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Welding-distortion:how to prevent and control.SOLUTIONS with Effective, Powerful Advice
Welding-distortion: why straight welds become crooked?
Visit the NEW Page on Welding Overview, for a thorough Here is a reminder about Welding distortion. Understanding these and related causes and effects will:
The effects of residual stresses Here we are dealing in some more detail on the subject of Welding-distortion, whose control finds an important place in well built procedures.
Welding-distortion or deformation or warping of weldments during welding is a natural outcome of intrinsic non uniform heating and cooling of the joint. Welding-distortion is the result of the action of internal stresses which are produced while welding. Residual Stresses are those remaining in the part or construction after heating is removed, as briefly explained in the following. Stresses are due to volume changes with heating and to decreasing yield strength at elevated temperature. Metal subject to thermal expansion while heating tends to be compressed by the surrounding cool structure. The expanding heated volume has now lower yield strength at high temperature, and therefore it is easily upset (deformed) to shorter dimensions. Upon cooling the same material tends to contract in all directions. But by now the yield strength is again higher, at lower temperature, so that the upset material cannot regain its original dimensions. The main dimension of the heated volume above, which was deformed (upset), is now shorter than it was before welding. But, while cooling down, it cannot reach the previous dimension because the attached cool structure, which did not move appreciably in the process, will not let it. Therefore that volume, that was first heated, shortened (upset) and then cooled, in now pulled in tension by the surrounding structure that was not heated. The result is the development of internal tension stresses in the weld. These residual stresses are the cause of Welding-distortion.
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How to decrease...? Sometimes to decrease the amount of Welding-distortion it is sufficient anticipate the deformation occurring, and to counter it by an opposite displacement before welding. If two plates being welded are rotating one toward the other while being welded, this amounts to place the elements at an angle before welding, in order to counter the movement by a certain opposite displacement.
Or (for butt welds) to weld a short length at one end and then start again from the other end, while the first end is already rigidly welded. Otherwise one can distribute short stretches of welding at distant places, by introducing sequences aimed at avoiding local concentration of heat input. Fixturing assemblies and sequencing short weld beads Or one can provide very rigid fixturing that will not allow any movement while welding is performed. Movement can be prevented: residual stresses cannot. One can easily prove that the part is now highly stressed (presenting a high level of residual stresses).
After taking the part out from the fixture if one removes some of the weld by partial asymmetric cutting or grinding, the remaining portion will deform considerably to rearrange the remaining internal stresses.
This Welding-distortion is an indirect proof of the high level of stresses which were present in the as welded part. Material removal is at the base of certain methods of residual stresses measurement. Occasionally when the residual stresses exceed material strength, cracks may appear in the weld at high temperature or while cooling near room temperature. See AWS standard C4.4/C4.4M:2007, Why stress relieve...? It is good practice to relieve the residual stresses of a constrained welded assembly. The reason is that these stresses can sum up with external stresses in service and exceed the material strength, producing failure, or further deformations. The most common method of stress relieving for reducing Welding-distortion, is performed by heating the welded assembly in a suitable furnace. Heating the welded structure uniformly in a furnace at elevated temperature, causes the remaining maximum stress to be reduced to the yield strength value of the material at that temperature, which is lower than it was at room temperature. During stress relieving some minor relative movements will take place. Upon cooling, the residual stresses will stay significantly reduced.
Another method uses mechanical peening of the weld either by hammering or by shot peening equipment: this is done sometimes on tool steels immediately after shielded metal arc welding and before cool down.
A short Article on Stress Relieving after Welding was published in the issue 8 of Practical Welding Letter, Section 7, for April 2004. Vibrational stress relief, uses sub-harmonic frequencies to reduce any residual stresses, by introducing mechanical energy into the work piece. A short note on Vibrational Stress Relief was published in the issue 15 of Practical Welding Letter (Section 9.5) for November 2004. Relieve stresses to stabilize structures The above is correct for a simple metal like plain low carbon steel where no metallurgical phase changes take place. In more complex cases, phase transformation comporting volume changes can intervene, complicating the issues and the solutions.
The general approach would be to reduce the causes of Welding-distortion by providing suitable pre-heating, if possible, so that there will be less expansion difference, between material at weld temperature and surrounding structure.
The actual determination of residual stresses after any of the above methods is done by specialized technologies. Among these are:
A short note on Welding Distortions was published (3) in Issue 66 of Practical Welding Letters for February 2009. The reference to a page on Residual Stresses was published (8) in Issue 80 of Practical Welding Letters for April 2010. A practical note on Straightening a Pipe Distorted by Welding was published (11) in Issue 83 of Practical Welding Letter for July 2010. An Article on Controlling Distortion was published (3) in Issue 104 of Practical Welding Letter for April 2012. An Article on Filler Metal for Controlling Welding Distortion was published (4) in Issue 132 of Practical Welding Letter for August 2014. An Article on Distortion Control was published (3) in Issue 162 of Practical Welding Letter for February 2017. An Article on Using flame to redress distortions in Stainless Steels was published (3) in Issue 164 of Practical Welding Letter for April 2017.
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