176521779-Welding-Metallurgy-Pc-1-Ppt-2.pdf

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Welding Metallurgy

Pallav Chattopadhyay

Manufacturing Technology- I

14-Oct-2005


Crystal Structure


Solid Solution

Substitutional

Interstitial


Solidification of Metal


Ie-C Diagram


Steel Structure as a function of %C


Structural Changes in 0.4%C Steel during Slow

Cooling


Typical Lamellar Pearlite

1500 X


0.25% C cooled from 870C

100X

Slow Cooled

Rapid Cooled

Oil Quenched

Proeutectoid Ferrite +

Pearlite

Less Proeutectoid Ferrite +

More Pearlite

Martensite + Ferrite +

Bainite + Pearlite


Welding Metallurgy

Welding

Complex Metallurgical Process involving:

Melting

Solidification

Gas-metal reaction

Slag-metal reaction

Surface phenomenon

Solid state reactions

Weld Joint consists of:

Weld metal

Heat Affected Zone (HAZ)

Unaffected Base Metal


Macro section of Weld


Weld & HAZ


Microstructure of Low C Steel Weld Metal

100 X


Weld Metal

Microstructure marked different from base material of same composition

different thermal & mechanical histories

Base material : Hot rolling Multiple recrystallization + Heat

Treatment

Weld Metal : No mechanical deformation As-solidified structure

No time for diffusion heterogeneous composition

Reactions with gases in the vicinity / non-metallic liquid phases (slag or flux) / after solidification


Weld Metal

Solidification: Unmelted portion of grains in HAZ act as nucleation site

Metals grow more rapidly in certain crystallographic

directions

Favorably oriented grains grow for substantial distance -

growth of others blocked by faster growing grains -

columnar grains


Weld Metal

Solidification: Micro-segregation of alloying and residual elements

formation of Dendrites

Solidification of primary dendrites more soluble solutes

in liquid rejected freezing point lowered


Weld Metal

Solidification:

Concentration of solute near solid-liquid interface arrest

crystal growth

Many dendrites grow simultaneously into liquid from

single grain

Same crystal orientation part of same grain

Weld structure appears coarse at low magnification

Fine dendritic structure at high magnification

Spacing between dendritic arms measure of alloy

segregation determined by rate of solidification


Typical Columnar Structure

Ingot

Weld Metal


Weld Metal

Gas-Metal reactions:

Depends on presence of O2, H2 or N2

O2 Comes from Shielding gas / Air

N2 Comes from Air

H2 Comes from Flux / coating / atmosphere / base metal


Weld Metal


Ferrous Material:

Diatomic gas molecule breaks down at high temp & dissolve in steel

O2 Reacts with de-oxidizers like Mn, Si, Al - Oxides taken out in

form of slag

Porosity (CO/CO2) formation in case of insufficient de-oxidizer

N2 content much lesser compared to O2 content raises transition

temp / introduces embrittlement & strain-ageing

H2 always present in arc atmosphere

Atomic hydrogen creates porosity

Dissolved hydrogen creates cracking tendency


Weld Metal


Non-Ferrous Material:

Solution, reaction & evolution of hydrogen or water vapor

Al & Mg alloys: H2 introduces in weld metal from work piece /

filler wire (present as hydrated oxides on the surface)

Cu & Ni Alloys: H2 reacts with O2 and form porosity add

deoxidizer in filler wire

Ti & Alloys: Embrittlement with N2, H2 & O2

: Weldment require inert gas protection till 260C

: Surface appearance indicates effectiveness of shielding

: H2 major cause of porosity


Weld Metal

Liquid-Metal reactions:

Non-metallic liquid phases (e.g. Al-Mn-Fe silicates)

produced Slag

Hot cracking:

Inter-dendritic liquid - substantially lower freezing temp than

previously solidified base metal

Presence of S, P, Pb

Mn:S ratio of >=30 for C-Mn & LAS

Presence of Delta Ferrite in microstructure for Austenitic SS

P tends to segregate readily cause harmful banding


Weld Metal

Solid State reactions:

Strengthening mechanisms

Solidification grain structure

Rapid freezing creating segregation / dendrites in each grain

Impeded plastic flow during Tensile test Higher YS / UTS ratio

Solid Solution Strengthening

Alloy additions

Substitutional / Interstitial

Precipitation hardening

Strengthening by ageing process after welding

Presence of over-aged weld metal

Not same level of strength as base metal


Weld Metal


Strengthening mechanisms

Transformation hardening

Formation of harder structure / Martensite

Formation of fine Ferrite Carbide aggregate


Weld Metal


Delayed / Cold Cracking

Solubility / Diffusivity decreases drastically during solidification

Atomic H try to escape settles in lattice imperfections

Molecular H2 formed -Tremendous internal pressure created

Hardened structure

Dissolved hydrogen in weld metal

Preheat to slower the cooling rate

Use of low hydrogen consumables


Solubility of Hydrogen in Iron


Diffusivity of Hydrogen in Iron



Adjacent to the base material

Portion of base material

Not melted

Microstructure altered

Mechanical properties changed

C-Mn steel : Above ~700C

Heat treated steel: Above 315C

Heat treated Al alloy: Above 120C



Strength & Toughness depends on Type of base metal,

welding process & welding parameters (Heat input)

Effect of welding parameters depends on types of

alloys:

1. Solid Solution Strengthened Alloys:

Hot rolled Low C steel, Al alloys, Cu alloys, Austenitic & Ferritic SS

Least HAZ problem largely unaffected by welding

Grain growth only few grains wide no major effect on mech prop.




alloys:

2. Strain Hardened Base Metal:

Recrystalize while heating above Recrystallization temp

Steel, Ti & other alloys show allotropic transformation

Two recrystallized zones Recrystallization of Cold worked Alpha

phase & Allotropic transformation to High temp phase




alloys:

3. Precipitation Hardened Alloys:

HAZ undergoes an Annealing cycle lowers strength

Relatively soft single phase solid solution with coarse grains near

fusion line can be hardened by post weld ageing treatment

Next to this region below Solution treatment temp overageing

post weld ageing do not have any effect


Transformation Hardening Alloys


Weld Metal vs. Base Metal

1. Columnar Grain

2. Segregation

3. Oxides - Sulphide Inclusions

4. Solidified Structure

5. Limitations On Heat Treatment

6. Entrapped Gases

7. Different Hardenability

8. Different Thermal Cycles

9. Weld Defects Higher Chances

WLED METAL BASE METAL

Polygonal Equiaxed

Homogeneous

Steel Making Process Benefits

Rolled / Forged Structure

Proper Heat Treatment

No Entrapments

_________

Uniform Heat Treatment Cycles

__________


Increase In Hardenability

Higher Dissolution Of Alloying Elements

Fusion Line Max Effect

Base Metal Chemistry

Very High Temperature

No Benefits Of Weld Chemistry Control

Fastest Cooling Rates

Consequences

Effects

WELD

FUSION LINE B. M.

Hardness, Ductility & Toughness

Loss Of B.M. Heat Treatment In HAZ (QT, NR, Solution Anneal)

Lower Delta Ferrite Retention In Austenitic Stainless Steel

Coarse Grains - Lower Room Temperature Strength

Grain Coarsening

RT-1000 : Retention Above 1000C


t8-5 Time To Cool From 800 500c

Faster Cooling Rate

Harder Structures

Lower Value

Base Metal Thickness / Joint Confg.

Heat Input

Preheat / Inter Pass

Affecting Parameters

t100 Time To Cool To 100c

Increased Hydrogen Diffusion

Reduced Probability Of Cold Cracking Longer Times

B.M. Thickness / Joint

Heat Input

Preheat / Interpass

Post Heat

Affecting Parameters

t8-5 & t100


Effect of Heat Input, Geometry & Preheat

on Cooling Rate


Effect of Weld Size on Cooling Rate

Higher Travel Speed greater portion

of energy input utilized in forming

weld bead & less in heating adjacet

area higher cross sectional area of

weld metal / penetrtaion


Preheat

Heating of weldment to a minimum predefined temp

before start of welding and maintaining the same

during welding

To reduce cooling rate of weldment softer structure

To avoid cracking

To reduce distortion

To remove Oil, Moisture etc

Increases with increasing thickness

Must be maintained at least 2 on either side of joint

Very critical for high strength / alloyed materials

Required for mainly Ferritic materials

Not required for Austenitic steel


Typical Preheat Temperature


Interpass Temperature

Maximum allowed temperature in the weldment in

between two subsequent passes

Reduces grain coarsening in Ferritic steel better impact

toughness

Reduces chance of IGC in Austenitic SS

Typical Interpass temp:

C-Mn Steel : 275C

Low Alloy steel : 250C

Austenitic SS : 175-200C


Effect of Preheat / Interpass Temp


De Hydrogenation Treatment (DHT)

Holding at 300-400C for 2-6hrs after welding and

before cooling down to room temperature

Allows Hydrogen to diffuse out (higher diffusivity at

high temp) from weldment and reduce chance of

Hydrogen cracking

Required mainly for Low Alloy Steel (e.g. Cr-Mo, Cr-

Mo-V steel) and QT steel

For highly restrained joint, DHT is replaced by an

Intermediate Stress Relieving (ISR) at 620-660C/2-

4hrs


PWHT / Stress Relieving

Normally below Tempering temp

Both mechanical & metallurgical effect in Steel

To relieve locked-up stresses

Tempered structure in some of the Steels

Both beneficial & detrimental effects on properties


PWHT / Stress Relieving

Larson Miller Parameter (LMP) =

T (20 + Log10 t), where T = Tempering Temp in K &

t = Tempering time in hrs


References

Welding Handbook AWS Volume-1 (Pg: 90-92, 103-111)

Weldability of Steels R D Stout WRC (Pg. 48-52, 84-103,

105-108)


Thank You

176521779-Welding-Metallurgy-Pc-1-Ppt-2.pdf

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