Uncoated Plain Carbon Steel Process Variables

Uncoated Steel – Process Variables Lesson Objectives When you finish this lesson you will understand: the effect that various welding parameter variables have on the weld quality of uncoated steels Learning Activities 1.Look up Keywords 2.View Slides; 3.Read Notes, 4.Listen to lecture 5.Do on-line workbook Keywords Weld Current Type, Heating Efficiency, Current Density, Weld Time, Hold Time, Pulsing, Electrode Force

Process Variables Process 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 Cycle Electrode Force Welding Current Welding Cycle Squeeze TimeWeld Time Hold Time Off Time [Reference: Welding Handbook, Volume 2, p.538, AWS]

Nugget Diameter vs. Weld Current Expulsion Small Nuggets Acceptable Nuggets Minimum Nugget Diameter Weld Current Nugget Diameter

Process Variables Process 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

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 Shear Strength Current Expulsion Begins [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 Variables Process 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 Nugget Diameter, inches [Reference: Welding in the Automotive Industry, p.175, D. W. Dickinson ]

Tensile-Shear Strength vs. Weld Time Weld Time Shear Strength Optimum [Reference: Welding Handbook, Volume 2, p.535, AWS]

Effect of Weld Time on Heat Input Low Current Long Time High Current Short Time Weld Time Nugget Diameter Weld Time Total Energy Input Minimum (a) (b) Steady State High Heat Input

Effect of Weld Time on Current Range (Weld Lobe)

Effect of Weld Time on Electrode Life High Current Short 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 Variables Process 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 Normal hold time Long hold time Interfacial Failures

Effect of Hold Time Hold Time, Cycles Max. Tensile-Shear Load, x 10 3 lbs Weld Hardness, Vickers Peel Test Fracture Mode Interfacial Failure Nugget Pull 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 Material With Faster Cooling Produce Harder Martensite With Cleavage Failure Mode

Weld-Metal Microstructures of Plain Carbon and Rephosphorized Steels for Different Hold Times 0.07C, 10 Cycle Hold0.07C, 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

Effect of Hold Time on Lobe Curve

Process Variables Process 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 1Pulse 2Pulse 3 Pulse Time

Without PulsingWith 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 Variables Process 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 Condition Steel Oils/Dirt Oxide Oils/Dirt (a) Pickled Conditions (b) Rusted Conditions Rusty Pickled Polished Electrode Force Resistivity

Resistance Varies with Pressure Low Pressure Medium Pressure High Pressure (a) (b) (c)

Relation of Electrode Force to Contact Resistance Resistance Greatly Dependent on Electrode Force Resistance Nearly Independent on Electrode Force FF FF Electrode Force (Linear Scale) Contact Resistance (Log Scale) R2R2 R1R1

Welding Force Surface Breakthrough Asperity Collapse Wire Brushed Pickled Electrode Force, lbs Resistance, Micro-ohms [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 Nugget Diameter Electrode Face Diameter Electrode Force, kg/mm 2

Very High Force Lobe moves to Higher Current Longer Times

Weld Force Effect on Current Range Current, kA Weld Force, lbs [Reference: “Forms of Dynamic Resistance Curves Generated During Resistance Spot Welding”, Watney & Nagel] Low Force Higher Force

Electrode Force Buildup with Time Time Electrode Force Time Adequate Squeeze Time Welding Current Initiated Adequate Squeeze Time Before 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?