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Uncoated Plain Carbon Steel Process Variables

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 Look up Keywords

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Uncoated Plain Carbon Steel Process Variables

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  1. Uncoated Plain Carbon Steel Process Variables

  2. 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 • Look up Keywords • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook Keywords Weld Current Type, Heating Efficiency, Current Density, Weld Time, Hold Time, Pulsing, Electrode Force

  3. 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 Process Variables

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

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

  6. 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 Process Variables

  7. Three Factors Responsible for AC Secondary Current Fluctuations • Fluctuation in Supply Voltage • Geometry of Throat • Magnetic Material in Throat DC not Significantly Affected

  8. Steel is Ferromagnetic High Impedance Low Impedance

  9. Current Density Shunting Effect Electrode Mushrooming

  10. Effect of Current on Spot Weld Shear Strength Expulsion Begins Shear Strength Current [Reference: Welding Handbook, Volume 2, p.535, AWS]

  11. Questions? • 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?

  12. 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 Process Variables

  13. Nugget Area vs. Weld Time 0.041” Bare & Galvanized Steel Nugget Diameter, inches [Reference: Welding in the Automotive Industry, p.175, D. W. Dickinson ] Weld Time, cycles

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

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

  16. Effect of Weld Time on Current Range (Weld Lobe)

  17. 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

  18. Effect of Weld Time on Softening Recovery Annealed Steels

  19. Questions? • 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?

  20. 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 Process Variables

  21. Hold Time Sensitivity Normal hold time Long hold time Interfacial Failures • 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

  22. Effect of Hold Time 500 Peel Test Fracture Mode Nugget Pull Out Interfacial Failure 400 Weld Hardness, Vickers 300 200 6 5 Max. Tensile-Shear Load, x 103 lbs 4 3 0 10 20 30 40 50 60 70 80 Hold Time, Cycles [Reference: Welding in the Automotive Industry, p.193, D. W. Dickinson]

  23. 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

  24. Higher Carbon Material With Faster Cooling Produce Harder Martensite With Cleavage Failure Mode Gould, Fracture Morphologies of RSW Exhibiting Hold Time, SMWC VIII, AWS, 1998

  25. Weld-Metal Microstructures of Plain Carbon and Rephosphorized Steels for Different Hold Times Some Pro-Eutectoid Ferrite Mostly Martensite 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]

  26. Effect of Hold Time on Lobe Curve

  27. 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 Process Variables

  28. Pulsing Pulse 1 Pulse 2 Pulse 3 Cool Time Pulse Time

  29. Without Pulsing With Pulsing • Tip Cooling Between Pulses • Heating At Electrode • Tip Skidding • Electrode Mushrooming

  30. Questions? • 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?

  31. 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 Process Variables

  32. Surface Condition (a) Pickled Conditions (b) Rusted Conditions Steel Steel Resistivity Rusty Steel Steel Oils/Dirt Oxide Oxide Oils/Dirt Polished Pickled Electrode Force

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

  34. Relation of Electrode Force to Contact Resistance Resistance Greatly Dependent on Electrode Force DR1 Contact Resistance (Log Scale) Resistance Nearly Independent on Electrode Force DR2 DF DF Electrode Force (Linear Scale)

  35. Welding Force 2800 Surface Breakthrough 2400 2000 Asperity Collapse 1600 Resistance, Micro-ohms Wire Brushed 1200 800 Pickled 400 1200 1400 1600 1800 2000 2400 Electrode Force, lbs [Reference: Welding in the Automotive Industry, p.139, D. W. Dickinson]

  36. Effect of Electrode Force on Nugget Diameter 1.8 1.6 1.4 Modified Electrode 1.2 1.0 Nugget Diameter Electrode Face Diameter Truncated Cone 0.8 0.6 0.4 Constant Weld Current Density 0.2 5 10 15 20 25 30 Electrode Force, kg/mm2

  37. Lobe moves to Higher Current Longer Times Very High Force

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

  39. Electrode Force Buildup with Time Adequate Squeeze Time Before Current Initiation Premature Initiation of Current Welding Current Initiated Electrode Force Electrode Force Adequate Squeeze Time Thermal Expansion & Contraction at Interfaces Time Time

  40. Questions? • 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?

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