Welding Distortion

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Teknologi Dan Rekayasa OXY-ACETYLENE WELDING

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Transcript of Welding Distortion

Page 1: Welding Distortion

Teknologi Dan Rekayasa

OXY-ACETYLENE WELDING

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DISTORTION DISTORTION

Objective :Objective :After studying this lesson, the students After studying this lesson, the students can be to identify the distortion.can be to identify the distortion.

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Distortion Caused Distortion Caused

If we heat a piece of metal, it gets larger, If we heat a piece of metal, it gets larger, and when cooled, it gets smaller. and when cooled, it gets smaller. Consider a cool piece of metal grasped Consider a cool piece of metal grasped lightly in a vice. If we heat this piece of lightly in a vice. If we heat this piece of metal, it expands in all directions except metal, it expands in all directions except the direction of the vice jaws.the direction of the vice jaws.

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Distortion Caused In a welded joint, distortion is classified as angular,

longitudinal or transverse.

Longitudinal distortion

Transverse distortion

Angular distortion

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Distortion Caused

Before welding

Angular distortion

90° 80° - 87°

After welding

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Distortion Caused Distortion Caused

Angular Distortion : reduces the angle Angular Distortion : reduces the angle between the plates.between the plates.

Longitudinal Distortion : causes shrinkage to Longitudinal Distortion : causes shrinkage to take place along the weld, bending the ends take place along the weld, bending the ends of the weldment upwardsof the weldment upwards

Transverse Distortion : the end of one plate Transverse Distortion : the end of one plate joins another continuing plate, and the joins another continuing plate, and the continuing plate bends or buckles in the continuing plate bends or buckles in the heated area.heated area.

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Distortion Caused Distortion Caused

Distortion caused by welding may be Distortion caused by welding may be minimised by :minimised by :1.1. Restraining the joint so it cannot moveRestraining the joint so it cannot move2.2. Positioning the plates to allow for Positioning the plates to allow for

shrinkageshrinkage3.3. Using heat to compensate for weld Using heat to compensate for weld

shrinkageshrinkage Steps to remedy distortion may be taken Steps to remedy distortion may be taken

before or after the weld is made. before or after the weld is made.

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Controlling Distortion Before Controlling Distortion Before WeldingWelding

1.1. Tack Welding Tack Welding The plates are “tacked” together with The plates are “tacked” together with small welds before complete welding of small welds before complete welding of the joint takes place.the joint takes place.Tack are usually at the ends of the joint, Tack are usually at the ends of the joint, and on long joints at points along the and on long joints at points along the joint about 50 mm apart.joint about 50 mm apart.

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Controlling Distortion Before Welding

Tack welding

“Tacks” of weld stop plates separating during welding

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Controlling Distortion Before Controlling Distortion Before WeldingWelding

2.2. OffsettingOffsettingThe plates are tacked together but are The plates are tacked together but are “offset”. That is to say, the angle between “offset”. That is to say, the angle between the plates where the joint is to take place the plates where the joint is to take place is a little larger than is required for the is a little larger than is required for the finished weld. The cooling weld metal finished weld. The cooling weld metal pulls the plates into their correct position.pulls the plates into their correct position.For example, two plates so be welded For example, two plates so be welded together at right angles would be offset so together at right angles would be offset so about 93° before commencement of the about 93° before commencement of the weld.weld.

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Controlling Distortion Before Welding

Offsetting. Coding weld draws plates together to produce the required 90° angle.

93°

Position of proposed weldment

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Controlling Distortion Before Controlling Distortion Before WeldingWelding

3.3. Jigs and FixturesJigs and FixturesVarious holding devices can be used to hold Various holding devices can be used to hold the work firmly so that it cannot move as the the work firmly so that it cannot move as the weld cools. G clamps and toggle clamps are weld cools. G clamps and toggle clamps are commonly used for this purpose.commonly used for this purpose.

4.4. Preheating Preheating Various methods of preheating are used to Various methods of preheating are used to combat distortion.combat distortion. Total preheatingTotal preheating Local preheatingLocal preheating

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Controlling Distortion Before Controlling Distortion Before WeldingWelding

5.5. Contra HeatingContra Heatingif a component is to be welded on one if a component is to be welded on one side, it may be heated on the other side side, it may be heated on the other side just before welding. This means that both just before welding. This means that both sides expand and contract at nearly the sides expand and contract at nearly the same rate, and distortion is minimised.same rate, and distortion is minimised.For example, if a branch is to be welded For example, if a branch is to be welded into a pipe, contra heating the opposite into a pipe, contra heating the opposite side of the main pipe will lessen side of the main pipe will lessen distortion. distortion.

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Controlling Distortion Before Welding Contra Heating

Weldment

Pipe is heated on opposite side to weld to straighten

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Controlling Distortion During Welding

1. Back step WeldingThe plates are tacked together but are not welded straight along from end to end. Instead, they are welded in short “stitches” each about 40 to 50 mm long. Each much finishes where the previous one started.

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Controlling Distortion During Welding

Back step Welding

Start first bead and weld to end

Start second bead and weld to beginning of first bead

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Controlling Distortion During Welding

2. Sequence WeldingA weld is made on one side, then another is made diagonally opposite to minimise uneven heating during welding.Sequence welding can be used on such things as joining pipes to plates, bosses to shafts, etc.

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Controlling Distortion During Welding

Weld first on one side – then diagonally opposite – then complete weld

Sequence Welding

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Controlling Distortion During Welding

3. Wedging or Tapered Gap WeldingPlates that are to be butt welded are positioned before welding with a tapering root gap.The root gap is the normal width where the weld is to start, but gets wider in a ratio varying between 1:25 and 1:50 (A pair of plates 50 cm long would have a root gap tapering from 12 mm to 2 mm).

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Controlling Distortion During Welding

Wedging or Tapered Gap Welding

Wedge is removed as weld progresses thus maintaining constant gap along weldment

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Controlling Distortion After Welding

1. Hammering or PeeningAs weldment cools, it shrink is hammered or peened during cooling, this will “stretch” the weld metal, tending to counteract the shrinkage.Peening is often used on the soft monel type deposits that are used to weld cast iron. Care should be taken, as excessive peening will cause the grain structure of the metal to break down and fracture.

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Controlling Distortion After Welding

2. Heat ShrinkageIf a flat plate is welded around its edge, these edge shrink and become shorter. However, the centre of the plate still unwelded has not shrank. This causes the centre of the plate to curve or form a dished configuration.

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THE ENDTHE ENDTHANK YOUTHANK YOU