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© 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy
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Page 1: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Chapter 25

Welding Metallurgy

Page 2: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Objectives

• List the crystalline structures of metals and explain how grains form

• Work with phase diagrams• List the five mechanisms used to strengthen

metals• Explain why steels are such versatile materials• Describe the types of weld heat-affected zones• Discuss the problems hydrogen causes during

steel welding

Page 3: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Objectives (cont’d.)

• Discuss the heat treatments used in welding• Explain the cause of corrosion in stainless steel

welds

Page 4: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Introduction

• Skilled welders – Need to understand the materials being welded

– Need to learn metallurgy

• Metals mechanical and chemical properties – Result from alloying and heat-treating

– Welding operations heat the metals• Change structure and properties

Page 5: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Heat, Temperature, and Energy

• Heat and temperature – Describe quantity and level of thermal energy

• Heat: quantity of thermal energy• Temperature: level of thermal activity

– Independent values• Material can have a large quantity of heat energy but

a low temperature• Material can be at a high temperature but have very

little heat

Page 6: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Heat

• Amount of thermal energy in matter– Measured in the British thermal unit (BTU)

• Two forms– Sensible (measurable)

• As it changes a change in temperature can be sensed or measured

– Latent• Absorbed by a material as it changes from one state

to another• Also occurs with a change in structure

Page 7: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

FIGURE 25-1 There is no change in temperature when there is a change in state. © Cengage Learning 2012

Page 8: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Temperature

• Measurement of frequency of atoms in matter– Matter becomes warmer: atoms vibrate at a higher

frequency

– Temperature: determined by frequency of light produced by vibrating atoms

FIGURE 25-4 Visible and invisible light.© Cengage Learning 2012

Page 9: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Mechanical Properties of Metal

• All of a metal's properties interact with one another• Significant mechanical properties

– Hardness: resistance to penetration

– Brittleness: ease metal cracks or breaks without noticeable deformation

– Ductility: ability of a metal to be permanently twisted, drawn out, bent, or changed in shape

– Toughness: allows a metal to withstand forces

– Strength: property of a metal to resist deforming• Tensile, compressive, shear, or torsional

Page 10: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Other Mechanical Concepts

• Include:– Strain: deformation caused by stress

– Elasticity: ability of a material to return to its original form

– Elastic limit: maximum load with a deformation directly proportional to the load

– Impact strength: ability of a metal to resist fracture under a sudden load

Page 11: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Structure of Matter

• Solid matter: two basic forms– Crystalline

• Orderly arrangement of atoms

– Amorphic• No orderly arrangement of atoms into crystals

• Both look and feel like solids– Sophisticated testing equipment is required to tell

the difference

Page 12: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Crystalline Structures of Metal

• Atoms arranged in very precise three-dimensional patterns are called crystal lattices– Smallest identifiable group of atoms is the unit cell

– Some metals change their lattice structure when heated above a specific temperature

– Crystal structures are studied by polishing and etching small pieces of metal

Page 13: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

FIGURE 25-9 Body-centered cubic unit cell.© Cengage Learning 2012

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© 2012 Delmar, Cengage Learning

FIGURE 25-10 Face-centered cubic unit cell.© Cengage Learning 2012

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© 2012 Delmar, Cengage Learning

FIGURE 25-11 Hexagonal close-packed cubic unit cell.© Cengage Learning 2012

Page 16: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Phase Diagrams

• Most engineering metals are alloys– Phases and temperatures at which alloys exist

• Summarized in phase diagrams• Also called equilibrium or constitution diagrams• Describe constituents present at temperature

equilibrium

Page 17: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Lead-Tin Phase Diagram

• Many similarities with iron-carbon phase diagram– Used for steel

• Chart areas– Liquid phase

– Solid phase

– Liquid-solid phase

– Solid-solution phase

• Eutectic composition – Lowest possible melting temperature of an alloy

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© 2012 Delmar, Cengage Learning

Iron-Carbon Phase Diagram

• More complex than lead-tin phase diagram– Very small changes in the percentage of carbon

produce major changes in the alloy's properties

– Iron is called an allotropic metal

– Pure iron forms body-centered cubic crystal below a temperature of 1675 degrees Fahrenheit

– Iron changes to face-centered cubic crystal above 1675 degrees Fahrenheit

Page 19: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

FIGURE 25-15 Iron-carbon phase diagram.© Cengage Learning 2012

Page 20: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Strengthening Mechanisms

• Metal strength– Most important physical characteristic

• Pure metals are relatively weak– Structures built with pure metals would be massive

and heavy

• Welders must understand numerous methods used to strengthen metals

Page 21: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Solid-Solution Hardening

• It is possible to replace atoms in crystal lattice with atoms of another metal– Not all metals have lattice dimensions that allow

substitution of other atoms

– Does not change lattice structure as a result of thermal treatments

– Alloys are generally weldable

Page 22: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Precipitation Hardening

• Solubility increases with temperature – Until alloy system reaches its limit

• Heat treatment involving three steps:– Heating alloy to dissolve the second phase

– Quenching alloy rapidly: producing a supersaturated solution

– Reheating alloy

• Process is used to strengthen many alloys

Page 23: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Mechanical Mixtures of Phases

• Two phases may exist in equilibrium– Depends on alloy’s temperature and composition

• Room temperature – Iron-carbon alloy has two forms

• Alpha iron ferrite: ductile but weak• Cementite: strong but brittle• In combination: cementite strengthens ferrite

Page 24: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

FIGURE 25-22 Change in mechanical properties caused by beta (silicon phase) in mechanical mixture with alpha (aluminum phase). © Cengage Learning 2012

Page 25: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Quench, Temper, and Anneal

• Quenching rapidly cools a metal– Methods

• Molten salt quenching• Air quenching• Oil quenching• Water quenching• Brine quenching

• Tempering reheats a part that has been hardened and quenched– Reduces some brittle hardness

Page 26: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Martensitic Reactions

• Martensite characteristics– Hardest of transformation products of austenite

– Has an acicular structure

– Formation can be minimized by preheating steel to slow cooling rates

– Can be tempered to a more useful structure

– Tempering time/temperature is increased: structure changes to spheroidized microstructure

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© 2012 Delmar, Cengage Learning

Cold Work

• Metals are deformed at room temperature – Grains are flattened and elongated

• Increases strength and decreases ductility

• Cold-worked structure– Can be annealed by heating above the

recrystallization temperature

• Final annealed structure – Weaker than cold-worked structure

Page 28: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Grain Size Control

• Grain growth – Common to all metals and alloys

– Growth rate increases with temperature and time

– Coarse grains are weaker and more ductile

– Allotropic transformation requires the creation of fresh grains

– Grain refinement: quickly heated above critical temperature and then quickly cooled

• Not all metals exhibit allotropic transformation

Page 29: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Heat Treatments Associated with Welding

• Welding specifications – Frequently call for heat treating joints before

welding or after fabrication

– Welders should understand the reasons for these heat treatments

Page 30: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Preheat

• Reduces the rate at which welds cool– Lowers residual stress

– Reduces cracking

• Amount of preheat– Increased when welding stronger platesor in

response to higher levels of hydrogen contamination

– Most commonly used preheat temperature range is between 250 and 400 degrees Fahrenheit

Page 31: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Stress Relief, Process Annealing

• Residual stresses are unsuitable in welded structures– Significant effects

• Yield strength of steels – Decreases at higher temperatures

• Temperature range for stress relief steel – 1100 to1150 degrees Fahrenheit

• Time at temperature – Important factor

Page 32: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Annealing

• Referred to as full annealing– Involves heating the structure of a metal to turn it

completely austenitic• After soaking to equalize temperature: cooled in

furnace at slowest possible rate• Austenite transforms to ferrite and pearlite• Metal is now its softest with small grain size

Page 33: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Normalizing

• Consists of heating steels to slightly above Ac3

– Holding for austenite to form

– Followed by cooling in still air

– On cooling: austenite transforms• Somewhat higher strength and hardness• Slightly less ductility than in annealing

Page 34: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Thermal Effects Caused by Arc Welding

• Liquid metal is deposited on base metal– Some base metal melts from contact with liquid

weld metal and arc, flame, etc.

• Metallurgic changes in heated region are inevitable– Lowest temperature at which such changes occur

defines the heat-affected zone (HAZ)

Page 35: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Thermal Effects Caused by Arc Welding (cont'd.)

• Exact size and shape of HAZ are affected by:– Type of metal or alloy

– Method of applying welding heat

– Mass of the part

– Pre- and postheating

• HAZ produces fine grains as a result of the allotropic transformation– Welder must control the HAZ

Page 36: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Gases in Welding

• Many welding problems and defects result from undesirable gases that can dissolve in weld metal– Gases that dissolve in the molten weld pool have a

high solubility in liquid metal

– During freezing process: dissolved gases try to escape

• High solidification rates: become trapped in the metal• Intermediate rates: trapped as bubbles

Page 37: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Hydrogen

• Many sources– Moisture in electrode coatings

– Fluxes

– Very humid air

– Damp weld joints

– Organic lubricants

– Rust on wire or joint surfaces

• Troublesome in aluminum and steel– Problems are avoidable

Page 38: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Nitrogen

• Comes from air drawn into the arc stream– GMAW: results from poor shielding or strong drafts

– SMAW: results from an excessively long arc

• Primary problems – Porosity

– Embrittlement

• Improves strength of stainless steel– Sometimes intentionally added

Page 39: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Oxygen

• Common source of oxygen contamination is air– Metallurgic changes cause most effects of oxygen

– Oxygen causes the loss of oxidizable alloys• Causes oxide formation on aluminum welds

– About two percent of oxygen is added intentionally to stabilize the GMAW process

• Amount of oxygen used is carefully controlled

Page 40: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Carbon Dioxide

• Oxygen substitute for stabilizing GMAW process using argon shields– Carbon in carbon dioxide is a potential contaminant

• Causes problems with corrosion resistance

– Carbon dioxide levels below five percent do not seem to increase carbon content of stainless steel

Page 41: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Metallurgic Defects

• Cold cracking – Result of hydrogen dissolving in weld metal

• Hot cracking– Caused by tearing metal along partially fused grain

boundaries of welds

• Carbide precipitation – Occurs when chromium carbides deplete steel of

free chromium• Carbon dioxide shield gases can cause a similar

problem, especially with ELC grades

Page 42: © 2012 Delmar, Cengage Learning Chapter 25 Welding Metallurgy.

© 2012 Delmar, Cengage Learning

Summary

• Understanding metallurgy – Enables a welding engineer to design better

weldments• Welding engineers know chemical elements that

make up a metal alloy

– As metals are thermally cycled their physical and mechanical properties change

• You must know the importance of controlling temperature cycles during welding

– Understanding metallurgy will aid you in avoiding welding problems