Defects in Materials
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The document discusses various defects in materials, including microscopic and macroscopic defects caused by inherent, processing, and service-related factors, which negatively impact physical and mechanical properties. It outlines specific casting defects such as inclusions, porosity, shrinkage, and defects from bulk deformation processes like forging and rolling, as well as other processing defects and service-related issues including corrosion and fatigue failure. Methods for detecting defects are also covered, highlighting destructive and non-destructive testing approaches.
Defects in Materials
- 2. DISCONTINUTIES & DEFECTS • NATURE OF DEFECTS – Microscopic Defects – Macroscopic Defects • ORIGIN OF DEFECTS – Inherent Defects – Processing Defects – Service Defects • DETERIORATES PHYSICAL and MECHANICAL PROPERTIES of MATERIALS • DETECTION of DEFECTS – Destructive Testing – Non-Destructive Testing
- 3. CASTING DEFECTS : Inclusions • Inclusions are nonmetallic materials (oxides or sulphides) • a lower melting point than the metal • relative brittleness • these defects occur near the surface as a “skin” effect. • occur at the centre of the casting • they tend to be irregular in shape, not spherical or oval.
- 4. CASTING DEFECTS : Porosity • gas trapped in the molten metal • formed by – release from the molten metal itself – release from the green sand mold, water vapour, or turbulence caused when pouring the metal • form of small pockets or voids appears as round, irregular or elongated shapes
- 5. CASTING DEFECTS : Shrinkage • Metal contracts or shrinks when changing from the liquid to the solid state and defects will occur in a casting unless sufficient molten metal is available to “feed” it. • Defects may take the form of cavities, branch-like tears • Shrinkage cavities occur usually at hot spots in the casting
- 6. CASTING DEFECTS • Piping : Central Cavity • As the casting solidifies, the metal contracts if there is not an adequate supply of molten metal to the centre of the casting • usually occurs in pure metals and alloys having narrow ranges of solidification temperature • causes lamination Hot Tears : If a section begins to shrink while still hot and there is not a sufficient supply of liquid metal, the resulting internal stress will tear the metal. This is because while hot, the metal has relatively low strength.
- 7. CASTING DEFECTS • Cold Shot • Cold Shut •Flash •Mold Shift •Short Casting
- 8. Bulk Deformation Processes • Forging – Open Die Forging – Impression Die Forging – Closed Die Forging • Rolling
- 9. Products of Bulk Deformation Processes
- 10. Open Die Forging Defects Fracture - – exhausted ductility – Intergranular fracture in hot working • Barreling - Friction • Solution - – limited deformation per step – Process anneal between steps
- 11. Closed Die Forging Defects • Laps
- 12. Closed Die Forging Defects • Flow Lines
- 13. Rolling Defects : Seams • Surface irregularities, such as cracks, on the slab or billet are stretched out and lengthened during rolling • Seams may also be caused by folding of the metal due to improper rolling • Seams are surface discontinuities and on finished bars will appear as either continuous or broken straight lines
- 14. Rolling Defects : Stringers • Nonmetallic inclusions in slabs or billets, that are thinned and lengthened in the direction of rolling, by the rolling process, are called stringers.
- 15. Rolling Defects • If the cold mill rolls or hot mill coils happen to have too much crown, the cold mill will roll out the center and create center buckle. • If cold mill rolls or hot mill have too little crown, the cold mill will roll out the edges and create edge wave.
- 16. Other Processing Defects • Heat-Treated Cracks •Grinding Cracks
- 17. Extrusion Defects Defects in Extrusion • Surface Crack – Lack of proper lubrication • Axial Hole or Funneling
- 18. Fatigue Failure Fractography of Fractured Surface direction of rotation final rupture crack origin dull, fibrous brittle failure smooth circular “beachmark” practical example of fatigue failure
- 19. Fatigue Failure Fractography of Fractured Surface Transmission electron fractograph showing fatigue striations.
- 20. Creep Failure Fractography of Fractured Surface Fractured surface showing oxide films
- 21. Service Defects • Corrosion Stress Corrosion Intergranular Corrosion Galvanic Corrosion
- 22. Service Defects Corrosion of boiler tube
- 23. Service Defects • Over Stress Improper Galvanisation
- 24. Service Defects Creep of Super heater tube Fish mouth effect
- 26. Creep – Overheating of Reformer Tubes
- 27. Service Defects Cracking during welding Failure of the weld