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WELDING METALLURGY
Welding of Steels
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High peak temperatures
High temperature gradients
Rapid heating and cooling
In welding the reactions take place within seconds in a small
volume of metal, characterized by.
HEAT FLOW IN WELDING
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Concept of moving heat sources
Most welding processes, heat source
not stationary, moves at constant
speed along a straight line
Power output constant with time
Consequence: Fused zone and
heat-affected zone(s) of constant width
Quasi-stationary heat source
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THE TRANSFER OF HEAT in the weldment isgoverned primarily by the time-dependent conduction of heat,
x = coordinate in welding direction, mm
y= coordinate transverse to weld, mm
z= coordinate normal to weldment surface, mmT = the temperature in the weldment, C
k(T) = thermal conductivity of the metal,J/mm-s-0C
p = density of the metal, g/mm3
C = specific heat of the metal, J/g' C
Q = rate of internal heat generation, W/mm3
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Rosenthals analysis of heat flow in welding
- Moving Heat Sources
Heat flow in a work piece of sufficient length is steady or quasi-
stationary, w.r.t the moving heat source except for the initial and final
transients of welding
Consider a schematic of stationary work piece.
i.e the temperature distribution and the pool geometry do not change with
time for an observer moving with the heat source
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Rosenthal's Two-Dimensional Equation
To calculate the temperature T(x, y) at any location in the workpiece (x, y)
with respect to the moving heat source
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Rosenthal's Three-Dimensional Equation
This eq. implies that on the transverse cross section of the weld all
isotherms, including the fusion boundary and the outer boundary of the
heat-affected zone, are semicircular in shape
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By converting distancexinto time tthrough t = (x - 0)/V one can get the
Temp.-Time curve i.e THERMAL CYCLE
material: 1018 steel.
Welding speed: 2.4mm/s;heat input: 3200 W:
Calculated results from Rosenthal's three-dimensional heat flow equation..
Thermal cycles
Isotherms
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Adams' Equations..for calculating peak temp.( Tp) adjacent to HA
Two -dimensional heat flow
For three-dimensional heat flow.
Adams' Equations differs from Rosenthal's by
As Rosenthal's Equations measures the temp. at any place in w/p w.r.tmoving heat source while Adams' Equations gives the peak temp.( Tp) at
any dist from the fusion boundary adjacent to HAZ
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COOLING RATES
Rthe cooling rate at the weld center line, C/s
kthermal conductivity of the metal, J/mms- C
To = the initial plate temperature
Tc
the temperature at which the cooling rate is calculated, C
h = thickness of the base metal, mm
= density of base metal, g/mm3
C = specific heat of base metal, J/g C
C = volumetric specific heat, 0.0044 j/mm3-C for steels
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Cooling rates in various processes
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Process Relative arc efficiency(%)
SMAW 65 - 85 (1.0, accg. To BS)GMAW 65 - 85 (1.0)
Cored wire 65 - 85 (1.0)
GTAW (dcen) 50 - 80 ( 0.8)(ac) 20 - 50
SAW 80 - 99 (1.25)
EBW 8095Laser W 0.5 - 70
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Cooling rate and welding conditions
Two-dimensional and three-dimensional cooling
Three-dimensional (thick plates,
requiring 6 or more passes) :
Cooling rate CR = 2k (TcT0)2 H , where
k = Thermal conductivityT0 = Initialplate temperature
Tc = Temp. at which CR is calculated
H = Arc energy
Note: Underwater welding Wet: very rapid cooling
Dry, hyperbaric welding: Also high CR
(conductivity of compressed atmosphere)
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Two-dimensional (thin plates,
requiring fewer than 4 passes) :
Cooling rate CR = 2kC (h/H)2 (TcT0)3 , where
h = Base metal thickness (Combined thickness)
= Density of base metal
C = Specific heat of base metal
Note: CR as section thickness , at constant heat input
Single and two-pass welds : Heat input proportional to h,so effectively CR is independent of thickness (approx.)
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Number of heat flow paths also to be considered
3 for fillet welds and 2 for butt welds
Combined thickness = Total thickness of those paths providing
the heat flow paths
Varying thickness : Averaged for a distance of
75 mm from weld line
Current practice : Cooling time between two temperatures
Common : t8-5
(between 800 and 5000C range of
transformation in most C- and C-Mn steels)
t 3-1 (Time for H to diffuse out of weld area)
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Weld thermal cycles
Max. temperature (and cooling rate)decreases on going away from weld metal
Faster heating Faster cooling
Higher heat input [(V x I)/s in arc welding)]
Slower cooling
Thick sections cool faster than thin ones
Fillet welds cool faster than butt welds
Preheating reduces cooling rate
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The iron-carbon phase diagram
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Iron-carbon diagram
Steels (< 2% C) and cast irons
Low-C, medium-C and high-C steels
Cooling of 0.8% C and 0.2% C steels
from liquid state to room temperature
Structural constituents at room temperature, lever rule
Effect of increasing carbon content
Significance of equilibrium
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Lamellar structure of pearlite
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Ferrite-pearlite microsturcture of medium-C steel
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Low carbon steel
(ferrite and pearlite)
Ferrite
in nearly pure iron
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Constitution of hypo-eutectoid steels
% C Ferrite:Pearlite Ferrite: Cementite
0.2
0.4
0.6
0.8
75:25
50:50
25:75
0:100
97:3
94:6
91:9
88:12
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Effects of rapid cooling
The eutectoid reaction : + Fe3C
Change in crystal structure and composition,
necessity for atom motion diffusion,
time requirement, slow cooling
Rapid cooling:
Lowering of transformation temperatures
Decrease of pearlite interlamellar spacing ( in H, strength)
Occurrence of other transformation types
Austenite to bainite transformation
Austenite to martensite transformation,
Ms and Mf temperatures
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Transformations in steel as a function of cooling rate
Austenite
Martensite
( single phase) Bainite Fine Pearlite
Coarse Pearlite
Cooling rate (CR) increases
Hardness and strength progressively
increase as CR increases
Temperature
Time
Mixtures of ferrite and iron carbide
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Properties of martensite
No composition change during martensitic
transformation, thus supersaturated
with carbon, high hardness and brittleness
Increasing carbon Increasing
hardness and brittleness
Increasing alloy content only marginal
effect on properties
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TTT and CCT diagrams TTT and CCT diagrams vary with steel composition
Alloying elements (and carbon) slow down
pearlitic and bainitic reactions,
shift CCT diagram to the right,
reduce critical cooling rate,martensite forms even on slower cooling,
more martensite forms under givencooling rate
Most alloying elements lower Ms & Mf temps
Concept of hardenability, industrial significance
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Isothermal transformation diagram for eutectoid steel
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CCT diagram for eutectoid steel
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CCT diagram and RT microstructures in eutectoid steel
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CCT diagram and RT microstructures
in medium-C low-alloy steel (4340)
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Tempering of martensite
Need to temper
Changes in structure and properties
Selection of tempering temperature
and time
Quenched and tempered steels, alsonormalized and tempered steels
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Hardness of plain-carbon steel
in various microstructural conditions
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Problem of cold cracking
Cracking due to welding stresses
acting on brittle microstructure,
e.g., martensite
Contributing factors
Residual stress (tensile!)
Martensite
Hydrogen
Terminology : Cold, underbead, hydrogen-
induced, or delayed cracking
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Underbead crack in low-alloy steel weldment
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Microstructures across the weld
Impose weld thermal cycle (i.e., cooling curve)on CCT diagram
Thus different microstructures under
different welding conditions
Possibility of undesirable microstructures,
especially martensite
Danger of cracking due to martensite
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Tendency to martensite formation
Depends on intersection of weld cooling curvewith CCT diagram of the steel
More the martensite formed, greater the danger of cracking
To modify microstructure, shift intersection Change composition (Lower the %C, alloy content)
Reduce cooling rate (Preheat, heat input control)
l
Weld metal : Both options
HAZ : Only cooling rate option
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Carbon Equivalent
Tendency of a HAZ to develop a hard microstructure(with a particular hardness)
under a particular cooling regime
can be related to a single compositional parameter
carbon equivalent
CE(IIW) =
C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
CE(IIW) < 0.42easy to weld w/o cracking
CE(IIW) > 0.5difficult to weld w/o cracking
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Need to recognise composition limits for
valid application of CE formula
Compare : Steel A (%) Steel B (%)
C 0.15 0.45
Mn 0.60 0.60
Cr 2.25, Mo 1.0, V 0.25 UnalloyedCE (IIW) 0.95 0.55
Compare also low-C, high-Mn steel
with higher-C, lower-Mn steel
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Carbon EquivalentSeveral other CE formulae also proposed:
CE(AWS): C + Mn/6 + Cr/5 + Mo/4 + Ni/15 +Cu/13 [Notice close similarity to CE(IIW)]
Ito and Bessyo (Japan):Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60
+ Mo/15 + V/10 + 5 B (Note importance of B)
Dren: CEq = C + Si/25 + (Mn+ Cu)/16+ Ni/40 + Cr/10 + Mo/15 + V/10
Note greater emphasis on C itselfThe latter two especially useful for low-C steels
(many modern steels, e.g., pipeline steels),for which CE (IIW) is not entirely suitable
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Carbon Equivalent
CE (IIW) - developed in the late 1960s -
and based on work from 1940 -
originally hardenability formula,
now used as hydrogen cracking formula
(Si ignored in formula, but affects hardenability same way asMn, but Si does not increase cracking tendency, unlike Mn)
(CE(IIW) cannot be used to find HAZ hardness of single-passweld containing Si!)
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Carbon EquivalentEmpirical formulae relating CE(IIW) to hardness and yield
strength
Applicability of CE to be modified by
1) Inclusion content
(Stray instances, e.g., low-S steel showed HIC,
but not similar steel with high Ssulphide inclusions nucleate ferrite at higher temperature
more crack-resistant than lower-temperature products)
2) Segregation, especially in concast plateshigher %C and alloying elements at centreline,
greater cracking tendency there
3) High scrap casts can have higher alloy content
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General Strategies to avoid HIC
Direct control of hydrogen level
Control of microstructure by
controlling cooling rate Temperature control
Microstructure control through
isothermal transformation Use of austenitic steel or
Ni base consumables
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Direct control of hydrogen level
Use of low hydogen consumables
Necessity for using basic fluxes !
Merit of gas shielded welding
SMAW electrodes, SAW fluxes to be
carefully stored (warm storing) and
baked up to 4500C
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Direct control of hyd. level (contd.)
Very low hyd. electrodes : Danger in hot,humid climates, hyperbaric chambers
Cored wire brands : Moisture pick up if reelskept on machines unprotected for long
(in humid conditions)
Parent steel cleanliness : Rust, oil, grease,
paint, even innate hydrogen
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Further lowering of hydrogen levels
Development of higher-strength steelsreduced tolerance for hydrogen
Enables reduction of preheat / interpass / postheat
temperature controls
Thus incentive for lowering benchmark hydrogen level
3 mL / 100 g weld metal in SMAW
2 mL / 100 g weld metal in FCAW
Several recent developments
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Hydrogen reduction strategies
Modifications in arc chemistry
1) Increasing slag basicity (B)
B = CaO + MgO + BaO + K2O + Li2O + CaF2 + 0.5(MnO+FeO)---------------------------------------------------------------------------------------------------------------
SiO2 + 0.5 (Al2O3 + TiO2 + ZrO2)
As B , weld metal oxygen level , also hydrogen level
(E.g., As B from 0 to 3, HD from 12 to 2 mL / 100 g)
Reason : Complex interaction based on water vapour solubility orhydroxyl capacity of the slag
Higher basicity higher hydroxyl capacity Lower HD level
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Also, dissociation of CaCO3 contributes additionally
to reduction of HD
( CaCO3 CaO + CO2, 2 CO2 2CO + O2
This in oxygen level in arc atmosphere suppresses
the moisture decomposition reaction H2O 2H + O )
Excess CaCO3 counter-productive, for complex reasons
Note also any such excess can adversely affect,
e.g., in SMAW, operational characteristics like arc stability,
arc forces, weld pool viscosity, weld bead shape, etc.
Hence optimal level of additions necessary
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2) Addition of fluorides to flux
a) Fluorine-containing compounds hydrogen content in weldmetal : F2 + H2 2 HF
HF insoluble in liquid iron (weld metal),
so escapes into the atmosphere,Thus, hydrogen availability reduced
Fluorine provided by adding fluorides, e.g., fluorspar (CaF2)
b) Also, if silica is present, CaF2 reacts with SiO2 to form SiF4 :2 CaF2 + SiO2 SiF4 (g) + 2 CaO
SiF4 provides shielding and hydrogen partial pressure
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c) Addition of CaF2 also slag basicity
and hydrogen level
CaF2 decomposes poorly in the arc, hence other fluorides alsotried : NaF, KF, K2SiF6, Na3AlF6, K2TiF6 , etc.
These dissociate more easily, so are more effective
than CaF2 in reducing hydrogen level
Proportion of additions to be optimized :
Too high an amount HD again
Excess CaF2 decreases arc stabilityExcess K2SiF6 and K2TiF6 slag basicity
Other more complex factors
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3) Concept and use of hydrogen traps
Hydrogen as solute in lattice and also segregated incrystal defects and second-phase particles
Mean residence time longer in these particles
than as solute hydrogentrapping
Specific rare earth and transition metal additions
compounds such as Ce2O3, TiC, Y2O3, etc. with
high binding energy (i.e., high affinity) for hydrogen
Addition of 1600 ppm (0.16 %) Y
HD reduces to 1-2 mL/100g
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Retained austenite (RA) as hydrogen trap
High solubility, but low diffusivity for hydrogenin austenite exact opposite in surroundingferrite, bainite or martensite thustrapping effect experimentally demonstrated
Caution:RA could transform to martensite on drop inservice temperature, high HD in martensite Embrittlement
Tailor RA content to % H pick-up &service conditions (especially temperature)
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Recent example: (Weld.J.: June 2007, 170-s-178-s)
Possibility of HIC in weld cladding
Fe-Cr-Al clads for high-temp. corrosion service(sulphur and oxygen-rich environments)
8-10% Al and up to 5% Cr common
However, brittle FeAl and Fe3Al intermetallicssusceptible to hyd. embrittlement & cracking
(Ductility of these alloys ~12% in high-vacuum or
pure oxygen, but 2-4% if water vapour present)
During welding, 2Al + 3H2O (from arc) Al2O3 + 6H
H + residual stress from welding cracking in cladding
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Cracks in Fe-Al cladding starting in weld spread
through FZ, but stop at the base metal
thus direct path for environment to attack
substrate steel, protection totally ineffective
Addition of Cr to Fe-Al composition beneficial
- ductility of clads increases
- corrosion resistance also improves
- hydrogen cracking susceptibilty decreases
[attributed to hydrogen trapping by(Fe,Cr)xCy and (Fe,Al)3C type carbides]
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Oxide inclusions more effective trapping sites than
dislocations, carbides more effective than oxides
In low-alloy Cr-Mo & Cr-Mo-V steels, M23C6 and M7C3
& V-carbides shown to be useful trapping sites
In other steels, Al2O3 also found effective
Thus, Fe-Cr and Fe-Al carbides useful in thecladding consumable
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Control of microstructure
Regulate cooling conditions to ensure
that HAZ is not too hard
Maintain high heat input
Use preheat
Use post-heating (different from final PWHT)
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Control of microstructure (contd.)
Heat input :Arc energy (per unit length of weld)
= Voltage x Amperage x 60 w.speed (mm/min.)
Multiarc welding (single pool) :
Add individual arc energies
AC welding : Use RMS values
Heat input (arc efficiency factored) and arc energy
P h t d i t t t
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Preheat and interpass temperatures
Preheat up to 2500C to avoid HIC
Max. interpass temperature (e.g., up to 3000C)to control weldment properties
Preheating reduces cooling rate, M formation
Note: Effect greater at lower temperatures
C-Mn steel: + P at higher temperature,
not much affected by preheat,
but hyd. diffusion (lower temperature) affected
Low-alloy steel: B or M at lower temperature,
so HAZ microstructure and hyd. diffusion
much influenced by preheat
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Measure the temp. parameters close to weld line
immediately before depositing a weld pass
If preheat from one side only, e.g., by gas flame,
measure preheat temperature from opposite side;
Otherwise, remove flame and measure temperature
immediately (waiting time of
I min / 25 mm thickness recommended
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Other benefits of preheating : Time to cool from Ms to Mf is increased, so martensitic
reaction (even if it occurs) is less abrupt
Residual stress magnitude is reduced (however,
significant only if high preheat temperatures are used)
Development of stresses also becomes slower(as temperature drops more slowly)
Escape of hydrogen faster
(steel remains in high-temp. range much longer),
so final hydrogen content much less
Thus, even if martensite forms, risk of HIC less, since at the
time it forms, both stress and H level are lower
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Welding diagrams (nomograms)
Example : TWI nomograms for C-Mn steels with CE from
0.32 up to 0.58 and different hydrogen levels
Steps: 1) Select CE axis or scale based on H level
> 15 ml/100 g (e.g., SMAW with non-basic coverings) : A;
10-15 ml/100 g : B; 5-10 ml/100 g : C;
< 5 ml/100 g : D (e.g., SMAW with baked low-H electrodes)
2) Select nomogram for the CE of the steel welded
and the expected hydrogen level of the process
3) For the combined thickness of base metaland heat input used, read off preheat required
Note: Different combinations of preheat and heat input possible
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Temperature control method
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Temperature control method
Mainly for a) steels of higher hardenability and
b) for C and C-Mn steels in very thick sectionspreheat alone not adequate to reduce hardening,
additionallypostheating at 150-2000C
(interrupting cooling for given period before
allowing it to continue) required for releasinghydrogen (at these temperatures, H will diffuse rapidly
out of the steel and weldment will not crack)
For steel type and %C,
determine HAZ hardness from lower partand use it to estimate minimum preheat,
interpass and post-heat temperatutres
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Benefits of postheating : Hydrogen content drops during postheating much faster
and reaches a low level if temp. is well chosen
Residual stresses do not rise during postheating ;
also, as cooling is resumed after postheating,
rise again more slowly (since temp. is equalized)Thus, at the time stresses rise to their max. value,
hydrogen level has dropped considerably,
so risk of cold cracking greatly reduced
Adequately high postheat temp. and time permits
reduction in preheat temperature
No structural change during postheating
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Temperature control ..(contd.)
Difficulty with steels of low weldability :
Build-up of hydrogen in multipass welds
(especially in short welds)
Remedy : Minimize hydrogen input +
allow interpass time
(Interpass time required can be estimated fromhydrogen diffusion data relevant to
the interpass temperature)
Selection of postheat temperature
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Postheat temperature should be < temperature for softening (tempering) andalso below Mf
If postheat temp. > Mf , retained austenite holds the hydrogen and releases iton cooling and transformation to cause cracking
(even if taken to PWHT temperature without intermediate cooling, danger
that RA will not transform fully during PWHT)
If Mfis too low (i.e.,< desired preheat level), weld at the
desired preheat level with scrupulous hydrogen control,
reduce temperature very slowly after welding so that
hydrogen released from the transforming RA has
enough time to escape
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Nomograms established by TWI are available
for different welding situations
Expected HAZ hardness obtainable based on
steel type and carbon content
For this hardness, minimum preheat, interpass
and post-heat temperatures can be read off,
depending on restraint and hydrogen levels
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Temper beads
To deal with a hard HAZ, if joint not given PWHT
problems of poor toughness and SC resistance
Deposit temper bead at controlled distance from
weld toe :
Tempers hard HAZ on parent steel of final weld run
Leaves its own HAZ in less hardenable weld metal
Grind off temper bead later, if necessary
Caution : Location of temper bead to be precise
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Isothermal transformation
Also for steels tending to form very hard HAZs
Carry out welding operation at a high temperature,
say 3600C, hold at that temperature long enough
to transform to bainite
Use CCT diagram for the parent steel, but use
holding time twice as longto allow for coarser-
grained HAZ
Longer time and higher temp. Hydrogen diffuses out to
safer levels
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Austenitic or nickel alloy consumables
Used when preheat levels necessary for other
methods are too highdamaging to the steel or
the welder
Up to 0.2 - 0.3 % C, no preheat required
Higher % C, 1500 C adequate
Preheat level = f (% C, restraint, hardenability,
hydrogen level)
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Principle :
1. ASS and Ni-base alloys dissolve appreciableamounts of hydrogen in the solid state
2. ASS and Ni-base alloys not susceptible to hydrogen
embrittlement3. Some hydrogen may diffuse during welding into the
HAZ while the latter is austenitic, but will migrate
back into the weld metal as the HAZ transforms
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Difficulties :
1. Ni alloys (also ASS, to a smaller extent) prone tosolidification cracking, especially if S is picked up
from the HAZ
2. ASS filler Hard martensite near fusion boundary
(weld metal due to incomplete mixing & BMHAZ)
cracking
3. ASS filler material Difference in CTE
4. HAZ still very hard
5. Difficulty of NDE, because of difference in crystal
structures, only visual, DPI possible
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Weld metal hydrogen cracking
Possible in mild steel, C-Mn and low-alloy steels
Same controlling factors as in HAZ
(Hydrogen level, strain/restraint, microstructure)
Mild steel and C-Mn steel : High restraint,
high hydrogen level, or both
Low-alloy (say Cr-Mo) steels : Lower levels of
restraint and hydrogen content sufficient to cause
FZ cracking, hence greater care needed
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Morphology of cracking
Longitudinal or transverse to weld axisLongitudinal crack often initiated at root or
toe of a pass in a multipass weld or
at a position where welding is interruptedand interpass temp. drops
Transverse cracking either normal to weld surface
or inclined at ~ 45
0
to itlatter often calledchevron crack (also 450 or staircase crack)
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Chevron cracking
Observed in SMA and SA welds
Earlier sometimes believed to be nucleated by hot cracks
or other types, but now established that chevron cracks are
indeed hydrogen cracks and hence avoidable by usual meansHowever, solidification and hyd.cracks may exist in same weld
Chevron cracks in heavy C-Mn steel sub-arc welds :
High heat input large bead sizes long diffusion paths
Compensates for reduction in cooling rate
Best to use very low hydrogen levels
FZ hydrogen cracks often with zig zag path
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FZ hydrogen cracks often with zig-zag path
Often transgranular in C-Mn steels,
but increasingly intergranular in Cr-Mo steels
Fracture surfaces vary depending on
microstructure, applied strain & hydrogen level
Often quasi-cleavage type, but occasionally also
microvoid coalescencetypical are also
features not fitting generally recogniseddescription
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Formation of fisheyes
Instance of hydrogen-induced cracking in weld metal
Small white spots on as-weldedtensile fracture faces
Fisheye surrounds discontinuity like gas pocket or
void associated with non-metallic inclusion (pupil)
Hyd. migrates to the voids triaxial stress, embrittlement
During necking in tensile test, more hyd. diffuses to voids,
these localized regions fracture in brittle manner,however no time for the usual interrupted cracking
Remainder of FZ section fractures with ductility
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Fisheyes not normally seen, because
All-weld tensile tests usually done after heat treating
for hydrogen removal
Cross-weld tensile tests Failure commonly in
base metal (overmatched weld metal)
Impacttesting : Even fisheyes cannot appear(Diffusion of hydrogen during test not possible)
Wh d ld t l h d ki ?
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When does weld metal hyd. cracking occur?
1)High levels of restraint & high H levels
Example: At ordinary restraint (say, 2400 N/mm.mm)
HAZ and FZ cracking avoided for arc energies > 2.0 kJ/mm,
but at higher restraint (6300 N/mm/mm) HAZ cracking
avoided at energy > 2.5 kJ/mm, but not weld metal HIC
(higher arc heat angular distortion & hence root strain)
(FZ requires allowable hardness levels lower than in HAZ)
Use of lower hydrogen content consumables
often mitigates the problem in such cases
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2) Welding C-Mn steels of low CE
Lower preheat or even no preheat needed for avoiding HAZhydrogen crackingno comparable change in weld (filler)
metal composition used
Example: Low-C, lean-alloyed steels of low CE
(such as HSLA steels)often weld metal with higher CErequired to achieve strength
Preheat may thus be required for avoiding FZ HIC
(full economic potential of low-CE not realised!)
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3) Using CrMo steel weld metals
Susceptibility to cracking already high (high CE)
(High degree of alloying required in the weld metal
for strength, resistance to creep, high-temperatureoxidation or hydrogen attack)
Also increased tendency to undergo intergranular
rupture (at least partially)
Recent example: (Welding J., Nov.2006, 28-30)
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p ( g , , )Thick sections (35-40 mm) of carbon steel (ASTM A36),
field welding, heavy restraint
Base metal carbon content ~0.25%, Mn ~1.0%, Si ~0.3%
FCAW used for welding, 75% Ar / 25% CO2 shielding
Deposited weld metal : C~ 0.07%, Mn ~1.5%, Si ~0.7%
Boron also present : ~ 0.005%Note the B addition & in %Mn, %Si to balance in %C
Weld metal cracks noticed when B level rose to > 0.006%,
and when 100% Ar was used for shielding100% Ar higher levels of Mn (1.85%) & Si (1.0%),
high hardness (even >350 HV), HAC
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Another example: Weld. J., Apr. 2002, 61-s67-s
Weathering (high-performance) steels :Cor-ten, A 485W
Low-C, low CE (Pcm=0.256) Bainitic structure in HAZ,
300-350 HV max., no preheat required
for thickness < 50 mm and HD < 4 mL/100g
However, weld metal likely to be more hardenable (lower-C,
balanced by suitable alloy additions to strength),
preheat required to avoid FZ HIC
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Gapped bead-on-plate (G-BOP) test two platesclamped together with a 5-10 mm gap machined
in one of the blocks, bead deposited over the gap,high stresses at root, delayed FZ root cracking,
measure min. preheat required to avoid cracking
Min. preheat levels required to avoid weld metal HIC
vary for different processes (SAW: ~500C,
GMAW: ~500C, FCAW & SMAW: ~1000C)
SMAW: Harder weld metal, higher % martensite,
also penetration stress concentration at root
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Precautions necessary :
Similar to measures for avoiding HAZ HIC,but few rational predictive formulae available, e.g.,
Preheat temp. required to avoid weld metal micro-
cracking = T0(
0
C) = 120 + 120 log (HD/3.5)+ 5.0 (h-20) + 8 ( B83),
where HD = Diffusible hyd. content of weld metal
(0.1 to 40 mL/100 g),
h = weld metal thickness (15 to 40 mm) and
B = UTS of weld metal (600 to 900 MPa)
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If clear formulation not available,
adopt scrupulous precautions to lower hyd. input :
Baking at highest temp. allowed by manufacturer
Warm storage , say at 1500C
SAW and GMAW wire (leaving weld nozzle)
to be clean, rust-freeno pick-up en route
Allowing adequate interpass time for H to escape
(especially during repair procedures)
Maintaining preheat temp. for some period
after welding - to reduce differential contraction
M h i / f h d ki
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Mechanism/s of hydrogen cracking
Hydrogen absorbed by liquid weld metal
: 30 mL/100g (if available!)
As temp. and solubility drop, some hyd. comes out of
solution Escape as gas bubbles or entrapped as pores
However, rapid cooling in welding
Excess hydrogen retained in solution (supersaturation)
Solute hydrogen is in atomic state, can diffuse quickly
From the FZ, hyd. diffuses a) out of the steel
b) into the HAZ (when hot!)
c) into discontinuities (2HH2)
Thus, at RT, both FZ & HAZ supersaturated with hydrogen
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Exce
ss
Solu
bilityofhydro
geninsteel
Temperature
RT MPWeld metaltemp.
Hydrogen absorption by weld metal
H d i i i / k
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Hydrogen in cavities / gas pockets enormous pressure
(2HH2, equilibrium const. K = (pH2 / aH2), Sieverts law)11 mL/100g of dissolved hyd. ~ 1450 MPa pressure!
Hydrostatic, hence triaxial
But hyd. in cavities in molecular form, cannot diffuse easily
Hydrogen supersaturated in FZ & HAZ in atomic state,
can diffuse rapidly, hence diffusible hydrogen
Both forms cause problems, but diffusible hydrogen
of much greater importance
F t ff ti h d b ittl t
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Factors affecting hydrogen embrittlement
Strength of the steel
(YS primary criterionlimits local peak stress build-up)
Microstructure:Order of embrittlement : Ferrite-pearlite, bainite,
bainite-martensite, martensite, twinned martensite)
(Martensite plateshigh short-range stresses)
Temperature of embrittlement : +200 to -1000C
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Strain rate : Time necessary for hydrogen to diffuse
prior to fracture by other means
(Not impact, not tension, but only stress rupture test)
Section size
Thicker plates: Higher shrinkage stresses, also triaxial(Compressive stresses due to martensite formation
only microscopic, so average out)
Lower surface-to-mass ratio
Longer diffusion distances for H escape(baking time proportional to D2 and t2 )
Coarser grain size (generally)
f h d ki
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Features of hydrogen cracking
Constant-load stress rupture testAbove upper limit stress, fracture without delay (not due to H)
Below lower limit stress, no damage due to hydrogen
Between these limits, brittle fracture due to hydrogenTime to rupture tRshorter for higher applied stress
( Static fatigue limit )
Incubation time before hydrogen cracking starts,followed by intermittent crack growth in several steps,
final catastrophic fracture by overload (section size )
Upper critical stressC H l l
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Time to fracture (log), h
Applie
dstress
Upper critical stress
Incubation
time
Time to fracture
Lower limit
stress
Const. H level
Const. temp.
Constant load rupture test
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Time
Com
plianceorcr
ackopening
ti
ti =Incubation time
Intermittent crack growth
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Theories of hydrogen embrittlement
Pressure theory (due to Zapffe and Tetelman)Hydrogen in supersaturated lattice escapes into tiny
voids (gas pockets, sub-microscopic rifts,
grain boundary imperfections, voids associated with
non-metallic inclusions)
Once inside cavity, atomic hydrogen changes to
molecular form, builds up enormous pressure
Triaxial state of stress a) adds to applied stress
b) increases crack susceptibility
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Sorption theory (due to Petch)
Hydrogen adsorbed on surface of internal
lattice imperfections and microcracks,
adsorption reduces surface energy,
facilitates crack propagation
(Griffith criterion)
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Lattice embrittlement theory (due to Troiano)
Hydrogen in lattice (solute) is the damaging specie,
reduces cohesive strength of the base metal bond
Embrittlement in 3 stages :
1)Incubation periodhydrogen migrates to high-stress
regions (e.g., stress raisers), accumulates to reach
a critical level when a crack is nucleated
( triaxiality YS, hydrogen cohesive strength)
2) Initial crack propagates some time,
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2) Initial crack propagates some time,
but soon stops as its tip reaches sound metalnot yet damaged by hydrogen
crack growth arrestedHowever, within a short time, more hydrogendiffuses to fresh crack tip (stress raiser),reduces strength, etc.,
crack propagation is resumed, cycle repeated intermittent crack growth (incubation periodsfollowed by rapid crack extensions)(crack growth rate just faster than
hydrogen diffusion rate!)
3) Crack advances so much that remainingsection too weak to sustain applied load catastrophic, ductile fracture
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Comparison between the theories : Baking removes hydrogen embrittlement
If embrittlement is due to molecular hydrogen,
it has first to dissociate into atomic hydrogen
before it can diffuse to the surface and escape.
Temperatures found to be effective for removing
hydrogen are too low for such dissociation
Tensile testing at liquid nitrogen temperatures (-1960C)
(nodiffusion) very low %RA in hyd.-embrittled samples
Void pressure too low to cause embrittlement
Residual hydrogen in lattice can still embrittle at -1960C
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