week 2 heat treatment and the effect of welding
TRANSCRIPT
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Heat Treatment and the Effect
of Welding
Week 2
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Heat Treatment of Steels
The basis of heat treatment is that FCC iron
can dissolve all carbon in steel (up to 2%
C), while BCC iron can dissolve practicallynone (
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Steel Phase Diagram
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Contd
On cooling the carbon will attempt to precipitateout of solution as Cementite
By controlling the mode of cooling thedistribution of Cementite & hence the mechanicalproperties can be controlled
Steels are heated slowly to the Austenite region (+30 to 50 C) to ensure it is fully Austenitic & thatthe grains are as small as possible
Final properties depend on the mode of cooling
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Cooling
Annealingusually on cast & hot workedsteels with coarse grain structures to obtain
grain refinement, stiffness & ductility
Particularly necessary on components
requiring additional work
Involves cooling slowly in the furnace orpacked in sand
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Contd
Hardeningquenching into oil, water or brinefrom the soak temperature fast enough to prevent
the formation of PearliteNew phase known as Martensite (supersaturated
solution of carbon in ferrite) very hard & as a
result the steels become very brittle
With water quenching the steel becomes too brittle
for use becomes necessary to tempersteel
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Contd
Tempering re-heating to the sub critical range(approx 650 C), where stresses set up on
quenching are relieved, so reducing the brittlenessSteel becomes tougher at the expense of hardness
Quenching & tempering are principally applied to
high carbon steels, where high hardness is
required or to alloy steels to achieve high strength
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Welding
Extensively used for joining materials together
Very complex geometries can be effectivelywelded
Produces cleaner lines and reduces painting costs
Cheaper, simpler & lighter than rivets or bolts
The material is heated locally to its melting
temperatureAdditional metal may be introduced and the jointis then allowed to cool naturally
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Contd
Allows greater freedom for design
Allows for continuous beams & girders
Easy & quick alterations
Additions can easily be made
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Methods Available
Arc welding
Gas welding
Friction weldingSpot welding
Soldering
Brazing
Electron beam
Laser
Diffusion bonding
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Ideal Requirements of Welding
A.) Complete continuity should be
maintained between parts to be joined
Joint should be indistinguishable from theparent metal
Practically the above is not always possible,
although satisfactory weld performance canbe achieved in most cases
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Contd
B.) The joining material should have
properties that are similar to the parent
metalCareful selection of welding rods etc. is
therefore essential
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Heat Affected Zone
Weld is basically a rapidly formed casting
surrounded by a heat affected zone (HAZ)
A temperature gradient is set up in thematerial during welding
Temperature gradient ranges from the
melting point at the point of fusion toambient temperature at some distance from
the weld
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Contd
High temperature followed by fairly rapid
cooling causes changes in the metallurgy of
the metal and the joint quality can beaffected by:
a.) Structure & quality of the weld metal
b.) Structure & properties of the part ofthe metal in the heat affected zone
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Rate of Cooling After Welding
The slower the rate of cooling, the closer
the structure to equilibrium
Cooling occurs mainly by conduction in theparent metal, depending upon the thermal
mass (thickness & size of parent material)
The greater the thermal mass, the faster therate of cooling
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Arc Welding
The main method employed for structuralsteelwork is arc welding
Principles Electrode or filler wire melts due topassage of welding current through the filler wire,Arc (plasma) & back to the power source via theearthed component
Typically arc temperature is 5000 to 30000K
The melt is transferred across the arc severalmechanisms droplets, spray etc.
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Welding Process
Basically require:
1.) Heat source to effect fusion
2.) Satisfactory metallurgical properties
3.) An efficient process
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Processes used
Manual metal arc
Automatic welding using continuous coated
electrodes
Submerged arc welding
Carbon dioxide shielded metal arc (MIG)
Electrostatic welding
Stud welding
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Typical Welds
Butt Weld
Full penetration
Partial penetration
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Contd
Fillet Weld
l1t
l2
t = throat
l1 = vertical leg
l2 = horizontal leg
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Defects
Residual stresses
Distortions
Undercut
Incomplete penetration
Porosity
Slag inclusion
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Weld Metal Solidification
Cracking
Weld metal solidification cracking hot
cracking longitudinal in a fillet weld
blue appearance (oxidised surface) due tomaterial composition and/or weld restrain &
bead shape
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Heat Affected Zone (HAZ)
Cracking
Heat Affected Zone (HAZ) Cracking heataffected zone due to weld adjacent to bead
affected by heat input & cooling cycle
depends on composition but cooling ratecan affect microstructure hardening more brittle carbide formation
Susceptibility also affected by hydrogen inthe weld metal introduced from the weldrod which is consumable
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Carbon Equivalent
Metal arc welding of carbon & carbon
manganese steels need to be checked by
reference to BS EN 1011 2: 2001 guidance on carbon equivalents suggests
suitable preheat levels to reduce cooling
rate for various thicknesses & limits onhydrogen levels sometimes need post heat
(heat treatment)
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Empirical Formula
15/)(5/)(6/ CuNiVMoCrMnCCE
C & Mn have a significant effect
Cr, Mo, Ni, Cu have little effect
Limited usually to
CE value
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HAZ Cracking
HAZ
HAZ crackWeld bead
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Lamellar Tearing
Associated with non-metallic manganese
sulphides & silicates when rolled material is
extended as planer type inclusions (likewrought iron)
Welds run parallel to inclusions & cracks
are induced through contractile stressingacross thickness of the plate
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Lamellar Tear Diagram
Lamellar tear
Inclusions
thin planer
types
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BS 4360 Steel (grade 50C)
Typical ladle analysis:
C = 0.21%
Mn = 1.50%
Cr = 0.025%
Mo = 0.015%
Ni = 0.04%
Cu = 0.04%
Determine the carbon
equivalent & comment on
weldability
C b E i l f BS 3460
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Carbon Equivalent of BS 3460
Steel
15/)(5/)(6/ CuNiVMoCrMnCCE
%473.0
005.008.025.021.0
15/)04.004.0(5/)015.0025.0(6/5.121.0
CE
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Comments on Weldability
Few problems are encountered at values
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Contd
If, of the elements in this formula, only
carbon and manganese are stated on the mill
sheet for carbon and carbon manganesesteels, then 0,03 should be added to the
calculated value to allow for residual
elements.
Where steels of different carbon equivalent
or grade are being joined, the higher carbon
equivalent value should be used
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Weld Decay in Stainless Steel
WeldHeat Affected Zone
Grain boundaries (scale ofgrains grossly exaggerated)
Region depleted of
chromium & no longer
stainless is attacked
preferentially by corrosion
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Welding & Structural Steels
Designed to be weldable
No serious loss of performance in the weld
or the HAZStructural engineers make allowance for
HAZ in the design process (typically a
20N/mm2 reduction in the yield strength isapplied)
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Electric Arc Welding
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Electric Arc Welding Equipment
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Use of Electric Arc Welding
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Metal Arc Inert Gas Shielded
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MIG Equipment
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Use of MIG Equipment
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Butt Weld
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Slag Inclusion
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X-Ray Testing