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© 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
© 2012 Delmar, Cengage Learning
Objectives (cont’d.)
• Discuss the heat treatments used in welding• Explain the cause of corrosion in stainless steel
welds
© 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
© 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
© 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
© 2012 Delmar, Cengage Learning
FIGURE 25-1 There is no change in temperature when there is a change in state. © Cengage Learning 2012
© 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
© 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
© 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
© 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
© 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
© 2012 Delmar, Cengage Learning
FIGURE 25-9 Body-centered cubic unit cell.© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
FIGURE 25-10 Face-centered cubic unit cell.© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
FIGURE 25-11 Hexagonal close-packed cubic unit cell.© Cengage Learning 2012
© 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
© 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
© 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
© 2012 Delmar, Cengage Learning
FIGURE 25-15 Iron-carbon phase diagram.© Cengage Learning 2012
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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)
© 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
© 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
© 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
© 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
© 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
© 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
© 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
© 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