As mentioned by Mr. Suresh there are a lot of steps which can be taken
for controlling distortion.
The peening method is one of the most valuable methods when applying
correctly, and for the best result you should combine this with
clamping and back-step welding.
I have executed several projects (one was a repair of a heat exchanger
hull where max. 1.5 mm distortion was allowed) where this combination
is used. The peening in these cases is executed with patented so
called high frequency hammering processes like Pneumatic Impact
Treatment (PIT). With ordinary hammering you will not get the same
result is my experience.
We also apply PIT successfully for repair procedures at deck-plates of
bridges when placing insert plates at fatigue damage locations. Here
also minimum shrinkage is allowed in order to provide arising of new
fatigue cracks on short time base.
Best Regards,
Herman Pieper
On 29 jan, 17:28, Suresh <bobsures...@gmail.com> wrote:
> Hii Gents ,
>
> First, let's discuss why distortion occurs. Weld metal is deposited at a
> high temperature, above the melting point of material. For steel, this is
> around 2,500°F (1,370°C). As the weld cools to room temperature, it shrinks,
> but is restrained from doing so by the adjacent cold base metal, resulting
> in high-residual tensile stress. The weld is now like a stretched rubber
> band, with the workpiece holding the ends. This is the reason that the base
> metal moves, or springs back, when the clamps holding the workpiece are
> removed, distorting the part.
>
> When the weld shrinks across its width, it causes groove welds to "wing-up"
> or fillet welds to close up. When the weld shrinks along its length, it
> causes base metal to twist around the weld.
>
> To minimize weld distortion, design and welding should be addressed. Weld
> shrinkage cannot be prevented, but it can be controlled. These are
> recommended steps for minimizing weld distortion:
>
> 1.
>
> *Avoid overwelding* — The bigger the weld, the greater the shrinkage.
> Correctly sizing a weld not only minimizes distortion, but also saves weld
> metal and time.
> 2.
>
> *Intermittent welding* — To minimize the amount of weld metal, use
> intermittent welds instead of continuous welds where possible.
> 3.
>
> *Fewer weld passes* — A fewer number of big passes results in less
> distortion than a greater number of small passes with small electrodes.
> Shrinkage accumulates from each weld pass.
> 4.
>
> *Place welds near the neutral axis, or the center of the part* —
> Distortion is reduced by providing less leverage for the shrinkage forces to
> pull the plates out of alignment.
> 5.
>
> *Balance welds around the neutral axis* — Welding on both sides of the
> plate offsets one shrinkage force with another, to minimize distortion.
> 6.
>
> *Use the backstep welding technique* — In the backstep technique, the
> general progression of welding may be left to right, but each bead segment
> is deposited from right to left. As each bead segment is placed, the heated
> edges expand, which temporarily separates the plates at B. As the heat moves
> out across the plate to C, expansion along outer edges CD brings the plates
> back together. This separation is most pronounced as the first bead. With
> successive beads, the plates expand less and less because of the restraint
> from the prior welds.
> 7.
>
> *Presetting the parts* — Presetting parts before welding can make
> shrinkage work for you. The required amount of preset can be determined from
> a few trial welds.
> 8.
>
> *Alternate the welding sequence* — A well-planned welding sequence
> involves placing weld metal at different points of the assembly so that, as
> the structure shrinks in one place, it counteracts the shrinkage forces of
> welds already made. An example of this is welding alternately on both sides
> of the neutral axis in making a complete joint penetration groove weld in a
> butt joint.
> 9.
>
> *Clamping* — Clamps, jigs, and fixtures that lock parts into a desired
> position and hold them until welding is finished are probably the most
> widely used means for controlling distortion in small assemblies or
> components. While there is some movement or distortion after the welded part
> is removed from the jig or clamps, it will be lower compared to the amount
> of movement that would occur if no restraint were used during welding.
> 10.
>
> *Peening* — Peening the weld bead stretches it and relieves the residual
> stresses. However, peening must be used with care. For example, a root bead
> should never be peened, because of the increased risk of concealing or
> causing crack. Also, peening is not permitted on the final pass, because it
> can cover a crack and interfere with visual inspection. Before peening is
> used on a job, engineering approval should be obtained.
> 11.
>
> *Thermal stress relieving* — Another method for removing shrinkage forces
> is thermal stress relieving, i.e., controlled heating of the weldment to an
> elevated temperature, followed by controlled cooling.
> 12.
>
> *I hope this information usefull for you ........................*
>
> --------------------------------------------------------------------------- --------------------------------------------------------------------------- -------------------------------------------
> On Sat, Jan 29, 2011 at 10:21 AM, yusufkhan pathan
> <yusufkhan...@gmail.com>wrote:
>
>
>
>
>
>
>
>
>
> > Dear All,
>
> > Please suggest me any information related to distortion control in
> > Stainless Steel welding. usually while welding lower thickness (bet 4 to 8
> > mm).
>
> > Thanks in advance.
>
> > Regards.
> > Yusufkhan.Pathan
> > 8008804165
>
> > --
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>
> --
> Thanks & Best Regards,
>
> Suresh
>
> Mobile No: 91-92 92 905 905
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