Uncoated Plain Carbon Steel Process Variables. Uncoated Steel – Process Variables Lesson...
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Transcript of Uncoated Plain Carbon Steel Process Variables. Uncoated Steel – Process Variables Lesson...
Uncoated Plain Carbon SteelProcess Variables
Uncoated Steel – Process Variables
Lesson ObjectivesWhen you finish this lesson you will understand:• the effect that various welding parameter variables have on the weld quality of uncoated steels
Learning Activities1. Look up Keywords2. View Slides; 3. Read Notes, 4. Listen to lecture5. Do on-line
workbook
KeywordsWeld Current Type, Heating Efficiency, Current Density, Weld Time, Hold Time, Pulsing, Electrode Force
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
Welding CycleElectrode Force
Welding Current
Welding Cycle
Squeeze Time Weld Time HoldTime
OffTime
[Reference: Welding Handbook, Volume 2, p.538, AWS]
Nugget Diameter vs. Weld Current
Expulsion
SmallNuggets
AcceptableNuggets
Minimum Nugget Diameter
Weld Current
Nug
get
Dia
met
er
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
AC DC
RMSIn
stan
tane
ous
Hea
t
Min Heat For Melting
Threshold for Expulsion
60 C
ycle
AC
Three Factors Responsible for AC Secondary Current Fluctuations
• Fluctuation in Supply Voltage• Geometry of Throat• Magnetic Material in Throat
DC not Significantly Affected
60 C
ycle
AC
60 C
ycle
AC
Steel is Ferromagnetic
High Impedance
Low Impedance
Current Density
Electrode Mushrooming
Shunting Effect
Effect of Current on Spot Weld Shear Strength
She
ar S
tren
gth
Current
ExpulsionBegins
[Reference: Welding Handbook, Volume 2, p.535, AWS]
Turn to the person sitting next to you and discuss (1 min.):• The ferromagnetic properties of steel effect the power factor of an AC welder when this material is concentrated within the throat. Will the same effects occur in DC current? Why or why not?
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
Nugget Area vs. Weld Time
0.041” Bare & Galvanized Steel
Weld Time, cycles
Nu
gge
t Dia
met
er,
inch
es
[Reference: Welding inthe Automotive Industry,p.175, D. W. Dickinson ]
Tensile-Shear Strength vs. Weld Time
Weld Time
She
ar S
tren
gth
Optimum
[Reference: Welding Handbook, Volume 2, p.535, AWS]
Effect of Weld Time on Heat Input
Low CurrentLong Time
High CurrentShort Time
Weld Time
Nu
gge
t Dia
met
er
Weld TimeT
ota
l En
erg
y In
pu
t
Minimum
(a) (b)
Steady State
High HeatInput
Weld Current
Wel
d Ti
me
Effect of Weld Time on Current Range (Weld Lobe)
Effect of Weld Time on Electrode Life
Time
Ele
ctro
de T
emp
electrode softening temp
High CurrentShort Time
Low Current Long Time
The Best Condition is Usually the One That Minimizes the Integrated Time at Temperature without exceeding
Tip Softening Temperature
Effect of Weld Time on Softening Recovery Annealed Steels
Turn to the person sitting next to you and discuss (1 min.):• We have already seen that weld time can effect the HAZ properties of Recovery Annealed Steels. Would you expect increased weld time to have any effect on the properties of HSLA or Dual Phase steels?
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
Hold Time Sensitivity
• Disadvantageous stress state(Yielding Tri-axial Condition at Peeled Edge)
• Presence of preferential crack paths (porosity)(Porosity Along Dendrite/Cell Boundaries - Low Melting Eutectics)
• Susceptible microstructure(Hard Brittle Martensites - Sensitive to Cleavage Fracture)
Gould, Fracture Morphologies of RSW Exhibiting Hold Time,SMWC VIII, AWS, 1998
Weld Current
We
ld T
ime
Normal hold time
Long hold time
Interfacial Failures
Effect of Hold Time
0 10 20 30 40 50 60 70 80 Hold Time, Cycles
500
400
300
200
6
5
4
3
Ma
x. T
ens
ile-S
hea
rL
oad
, x 1
03 lb
sW
eld
Ha
rdn
ess
,V
icke
rs
Peel Test Fracture ModeInterfacialFailure
NuggetPull Out
[Reference: Welding in the Automotive Industry, p.193, D. W. Dickinson]
Rephosphorized Steels have low melting Eutectics and Form Porosity at Dendrite & Cell Boundaries & Weld Centerline
Gould, Fracture Morphologies of RSW Exhibiting Hold Time,SMWC VIII, AWS, 1998
Gould, Fracture Morphologies of RSW Exhibiting Hold Time,SMWC VIII, AWS, 1998
Higher Carbon MaterialWith Faster Cooling
Produce Harder MartensiteWith Cleavage Failure Mode
Weld-Metal Microstructures of Plain Carbon and Rephosphorized Steels for Different Hold
Times
0.07C, 10 Cycle Hold 0.07C, 30 Cycle Hold
0.06C-0.09P, 10 Cycle Hold 0.06C-0.09P, 30 Cycle Hold
[Reference: “Spot Weldability of High-Strength Sheet Steels”, Welding Journal 59 (January 1980), Baker & Sawhill]
Some Pro-Eutectoid Ferrite
Mostly Martensite
Weld Current
Wel
d Ti
me
5 CycleHold Time
60 CycleHold Time
BrittleButtons
Effect of Hold Time on Lobe Curve
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
Pulsing
Cool Time
Pulse 1 Pulse 2 Pulse 3
PulseTime
Without Pulsing With Pulsing
• Heating At Electrode• Tip Skidding• Electrode Mushrooming
• Tip Cooling Between Pulses
Turn to the person sitting next to you and discuss (1 min.):• Pulsing cycles of 3 or 4 cycles current flow followed by 1 or 2 cycles off are very effective when welding steels to allow the electrodes to cool while not seriously effecting the steel nugget growth. Would this be an effective procedure for aluminum alloys? Why or why not?
Process VariablesProcess
Parameters:(Manufacturer’s Control)
• Weld Current Type
• Weld Time
• Hold Time
• Pulsing
• Electrode Force
• Electrode Material
• Electrode Designs
• Electrode Cooling
• Postweld Temper
Material Parameters:(Steel Company Control)
• Chemistry• Cleanliness• Surface Condition• Material Processing• Thickness
Surface ConditionSteel
Steel
Steel
Steel
Oils/DirtOxide
OxideOils/Dirt
(a) Pickled Conditions
(b) Rusted ConditionsRusty
Pickled
Polished
Electrode ForceR
esis
tivity
Resistance Varies with Pressure
Low Pressure Medium Pressure High Pressure
(a) (b) (c)
Relation of Electrode Force to Contact Resistance
Resistance Greatly Dependenton Electrode Force
Resistance Nearly Independenton Electrode Force
F F
Electrode Force (Linear Scale)
Co
nta
ct R
esi
stan
ce (
Lo
g S
cale
)
R2
R1
Welding Force
Surface Breakthrough
Asperity Collapse
Wire Brushed
Pickled
Electrode Force, lbs1200 1400 1600 1800 2000 2400
2800
2400
2000
1600
1200
800
400Res
ista
nce,
Mic
ro-o
hms
[Reference: Welding in the Automotive Industry, p.139, D. W. Dickinson]
Effect of Electrode Force on Nugget Diameter
Modified Electrode
Truncated Cone
Constant Weld Current Density
Nu
gge
t Dia
met
er
Ele
ctro
de
Fa
ce D
iam
ete
r
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
5 10 15 20 25 30
Electrode Force, kg/mm2
Low Force
Higher Force
Weld Current
Wel
d Ti
me
Indentation, UnstableVery High Force
Lobe moves toHigher CurrentLonger Times
Weld Force Effect on Current Range
Current, kA
We
ld F
orc
e, l
bs
[Reference: “Formsof Dynamic Resistance CurvesGenerated DuringResistance SpotWelding”, Watney& Nagel]
Low Force
Higher Force
Electrode Force Buildup with Time
Time
Ele
ctro
de
Fo
rce
Ele
ctro
de
Fo
rce
Time
AdequateSqueezeTime
Welding CurrentInitiated
Adequate Squeeze TimeBefore Current Initiation
Premature Initiation of Current
Thermal Expansion& Contraction at Interfaces
Turn to the person sitting next to you and discuss (1 min.):• Several companies are developing in the electrode real time force sensing devices. Do you think they will be able to use these to determine spot weld quality? Develop feedback control? What problems might they have when welding steel?