WELDABILITY
KDJ20203
TECHNOLOGY OF METAL WELDING
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GTAW is usually used to weld expensive metals or
alloys, such as copper, aluminum, titanium, etc. What
do we use to weld cheap 1mm thickness mild steel
plates?
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INTRODUCTION
o Weldability: “Capability of a metal to be welded under imposed
fabrication conditions, into a specific designed structure that performs
satisfactorily in the intended service.”
o Factors that affect the weldability of metals must be considered to
ensure the desired welding quality.
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CARBON & ALLOY STEELS
o Carbon & alloying elements can affect the weldability, preheat & post
heating of steels
o Carbon steels: iron, carbon & manganese
o Groups: low, medium & high carbon steels
o Carbon content increases weldability decreases
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CARBON & ALLOY STEELS
o Low carbon steel (≤ 0.3%C, ≤ 1.2% Mn):
o Not strengthened by heat treatment
o Surface hardened by carburizing
o Structural applications: building framework, pressure vessels &
automotive bodies.
o Nickel steels 2-9% Ni: storage tanks for liquefied hydrocarbon gases
and machines for use in cold climates.
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CARBON & ALLOY STEELS
o Medium carbon steels (0.3-0.5%C, 0.6-1.65%Mn):
o Stronger than low carbon steels
o Form martensite in Heat Affected Zone (HAZ) when rapidly cooled.
o Susceptible to hydrogen cracking.
o Require heat treatment after welding to achieve desired strength &
hardness.
o Improve wear resistance through chrome plating or nitriding.
o Used in machinery parts: tractors & pumps.
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CARBON & ALLOY STEELS
o High carbon steels (> 0.6%C, not welded):
o High hardness & strength: drill bits & files.
o Free-machining steels: small amount of S,P & Pb to improve
machinability.
o Not recommended to weld
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CARBON & ALLOY STEELS
o Graphitization: Steel break down to form iron and carbon (graphite).
Carbon migrates to the material’s grain boundaries forming graphite
nodules, which cause the metal to become brittle, losing strength,
toughness, creep resistance, and ductility.
o Carbon content <0.2% to maintain weldability.
o High temp piping & vessels in petroleum refining & steam power
generator (up to 1000F @ 537C)
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CARBON & ALLOY STEELS
o Rapid cooling after welding causes metallurgical transformation similar
as in quenching
o Martensite transformation in HAZ: %C ↑, hardness ↑ toughness ↓
o Increases susceptibility to cold cracking @ hydrogen cracking from
residual stress
o Preheat to reduce cold cracking by reducing the cooling rate &
formation of martensite
o Postheat to improve martensite toughness by reducing residual stress
& eliminate hydrogen
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CARBON & ALLOY STEELS
o Alloy steels form martensite or bainite depending on the cooling rate
o Slower cooling rate bainite
o Preheat is required to slow cooling rate
A B
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HYDROGEN CRACKING
o Caused by hydrogen atom resides in:
o Grease
o Absorbed water in electrode coating
o Moisture on work piece surface
o Hydrogen atoms created at welding temp
o Diffuse rapidly into molten weld metal
o As weld metal solidifies, most hydrogen escape
but some trapped inside HAZ
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HYDROGEN CRACKING
o Brittle martensite failed to yield to accommodate the residual stresses
that develop due to hydrogen entrapment hydrogen cracking
o Hydrogen crack may occur days after welding
o Often located below the welding surface undetectable by common
NDT techniques
o Preventive methods:
o Low hydrogen electrodes
o Electrode storage oven
o Preheating & postheating
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HEAT REQUIREMENT
o Include preheat, interpass temp control & postheating.
o Preheat lower the cooling rate of weld thus lessens the tendency to
form martensite
o Prevents hardness in HAZ, residual stresses, distortion.
o Burns grease, oil & scale clean surface
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HEAT REQUIREMENT
o Accomplished by oxyacetylene flame
o 200°F - 700°F (93°C - 371°C)
o %C ↑ preheat temp ↑
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HEAT REQUIREMENT
o Interpass temp is temp between passes of multi-pass welding.
o Large volume of heat during welding can maintain interpass
temperature.
o High current & low travel speed cause heat to build up in the metal
thus slow the cooling rate
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HEAT REQUIREMENT
o Postheating is a stress-relief treatment for welding medium carbon
steel
o Necessary for thick base metal
o 900°F - 1250°F (482°C - 677°C)
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CAST IRONS
o Groups:
o Gray
o White
o Malleable
o Ductile
o compacted graphite
o alloy irons
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CAST IRONS
o Gray irons: pearlite (iron carbide & ferrite), ferrite or martensite
o All 3 structures contain graphite flakes
o High temp scaling resistance & thermal shock resistance
Easy to arc weld.
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CAST IRONS
o White irons: carbon does not precipitate as graphite during
solidification
o Combines with iron or alloys elements Cr, Mo & Va to form iron carbide
or alloy carbide
o Carbides make white iron extremely hard, wear resistant & brittle
o Not recommended for welding
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CAST IRONS
o Malleable irons: high ductility due to heat treating of white iron
o Can be welded but cannot be heated > critical temp 1382°F (750°C)
or reverts to white iron
o Heat treatment turns graphite flakes into nodules ductility ↑
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CAST IRONS
o Ductile irons: %C & Si similar to gray iron but differ graphite shape –
spheroidal (nodular)
o Can be arc weld provided adequate preheat & postheating are used or
properties might be lost
23
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STAINLESS STEELS
o Corrosion resistance due to %Cr ↑↑ 12-30%
o Up to 25%Ni & 6.5%Mo
o Heat resistance, corrosion resistance & low temp toughness
o Groups:
o Austenitic
o Martensitic
o Ferritic
o Duplex
o Precipitation hardening
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STAINLESS STEELS
o Austenitic: largest groups & widest usage
o Excellent corrosion resistance, weldability, high temp strength & low
temp toughness
o Basic composition 16-26%Cr, 3.5-37%Ni
o Standard grade: 200 & 300 series
o CTE of Austenitic 50-60% higher than carbon steels, thus prone to
distortion during welding
o Jig & fixtures to prevent warping & distortion
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STAINLESS STEELS
o Martensitic: up to 18%Cr & 1.5%C
o High strength & wear resistance via air hardening & tempering
o Basic martensitic stainless steel is type 410
o CTEs of martensitic are similar to carbon steels, thus their thermal
expansion are practically equal weldability equivalent to carbon
steels
o Thermal conductivity of martensitic are 50% higher than carbon steels
higher current
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STAINLESS STEELS
o Ferritic: %Cr is higher than martensitic
o Higher corrosion resistance than martensitic
o Basic ferritic stainless steel is type 430
o CTE and thermal conductivity are similar to martensitic.
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STAINLESS STEELS
o Duplex: composite of equal austenite & ferrite
o Higher strength & chloride stress-cracking resistance than austenitic
o Basic duplex stainless steel is type 329
o Intermittent welding or back step welding to minimize heat input
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STAINLESS STEELS
o Precipitation hardening: Via heat treatment to much higher strengths
o Relatively weak & soft when quenched from solution annealing heat
treatment temp
o Filler metals: depend on the type of base metal
o Alloy content ≥ base metal to compensate for expected alloy loss
during welding.
o Cr-Ni filler metals for Cr-grade stainless steels to provide ductile weld
metal.
o Low hydrogen filler metals must be heated in the oven due to
susceptibility to moisture pickup
30
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welding 304 stainless steel
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NONFERROUS
o Ni-alloys generally easy to weld
o Extremely sensitive to cracking due to contamination.
o Sulfur from grease & oil is harmful.
o Oxides inhibit wetting which prevents fusion between base metal &
filler metal subsurface inclusions & poor bead contour.
o Heat retained in weld due to low thermal conductivity of Ni-alloys
prone to distortion
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NONFERROUS
o Cu & Cu alloys are difficult to weld due to high thermal conductivity,
CTE, fluidity & hot cracking susceptibility.
o High electrical & thermal conductivities difficult for resistance spot
& seam welding
o Best joined by brazing & soldering.
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NONFERROUS
o Wrought Aluminum or cast aluminum alloys have low density, good
corrosion resistance & weldability.
o Cleaning requirements are stringent due to the Aluminum oxide
surface
o Al2O3 particles might be trapped in the weld – low ductility, lack of
fusion & weld cracking
o Heat requirements is high due to high thermal conductivity & CTE
o High heat input & welding speed is preferred such as GMAW
o Distortion is twice as great as steel – high speed welding is required.
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NONFERROUS
o Welding processes: GTAW, GMAW & resistance welding
o GTAW: thin joints, AC current @ cleaning action
o Ar is commonly used for shielding
o He to increase penetration
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THE END
