8-Welding Distortion & Control

transcript

WeldingDistortion Control

What is distortion ?

Undesirable change in Undesirable change in

original shape is called original shape is called

DISTORTIONDISTORTION

Before distortion

After distortion Distortion occurs due to heat input and

mechanical forces.

• Uniform heating of a steel bar through out of its entire volume -

considerable expansion take place in all direction.

• Now, if cooling of the bar is allowed evenly - retain its original

shape and size without distortion.

DURING HEATED CONDITION

X + 9X

BEFORE HEATING AND AFTER COOLING

Experiment No 1:

So, we can say that,

“ Uniform heating and cooling of a

component that can expand and

contract does not cause any

appreciable distortion ”

• Repeat experiment no:1 but heat the steel bar in

clamp condition and see the changes in shape and size

after cooling.

STEEL BAR AFTER HEATING & COOLING DOWN

STEEL BAR BEFORE HEATING

CLAMPING JAWS

Experiment No 2 :

RKS,HZW

So, we can conclude that,

• Restraint hinders free expansion and

contraction and causes material to

deform resulting in

Distortion

Gas cutting/heating welding

Heat input

HEATING Heated area expands

Expansion restrained by surrounding solid area

Compressive stresses are developed

Further compressive stress leads to plastic deformation

Material bulges at the spot towards heat source side

HEAT SOURCE

Distortion in case of spot heating?

COOLING

Spot area tends to contract.

Contraction restrained by surrounding hot area.

Material goes back to original position with plastic deformation.

Resulting distortion

Distortion in case of spot heating?

WELD BEAD

ORIGINAL POSITION

AFTER WELDING

Longitudinal distortion WELD BEAD

LONGITUDINALDISTORTION

LONGITUDINAL SHRINKAGE• (A) BUTT WELDS IN CS/LAS

LS = 3. I .L / 100,000 t

LS = longitudinal shrinkage (mm) I = welding current(amp) L = length of weld (mm) t = plate thickness (mm)

It is contraction along the length of weld bead

It is maximum along weld bead and decreases at points away from the bead.

In C/S of shell it lead to reduction in diameter at

Distortion in Butt welds

Longitudinal Distortion

EXAMPLE (LS IN BUTT WELDS)• Calculate LS for 6mm thick CS plate

welded by SMAW using 200 A current.

• Solution : LS = 3. 200. L / 100,000 x 6

= L/1000 mm

LONGITUDINAL SHRINKAGE

LS = 25 Aw/ Ap

Aw = Weld X-sectional area

Ap = Resisting X-sectional area

• (B) FILLET WELD

EXAMPLE OF LS IN FILLET WELD

All dimensions in mm

LS = 1.52 mm

It is the shrinkage perpendicular to the weld.

It leads to the development of high residual stress and

also cracking in case of highly restrained joint.

It is not uniform along the length of the plate

It is lesser at that end of plate where bead is started.

Distortion in Butt welds

Transverse Distortion

ORIGINAL POSITION

AFTER WELDING

Transverse distortion

WELD BEAD

WELD BEAD TRANSVERSE DISTORTION

TRANSVERSE SHRINKAGE IN SINGLE PASS BUTT JOINTS

S = 0.2 Aw / t + 0.05 d

S = Transverse Shrinkage (mm)

Aw = Cross sectional Area of Weld (mm2)

t = Thickness of Plates (mm)

d = Root Opening (mm)

TRANSVERSE SHRINKAGE DURING MULTIPASS WELDING

TS = TS0 + b (log w - log w0)

TS = Total Transverse Shrinkage

TS0 = Transverse Shrinkage after first pass

w = Total weight of weld metal

w0 = weight of first pass weld metal

b = a coefficient

Effect of Various Procedures on Transverse Shrinkage of Butt

WeldsProcedures

Root Gap

Joint design

Electrode dia.

Degree of constraint

Peening

Gouging & repairs

Effect on TSTS increases with increase in RGSingle Vee produces more TS thandouble VTS decreases with increase in electrodedia.TS decreases with Degree of constraint

TS decreases by peeningTS increases by these operations.

TRANSVERSE SHRINKAGE IN FILLET JOINTS

1. For T joints with two continuous fillets.

TS = Leg of fillet Weld (l) x 1.02

Bottom Plate thickness (tb)

All dimensions in mm.tb

2. For intermittent fillet welds , a correcting

factor of proportional length of fillet weld to

total length of joint should be used.

(3) For fillet welds in a lap joints between

plates of equal thickness (two welds)

TS = Leg of fillet Weld (l) x 1.52

Plate thickness (t)All dimensions in mm.

ORIGINAL POSITION

AFTER WELDING

Angular distortion

It is the bending transverse to the weld.

Due to non-uniform heating and cooling along

the thickness of plate.

This is the main source of mismatch and

dimensional inaccuracy in large welded

structures

Distortion in Butt weldsAngular Distortion

Angular Distortion in Butt Joints

g = 3 mmt3 = 2 mmt

t1 + 1/2 t3

t= 0.6

1. Use Both Side Welding Technique in place of Single Side Welding

AD = 0.0076 . W . l1.3

AD= Angular Distortion, mm

W=flange width, mm

l = weld leg length, mm

t = flange thickness, mm

Angular Distortion in Fillet Welds

RKS,HZW

Example of Angular Distortion in Fillet Welds

Find the angular distortion in a double fillet weld of a T-joint between a flange 1000 mm wide and a vertical member when the thickness of both the members is 6 mm and the weld leg length = 8 mm

Solution. AD = =

0.0076 x 1000 x (8)1.3

(6)23.15 mm.

Multiple Restrained Fillet Welds

AD = Angular distortion, mm.L = span length, mm.Ø = angular change, radians

x = distance from weld to the point where distortion is to be determined, mm.

RKS,HZW

Example of AD in Multiple Restrained Fillet Welds

In multiple restrained fillet welds the span length is 1 m and the angular change is 90 at a distance of 400 mm from the span end, find the distortion.

Solution.

By putting L = 1000 mm, Ø = 90 = 0.1571 rad.

x = L/2 - 400 = 100 mm in the Formula,

AD = 14.164 mm.

Distortion in ‘T’-jointsAngular distortion

Before welding After welding

Distortion in ‘T’-jointsLongitudinal distortion(a) pulling effect towards neutral axis

ASection A - A

Distortion in ‘T’-StiffenerLongitudinal distortion(b) pulling effect of welds above neutral axis.

Section A - A A

To prevent distortion :-

Reduce the effective shrinkage force.

Reduce effective shrinkage force (A-1) Keep the angle of weld joint to the

barest minimum.

keep the angle of weld joint 45 deg.

MINIMUM ANGLE, LESS WELDING , LESS HEAT INPUT

Hence less distortion

50 deg. +/- 5 deg.

keep the angle of weld joint 45 deg.

keep fillet size 18 mm/6 mm

19 mm +3/-1

7 mm +3/-1

(A-4) Minimize no of passes larger size of electrodes

Reduce effective shrinkage force

MORE NO OF PASSES LESS NO OF PASSES

(A-5) Place welds near the neutral axis

Reduce effective shrinkage force

Make shrinkage work for us

CLAMPS ALONG EDGE

Make shrinkage work for us

(B.1) Pre cambering OR Pre bending in plate

Make shrinkage work for us(B-2) Keep over dimensions OR over

bend before welding

Balance shrinkage force with other forces

Do Sequence welding

Balance shrinkage forces with other forces.

Balance shrinkage forces with other forces

(C-2) Back step welding

1 2 3 4

Welding progresion

Two identical parts should be tacked back to back together before welding as shown

PART -II

PART -IEND PLATES TACKED

(C-3) Back to back clamping for welding

Balance shrinkage forces with other forces

SADDLE-I

SADDLE-IIWELDING TACKS

Back to back welding of saddles

Good working

methods for

welding distortion

in our routine work

Bulging of tube sheet of heat exchanger

TUBE SHEET BULGES DURING SHELL TO TUBE SHEET WELDING

• Welding of shell to tube sheet

LEADS TO

• Improper seating of gasket and leakage

• Non uniform projection of tube ends from tube sheet face

CONTROLLED BY • Back to back

• Weld optimum fillet size

TUBE SHEET

Distortion of shell long seams Typical weld sequence and distortion observed

58T MIN LAS.

INSIDE

OUTSIDE

6000.2mm GAP

JOINT DETAIL WELD SEQUENCE

BACKGOUGING

Distortion of shell long seams

1 2 31 SET-UP STAGE 4 + 2 + 2 +2 AFTER SEAL RUN 6 + 4 + 5 +3 AFTE R O/S WELDING 8 + 6 + 8 +4 AFTER BACK GOUGING 6 + 5 + 5 +5 AFTER I/S WELDING 4 + 2.5 + 4 +

LOCATION STAGE

( D/4 TEMPLATE READING

• Caused by longitudinal shrinkage of weld

• Reduction in diameter around circumferential seam

• Reduction in shell length

Controlled by • Provide compression spiders on both sides of C/S• Design weld joint to have minimum weld metal deposit • Use restricted heat input ( minimum no. of passes )

CIRCSEAM JIINT SHELL

SUGARCANE EFFECT

Distortion of circumferential seams in shell

Gauge for checking long seam distortion in plate stage welding

PICK IN OR PICK OUT = A-B OR C-D

(MAXIMUM DIFFERENCE TO BE CONSIDERED)

GAUGE FOR CHECKING

Gauge for checking distortion of ‘T’- joint welding

PRE-TILT OF T-STIFFENER = A - C

SAGGING OF T-STIFFENER = A - B

GAUGE FOR CHECKING

Distortion in flange to pipe welding

FLANGE

BEFORE WELDING

AFTER WELDING

FLANGE WARPS

FLANGE FLANGE

PIPE PIPE

TEMP. SUPPORTS

• Heavy fillet weld on flange to pipe joint leads to warping of flange

• Causing no machining allowance on flange face thickness CONTROL : back to back welding

• Temporary set up two flanges back to back as shown

Sinking in of nozzle on shell Controlling sinking

• Provide rigid internal jacks

/supports with moon plates

/compression spider

• Maintain optimum weld

preparation and fit up to avoid

extra weld deposit

• Keep excess nozzle projection

at set up stage to compensate

for sinking

STRIP CUTTING FROM PLATE

The strip tends to bow outwards as shown

Distortion ( bow ) results due to unequal heating of

the metal

During cutting when hot, the bow is more on cooling

& the bow diminishes slightly

Finally the strip never returns to it’s intended shape

Distortion During Oxy-acetylene Cutting

Controlling distortion during oxy-acetylene cutting

METHOD I

Two Torches Technique

• Mark strip of required width leaving 10 mm distance

• Move two torches simultaneously carrying out cutting operation

SCRAP 10 mm

TORCH I

TORCH II

DIRECTION OF MOVEMENT FOR TORCHES

Controlling distortion during oxy - acetylene cutting

Method II

• Mark the strips with kerf allowance on the plate

• Drill small hole in kerf allowance at distance 20 mm away from the edge

• Start cut from drilled hole in kerf to the end such that the strip is attached to main plate

• Cut the balance strip attached to the plate

STRIPS

Controlling distortion during oxy - acetylene cutting

Aim : To get undistorted segment from the plate of size as shownSpecific Steps • Mark leaving 30mm Dist. from edge• Start with pierce cut as shown instead of starting from the edge • Follow the path as shown

12 mm THK PLATE

50 mmPIERCE START

R250 mm

Reduction in distortion• Less weld edge preparation.

• Less welding current as per WPS.

• Higher base metal thickness.

• Lesser welding passes

• Do not over weld

• More distortion in stainless steel then carbon

steel.

• Less offset-Lesser welding-Lower distortion

• Provide intermittent welding

• Place weld near the neutral axis

• Balancing weld around neutral axis

• Back-step welding

• Sequence welding

• Pre bending OR Pre cambering

• Back to back clamping

• Double operator welding technique

Reduction in distortion

8-Welding Distortion & Control

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