7/30/2019 Weld. Met.

1/111

WELDING METALLURGY

Welding of Steels

7/30/2019 Weld. Met.

2/111

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

7/30/2019 Weld. Met.

3/111

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

7/30/2019 Weld. Met.

4/111

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

7/30/2019 Weld. Met.

5/111

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

7/30/2019 Weld. Met.

6/111

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

7/30/2019 Weld. Met.

7/111

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

7/30/2019 Weld. Met.

8/111

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

7/30/2019 Weld. Met.

9/111

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

7/30/2019 Weld. Met.

10/111

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

7/30/2019 Weld. Met.

11/111

Cooling rates in various processes

7/30/2019 Weld. Met.

12/111

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

7/30/2019 Weld. Met.

13/111

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)

7/30/2019 Weld. Met.

14/111

7/30/2019 Weld. Met.

15/111

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.)

7/30/2019 Weld. Met.

16/111

7/30/2019 Weld. Met.

17/111

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)

7/30/2019 Weld. Met.

18/111

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

7/30/2019 Weld. Met.

19/111

The iron-carbon phase diagram

7/30/2019 Weld. Met.

20/111

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

7/30/2019 Weld. Met.

21/111

Lamellar structure of pearlite

7/30/2019 Weld. Met.

22/111

Ferrite-pearlite microsturcture of medium-C steel

7/30/2019 Weld. Met.

23/111

Low carbon steel

(ferrite and pearlite)

Ferrite

in nearly pure iron

7/30/2019 Weld. Met.

24/111

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

7/30/2019 Weld. Met.

25/111

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

7/30/2019 Weld. Met.

26/111

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

7/30/2019 Weld. Met.

27/111

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

7/30/2019 Weld. Met.

28/111

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

7/30/2019 Weld. Met.

29/111

Isothermal transformation diagram for eutectoid steel

7/30/2019 Weld. Met.

30/111

CCT diagram for eutectoid steel

7/30/2019 Weld. Met.

31/111

CCT diagram and RT microstructures in eutectoid steel

7/30/2019 Weld. Met.

32/111

CCT diagram and RT microstructures

in medium-C low-alloy steel (4340)

7/30/2019 Weld. Met.

33/111

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

7/30/2019 Weld. Met.

34/111

Hardness of plain-carbon steel

in various microstructural conditions

7/30/2019 Weld. Met.

35/111

7/30/2019 Weld. Met.

36/111

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

7/30/2019 Weld. Met.

37/111

Underbead crack in low-alloy steel weldment

7/30/2019 Weld. Met.

38/111

7/30/2019 Weld. Met.

39/111

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

7/30/2019 Weld. Met.

40/111

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

7/30/2019 Weld. Met.

41/111

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

7/30/2019 Weld. Met.

42/111

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

7/30/2019 Weld. Met.

43/111

7/30/2019 Weld. Met.

44/111

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

7/30/2019 Weld. Met.

45/111

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!)

7/30/2019 Weld. Met.

46/111

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

7/30/2019 Weld. Met.

47/111

7/30/2019 Weld. Met.

48/111

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

7/30/2019 Weld. Met.

49/111

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

7/30/2019 Weld. Met.

50/111

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

7/30/2019 Weld. Met.

51/111

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

7/30/2019 Weld. Met.

52/111

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

7/30/2019 Weld. Met.

53/111

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

7/30/2019 Weld. Met.

54/111

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

7/30/2019 Weld. Met.

55/111

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

7/30/2019 Weld. Met.

56/111

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

7/30/2019 Weld. Met.

57/111

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)

7/30/2019 Weld. Met.

58/111

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

7/30/2019 Weld. Met.

59/111

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]

7/30/2019 Weld. Met.

60/111

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

7/30/2019 Weld. Met.

61/111

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)

7/30/2019 Weld. Met.

62/111

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

7/30/2019 Weld. Met.

63/111

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

7/30/2019 Weld. Met.

64/111

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

7/30/2019 Weld. Met.

65/111

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

7/30/2019 Weld. Met.

66/111

7/30/2019 Weld. Met.

67/111

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

7/30/2019 Weld. Met.

68/111

7/30/2019 Weld. Met.

69/111

Temperature control method

7/30/2019 Weld. Met.

70/111

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

7/30/2019 Weld. Met.

71/111

7/30/2019 Weld. Met.

72/111

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

7/30/2019 Weld. Met.

73/111

7/30/2019 Weld. Met.

74/111

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

7/30/2019 Weld. Met.

75/111

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

7/30/2019 Weld. Met.

76/111

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

7/30/2019 Weld. Met.

77/111

7/30/2019 Weld. Met.

78/111

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

7/30/2019 Weld. Met.

79/111

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

7/30/2019 Weld. Met.

80/111

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)

7/30/2019 Weld. Met.

81/111

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

7/30/2019 Weld. Met.

82/111

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

7/30/2019 Weld. Met.

83/111

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

7/30/2019 Weld. Met.

84/111

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)

7/30/2019 Weld. Met.

85/111

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

7/30/2019 Weld. Met.

86/111

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

7/30/2019 Weld. Met.

87/111

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

7/30/2019 Weld. Met.

88/111

7/30/2019 Weld. Met.

89/111

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 ?

7/30/2019 Weld. Met.

90/111

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

7/30/2019 Weld. Met.

91/111

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!)

7/30/2019 Weld. Met.

92/111

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)

7/30/2019 Weld. Met.

93/111

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

7/30/2019 Weld. Met.

94/111

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

7/30/2019 Weld. Met.

95/111

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

7/30/2019 Weld. Met.

96/111

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)

7/30/2019 Weld. Met.

97/111

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

7/30/2019 Weld. Met.

98/111

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

7/30/2019 Weld. Met.

99/111

Exce

ss

Solu

bilityofhydro

geninsteel

Temperature

RT MPWeld metaltemp.

Hydrogen absorption by weld metal

H d i i i / k

7/30/2019 Weld. Met.

100/111

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

7/30/2019 Weld. Met.

101/111

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

7/30/2019 Weld. Met.

102/111

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

7/30/2019 Weld. Met.

103/111

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

7/30/2019 Weld. Met.

104/111

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

7/30/2019 Weld. Met.

105/111

Time

Com

plianceorcr

ackopening

ti

ti =Incubation time

Intermittent crack growth

7/30/2019 Weld. Met.

106/111

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

7/30/2019 Weld. Met.

107/111

Sorption theory (due to Petch)

Hydrogen adsorbed on surface of internal

lattice imperfections and microcracks,

adsorption reduces surface energy,

facilitates crack propagation

(Griffith criterion)

7/30/2019 Weld. Met.

108/111

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,

7/30/2019 Weld. Met.

109/111

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

7/30/2019 Weld. Met.

110/111

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

7/30/2019 Weld. Met.

111/111