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,
Introduction Materials Behavior • 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 0.1.1.5.0.T1.95.12
Introduction Basic Regions of a Weld • 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 0.1.1.5.0.T2.95.12
Cracking in Welds Solidification (Hot) Cracking • 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 0.1.1.5.2.T13.95.12
Introductory Welding Metallurgy, AWS, 1979
Steel Perils of Welding Free-Machining Steels • Solidification cracking due to impurity elements • Sulfur, phosphorus, boron • Lead doesn’t seem to cause a problem, e.g. 12L14 • Impurity segregation at weld centerline creates low ductility area • Combines with shrinkage stress to cause cracking 0.1.1.5.2.T13.95.12
Steel Manganese Can Prevent Solidification Cracking • Manganese combines with sulfur to form MnS particles • Use a filler metal with higher manganese to absorb sulfur
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?
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?