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Metallurgy of the Welded Joint
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1Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Basics of welding metallurgy
2Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Basic metallurgy
Amorphous microstructure Cubic microstructure
3Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Body cubic centered microstructures
4Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Face cubic centered microstructures
5Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Hexagonal close packed crystal structure
6Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
BCC
CrMoNbWV
Fe, Fe Ti Zr
CFC
AlAgNiPbCu
Fe Co
HCP
MgSnZn
Ti Co Zr
Monomorphous metals
Polimorphous metals
Pure metals and allotropy
7Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Metallic alloys
Insertional alloys Substitutional alloys
8Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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)
9Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Iron-base alloys (steels): microstructures
Cementite (Fe3C)
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Iron-base alloys (steels): microstructures
Ferrite
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Iron-base alloys (steels): microstructures
Austenite
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Iron-base alloys (steels): microstructures
Pearlite
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Iron-base alloys (steels): microstructures
Martensite
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Heat distribution
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Metallurgical effects: structure of the welded joint
FUSED ZONE or WELD METAL (WM)
BASE MATERIAL
HEAT AFFECTED ZONE (HAZ)
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Metallurgical structure of the weld metal
Weld metal dendritic microstructure
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Aluminum alloys HAZ Softening
WELD METAL
HARDNESS
DISTACE FROM THE JOINT CENTERLINE
HEAT AFFECTED ZONE
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Angular shrinkage Due to transversal shrinkage, also angular distortion is
provoked Joint welded from one side
T joints
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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 Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
Welding residual stresses
Welding transversal residual stresses Welding longitudinal residual stresses
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Welding Technology Module
IIS ProgressGruppo Istituto Italiano della Saldatura
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