Phase Transformations

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Welding Metallurgy
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• Learning Activities
• Read Handbook pp 103-123
• View Slides
• Read Notes,
• Listen to lecture
• Do on-line workbook
• Do homework

• Lesson Objectives
• When you finish this lesson you will understand
• The various region of the weld where liquid
  forms
• Mechanisms of cracking associated with these
  regions

Keywords Composite Zone, Hot Cracking,
Constitutional Supercooling, Unmixed Zone,
Partially Melted Zone, Constitutional Liquation,
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Materials Behavior
Introduction

• Weldability is the capacity of a material to be
  welded under the fabrication conditions imposed,
  into a specific, suitably designed structure, and
  to perform satisfactorily in the intended
  service.
• Materials compatibility
• Process
• Response to stress and strain during welding

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Basic Regions of a Weld
Introduction

• Fusion Zone - area that is completely melted
• Heat-Affected Zone - portion of the base metal
  not melted but whose mechanical properties and
  microstructure were affected by the heat of the
  joining process
• Base Metal

Fusion zone
Base metal
Heat-affected zone
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Composite Zone Concerns
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Solidification (Hot) Cracking
Cracking in Welds

• Solidification (hot) cracking requires
• Low ductility material
• High tensile contraction stress
• Solidification occurs over a range of temperature
• Low melting point intergranular films
• Sulfur, phosphorus, boron
• Prevention by
• Low C, S, P levels
• Increased Mn

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Introductory Welding Metallurgy, AWS, 1979
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Phase Diagram When Interstitial Carbon Alloys
with Iron
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Perils of Welding Free-Machining Steels
Steel

• Solidification cracking due to impurity elements
• Sulfur, phosphorus, boron
• Lead doesnt seem to cause a problem, e.g. 12L14
• Impurity segregation at weld centerline creates
  low ductility area
• Combines with shrinkage stress to cause cracking

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Manganese Can Prevent Solidification Cracking
Steel

• Manganese combines with sulfur to form MnS
  particles
• Use a filler metal with higher manganese to
  absorb sulfur

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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• Constitutional supercooling works for alloys
  with K values less than 1.0 but what happens for
  alloys with K greater than 1.0 (I.e. rising
  liguidus with increasing temperature)? Can you
  draw the three corves?

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Unmixed Zone Concerns
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Questions?
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Partially Melted Zone Concerns
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• What sequence of event needs to take place for
  constitutional liquation to result in liquid
  films? Can we use a phase diagram to predict if
  this will happen or not?