1. WELDING METALLURGY
Welding of Steels

2. High peak temperatures High temperature gradients
HEAT FLOW IN WELDING
In welding the reactions take place within seconds in a small volume of metal, characterized by….
High peak temperatures
High temperature gradients
Rapid heating and cooling

3. 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

4. THE TRANSFER OF HEAT in the weldment is
governed 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, mm
T = the temperature in the weldment, °C
k(T) = thermal conductivity of the metal,J/mm-s-0C
p = density of the metal, g/mm C = specific heat of the metal, J/g' °C Q = rate of internal heat generation, W/mm3

5. Rosenthal’s analysis of heat flow in welding - Moving Heat Sources
Consider a schematic of stationary work piece…….
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
i.e the temperature distribution and the pool geometry do not change with time for an observer moving with the heat source

6. 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

7. 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

8. By converting distance x into time t through t = (x - 0)/V one can get the Temp.-Time curve i.e THERMAL CYCLE
Calculated results from Rosenthal's three-dimensional heat flow equation………..
Thermal cycles
Isotherms
material: 1018 steel.
Welding speed: 2.4mm/s;
heat input: 3200 W:

9. As Rosenthal's Equations measures the temp. at any place in w/p w.r.t
Adams' Equations ……..for calculating peak temp.( Tp) adjacent to HAZ
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.t
moving heat source while Adams' Equations gives the peak temp.( Tp) at
any dist from the fusion boundary adjacent to HAZ

10. COOLING RATES R — the cooling rate at the weld center line, °C/s
k — thermal 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/mm C = specific heat of base metal, J/g °C
C = volumetric specific heat, j/mm3-°C for steels

11. Cooling rates in various processes

12. Process Relative arc efficiency(%) SMAW 65 - 85 (1.0, accg. To BS)
GMAW (1.0)
Cored wire (1.0)
GTAW (dcen) ( 0.8)
(ac)
SAW (1.25)
EBW – 95
Laser W

13. Two-dimensional and three-dimensional cooling
Cooling rate and welding conditions
Two-dimensional and three-dimensional cooling
Three-dimensional (thick plates,
requiring 6 or more passes) :
Cooling rate CR = 2πk (Tc –T0)2 ∕ H , where
k = Thermal conductivity
T0 = Initial plate 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)

15. Two-dimensional (thin plates,
requiring fewer than 4 passes) :
Cooling rate CR = 2πkρC (h/H)2 (Tc –T0)3 , where
h = Base metal thickness (Combined thickness)
ρ = 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.)

17. 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 : ∆t 8-5 (between 800 and 5000C – range of
γ transformation in most C- and C-Mn steels)
∆t 3-1 (Time for H to diffuse out of weld area)

18. 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

19. The iron-carbon phase diagram

20. 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

21. Lamellar structure of pearlite

22. Ferrite-pearlite microsturcture of medium-C steel

23. Low carbon steel (ferrite and pearlite)
in nearly pure iron
Low carbon steel (ferrite and pearlite)

24. Constitution of hypo-eutectoid steels
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

25. Effects of rapid cooling
The eutectoid reaction : γ → α + 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

26. Transformations in steel as a function of cooling rate
Austenite
Cooling rate (CR) increases
Temperature
Coarse Pearlite
Martensite
( single phase)
Bainite
Fine Pearlite
Mixtures of ferrite and iron carbide
Time
Hardness and strength progressively increase as CR increases
Transformations in steel as a function of cooling rate

27. 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

28. 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 given cooling rate
Most alloying elements lower Ms & Mf temp’s
Concept of hardenability, industrial significance

29. Isothermal transformation diagram for eutectoid steel

30. CCT diagram for eutectoid steel

31. CCT diagram and RT microstructures in eutectoid steel

32. CCT diagram and RT microstructures in medium-C low-alloy steel (4340)

33. Tempering of martensite
Need to temper
Changes in structure and properties
Selection of tempering temperature
and time
Quenched and tempered steels, also
normalized and tempered steels

34. Hardness of plain-carbon steel in various microstructural conditions

36. 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

37. Underbead crack in low-alloy steel weldment

39. 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

40. Tendency to martensite formation
Depends on intersection of weld cooling curve with 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

41. 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) < 0.42 – easy to weld w/o cracking
CE(IIW) > 0.5 – difficult to weld w/o cracking

42. Need to recognise composition limits for
valid application of CE formula
Compare : Steel A (%) Steel B (%) C Mn
Cr 2.25, Mo 1.0, V Unalloyed
CE (IIW)
Compare also low-C, high-Mn steel
with higher-C, lower-Mn steel

44. Carbon Equivalent Several 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/ B (Note importance of B)
Düren: CEq = C + Si/25 + (Mn+ Cu)/16
+ Ni/40 + Cr/10 + Mo/15 + V/10
Note greater emphasis on C itself
The latter two especially useful for low-C steels
(many modern steels, e.g., pipeline steels),
for which CE (IIW) is not entirely suitable

45. Carbon Equivalent CE (IIW) - developed in the late 1960s -
and based on work from
originally hardenability formula,
now used as hydrogen cracking formula
(Si ignored in formula, but affects hardenability same way as Mn, but Si does not increase cracking tendency, unlike Mn)
(CE(IIW) cannot be used to find HAZ hardness of single-pass weld containing Si!)

46. Carbon Equivalent
Empirical 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 S –
sulphide inclusions nucleate ferrite at higher temperature
more crack-resistant than lower-temperature products)
2) Segregation, especially in concast plates –
higher %C and alloying elements at centreline,
greater cracking tendency there
3) High scrap casts can have higher alloy content

48. 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

49. 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

50. Direct control of hyd. level (contd.)
Very low hyd. electrodes : Danger in hot,
humid climates, hyperbaric chambers
Cored wire brands : Moisture pick up if reels
kept on machines unprotected for long
(in humid conditions)
Parent steel cleanliness : Rust, oil, grease,
paint, even innate hydrogen

51. Further lowering of hydrogen levels
Development of higher-strength steels –
reduced 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

52. Hydrogen reduction strategies Modifications in arc chemistry
1) Increasing slag basicity (B)
B = CaO + MgO + BaO + K2O + Li2O + CaF (MnO+FeO)
SiO (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 or hydroxyl capacity of the slag
Higher basicity → higher hydroxyl capacity → Lower HD level ↔

53. Also, dissociation of CaCO3 contributes additionally
to reduction of HD
( CaCO3 ↔ CaO + CO2, 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

54. 2) Addition of fluorides to flux
a) Fluorine-containing compounds ↓ hydrogen content in weld metal : 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

55. Addition of CaF2 also ↑ slag basicity
and ↓ hydrogen level
CaF2 decomposes poorly in the arc, hence other fluorides also tried : 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 stability
Excess K2SiF6 and K2TiF6 ↓ slag basicity
Other more complex factors

56. 3) Concept and use of hydrogen traps
Hydrogen as solute in lattice and also segregated in
crystal defects and second-phase particles
Mean residence time longer in these particles
than as solute – “hydrogen trapping”
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

57. Retained austenite (RA) as hydrogen trap
High solubility, but low diffusivity for hydrogen
in austenite – exact opposite in surrounding ferrite, bainite or martensite – thus
trapping effect – experimentally demonstrated
Caution:
RA could transform to martensite on drop in service temperature, high HD in martensite → Embrittlement
Tailor RA content to % H pick-up &
service conditions (especially temperature)

58. 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 intermetallics
susceptible 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

59. 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]

60. 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 the
cladding consumable

61. 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)

62. 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

63. 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: γ → α + P at higher temperature,
not much affected by preheat,
but hyd. diffusion (lower temperature) affected
Low-alloy steel: γ → B or M at lower temperature,
so HAZ microstructure and hyd. diffusion
much influenced by preheat

64. 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

65. 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

67. 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; 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 metal
and heat input used, read off preheat required
Note: Different combinations of preheat and heat input possible

70. Mainly for a) steels of higher hardenability and
Temperature control method
Mainly for a) steels of higher hardenability and
b) for C and C-Mn steels in very thick sections
– preheat alone not adequate to reduce hardening,
additionally postheating at C
(interrupting cooling for given period before
allowing it to continue) required for releasing
hydrogen (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 part
and use it to estimate minimum preheat,
interpass and post-heat temperatutres

72. 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

74. 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 from
hydrogen diffusion data relevant to
the interpass temperature)

75. If Mf is too low (i.e.,< desired preheat level), weld at the
Selection of postheat temperature
Postheat temperature should be < temperature for softening (tempering) and also below Mf
If postheat temp. > Mf , retained austenite holds the hydrogen and releases it on 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 Mf is 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

76. 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

78. 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

79. Also for steels tending to form very hard HAZs
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 long – to allow for coarser-
grained HAZ
Longer time and higher temp. → Hydrogen diffuses out to
safer levels

80. Austenitic or nickel alloy consumables
Used when preheat levels necessary for other
methods are too high – damaging to the steel or
the welder
Up to % C, no preheat required
Higher % C, 1500 C adequate
Preheat level = f (% C, restraint, hardenability,
hydrogen level)

81. Principle :
ASS and Ni-base alloys dissolve appreciable amounts of hydrogen in the solid state
2. ASS and Ni-base alloys not susceptible to hydrogen embrittlement
3. 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

82. Difficulties :
Ni alloys (also ASS, to a smaller extent) prone to solidification cracking, especially if S is picked up from the HAZ
ASS filler → Hard martensite near fusion boundary (weld metal due to incomplete mixing & BMHAZ) → cracking
ASS filler material → Difference in CTE
HAZ still very hard
Difficulty of NDE, because of difference in crystal structures, only visual, DPI possible

83. 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

84. Morphology of cracking
Longitudinal or transverse to weld axis
Longitudinal crack often initiated at root or
toe of a pass in a multipass weld or
at a position where welding is interrupted
and interpass temp. drops
Transverse cracking either normal to weld surface
or inclined at ~ 450 to it – latter often called
chevron crack (also 450 or staircase crack)

85. Observed in SMA and SA welds
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 means
However, 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

86. 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 coalescence – typical are also
features not fitting generally recognised
description

87. Formation of fisheyes
Instance of hydrogen-induced cracking in weld metal
Small white spots on as-welded tensile 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

89. 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)
Impact testing : Even fisheyes cannot appear
(Diffusion of hydrogen during test not possible)

90. 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

91. 2) Welding C-Mn steels of low CE
Lower preheat or even no preheat needed for avoiding HAZ hydrogen cracking – no 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 CE
required to achieve strength
Preheat may thus be required for avoiding FZ HIC
(full economic potential of low-CE not realised!)

92. 3) Using Cr – Mo 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-temperature
oxidation or hydrogen attack)
Also increased tendency to undergo intergranular
rupture (at least partially)

93. Recent example: (Welding J., Nov.2006, 28-30)
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 shielding
100% Ar → higher levels of Mn (1.85%) & Si (1.0%),
high hardness (even >350 HV), HAC

94. Another example: Weld. J., Apr. 2002, 61-s – 67-s
Weathering (high-performance) steels :Cor-ten, A 485W
Low-C, low CE (Pcm=0.256) → Bainitic structure in HAZ,
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

95. Gapped bead-on-plate (G-BOP) test – two plates clamped 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

96. 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(0C) = log (HD/3.5)
+ 5.0 (h-20) + 8 ( σB – 83),
where HD = Diffusible hyd. content of weld metal
(0.1 to 40 mL/100 g),
h = weld metal thickness (15 to 40 mm) and
σB = UTS of weld metal (600 to 900 MPa)

97. 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-free – no 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

98. 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 (2H→H2)
Thus, at RT, both FZ & HAZ supersaturated with hydrogen

99. Solid
Liquid
Excess
Solubility of hydrogen in steel
Temperature RT MP
Weld metal temp.
Hydrogen absorption by weld metal

100. (2H→H2, equilibrium const. K = (pH2 / aH2), Sievert’s law)
Hydrogen in cavities / gas pockets → enormous pressure
(2H→H2, equilibrium const. K = (pH2 / aH2), Sievert’s 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

101. Factors affecting hydrogen embrittlement
Strength of the steel
(YS primary criterion – limits local peak stress build-up)
Microstructure:
Order of embrittlement : Ferrite-pearlite, bainite,
bainite-martensite, martensite, twinned martensite)
(Martensite plates – high short-range stresses)
Temperature of embrittlement : +200 to -1000C

102. 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)

103. Features of hydrogen cracking
Constant-load stress rupture test
Above 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 hydrogen –
Time to rupture tR shorter 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 ↓)

104. Time to fracture (log), h
Applied stress
Upper critical stress
Incubation time
Time to fracture
Lower limit stress
Const. H level Const. temp.
Constant load rupture test

105. Compliance or crack opening
ti = Incubation time
Compliance or crack opening ti
Time
Intermittent crack growth

106. 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

107. Sorption theory (due to Petch)
Hydrogen adsorbed on surface of internal
lattice imperfections and microcracks,
adsorption reduces surface energy,
facilitates crack propagation
(Griffith criterion)

108. 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 period – hydrogen 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)

109. 2) Initial crack propagates some time,
but soon stops as its tip reaches sound metal
not yet damaged by hydrogen –
crack growth arrested
However, within a short time, more hydrogen
diffuses to fresh crack tip (stress raiser),
reduces strength, etc.,
crack propagation is resumed, cycle repeated –
intermittent crack growth – (incubation periods
followed by rapid crack extensions)
(crack growth rate just faster than
hydrogen diffusion rate!)
3) Crack advances so much that remaining
section too weak to sustain applied load →
catastrophic, ductile fracture

110. 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)
(no diffusion)→ very low %RA in hyd.-embrittled samples
Void pressure too low to cause embrittlement
Residual hydrogen in lattice can still embrittle at -1960C