A thin sheet component is usually understood as a workpiece obtained by the weld of steel sheets no thicker than 4 mm (such as stainless steels, zinc sheets, tins), for instance, hangars for roller installation, driver’s chambers, excavator chambers manufactured in our factories.
High technologies are required to monitor and escape weld distortions of thin sheets. The arguments delivered beneath can be considered for reference.
Causes of Weld Distortions
Arch welding is an irregular process of rapid over-heat and cooling, in which distortions are possible during or after the welding process.
The major factors of weld distortions are the thermal one and the hard state of the weld components.
The thermal distortions during the welding process are constrained by the hard state of components, compression plastic distortions occur leading to the weld residual distortions.
1. Factors that affect the weld thermal distortions.
1. Methods to weld.
Each weld method produces a specific thermal field, producing certain thermal distortions.
Generally, the automatic weld is of higher concentrations compared to the weld handled manually. The latter is narrower and inclined to less deformity. Thinner welding wire, high-current densities, heat concentrations, CO2 protective gas cause slight distortions.
2. Weld specifications
These include weld current, arch voltages and weld rate.
The great energy of the line causes more severe weld distortions.
The weld distortions increase according to the rise of the weld current and arch voltages and decrease according to the rise of the weld rates.
Among the 3 specifications, the influence of arch voltages is evident. The automatic weld of lower voltages, higher rates, higher current densities results in slight weld distortions.
3. Amounts of weld seams and cross-sectional sizes.
The sizes of each cross-section depend on the weld seams. The more they are, the more serious are the weld distortions.
4. Techniques to operate.
Since uninterrupted and discontinuous welding produces dissimilar heat ranges, thermal distortions differ respectively.
Mostly, uninterrupted weld distortions are more serious and the intermittent weld distortions are slight.
5. Thermo-physical characteristics of materials.
Each material has got particular thermal conductibility, dissimilar heating and extension factors leading to certain thermal and weld distortions.
2. Factors affecting the stiffness coefficient of welded parts
1. Sizes and shapes of parts.
The more rigid is the part, the less is the weld distortion.
2. Applying the tire clamping.
Application of tire clamps makes the parts more rigid, consequently reduces the weld distortions.
3. Assembly of the weld procedures.
The weld during assembly leads to changes in stiffness and the location of the gravity center of gravity of parts at each phase of assembly, which has a large effect on distortions in weld controlling components.
Typically, in harsh circumstances weld elements bear larger distortions.
Kinds of Steel Sheet Weld Distortions
Distortions of steel sheets fall in 2 categories: entire and partial.
In case of entire distortion in the aftermath of welding, component sizes and shapes change. This includes the lateral and cross shrink (decrease of overall size), bend distortions etc.
Rules and methods of controlling weld distortions of steel sheets.
Thermal distortions and the hard state of weld elements in the weld process are the major factors affecting the weld residual distortions, which seem unavoidable. Thus, the complete elimination of weld distortions is impossible.
Controlling residual distortion during welding implies taking into account the sheet designs of the components and the manufacturing methods meanwhile.
While developing thin-sheet components, besides durability and operational characteristics, minimal distortion during welding and the least labor costs should also be observed.
Consequently, optimizing the scheme of plate connection becomes crucial.
When designing the plate connection scheme, technical properties are not often taken into account, which might easily bring on distortion during welding.
The welding process is a determining process to construct steel structures. A smart weld process will minimize weld distortions reducing stress build-up.
The following steps offer efficiency in controlling the weld distortions:
- Components fall in two groups: smaller parts and sections of components, transmitting weld distortions to a range of parts, promoting the control and corrections of distortions.
- To escape the distortions and extreme bend distortions in the aftermath of welding, the scheme of part seams must be symmetric or near the neutral axle of cross-sectional components.
- For major weld seams decide on smaller weld foot sizes and the most possible short welding.
- Escape extreme build-up and lateral position of weld seams.
- Apply wider and longer steel sheets possibly more or minimize the amount of weld seams.
Methods controlling the weld distortions of thin sheets include:
1. Assembly elements with no assembly strain.
2. Applying automatic weld and gas protective weld methods, for instance, the most progressive Ar+CO2 mixed gas MAG shield weld.
3. Smart adoption of welding specifications and succession to assemble.
Welding wire feed, current voltages should be reduced, polarity should be changed (typically dc reverse, dc positive).
Shorter seams have to be welded first, only then the longer ones, weld backward from inner side to outwards.
4. Fixed methods and distortion prevention techniques could be put in use rationally.
Elimination of Weld Distortions of Thin Parts
In the formation of steel structures, though arrangements are made from the point of view of designing components and construction techniques to keep welding distortions in control, due to the complicacy of weld and construction techniques, weld distortions are still unavoidable, so weld distortions that exceed design specifications should be corrected.
Correction is constricted to eliminate certain distortions of welding elements referring to distortions of angularity, bending, wave, etc., for general component distortion, such as lateral and cross compression (reduction in overall size), it is only possible to compensate using a workpiece or assembly allowances.
It is simple to accept mechanical correction for straightening steel structures to make metals harden, apply particular amounts of material reserve, consequently it is applicable just for material with fine ductility.
In real productions, the mechanical correction may use good-sized hydraulic and friction presses.
Through flame correction exposed to steel structures, in the following phase of cooling, welded metals bear permanent contraction and ductile distortion, thus, the overall weld component distortions can be eliminated.
Flame correction needs certain ductility. This fact has to be considered if it is used for fragile or plastic material.
The temperature of flame heat must be kept in control, as extremely high temperatures affect the material mechanical features reducing them, and extremely low temperatures deliver inefficiency in corrections.
The cool speed does not affect the correction, so while constructing you may spray water to cool improving work effectiveness and correction effects.
Summing up, we can say that in the manufacture of steel structures, distortions during the welding process is unavoidable. This is possible to keep in control by adopting techniques and arrangements that ensure effectiveness and correcting from the point of view of welding distortions surpassing the permissible requirements. Thus, both the quality of the steel structures and the requirements for the economy are achieved.