines:
Currently, 2 types of weld techniques are applied to weld stainless steel in most factories.
1. Electrode Arch Welding
This technique is a conventional one. It is highly required by the majority of welders. It influences the part heat greatly. This takes a long time to process after welding. Moreover, it is rather difficult to control the weld quality.
Still, the equipment offers simplicity in use and suitability for a variety of material through pliable rods.
2. Gas Shielded Weld
This technique in its turn includes some other methods, one of which is the argon arc weld, suitable for stainless steel. It combines argon and mixed gases (MAG weld) as a protecting gas.
Gas-shield weld is advantageous as it is faster, produces a small heat-exposure zone and provides simplicity while processing in the aftermath of welding.
To decrease the heat exposure on parts while welding stainless steel, it is advisable to apply gas protective weld possibly more.
While designing the welding process, the right-side, and left-side alternation technique as well as symmetric weld, back-set weld methods should be adopted. The principle is first inward then outward, from less to more and from shorter to longer.
Weld specifications like weld current as well as arch voltages influence weld deformity, too.
In the welding process of stainless steel parts, the weld current has to be added in accordance with increasing sizes of parts. Besides, the weld current needs controlling to distribute the heat evenly. In the case of extremely little weld current, the weld performance can suffer. On the other hand, extremely big weld currents can cause even more severe deformity.
That is why Weld specifications like current and arch voltages have to be set in accordance with the thicknesses and weld requirements of materials.
1. Small-sized, plain parts
For instance, in the case of L-shaped, T-shaped weld methods or lap parts, copper plates are recommended to add underparts (as thicknesses > 8 mm). See Figure 1.
Copper plates are capable of transferring heat more efficiently than steel plates. Thus, they can take the weld heat off, decreasing the part overheats deformity.
If shapes appear too convex making close contact with copper plates, puffy cotton or absorbing mats could be applied under the welding beads. This will efficiently decrease the part deformity.
2. Big, intricate parts
The mentioned above solutions are not applicable for intricate shapes. Here, it is advisable to adopt a water refrigerant technique. See Figure 2.
Water refrigerant method offers 2 options:
1. Spray cooling, which is suitable for large parts. This option should be applied to the backside of welding beads, to T-shaped or L-shaped laps to prevent water flowing to the welding position. Water flow angularity has to be adjusted.
This option is advantageous, as it provides efficient cooling and convenience for mass production. However, this method requires particular equipment and offers a single type of part processing.
2. Wet-sand cooling is the second option. The mentioned above method isn`t suitable for weld paths of planar-joint forms, as it may fail to prevent water flowing into weld passing position.
At this point, wet-sand cooling option is recommended. Take a container, which has to be of larger sizes than the weld part, inject water to completely saturated sand, put the part upon the wet sand while welding. Make sure that the backside of welding beads are in full contact with the sand, then go on to weld.
This option provides simplicity and suitability for various types of intricate parts. Still, fabricating large-sized shapes is somewhat challenging.
3. Welding of large-sized plates
This is mostly related to parts thicker than 6 mm.
Large sizes, long welding lengths as well as high weld foot (large zone of the melting pool and heat exposure zone) are more likely to be exposed to over-heat deformity in the welding process.
The following aspects have to be considered to deal with the mentioned trouble.
1. Measurements of cooling must be carried out prior to the welding process.
2. Welding allowance of deformity.
It is rather challenging to make symmetrical welds and hence, more possibility of heat uneven spread.
Consequently, parts have to be shifted in the opposite direction of the deformity in accordance with the part lengths, thicknesses of materials and shapes.
This task should be realized by skillful engineers and technicians, as it requires great experience. Parts can be fixed through a fixture. Adjustments must be made in accordance with actual effects.
Relieving Stress After Part Welding
The coefficient of heat conductibility of stainless steel is less in comparison with regular carbon steel welds. Moreover, stainless steels are more electric resistant, thus heat transferring is slower, while the over-heat deformity is large.
When the whole fabrication is completed, the part face deformity becomes insignificant. Some changes may occur while transporting or as a result of vibrations, strokes or temperature. Part appearance, sizes, usage effects bear direct effect.
This is why stress for big-sized parts should be removed in the aftermath of finishing fabrication, particularly in case of parts of considerable thickness and multiple-weld beads.
Stress relief is possible to adopt through natural and simulated aging.
The first one is commonly applied in a big casting. It isn`t proper for common weld parts. The age duration is longer and difficult to monitor.
The simulated one includes the heat process and vibrancy aging.
Heat-processing heats parts at 550~650◦ temperatures to realize the stress anneal. Compared to natural aging this type saves more time and provides effectiveness. Still, most factories possess no treatment conditions. The out-source treatment increases the cost of transport, thus making it less suitable to adopt.
Vibrancy aging is suitable to remove the residual inner stress for material in vibration to decrease the deformity brought about by the inner residual stress. This will result in the elimination of stress.
It is recommended to set motor systems of eccentric blocks or vibrators upon artifacts and carrying elements like rubber pads, then start the motor through controllers, meanwhile adjusting the speeds to get the artifacts into resonance.
The vibrancy processing of 20~30 minutes adjusts the inner stress. The overall vibration duration must not surpass 40 minutes.
This technique delivers simplicity in operation. That is why many factories adopt this method.
Part shapes and structures do not change because of the outer surroundings. Hence, part stability is ensured.
To round up, weld deformity is something inevitable while welding stainless steel. This influences the processing and actual usage of stainless steel parts.
To escape this trouble the welding technology has to be considered more thoroughly. This includes the weld methods, technical specifications, continuity, positioning of components, finishing fabrication, etc. Having done this, the weld deformation will keep to the minimum.
While controlling the machining process, technicians and operators should cooperate, put together the theory and actual circumstances, draw a sensible scheme to carry out full control of the weld distortion. All of this will lead to the production of fine parts.