distorsion corrective techniques

8/11/2019 Distorsion Corrective Techniques

1/4


2/4

(se packing pieces which will over correct thedistortion so that spring-back will return the component to the correct shape

)heck that the component is adequately supported during pressing to prevent buckling (se a former *or rolling+ to achieve a straight component or produce a curvature

As unsecured packing pieces may fly out from the press, the following safe practice must

be adopted'

- bolt the packing pieces to the platen- place a metal plate of adequate thickness to intercept the missile- clear personnel from the haard area

!hermal techniques

!he basic principle behind thermal techniques is to create sufficiently high local stresses sothat, on cooling, the component is pulled back into shape.

Fig. $ Localised heating to correct distortion

!his is achieved by locally heating the material to a temperature where plastic deformation willoccur as the hot, low yield strength material tries to e&pand against the surrounding cold, higheryield strength metal. n cooling to room temperature the heated area will attempt to shrink to asmaller sie than before heating. !he stresses generated thereby will pull the component intothe required shape. *ee #ig. /+

0ocal heating is, therefore, a relatively simple but effective means of correcting weldingdistortion. hrinkage level is determined by sie, number, location and temperature of theheated ones. !hickness and plate sie determines the area of the heated one. 1umber andplacement of heating ones are largely a question of e&perience. #or new 2obs, tests will oftenbe needed to quantify the level of shrinkage.

%pot& line&or wedge-shapedheating techniques can all be used in thermal correction ofdistortion.

%pot heating

Fig. ' %pot heating for correcting buckling

pot heating *#ig. 3+, is used to remove buckling, for e&ample when a relatively thin sheet hasbeen welded to a stiff frame. 4istortion is corrected by spot heating on the conve& side. If thebuckling is regular, the spots can be arranged symmetrically, starting at the centre of the buckle

and working outwards.

Line heating


3/4

Fig. ( Line heating to correct angular distortion in a fillet weld

Heating in straight lines is often used to correct angular distortion, for e&ample, in fillet welds*#ig. 5+. !he component is heated along the line of the welded 2oint but on the opposite side tothe weld so the induced stresses will pull the flange flat.

)edge-shaped heating

!o correct distortion in larger comple& fabrications it may benecessary to heat whole areas in addition to employing lineheating. !he pattern aims at shrinking one part of thefabrication to pull the material back into shape.

Fig. * se of wedge shaped heating to straighten plate

Apart from spot heating of thin panels, a wedge-shaped heating one should be used, *#ig. 6+from base to ape& and the temperature profile should be uniform through the plate thickness.

#or thicker section material, it may be necessary to use two torches, one on each side of theplate.

As a general guideline, to straighten a curved plate *#ig. 6+ wedge dimensions should be'

$. 0ength of wedge - two-thirds of the plate width/. 7idth of wedge *base+ - one si&th of its length *base to ape&+

!he degree of straightening will typically be 6mm in a 3m length of plate.

7edge-shaped heating can be used to correct distortion in a variety of situations, *#ig. 8+'

$. tandard rolled section which needs correction in two planes,*#ig. 8a+/. 9uckle at edge of plate as an alternative to rolling *#i.g 8b+3. 9o& section fabrication which is distorted out of plane *#ig. 8c+

Fig. + )edge shaped heating to correct distortion

a) standard rolled

steel section

b) buckled edge of plate c) box fabrication


4/4

,eneral precautions

!he dangers of using thermal straightening techniques are the risk of over-shrinking toolarge an area or causing metallurgical changes by heating to too high a temperature. Asa general rule, when correcting distortion in steels the temperature of the area should berestricted to appro&imately to 8:; - 86:;) - dull red heat.

If the heating is interrupted, or the heat lost, the operator must allow the metal to cooland then begin again.

#est practice for distortion correction b thermal heating

!he following should be adopted when using thermal techniques to remove distortion'

use spot heating to remove buckling in thin sheet structures

other than in spot heating of thin panels, use a wedge-shaped heating technique

use line heating to correct angular distortion in plate restrict the area of heating to avoid over-shrinking the component

limit the temperature to 8:; to 86:;) *dull red heat+ in steels to prevent

metallurgical damage in wedge heating, heat from the base to the ape& of the wedge, penetrate evenly

through the plate thickness and maintain an even temperature

Article prepared by 9ill 0ucas with help from athers.

)opyright ?/::5 !7I 0td

distorsion corrective techniques

Documents