Stainless Steel

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Stainless Steel
High Ni Cr Content Low (Controlled)
Interstitials
Nitrogen Strengthened Austenitic
Austenitic
Martensitic
Ferritic
Super Austenitic
Precipitation Hardened
Duplex
Super Ferritic
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Resistance Welding

• Learning Activities
• View Slides
• Read Notes,
• Listen to lecture
• Do on-line workbook

• Lesson Objectives
• When you finish this lesson you will understand

Keywords
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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
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AMartensitic Alloys BSemi-Ferritic CFerritic
Castro Cadenet, Welding Metallurgy of
Stainless and Heat-resisting Steels Cambridge
University Press, 1974
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We will look at these properties in next slide!
AWS Welding Handbook
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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Static Resistance Comparison
Plain-carbon Steel
Electrode Electrode
Stainless Steel
Higher Bulk Resistance Alloy Effect
Workpieces
Higher Surface Resistance Chromium Oxide
Class 3 Electrode Higher Resistance
Resistance
Higher Resistances Lower Currents Required
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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Conduction in Plain Carbon
Conduction in SS
Base Metal Base Metal
Weld Nugget
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)
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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Melting Temp of Plain Carbon
Base Metal Base Metal
Weld Nugget
Melting Temp of SS
Melting Temp of SS is lower Nugget Penetrates
More Therefore Less Current and Shorter Time
Required
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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Ferritic, Martensitic, Ppt. 6 - 11 greater
expansion Austenitic 15 greater expansion than
Plain Carbon Steel Therefore Warpage occurs
especially in Seam Welding Hot Cracking can Occur
Dong et al, Finite Element Modeling of Electrode
Wear Mechanisms, Auto Steel Partnership, April
10, 1995
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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Force
High Strength High Hot Strength

• Need Higher Electrode Forces
• Need Stronger Electrodes (Class 3, 10 14
  Sometimes Used)

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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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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

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General Properties of Stainless Steels

• Coefficient of Thermal Expansion
• Greater coefficient than plain-carbon 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

• Electrical Resistivity
• Surface bulk resistance is higher than that for
  plain-carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain-carbon steel
• Melting Temperature
• Plain-carbon1480-1540 C
• Martensitic 1400-1530 C
• Ferritic 1400-1530 C
• Austenitic 1370-1450 C

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Look at Each Grade Its Weldability
Austenitic
Super Austenitic
Nitrogen Strengthened Austenitic
Martensitic
Ferritic
Super Ferritic
Precipitation Hardened
Duplex
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• 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.
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AWS Welding Handbook
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AWS Welding Handbook
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Pseudobinary Phase Diagram _at_ 70 Iron
AWS Welding Handbook
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Prediction of Weld Metal Solidification Morphology
Schaeffler Diagram
WRC Diagram
AWS Welding Handbook
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Hot Cracking
PS
A few Ferrite Reduces Cracks But PS Increase
Cracks
AWS Welding Handbook
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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 lt4 X Thickness of
  thinner piece
• More small diameter spots preferred to fewer
  Large Spots

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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)

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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

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Chromium Carbide Precipitation Kinetics Diagram
1500 F
1500 F
M23C6 Precipitation
1200 F
800 F
Temperature
Chromium Oxide
800 F
M23C6 Chromium-Rich Carbides
Intergranular Corrosion
Time
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• 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)

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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

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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

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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

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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

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AWS Welding Handbook
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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

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• 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
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• 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
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• 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
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From a Metallurgical Standpoint, Martensitic SS
is similar to Plain Carbon
AWS Welding Handbook
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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

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Effect of Tempered Martensite on Hardness
As Quenched
Loss of Hardness and Strength
Hardened Martensite Tempered Martensite
Hardness
Fusion Zone
SS with carbon content above 0.15 Carbon (431,
440) are susceptible to cracking and need Post
Weld Heat Treatment
HAZ
Distance
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• 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
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AWS Welding Handbook
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FERRITIC STAINLESS STEELS
Spot Seam Welding
Because No Phase Change, Get Grain Growth
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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

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Super Ferritic

• Lower than ordinary interstitial (CN)
• Higher Cr Mo

AWS Welding Handbook
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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
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• 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
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AWS Welding Handbook
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• 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

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RCRapid Cool to RT SZC Rapid cool to -100F
ACAir cooled WQWater Quenched
AWS Welding Handbook
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Effect on Aging on the Nugget Hardness in
Precipitation-Hardened Stainless Steels
Aged
Hardness

• When Welded in the Aged Condition
• Higher Electrode Forces
• Post Weld Treatment

Annealed
Weld Centerline
Distance
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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

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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
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AWS Welding Handbook
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• 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.
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Some Applications
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Method of Making an Ultra Light Engine Valve
Deep Drawing of Plain Carbon Steel
or Stainless Steel
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
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Homework