Upload
erick-hogan
View
271
Download
9
Tags:
Embed Size (px)
Citation preview
This document is a property of L&T and must not be copied or lent without their permission in writing. 1
Welding Metallurgy
Pallav Chattopadhyay
Manufacturing Technology- I
14-Oct-2005
This document is a property of L&T and must not be copied or lent without their permission in writing. 2
Crystal Structure
This document is a property of L&T and must not be copied or lent without their permission in writing. 3
Solid Solution
Substitutional
Interstitial
This document is a property of L&T and must not be copied or lent without their permission in writing. 4
Solidification of Metal
This document is a property of L&T and must not be copied or lent without their permission in writing. 5
Ie-C Diagram
This document is a property of L&T and must not be copied or lent without their permission in writing. 6
Steel Structure as a function of %C
This document is a property of L&T and must not be copied or lent without their permission in writing. 7
Structural Changes in 0.4%C Steel during Slow
Cooling
This document is a property of L&T and must not be copied or lent without their permission in writing. 8
Typical Lamellar Pearlite
1500 X
This document is a property of L&T and must not be copied or lent without their permission in writing. 9
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 10
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 11
Macro section of Weld
This document is a property of L&T and must not be copied or lent without their permission in writing. 12
Weld & HAZ
This document is a property of L&T and must not be copied or lent without their permission in writing. 13
Microstructure of Low C Steel Weld Metal
100 X
This document is a property of L&T and must not be copied or lent without their permission in writing. 14
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 15
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 16
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 17
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 18
Typical Columnar Structure
Ingot
Weld Metal
This document is a property of L&T and must not be copied or lent without their permission in writing. 19
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 20
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 21
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 260C
: Surface appearance indicates effectiveness of shielding
: H2 major cause of porosity
This document is a property of L&T and must not be copied or lent without their permission in writing. 22
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 23
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 24
Weld Metal
Solid State reactions:
Strengthening mechanisms
Transformation hardening
Formation of harder structure / Martensite
Formation of fine Ferrite Carbide aggregate
This document is a property of L&T and must not be copied or lent without their permission in writing. 25
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 26
Solubility of Hydrogen in Iron
This document is a property of L&T and must not be copied or lent without their permission in writing. 27
Diffusivity of Hydrogen in Iron
This document is a property of L&T and must not be copied or lent without their permission in writing. 28
This document is a property of L&T and must not be copied or lent without their permission in writing. 29
This document is a property of L&T and must not be copied or lent without their permission in writing. 30
Heat Affected Zone (HAZ)
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 31
Heat Affected Zone (HAZ)
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.
This document is a property of L&T and must not be copied or lent without their permission in writing. 32
Heat Affected Zone (HAZ)
Effect of welding parameters depends on types of
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 33
Heat Affected Zone (HAZ)
Effect of welding parameters depends on types of
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 34
Transformation Hardening Alloys
This document is a property of L&T and must not be copied or lent without their permission in writing. 35
This document is a property of L&T and must not be copied or lent without their permission in writing. 36
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
__________
This document is a property of L&T and must not be copied or lent without their permission in writing. 37
This document is a property of L&T and must not be copied or lent without their permission in writing. 38
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 39
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 40
Effect of Heat Input, Geometry & Preheat
on Cooling Rate
This document is a property of L&T and must not be copied or lent without their permission in writing. 41
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 42
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 43
Typical Preheat Temperature
This document is a property of L&T and must not be copied or lent without their permission in writing. 44
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 45
Effect of Preheat / Interpass Temp
This document is a property of L&T and must not be copied or lent without their permission in writing. 46
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 47
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
This document is a property of L&T and must not be copied or lent without their permission in writing. 48
PWHT / Stress Relieving
Larson Miller Parameter (LMP) =
T (20 + Log10 t), where T = Tempering Temp in K &
t = Tempering time in hrs
This document is a property of L&T and must not be copied or lent without their permission in writing. 49
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)
This document is a property of L&T and must not be copied or lent without their permission in writing. 50
Thank You