Welding-distortion:how to prevent and control.SOLUTIONS with Effective, Powerful AdviceWelding-distortion, deformation, warping, residual welding stresses, fixturing, welding distortion causes, welding localized heat input, reduced yield strength at high temperature, straightening, compensating distortion, stress relieving, welding links, welding tips, improving deformation outcome, joining questions needing answers: these are some of the items developed in this Site for the benefit of interested readers. What is in here for me? The unavoidable nature of Welding-distortion and its causes, explained.
How to reduce Welding-distortion?
What happens to fixtured welds? If you know all you need about Welding-distortion then you can skip this page. Otherwise, understanding these and related causes and effects will improve your mastering of the processes, will save your time by avoiding predictable distortions, will let you find practical solutions, will improve quality, will reduce scrap and rework, will increase productivity.
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In previous pages we saw how Welding Information is transmitted to all involved in industrial settings and how Welding Procedures are prepared to specify the essential steps to follow in a well organized welding operation, to ensure successful results every time. Here we are dealing in some more detail on the subject of distortion, whose control finds an important place in well built procedures. Why Welding-distortion occur, causing warping? Welding-distortion or deformation or warping of weldments during welding is a natural outcome of intrinsic non uniform heating and cooling of the joint. Distortion is the result of the action of internal stresses which are produced while welding, and remain in the part 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 heated volume has now lower yield strength at high temperature, and then it is easily upset to shorter dimensions. Upon cooling the same material tends to contract in all directions and is now stressed in tension by the attached cool structure which did not move appreciably in the process. By now the yield strength is again higher, at lower temperature, so that the upset material cannot regain its original dimensions. How to decrease...? Sometimes to decrease the amount of Welding-distortion it is sufficient to place the elements at an angle before welding in order to counter the movement by a certain opposite displacement to be introduced before welding. 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. 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. 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 is performed by heating the welded assembly in a suitable furnace. By heating the welded structure uniformly in a furnace at elevated temperature, the remaining maximum stress will be reduced to the lower yield strength value which the material exhibits at that 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. To see the article click on PWL#008. 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. To see the note Click on PWL#015. 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 strain gages application (a destructive method that provides precise measuring of tiny movements or strains subsequent to strategic cuts in the material), x-ray diffraction (a nondestructive method that measures the deformation of metallic crystals), and ultrasonic measurement of residual stresses (study of acusto-elastic properties).
A short note on Welding Distortions was published (3) in Issue 66 of Practical Welding Letters for February 2009. Click on PWL#066 to see it.
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