1. Chapter 25
Welding Metallurgy

2. 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

3. Objectives (cont’d.) Discuss the heat treatments used in welding
Explain the cause of corrosion in stainless steel welds

4. Introduction Skilled welders Metals mechanical and chemical properties
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

5. 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

6. Heat Amount of thermal energy in matter Two forms
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

7. FIGURE 25-1 There is no change in temperature when there is a change in state.
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8. 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.
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9. 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

10. 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

11. 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

12. 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

13. FIGURE 25-9 Body-centered cubic unit cell.
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14. FIGURE 25-10 Face-centered cubic unit cell.
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15. FIGURE 25-11 Hexagonal close-packed cubic unit cell.
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16. 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

17. 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

18. 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

19. FIGURE 25-15 Iron-carbon phase diagram.
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20. 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

21. 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

22. 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

23. 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

24. FIGURE Change in mechanical properties caused by beta (silicon phase) in mechanical mixture with alpha (aluminum phase).
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25. 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

26. 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

27. 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

28. 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

29. 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

30. Preheat Reduces the rate at which welds cool Amount of preheat
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

31. 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

32. 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

33. 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

34. 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)

35. 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

36. 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

37. Hydrogen Many sources Troublesome in aluminum and steel
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

38. Nitrogen Comes from air drawn into the arc stream Primary problems
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

39. 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

40. 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

41. Metallurgic Defects Cold cracking Hot cracking Carbide precipitation
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

42. 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