Welding Metallurgy Pc[1].Ppt 2

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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 870°C

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

• Gas-Metal reactions:– 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

• Gas-Metal reactions:– 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 260°C

: 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

• Solid State reactions:– Strengthening mechanisms

• Transformation hardening– Formation of harder structure / Martensite

– Formation of fine Ferrite – Carbide aggregate


Weld Metal

• Solid State reactions:– 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




Heat Affected Zone (HAZ)

• Adjacent to the base material• Portion of base material

– Not melted

– Microstructure altered

– Mechanical properties changed

• C-Mn steel : Above ~700°C• Heat treated steel: Above 315°C• Heat treated Al alloy: Above 120°C



• 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.




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




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 LINEB. 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 1000°C


t8-5Time To Cool From 800 – 500°c

Faster Cooling Rate Harder Structures

Lower Value

Base Metal Thickness / Joint Confg. Heat Input Preheat / Inter Pass

Affecting Parameters

t100Time To Cool To 100°c

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 : 275°C

– Low Alloy steel : 250°C

– Austenitic SS : 175-200°C


Effect of Preheat / Interpass Temp


De Hydrogenation Treatment (DHT)

• Holding at 300-400°C 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 620-660°C/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

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Welding Metallurgy Pc[1].Ppt 2