Uncoated Plain Carbon Steel Material Variables

Uncoated Plain Carbon – Material variables Lesson Objectives When you finish this lesson you will understand: the relationship between steel manufacturing variables and spot weldability Learning Activities 1.Look up Keywords 2.View Slides; 3.Read Notes, 4.Listen to lecture 5.Do on-line workbook Keywords Chemistry, Carbon Equivalent, Steel Cleanliness, Surface Condition, Solid State Bond, Thickness

Material Variables Process Parameters: Weld Current Weld Time Hold Time Upslope/Downslope Pulsing Electrode Force Postweld Temper Electrode Designs Material Parameters: Chemistry Cleanliness Surface Condition Material Processing Thickness

Chemistry Material Thickness Carbon Content, % Maximum Hardness, DPH [Reference: Welding in the Automotive Industry, p.153, D. W. Dickinson ] To avoid weld problems: C < 0.10% t Thin Material = 100% Martensite (interface tears) Thick Material = Slower Cooling= Ferrite + Pearlite

Weldability Lobes for Uncoated Mild & Interstitial-Free Steel Interstitial Free Steel Mild Steel Welding Current (kA) Weld Time (Cycles) [Reference: “Challenges in Welding New Sheet Steel Products”, Gould & Kimchi] IF Steels have lower bulk Resistance IF Mild Steel

Chemistry (CONT.) Spot Weld Failure Acceptable Spot Weld C, % P + 3S, % [Reference: Welding in the Automotive Industry, p.154, D. W. Dickinson ] (Centerline Cracking)

Base Metal Microstructures for Killed Plain Carbon and Rephosphorized Steels [Reference: “Spot Weldability of High-Strength Sheet Steels”, Welding Journal 59 (January 1980), Baker & Sawhill] Since P in solution = Very little difference in microstructure 100/110 HV140/170 HV Result: Hardness & Centerline Cracking

Effect of Carbon and Phosphorous on Current Range 0.31 in in %C + %P Current Range, Amp (0.15 in. Minimum Button) [Reference: “Spot Weldability of High-Strength Sheet Steels”, Welding Journal 59 (January 1980), Baker & Sawhill] 0.25 in Electrode Drop due to interfacial nugget tears

Hardness Transverses in a Spot Weld between Rephosphorized and Plain Carbon Steels HAZ Distance from Fusion Line, in. Hardness, HV1.0 [Reference: “Spot Weldability of High-Strength Sheet Steels”, Welding Journal 59 (January 1980), Baker & Sawhill] Base Metal

Chemistry (CONT.) Max. Tensile Shear Stress Max. Cross- Tension Strength Max. Button Dia. in Peel Test Weight, %Ti Maximum Strength Maximum Button Diameter Weight, %Ti Tensile-Shear Test Peel Test Welding Range, % [Reference: Welding in the Automotive Industry, p.156, D. W. Dickinson ] Centerline Tears

OTHER ELEMENTS NITROGEN Promotes Interfacial Tears More Critical in Unkilled Cold-Rolled Gages N Tied up by Al in Killed Steels OXYGEN Promotes Interfacial Tears Kill Heats to Reduce Get Rid of Rust HYDROGEN Usually not a problem except in High Carbon Remove Surface Oils

CE = C + Mn/36 + (Cr + Mo + Zr)/10 + Ti/2 + Cb/3 + V + T.S.(ksi)/900 + t(in.)/20 Empirical Carbon Equivalent Equation For Best Results CE < 0.30

Effect of Boron Babu, S et Al, “Effect of Boron on the Microstructure of Low-Carbon Steel Resistance Seam Welds” Welding Journal, 1997

Turn to the person sitting next to you and discuss (1 min.): The chemistry effects in spot and seam welding of carbon steels are similar to those in GTAW at high travel speeds but somewhat more exaggerated. Considering solidification morphology, why should this be?

Process Variables Process Parameters: Weld Current Weld Time Hold Time Upslope/Downslope Pulsing Electrode Force Postweld Temper Electrode Designs Material Parameters: Chemistry Cleanliness Surface Condition Material Processing Thickness

Steel Cleanliness [Reference: Welding in the Automotive Industry, p.160, D. W. Dickinson ]

Process Variables Process Parameters: Weld Current Weld Time Hold Time Upslope/Downslope Pulsing Electrode Force Postweld Temper Electrode Designs Material Parameters: Chemistry Cleanliness Surface Condition Material Processing Thickness

Surface Condition Oils/Dirt Oxide Steel Hot Spots Alloying & Cavitation Surface Expulsion Electrode Eroding

THICK SHEET - High Currents THIN SHEET Lower Current Weld on Scale Scale Cleaned Weld on Scale - No Upslope With Upslope Scale Cleaned Effect of Surface Oxide on Electrode

Effect of Surface Oxide on Lobe

Effect of Surface Carbon on Button Tear Dirty Clean HAZ Solid State Bond Nugget Interface Failure

Weld Lobes of Two HSLA Steels Batch Annealed (high surface C) vs Continuous Annealed (low surface C) Current, kA Weld Time, cycles [Reference: “Forms of Dynamic Resistance Curves Generated During Resistance Spot Welding”, Watney & Nagel] Accu-form 50XK Batch-Annealed B50XK Lower Surface Carbon No Partial Nuggets

Turn to the person sitting next to you and discuss (1 min.): What are some ways that a steel company can get cleaner steels both internally and on the surface?

Process Variables Process Parameters: Weld Current Weld Time Hold Time Upslope/Downslope Pulsing Electrode Force Postweld Temper Electrode Designs Material Parameters: Chemistry Cleanliness Surface Condition Material Processing Thickness

Heat-Affected Zone Property Loss in HSLA Steels Knoop Hardness Distance From Fusion Line Base MetalAged Zone Recrystallized Ferritic Zone Martensitic Zone [Reference: Welding in the Automotive Industry, p.162, D. W. Dickinson ] Plain Carbon HSLA Controlled Rolled Grain Refinement Ppt Strength Transformation Martensite Tempering

Microstructure Near Outside Edge of HAZ in SRA Steel Spot Weld [Reference: “Spot Weldability of High -Strength Sheet Steels”, Welding Journal 59 (January 1980), Baker & Sawhill]

Process Variables Process Parameters: Weld Current Weld Time Hold Time Upslope/Downslope Pulsing Electrode Force Postweld Temper Electrode Designs Material Parameters: Chemistry Cleanliness Surface Condition Material Processing Thickness

High Hardness at Weld Edge Water Cooled Copper Electrode

THICKNESS HARDNESS martensitem + p + fp + f carbon %

Thickness Time (cycles) Time (cycles) Time (cycles) Temperature 0.5 mm Steel 1.1 mm Steel 1.5 mm Steel

Spot Welding of Extra Heavy Gage Mild Steel Plate (12 mm) Machine Characteristics Power Source: 3 phase Freq Convert Rated Capacity : 150 kVA Max Capacity : 1,000 kVA Max Current : 150 KA Max Force : 20 Tons Uses Architecture Bridges Off Highway Vehicles Yamamoto, T “A study of spot welding of heavy gauge mild steel”, Welding In the World, July/Aug, 1971

Spot Welding of Extra Heavy Gage Mild Steel Plate (12 mm) Welding Procedure Modified Squeeze time heat and Pre-heat, High Squeeze Force added to Set Parts. (20 Ton Force Press) Welding Done With Frequency Changer – Very Long Weld Times Forge and Q&T Added Yamamoto, T “A study of spot welding of heavy gauge mild steel”, Welding In the World, July/Aug, 1971

Spot Welding of Extra Heavy Gage Mild Steel (12 mm) Corona Diameter Nugget Diameter Indentation Penetration 1mm Thick steel reaches peak in nugget diameter in 12 cycles But, 12 mm Thick Steel peak is 150 – 5 cycle pulses = 750 cycles Weld Strength = f (Nugget dia + Corona Dia)

Yamamoto, T “A study of spot welding of heavy gauge mild steel”, Welding In the World, July/Aug, 1971 Effect of Electrode Force on Spot Welding of Extra Heavy Gage Mild Steel (12 mm) Increased Force Reduces Nugget Diameter (lower R) Almost no effect on Corona Dia Only Slightly Lowers TSS The Corona Diameter Plays a large Role in Strength of Very Thick Materials >F

Spot Welding of Extra Heavy Gage Mild Steel (12 mm) The thermal time constant for 12 mm thick steel plate is remarkably high (10 sec. vs 1 mm at 0.1 sec) Increased electrode force leads to decreased heat The corona bond around the weld contributes a great deal to mechanical strength The incidence of blow holes or shrinkage cavities decreases as electrode force increases Yamamoto, T “A study of spot welding of heavy gauge mild steel”, Welding In the World, July/Aug, 1971

Turn to the person sitting next to you and discuss (1 min.): What factors might limit the thickness of shet or plate steel that can be spot welded?

RW Hot Rolled Steel

RW Cold Roiled Steel

Structure Property