Stainless Steel
High Ni & Cr Content
Low (Controlled) Interstitials
Austenitic
Nitrogen Strengthened
Austenitic
Martensitic
Precipitation Hardened
Super Ferritic
Ferritic
Super Austenitic
Duplex
Resistance Welding
Lesson Objectives
When you finish this lesson
you will understand:
•
Keywords
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line workbook
AOD Furnace
Argon & Oxygen
Today, more than 1/2 of the high chromium steels
are produced in the AOD Furnace
Linnert, Welding Metallurgy
AWS, 1994
A=Martensitic Alloys
B=Semi-Ferritic
C=Ferritic
Castro & Cadenet, Welding Metallurgy of
Stainless and Heat-resisting Steels
Cambridge University Press, 1974
We will look at these properties in next slide!
AWS Welding Handbook
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Static Resistance Comparison
Electrode
Plain-carbon Steel
Stainless Steel
Higher Bulk Resistance
Alloy Effect
Workpieces
Higher Surface Resistance
Chromium Oxide
Electrode
Class 3 Electrode
Higher Resistance
Resistance
Higher Resistances = Lower Currents Required
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Conduction in Plain Carbon
Conduction in SS
Base Metal
Weld Nugget
Base Metal
Only 40 - 50% Heat conduction in SS
Less Heat Conducted Away
Therefore
Lower Current Required
Less Time Required (in some cases less than 1/3)
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Melting Temp of Plain Carbon
Base Metal
Weld Nugget
Base Metal
Melting Temp of SS
Melting Temp of SS is lower
Nugget Penetrates More
Therefore
Less Current and Shorter Time Required
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Ferritic, Martensitic, Ppt. = 6 - 11% greater expansion
Austenitic = 15% greater expansion than Plain Carbon Steel
Therefore
Warpage occurs especially in Seam Welding Dong et al, Finite Element Modeling of
Electrode Wear Mechanisms,
Hot Cracking can Occur
Auto Steel Partnership, April 10, 1995
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Force
High Strength
High Hot Strength
• Need Higher Electrode Forces
• Need Stronger Electrodes (Class 3, 10 & 14 Sometimes Used)
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Oxide from Hot Rolling
Oxide Protective Film
• Chromium Oxide from Hot Rolling must be removed by Pickle
• Ordinary Oxide Protective Film is not a Problem
General Properties of Stainless Steels
• Coefficient of Thermal
Expansion
– Surface & bulk resistance is
• Electrical Resistivity
higher than that for plaincarbon steels
• Thermal Conductivity
– About 40 to 50 percent that
of plain-carbon steel
• Melting Temperature
– Plain-carbon:1480-1540 °C
– Martensitic: 1400-1530 °C
– Ferritic: 1400-1530 °C
– Austenitic: 1370-1450 °C
– Greater coefficient than plaincarbon steels
• High Strength
– Exhibit high strength at room
and elevated temperatures
• Surface Preparation
– Surface films must be
removed prior to welding
• Spot Spacing
– Less shunting is observed
than plain-carbon steels
Look at Each Grade & Its Weldability
Austenitic
Super Austenitic
Nitrogen Strengthened Austenitic
Martensitic
Ferritic
Super Ferritic
Precipitation Hardened
Duplex
Austenitic
• Contain between 16 and 25 percent
chromium, plus sufficient amount of nickel,
manganese and/or nitrogen
• Have a face-centered-cubic (fcc) structure
• Nonmagnetic
• Good toughness
• Spot weldable
• Strengthening can be accomplished by cold
work or by solid-solution strengthening
Applications:
Fire Extinguishers, pots & pans, etc.
AWS Welding Handbook
AWS Welding Handbook
Pseudobinary
Phase Diagram
@ 70% Iron
AWS Welding Handbook
Prediction of Weld
Metal Solidification
Morphology
Schaeffler
Diagram
WRC
Diagram
AWS Welding Handbook
Hot Cracking
A few % Ferrite Reduces Cracks
But P&S Increase Cracks
AWS Welding Handbook
Spot Welding Austenitic Stainless Steel
Some Solidification Porosity Can Occur:
• As a result of this tendency to Hot Crack when Proper
Percent Ferrite is not Obtained
• Because of higher Contraction on Cooling
Suggestions:
• Maintain Electrode Force until Cooled
• Limit Nugget Diameter to <4 X Thickness of thinner piece
• More small diameter spots preferred to fewer Large Spots
Spot Welding Austenitic Stainless Steel
Some Discoloration May Occur Around Spot Weld
Oxide Formation in HAZ
Nugget
Solutions
•Maintain Electrode Force until weld cooled below oxidizing
Temperature
• Post weld clean with 10% Nitric, 2% Hydrofluoric Acid
(Hydrochloric acid should be avoided due to chloride
ion stress-corrosion cracking and pitting)
Seam Welding Austenitic Stainless Steel
Somewhat more Distortion Noted
Because of Higher Thermal
Contraction
Solution
• Abundant water cooling to remove heat
Knifeline Corrosion Attack in
Austenitic Stainless Steel Seam Welds
Solution
• See Next Slide for more description
Chromium Carbide Precipitation
Kinetics Diagram
1500 °F
Temperature
1500 F
1200 °F
M23C6
Precipitation
800 F
Chromium Oxide
800 °F
Intergranular
Corrosion
Time
M23C6
Chromium-Rich
Carbides
Preventative Measures






Short weld times
Low heat input
Lower carbon content in the base material
 304L, 316L
Stabilization of the material with titanium additions
 321 (5xC)
Stabilization with columbium or tantalum additions
 347, 348 (10xC)
Lower nitrogen content (N acts like C)
Projection Welding Austenitic Stainless Steel
Because of the Greater Thermal Expansion and Contraction,
Head Follow-up is critical
Solution
• Press Type machines with low inertia heads
• Air operated for faster action
In Welding Tubes with Ring projections for leak tight
application, electrode set-up is critical
Solution
• Test electrode alignment
Cross Wire Welding Austenitic Stainless Steel
Often used for grates, shelves, baskets, etc.
• Use flat faced electrodes, or
• V-grooved electrodes to hold wires in a fixture
• As many as 40 welds made at one time
Flash Welding Austenitic Stainless Steel
• Current about 15% less than for plain carbon
• Higher upset pressure
• The higher upset requires 40-50% higher clamp force
• Larger upset to extrude oxides out
Super Austenitic
Alloys with composition between standard 300 Austenitic SS
and Ni-base Alloys
• High Ni, High Mo
• Ni & Mo- Improved chloride induced Stress Corrosion
Cracking
Used in
• Sea water application where regular austenitics suffer
pitting, crevice and SCC
AWS Welding Handbook
The Super Austenitic Stainless Steels are susceptible
to copper contamination cracking. RESISTANCE
WELDING NOT NORMALLY PERFORMED
Copper and Copper Alloy Electrodes can cause cracking:
• Flame spray coated electrodes
• Low heat
Nitrogen-Strengthened Austenitic
•High nitrogen levels, combined with
higher manganese content, help to
increase the strength level of the material
•Consider a postweld heat treatment for
an optimum corrosion resistance
Little Weld Data Available
Martensitic
• Contain from 12 to 18 percent chromium and
0.12 to 1.20 percent carbon with low nickel
content
• Combined carbon and chromium content gives
these steels high hardenability
• Magnetic
• Tempering of the low-carbon martensitic
stainless steels should avoid the 440 to 540 °C
temperature range because of a sharp reduction
in notch-impact resistance
Applications:
Some Aircraft & Rocket Applications
Cutlery
Martensitic SS Wrought Alloys are divided into two groups
• 12% Cr, low-carbon engineering grades (top group)
• High Cr, High C Cutlery grades (middle group)
AWS Welding Handbook
From a Metallurgical Standpoint, Martensitic SS
is similar to Plain Carbon
AWS Welding Handbook
Martensitic
Spot Welding
• HAZ Structural Changes
• Tempering of hard martensite at BM side
• Quench to hard martensite at WM side
• Likelihood of cracking in HAZ increases with Carbon
• Pre-heat, post-heat, tempering helps
Flash Weld
• Hard HAZ
• Temper in machine
• High Cr Steels get oxide entrapment at interface
• Precise control of flashing & upset
• N or Inert gas shielding
Effect of Tempered Martensite on
Hardness
As Quenched
Hardness
Loss of Hardness and Strength
Hardened Martensite
Tempered Martensite
Fusion
Zone
HAZ
SS with carbon content above
0.15% Carbon (431, 440) are
susceptible to cracking and need
Post Weld Heat Treatment
Distance
Ferritic
• Contain from 11.5 to 27 percent
chromium, with additions of manganese
and silicon, and occasionally nickel,
aluminum, molybdenum or titanium
• Ferritic at all temperatures, no phase
change, large grain sizes
• Non-hardenable by heat treatment
• Magnetic (generally)
Applications:
Water Tanks in Europe
Storage Tanks
AWS Welding Handbook
FERRITIC STAINLESS STEELS
Spot & Seam Welding
Because No Phase Change, Get Grain Growth
large
HAZ
Base
Grain
Size
fine
Strength
Toughness
885 Embrittlement
DISTANCE
FERRITIC STAINLESS STEELS
Flash Weld
• Lower Cr can be welded with standard flash weld techniques
• loss of toughness, however
• Higher Cr get oxidation
• Inert gas shield recommended
• long flash time & high upset to expel oxides
Super Ferritic
• Lower than ordinary interstitial (C&N)
• Higher Cr & Mo
AWS Welding Handbook
Increased Cr & Mo
promotes Embrittlement
• 825F Sigma Phase (FeCr)
precipitation embrittlement
•885F Embrittlement
(decomposition of iron-chromium
ferrite)
• 1560F Chi Phase (Fe36Cr12Mo10)
precipitation embrittlement
Because of the Embrittlement,
Resistance Welding is Usually
Not Done on These Steels
large
HAZ
Base
Grain
Size
fine
Strength
Toughness
885 Embrittlement
DISTANCE
Precipitation-Hardened
• Can produce a matrix structure of
either austenite or martensite
• Heat treated to form CbC, TiC, AlN,
Ni3Al
• Possess very high strength levels
• Can serve at higher temperature
than the martensitic grades
Applications:
High Strength Components in Jet & Rocket Engines
Bombs
AWS Welding Handbook
Martensitic
• Solution heat treat above 1900F
• Cool to form martensite
• Precipitation strengthen
• Fabricated
Semiaustenitic
• Solution heat treat (still contain 5-20% delta ferrite)
• Quench but remain austenitic (Ms below RT)
• Fabricate
• Harden (austenitize, low temp quench, age)
Austenitic
• Remain austinite
• Harden treatment
AC=Air cooled
WQ=Water Quenched
RC=Rapid Cool to RT
SZC= Rapid cool to -100F
AWS Welding Handbook
Effect on Aging on the Nugget Hardness in
Precipitation-Hardened Stainless Steels
Hardness
Aged
When Welded in the Aged Condition
• Higher Electrode Forces
• Post Weld Treatment
Annealed
Weld
Centerline
Distance
Precipitation-Hardened
Spot Welding
• 17-7PH, A-286, PH15-7Mo, AM350 & AM355 have been welded
• Generally welded in aged condition, higher forces needed
• Time as short as possible
Seam Welding
• 17-7PH has been welded
• Increased electrode force
Flash Welding
• Higher upset pressure
• Post weld heat treatment
Duplex
• Low Carbon
• Mixture: {bcc} Ferrite + {fcc} Austenite
• Better SCC and Pitting Resistance than Austenitics
• Yield Strengths twice the 300 Series
Early grades had 75-80% Ferrite (poor weldability due to ferrite)
Later grades have 50-50
AWS Welding Handbook
Due to the Ferrite:
• Sensitive to 885F embrittlement
• Sigma Phase embrittlement above 1000F
• High ductile to brittle transition temperatures (low toughness)
• Solidifies as ferrite, subsequent ppt of nitrides, carbides which
reduces corrosion resistance
• Rapid cooling promotes additional ferrite
• Not Hot Crack Sensitive
Resistance Welds generally not recommended
because low toughness and low corrosion resistance
Unless post weld solution anneal and quench.
Some
Applications
Deep Drawing of Plain Carbon Steel
or Stainless Steel
Method of
Making an Ultra
Light Engine
Valve
Stainless Steel Cap
Resistance
Weld
Larson, J & Bonesteel, D “Method of Making an Ultra
Light Engine Valve” US Patent 5,619,796 Apr 15, 1997