Transcript No Slide Title

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:
Material Parameters:
•
•
•
•
•
•
•
•
•
•
•
•
•
Weld Current
Weld Time
Hold Time
Upslope/Downslope
Pulsing
Electrode Force
Postweld Temper
Electrode Designs
Chemistry
Cleanliness
Surface Condition
Material Processing
Thickness
Chemistry
To avoid weld problems: C < 0.10% + 0.3 t
Material Thickness
Maximum Hardness, DPH
1000
Thin Material =
100% Martensite
(interface tears)
800
600
Thick Material =
Slower Cooling=
Ferrite + Pearlite
400
200
0
0
0.2
0.4
0.6
0.8
Carbon Content, %
1.0
[Reference: Welding in the Automotive Industry, p.153, D. W. Dickinson ]
Weldability Lobes for Uncoated
Mild & Interstitial-Free Steel
Weld Time (Cycles)
15
Mild
Steel
10
IF Steels have
lower bulk
Resistance
IF
5
Interstitial Free
Steel
Mild Steel
0
6
7
8
9
10 11
Welding Current (kA)
[Reference: “Challenges in
Welding New Sheet Steel
Products”, Gould & Kimchi]
12
Chemistry (CONT.)
(Centerline Cracking)
0.20
P + 3S, %
0.15
Spot Weld Failure
0.10
Acceptable
Spot Weld
0.05
0
0
0.05
0.10
0.15
C, %
0.20
0.25
[Reference: Welding in the Automotive Industry, p.154, D. W. Dickinson ]
0.30
Base Metal Microstructures for Killed
Plain Carbon and Rephosphorized Steels
Since P in solution = Very little difference in microstructure
100/110 HV Result: Hardness &140/170 HV
Centerline Cracking
[Reference: “Spot Weldability of High-Strength Sheet Steels”, Welding Journal 59
(January 1980), Baker & Sawhill]
Current Range, Amp
(0.15 in. Minimum Button)
Effect of Carbon and Phosphorous
on Current Range
4000
0.31 in.
3000
0.28 in
Drop due to
interfacial nugget
tears
2000
0.25 in
Electrode
1000
0.05
0.10
0.15
%C + %P
[Reference: “Spot Weldability of
High-Strength Sheet Steels”,
Welding Journal 59 (January
1980), Baker & Sawhill]
Hardness, HV1.0
Hardness Transverses in a Spot Weld between
Rephosphorized and Plain Carbon Steels
HAZ
Base Metal
[Reference: “Spot Weldability of
High-Strength Sheet Steels”,
Welding Journal 59 (January
1980), Baker & Sawhill]
Distance from Fusion Line, in.
Max. CrossTension Strength
Max. Button Dia.
in Peel Test
Weight, %Ti
Welding Range, %
Maximum Strength
Max. Tensile Shear Stress
Maximum Button Diameter
Chemistry (CONT.)
Centerline Tears
Tensile-Shear Test
Peel Test
Weight, %Ti
[Reference: Welding in the Automotive Industry, p.156, D. W. Dickinson ]
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
Empirical Carbon Equivalent Equation
CE = C + Mn/36 + (Cr + Mo + Zr)/10 + Ti/2
+ Cb/3 + V + T.S.(ksi)/900 + t(in.)/20
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:
Material Parameters:
•
•
•
•
•
•
•
•
•
•
•
•
•
Weld Current
Weld Time
Hold Time
Upslope/Downslope
Pulsing
Electrode Force
Postweld Temper
Electrode Designs
Chemistry
Cleanliness
Surface Condition
Material Processing
Thickness
Steel Cleanliness
[Reference: Welding in the Automotive Industry, p.160, D. W. Dickinson ]
Process Variables
Process Parameters:
Material Parameters:
•
•
•
•
•
•
•
•
•
•
•
•
•
Weld Current
Weld Time
Hold Time
Upslope/Downslope
Pulsing
Electrode Force
Postweld Temper
Electrode Designs
Chemistry
Cleanliness
Surface Condition
Material Processing
Thickness
Surface Condition
Hot Spots Alloying & Cavitation
Surface Expulsion
Electrode Eroding
Oils/Dirt
Oxide
Steel
Effect of Surface Oxide on Electrode
Electrode Face
Diameter
THICK SHEET - High Currents
Weld on Scale
Scale Cleaned
Weld on Scale - No Upslope
With Upslope
THIN SHEET
Lower Current
Scale Cleaned
5K
10K
15K
Number of Welds
20K
Effect of Surface Oxide on Lobe
Effect of Surface Carbon on Button Tear
Interface
Failure
Lobe Curve Size
WT
WC
WT
WC
Dirty
1.2
0.6
Sur face Carbon (m g/s q ft)
Solid State
Bond
HAZ
Nugget
Clean
1.8
Weld Lobes of Two HSLA Steels
Batch Annealed (high surface C) vs
Continuous Annealed (low surface C)
Accu-form 50XK
Weld Time, cycles
Batch-Annealed B50XK
Lower Surface Carbon
No Partial Nuggets
Current, kA
[Reference: “Forms
of Dynamic
Resistance Curves
Generated During
Resistance Spot
Welding”, Watney
& Nagel]
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:
Material Parameters:
•
•
•
•
•
•
•
•
•
•
•
•
•
Weld Current
Weld Time
Hold Time
Upslope/Downslope
Pulsing
Electrode Force
Postweld Temper
Electrode Designs
Chemistry
Cleanliness
Surface Condition
Material Processing
Thickness
Recrystallized Ferritic Zone
Knoop Hardness
Martensitic
Zone
Heat-Affected Zone Property
Loss in HSLA Steels
Aged Zone
Base Metal
Transformation
Martensite
Tempering
HSLA Controlled Rolled
Grain Refinement
Ppt Strength
Plain Carbon
[Reference: Welding in
the Automotive Industry,
p.162, D. W. Dickinson
]
Distance From Fusion Line
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:
Material Parameters:
•
•
•
•
•
•
•
•
•
•
•
•
•
Weld Current
Weld Time
Hold Time
Upslope/Downslope
Pulsing
Electrode Force
Postweld Temper
Electrode Designs
Chemistry
Cleanliness
Surface Condition
Material Processing
Thickness
High Hardness at Weld Edge
Water Cooled
Copper Electrode
300
250
350
400
230
m+p+f
HARDNESS
martensite
p+f
carbon
%
0.5
0.3
0.1
THICKNESS
Temperature
Temperature
Temperature
Thickness
Time (cycles)
0.5 mm Steel
Time (cycles)
1.1 mm Steel
Time (cycles)
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
Yamamoto, T “A study of spot welding of
heavy gauge mild steel”, Welding In the
World, July/Aug, 1971
Uses
• Architecture
• Bridges
• Off Highway Vehicles
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)
• 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
Indentation
Penetration
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
Yamamoto, T “A study of spot welding of
heavy gauge mild steel”, Welding In the
World, July/Aug, 1971
>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?
Typical Resistance Welding Properties for Hot Rolled Steels
Weld Time
CQ
Mild Steel
80 XLF
HSLA
50XLF
HSLA
35XLF
DQ
120XF
Dual Phase
135 XF
Transformation
140T
55XLF
HSLA
M190HT
Full Mart.
Weld Current
W eldable Over W ide
Range of Parameters
W eldable W ith Parameter
Variations Expected
RW Hot
Rolled
Steel
Typical Resistance Welding Properties for Cold
Rolled Steels
Weld Time
CQ
80 XLF
DQ
120XF
50XLF
55XLF
140T
M190HT
35XLF
135 XF
Weld Current
Weldable Ove r Wide
Range of Param e te rs
Weldable With Param e ter
Variations Expe cte d
RW Cold
Roiled Steel
Interstitial
Free
Plain
Carbon
Bake
Hardened
Microalloyed
Dual Phase
As-Rolled
Batch
Hardness
Continuous
Annealed
Baked
As Welded
Multi-Phase
No
Data
TRIP
Recovery
Annealed
Cold
Rolled
Martensitic
Hard
Hi C
Tempered
No
Data
Low C
Distance
Structure
Property