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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. Resistance Welding . Learning Activities View Slides; Read Notes,

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slide1

Stainless Steel

High Ni & Cr Content

Low (Controlled) Interstitials

Nitrogen Strengthened

Austenitic

Austenitic

Martensitic

Ferritic

Super Austenitic

Precipitation Hardened

Duplex

Super Ferritic

slide2

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

slide3

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

slide4

A=Martensitic Alloys

B=Semi-Ferritic

C=Ferritic

Castro & Cadenet, Welding Metallurgy of

Stainless and Heat-resisting Steels

Cambridge University Press, 1974

general properties of stainless 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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
static resistance comparison
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

general properties of stainless steels9
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide10

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)

general properties of stainless steels11
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide12

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

general properties of stainless steels13
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide14

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

general properties of stainless steels15
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide16

Force

High Strength

High Hot Strength

  • Need Higher Electrode Forces
  • Need Stronger Electrodes (Class 3, 10 & 14 Sometimes Used)
general properties of stainless steels17
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide18

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 steels19
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-carbon:1480-1540 °C

Martensitic: 1400-1530 °C

Ferritic: 1400-1530 °C

Austenitic: 1370-1450 °C

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

General Properties of Stainless Steels
slide20

Look at Each Grade & Its Weldability

Austenitic

Super Austenitic

Nitrogen Strengthened Austenitic

Martensitic

Ferritic

Super Ferritic

Precipitation Hardened

Duplex

slide21

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.

slide24

Pseudobinary

Phase Diagram

@ 70% Iron

AWS Welding Handbook

slide25

Prediction of Weld

Metal Solidification

Morphology

Schaeffler

Diagram

WRC

Diagram

AWS Welding Handbook

slide26

Hot Cracking

P+S

A few % Ferrite Reduces Cracks

But P&S Increase Cracks

AWS Welding Handbook

slide27

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
slide28

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)
slide29

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
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

slide31

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)
slide32

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
slide33

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
slide34

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
slide35

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
slide37

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
slide38

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

slide39

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

slide40

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

slide41

From a Metallurgical Standpoint, Martensitic SS

is similar to Plain Carbon

AWS Welding Handbook

slide42

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
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

slide44

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

slide46

FERRITIC STAINLESS STEELS

Spot & Seam Welding

Because No Phase Change, Get Grain Growth

slide48

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
slide49

Super Ferritic

  • Lower than ordinary interstitial (C&N)
  • Higher Cr & Mo

AWS Welding Handbook

slide50

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

slide51

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

slide53

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
slide54

RC=Rapid Cool to RT

SZC= Rapid cool to -100F

AC=Air cooled

WQ=Water Quenched

AWS Welding Handbook

effect on aging on the nugget hardness in precipitation hardened stainless steels
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

slide56

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
slide57

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

slide59

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.

slide60

Some

Applications

slide61

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