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Stainless Steel

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Title: Stainless Steel


1
Stainless Steel
High Ni Cr Content Low (Controlled)
Interstitials
Nitrogen Strengthened Austenitic
Austenitic
Martensitic
Ferritic
Super Austenitic
Precipitation Hardened
Duplex
Super Ferritic
2
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
3
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
4
AMartensitic Alloys BSemi-Ferritic CFerritic
Castro Cadenet, Welding Metallurgy of
Stainless and Heat-resisting Steels Cambridge
University Press, 1974
5
(No Transcript)
6
We will look at these properties in next slide!
AWS Welding Handbook
7
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

8
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
9
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

10
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)
11
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

12
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
13
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

14
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
15
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

16
Force
High Strength High Hot Strength
  • Need Higher Electrode Forces
  • Need Stronger Electrodes (Class 3, 10 14
    Sometimes Used)

17
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

18
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

19
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

20
Look at Each Grade Its Weldability
Austenitic
Super Austenitic
Nitrogen Strengthened Austenitic
Martensitic
Ferritic
Super Ferritic
Precipitation Hardened
Duplex
21
  • 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.
22
AWS Welding Handbook
23
AWS Welding Handbook
24
Pseudobinary Phase Diagram _at_ 70 Iron
AWS Welding Handbook
25
Prediction of Weld Metal Solidification Morphology
Schaeffler Diagram
WRC Diagram
AWS Welding Handbook
26
Hot Cracking
PS
A few Ferrite Reduces Cracks But PS Increase
Cracks
AWS Welding Handbook
27
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

28
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)

29
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

30
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
31
  • 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)

32
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

33
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

34
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

35
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

36
AWS Welding Handbook
37
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

38
  • 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
39
  • 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
40
  • 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
41
From a Metallurgical Standpoint, Martensitic SS
is similar to Plain Carbon
AWS Welding Handbook
42
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

43
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
44
  • 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
45
AWS Welding Handbook
46
FERRITIC STAINLESS STEELS
Spot Seam Welding
Because No Phase Change, Get Grain Growth
47
(No Transcript)
48
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

49
Super Ferritic
  • Lower than ordinary interstitial (CN)
  • Higher Cr Mo

AWS Welding Handbook
50
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
51
  • 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
52
AWS Welding Handbook
53
  • 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

54
RCRapid Cool to RT SZC Rapid cool to -100F
ACAir cooled WQWater Quenched
AWS Welding Handbook
55
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
56
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

57
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
58
AWS Welding Handbook
59
  • 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.
60
Some Applications
61
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
62
Homework
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