Uncoated Plain Carbon SteelMaterial Variables

Uncoated Plain Carbon – Material variables

Lesson ObjectivesWhen you finish this lesson you will understand:• the relationship between steel manufacturing variables and spot weldability

Learning Activities1. Look up Keywords2. View Slides; 3. Read Notes, 4. Listen to lecture5. Do on-line

workbook

KeywordsChemistry, 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, %

Ma

xim

um

Ha

rdn

ess

, DP

H1000

800

600

400

200

00 0.2 0.4 0.6 0.8 1.0

[Reference: Welding in the Automotive Industry, p.153, D. W. Dickinson ]

To avoid weld problems: C < 0.10% + 0.3 t

Thin Material =100% Martensite(interface tears)

Thick Material =Slower Cooling=Ferrite + Pearlite

Weldability Lobes for Uncoated Mild & Interstitial-Free Steel

15

10

5

06 7 8 9 10 11 12

Interstitial FreeSteelMild Steel

Welding Current (kA)

We

ld T

ime

(Cyc

les)

[Reference: “Challenges inWelding New Sheet SteelProducts”, Gould & Kimchi]

IF Steels havelower bulk ResistanceIF

MildSteel

Chemistry (CONT.)

Spot Weld Failure

AcceptableSpot Weld

0 0.05 0.10 0.15 0.20 0.25 0.30

C, %

0.20

0.15

0.10

0.05

0

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 HV 140/170 HVResult: Hardness &Centerline Cracking

Effect of Carbon and Phosphorous on Current Range

0.31 in.

0.28 in

4000

3000

2000

1000

0.05 0.10 0.15%C + %P

Cu

rre

nt R

an

ge, A

mp

(0.1

5 in

. Min

imu

m B

utto

n)

[Reference: “Spot Weldability ofHigh-Strength Sheet Steels”, Welding Journal 59 (January1980), Baker & Sawhill]

0.25 inElectrode

Drop due to interfacial nugget tears

Hardness Transverses in a Spot Weld between Rephosphorized and Plain Carbon Steels

HAZ

Distance from Fusion Line, in.

Ha

rdne

ss, H

V1

.0

[Reference: “Spot Weldability ofHigh-Strength Sheet Steels”, Welding Journal 59 (January1980), Baker & Sawhill]Base Metal

Chemistry (CONT.)

Max. Tensile Shear Stress

Max. Cross- Tension Strength

Max. Button Dia. in Peel Test

Weight, %Ti

Ma

xim

um

Str

en

gth

Ma

xim

um

But

ton

Dia

me

ter

Weight, %Ti

Tensile-Shear Test

Peel Test

We

ldin

g R

ang

e, %

[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 ResultsCE < 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 ExpulsionElectrode Eroding

Number of Welds

5K 10K 15K 20K

Elec

trod

e Fa

ceD

iam

eter

THICK SHEET - High Currents

THIN SHEETLower Current

Weld on ScaleScale 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 TearL

ob

e C

urv

e S

ize

Surface Carbon (mg/sq ft)

WC

WT

WC

WT

0.6 1.2 1.8Dirty

Clean

HAZSolid State

Bond

Nugget

InterfaceFailure

Weld Lobes of Two HSLA SteelsBatch Annealed (high surface C) vs

Continuous Annealed (low surface C)

Current, kA

We

ld T

ime,

cyc

les

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

Accu-form 50XK

Batch-Annealed B50XK

Lower Surface CarbonNo 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

Kn

oop

Har

dn

ess

Distance From Fusion Line

Base MetalAged Zone

Rec

ryst

alli

zed

Fer

riti

c Z

on

e

Mar

tens

itic

Zon

e

[Reference: Welding inthe Automotive Industry,

p.162, D. W. Dickinson ]

Plain Carbon

HSLA Controlled Rolled

Grain RefinementPpt Strength

TransformationMartensiteTempering

Microstructure Near Outside Edge of HAZ in SRA Steel Spot Weld

[Reference: “Spot Weldability of High-Strength Sheet Steels”, WeldingJournal 59 (January1980), 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 EdgeHigh Hardness at Weld Edge High Hardness at Weld EdgeHigh Hardness at Weld Edge

230250

400350

300

Water Cooled Copper Electrode

THICKNESS

HA

RD

NE

SS

martensite m + p + f p + f

carbon %

0.5

0.3

0.1

Thickness

Time (cycles) Time (cycles) Time (cycles)

Te

mp

era

ture

Te

mp

era

ture

Te

mp

era

ture

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

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 Diamete

r

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?

Weld Current

Wel

d T

ime

CQMild Steel DQ 35XLF

50XLFHSLA

55XLFHSLA

80 XLFHSLA

120XFDual Phase

135 XFTransformation 140T

M190HTFull Mart.

Weldable Over WideRange of Parameters

Weldable With ParameterVariations Expected

Typical Resistance Welding Properties for Hot Rolled Steels

RW Hot Rolled Steel

Weld Current

Wel

d T

ime

CQ DQ 35XLF 50XLF 55XLF

80 XLF 120XF 135 XF 140T M190HT

Weldable Over WideRange of Parameters

Weldable With ParameterVariations Expected

Typical Resistance Welding Properties for ColdRolled Steels

RW Cold Roiled Steel

Distance

Har

dn

ess

InterstitialFree

PlainCarbon

BakeHardened

Microalloyed Dual Phase

Multi-Phase TRIPRecoveryAnnealed

ColdRolled

Martensitic

As Welded

Baked

Hard

TemperedHi C

Low C

NoData

NoData

As-Rolled

Batch

ContinuousAnnealed

Structure Property