metallurgy of the welded joint

1Welding Technology Module

IIS ProgressGruppo Istituto Italiano della Saldatura

Basics of welding metallurgy



Basic metallurgy

Amorphous microstructure Cubic microstructure



Body cubic centered microstructures



Face cubic centered microstructures



Hexagonal close packed crystal structure



BCC

CrMoNbWV

Fe, Fe Ti Zr

CFC

AlAgNiPbCu

Fe Co

HCP

MgSnZn

Ti Co Zr

Monomorphous metals

Polimorphous metals

Pure metals and allotropy



Metallic alloys

Insertional alloys Substitutional alloys



Delta iron, BCC (1535 -1390C)

Gamma iron, FCC (1390 -910C)

Beta iron BCC (910 -770C non-magnetic)

Alfa iron, BCC (910C -0K, magnetic)

Liquid

Fe

Fe

Fe

Fe 770C

1535C

910C

1390C

TC

Iron-base alloys (steels)



Iron-base alloys (steels): microstructures

Cementite (Fe3C)

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Welding Technology Module



Ferrite

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Austenite

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Pearlite

13




Martensite

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Iron-base alloys (steels): basic heat treatmentsAnnealing The steel is fully austenitized,

then slowly cooled up to room temperature

This treatment improves the ductility but reduces the tensile properties, the hardness and the fracture toughness (coarse grain)

Normalizing The steel is again fully

austenitized, but this time cooled up to room temperature with higher cooling rates

This treatment improves the tensile properties, the hardness and the fracture toughness, but reduces the ductility (grain refinement)

Quench The steel is once more fully austenitized,

then cooled very quickly up to room temperature

This treatment strongly improves the thetensile properties, the hardness but reduced the fracture toughness and the ductility

Tempering The steel is treated in the ferritic

temperature range, then cooled up to room temperature

This treatment improves the fracture toughness and reduces the peak hardness

It is typical of hardening steels, after a quench (sometimes after a normalizing)

During PWHT, promotes the weld stress relieving

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Factors influencing thethermal cycle:

heat input

thickness preheat temperature

Consequences of the heat treatment imposed by the welding thermal sources:

metallurgical structure of welded zone mechanical effects (stresses and

distortions)

Welding thermal cycle

60=vIVHI

T

E

M

P

E

R

A

T

U

R

E

TIME

Welding cycles (as a function of the distance to the weld axis)

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Heat distribution

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Metallurgical effects: structure of the welded joint

FUSED ZONE or WELD METAL (WM)

BASE MATERIAL

HEAT AFFECTED ZONE (HAZ)

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Weld metal composition

Dilution ratio (Rd), is used to evaluate chemical composition of the weld metal

100+= bab

d VVVR

Va+Vb

Vb

Examples of typical Dilution Ratio for different welding processes:

SMAW: First pass Rd=30% Fill passes Rd=10%

TIG: Rd=20-40% MIG/MAG:

First passes Rd=10-40% Fill passes Rd=5-20%

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Metallurgical structure of the weld metal

WELD DEPOSIT

HEAT SOURCE

HEAT FLOW

Welding direction

Welding directionThe final microstructure of a welded joint is influenced by several factors:

Thermal cycle severity (cooling speed)

t8/5 is assumed as the most significant parameter for low alloyed steels;

Heat input and number of passes strongly affect the grain growth in the weld metal

Number of the material allotropic transformations;

Grain dimension of the base metal.

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Metallurgical structure of the weld metal

Weld metal dendritic microstructure

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Heat Affected Zone (HAZ)The heat-affected zone includes those regions that are measurably influenced by the heat of the welding process:

For a plain carbon steel, the heat-affected zone may not include regions of the base metal heated to less than approximately 700C since the welding heat has little influence on those regions

In a hardened steel that has been quenched to martensite and tempered at 315C, any area heated above 315C during welding would be considered part of the heat affected zone

Heat-affected zones can be defined by a changes in microstructure close to the welded joint. The various effects of welding heat on the heat-affected zone, can be therefore considered in terms of four different types of alloys that may be welded:

1. Alloys strengthened by solid solution2. Alloys strengthened by cold work3. Alloys strengthened by precipitation hardening4. Alloys strengthened by transformation (martensite)

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C(%)T

(

C

)

Liquid

+ Fe3C

+

M

a

x

i

m

u

m

t

e

m

p

e

r

a

t

u

r

e

r

e

a

c

h

e

d

d

u

r

i

n

g

w

e

l

d

i

n

g

WM

HAZ

Grain coarsened zone

Tempered zone

Partly austenitized zone

Plain carbon steels

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Stainless steelsWelding influences the metallurgical behavior of stainless Cr-Ni steels:

- a grain coarsened region can be individuated

- Intergranular corrosion resistance of the HAZ can significantly be reduced (sensitizing)

More complex phenomena are involved in the HAZ of stainless chromium steels.

Sensitizedzone

18%

13%

Grain boundary

1 m

Chromium %

Tmax1300 850 400400 850 1300

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Aluminum alloys HAZ Softening

WELD METAL

HARDNESS

DISTACE FROM THE JOINT CENTERLINE

HEAT AFFECTED ZONE

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Origin of residual stresses and distortion During thermal welding, the

weld region is heated up strongly incomparison with the surroundingregion and is fused locally. Thematerial expands as a result of being heated.

The thermal expansion is restrainedby the colder surrounding region,thus leading to thermal stresses.

The thermal stresses partly exceed the yield limit which is lowered at elevated temperatures.

Consequently, the weld region is upset plastically and, after cooling-down, is too short, too narrow or too small in relation to the surrounding region. It thus displays tensile residual stresses while the surrounding region exhibits compressive residual stresses.

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Transversal shrinkage Depending on the position of

restraints, welding speed and heat input, the rotational distortion can result in opening or closing the of the finishing end of the joint

In the case of multipass welding, shrinkage is accumulated

Tack welding can reduce the distortion, but residual stresses are increased

600C

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Angular shrinkage Due to transversal shrinkage, also angular distortion is

provoked Joint welded from one side

T joints

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Longitudinal shrinkage The longitudinal shrinkage of

the weld during cooling, following longitudinal upsetting during heating, results in a longitudinal shortening of the component, notably in the weld zone. Where the weld is arranged eccentrically, this produces the unwanted bending deformation of girders and plates (bending distortion).

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Welding residual stresses

Welding transversal residual stresses Welding longitudinal residual stresses

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Multipass welding

Weld metalHAZ of each pass

During multipass welding: The total heat input is lower Every pass produces a heat

treatment effect on the previous passes

As a consequence, two important parameters need to be defined:

Preheat temperature Interpass temperature

metallurgy of the welded joint

Documents

room temperature

temperature consequences

typical of hardening

c beta iron bcc

c gamma iron

ferritic temperature

alfa iron

peak hardness