20053429

8/18/2019 20053429

http://slidepdf.com/reader/full/20053429 1/27

~

i

Welding Metallurgy

istortion

Residual Stress _

Post Weld Heat

Treatment

cture 10

_ ~ ~

8/18/2019 20053429


ecture Sc ope

• Typical patt€ rns

distortion residual stress in

welded assemblies

• Effects residual stress

• Methods for prevention and correction

distortion

• Heat treatmemt steel weldments

-

8/18/2019 20053429


r

I

~

Residual Stress efinitions

• Residual stresses are the stresses that exist in a structure

in the absence

external loads

• Residual stresses can be produced metal structures by

many processes in luding casting heat treatmenf

forming and bending and thermal cutting

• Welding produces residual stress and distortion as a

result localised heating and cooling the work material

_ _ _ _

_ _ _ _ _

ecture 10 P 3

8/18/2019 20053429


auses

esidual stress

• During welding the weld and HAZ are heated to

temperatures

w f ~

above those the surrounding material

• The weld and HAZ deform plastically because their

thermal expansion is resisted by the surrounding material

• As the weld cools and contracts tensile stresses develop

elastically

• Welds contain tensile stresses that approach the yield

stress.

p4

8/18/2019 20053429


8/18/2019 20053429


xial Residual Stress attern

The tensile stress in the

weld is balanced by

compressive stresses

o

lower magnitude in the

surrounding plate

L

t

Axial

Stress

8/18/2019 20053429


Transverse Stresses utt oint

x

y

y

tension compression

x

Axial stress

Distribution o y stress

component along x x

Lecture 1

_ _

_

_ _ _ _ ~

p7

8/18/2019 20053429


Pipe irth Weld isplacements

_ · _ _ · ~

I

Tensile hoop

stress

Displaced sh p due to

circumferential shrinkage

strains exaggerated

_ _

p

8/18/2019 20053429


Typical Pipe irth Weld Stresses

Longitudinal Stresses

Circumferential St[esses

6

6

20

e uter Surface

e nner Surface

e

ueer

Surface

e

nner

Surface

-20

4

lO

4 6 8

1

4

6 8

1

Distmce

from weld

CL

in

Distance from

weld

CL in

Lecture

8/18/2019 20053429


i

estraint tresses

• When the workpiece is free to expand or

contract residual stresses are confined to the -

region of t ~ weld

• When the workpiece is restrained e.g.

between rigid anchors long range reaction

stresses develop

P

8/18/2019 20053429


estraint tresses

: :

support

reaction

Rigid

support

Rigid

sU rt

~ I ~

support

reaction

Unwelded

Free

Restrained

Examples:

• a short run pipe welded between a rigidly mounted pump and a vessel

• a beam welded between two sti ff columns

ure

P

8/18/2019 20053429


ffects

R esidual Stress

• Residual stresses do not affect the load

carrying capacity

of

ductile materials

th t

fail

by yielding

• Residual stresses may promote failure

mechanisms that are sensitive to localized

stresses

f tigue

brittle fracture

stress corrosion cracking

creep cracking

P

8/18/2019 20053429


~ ~ .

ontroll

esidual Stress

• Use minimum required weld size and heat input

U prE parations give smaller weld areas

• Minimise constraint during welding

• Stress re lief

Heat treatment

Mechanical e.g. vibratory

~ ~

_

P

3

8/18/2019 20053429


stort on

• Distortion in welded fabrications is caused by

movements to accommodate thermal stresses

• Distortion in welded fabrications consists of:

- transverse shrinkage

-longitudinal shrinkage

- angular rotations

• Distortion is

problem mostly with thinner

materials

.._

_

P

8/18/2019 20053429


~ ~

owing

ypical distortion pattern

e x a g g e r a ~ ~ d < ~ ~ ~ ~ k a g e _

0.1 of

length _

Transverse

t

hrinkage

1 5 3

mm

1..---=_ _=-= :_= _ J

1

ngular

Rotation

1_3°

10 P

8/18/2019 20053429


Angular Rotation

illet Weld istortion

~ N e u t r a I x i ~ S

2

Bending caused

weld shrinkage offset

from neutral axis

section

P 6

8/18/2019 20053429


r ·· ·· ·· ·

eaking tiffened anel

Examples: bridge deck ship hull

= = ~ = = = L = = ~ =

_ _ _

_

..

_ _ _ _ _

_

_ _ _ _

...

_ _

_ _

cture 10

pH

8/18/2019 20053429


ontrol istortion

Minimise heat input

- Maximise melting efficiency, i.e. welding processes with high energy density

2

Use minimum required weld sizes,

- e.g. intermittent rather than continuous fillet welds, or U preparations

3

Balance welds about neutral axis

- e.g. double V preparation, simultaneously weld top and bottom

plate girder

4

Use clamps, jigs

fixtures

5 Preset the workpiece to compensate for anticipated

changes, for instance:

FilletWelds

P 18

8/18/2019 20053429


ontrol distortion

• Avoid using welding for precision assembly

• achine

to final dimensions after welding

__ ....ll

« « «

~ i l l t welds

c <.<.<.<.<.<.<.<.

machine bore to final

dimensions after welding

1

P 19

8/18/2019 20053429


8/18/2019 20053429


l

Correction

distortion

• Mechanical straightening

~

~ ~ ~ ~

~

~ ~ _ _ ~ ~

~ ~ ~ ~

1

p21

8/18/2019 20053429


at

Treatment Welds

• eat treatment

is costly

nd should be

voided unless necessary

for

s tisf ctory

performance

• ay e

required y pplic le codes

and

st nd rds

_ _

8/18/2019 20053429


Weld

H E ~ a t

Treatments

• reheating

- Heating prior to welding usually to t m p r t u r ~ less

than 200C

- Applied in welding C-Mn steels to decrease cooling

rates and reduce HAZ hardness

- <'O ''''; ,,,,, I\I r '1llirAr

nr

: t ~ i o l e s s steels. nickel

alloys titanium zirconium or aluminum

• ost iel eat Treatment

- Heating after welding to relieve stresses refine weld

grain structure or improve weld properties

I

1 _

Lecture 10

p

8/18/2019 20053429


ost eld I eat re tment

• efinitions

- Stress Relief

• Heating to a tl3mperature usually around 600C at

which the yield strength is reduced such that residual

stresses are relieved

- Normalizing

• Heating to above the temperature for transformation to

austenite A3 and slow cooling for to refine and

homogenize the grain structure

Quenching

• Heating as in normalizing and rapid cooling in water,

brine or air to develop high strength.

p 4

8/18/2019 20053429


~ ~

ost ~ l d eat Treatment

• Definitions cont d)

- Tempering .

Reheating after quenching to below the transformation

temperature to reduce hardness and improve .ductility

- Solution Treating

Heating to take into solution elements which will be

precipitated later in a controlled manner to produce

the desired properties

geing

Reheslting after solution treating to allow formation

of

preciplitates which strengthen the material.

~

~

cture 10 p

8/18/2019 20053429


PW T ethods

•

urnace eating

- Gas

or

electric most common

- Restricted to parts that can be contained in a furnace

- Optimum efficielncy and control

• Local

e a t i n ~

- Gas burners or electric resistance heaters most common

- Mostly limited to relatively simple treatments typically

stress relief o 8teel weldments

- Main advantagl3 is capability to treat large or immovable

structures

t

low cost.

p26

8/18/2019 20053429


I

,

PW T ffects on Steels

Stress re efreduces residual stress and tempers

hardened

heat

affected zones. In

most

grades

of

steel

but notall it improves ductility and toughness.

orm lizing is used to refine the grain structure r ~ s u l t n

from welding

when

optimum properties

are

required, e.g.

electrosla£1 welded pressure vessels

Quench Temper omesteels require Q T treatments

to

develop

high strength. Sometimes

such

steels can

be

welded

as··quenched followed by a combined

temperlstr1ass relief treatment.

-

The

metallurgical aspects

of

welding

Q

T steels

must be

carefully assessed.

- - - - - - - - - - - -

_._

_ . _ - . _ - - - - - - - - - - - - .

20053429

Documents