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

• Weld Time

• Hold Time

• Pulsing

• Electrode Force

• Postweld Temper

Min Heat For Melting

Threshold for Expulsion

Three Factors Responsible for AC Secondary Current Fluctuations

• Fluctuation in Supply Voltage• Geometry of Throat• Magnetic Material in Throat

DC not Significantly Affected

Steel is Ferromagnetic

High Impedance

Low Impedance

Current Density

Electrode Mushrooming

Shunting Effect

Effect of Current on Spot Weld Shear Strength

Current

ExpulsionBegins

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?

• Weld Time

• Hold Time

• Pulsing

• Electrode Force

• Postweld Temper

Nugget Area vs. Weld Time

0.041” Bare & Galvanized Steel

Weld Time, cycles

[Reference: Welding inthe Automotive Industry,p.175, D. W. Dickinson ]

Tensile-Shear Strength vs. Weld Time

Weld Time

Optimum

Effect of Weld Time on Heat Input

Low CurrentLong Time

High CurrentShort Time

Weld Time

Weld TimeT

Minimum

(a) (b)

Steady State

High HeatInput

Weld Current

Effect of Weld Time on Current Range (Weld Lobe)

Effect of Weld Time on Electrode Life

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?

• Weld Time

• Hold Time

• Pulsing

• Electrode Force

• Postweld Temper

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

Normal hold time

Long hold time

Interfacial Failures

Effect of Hold Time

0 10 20 30 40 50 60 70 80 Hold Time, Cycles

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

Higher Carbon MaterialWith Faster Cooling

Produce Harder MartensiteWith Cleavage Failure Mode

Weld-Metal Microstructures of Plain Carbon and Rephosphorized Steels for Different Hold

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

5 CycleHold Time

60 CycleHold Time

BrittleButtons

Effect of Hold Time on Lobe Curve

• Weld Time

• Hold Time

• Pulsing

• Electrode Force

• Postweld Temper

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?

• Weld Time

• Hold Time

• Pulsing

• Electrode Force

• Postweld Temper

Surface ConditionSteel

Oils/DirtOxide

OxideOils/Dirt

(a) Pickled Conditions

(b) Rusted ConditionsRusty

Pickled

Polished

Electrode ForceR

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

Electrode Force (Linear Scale)

Welding Force

Surface Breakthrough

Asperity Collapse

Wire Brushed

Pickled

Electrode Force, lbs1200 1400 1600 1800 2000 2400

400Res

[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

5 10 15 20 25 30

Electrode Force, kg/mm2

Low Force

Higher Force

Weld Current

Indentation, UnstableVery High Force

Lobe moves toHigher CurrentLonger Times

Weld Force Effect on Current Range

Current, kA

[Reference: “Formsof Dynamic Resistance CurvesGenerated DuringResistance SpotWelding”, Watney& Nagel]

Low Force

Higher Force

Electrode Force Buildup with 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?

Uncoated Plain Carbon Steel Process Variables. Uncoated Steel – Process Variables Lesson...

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