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  1. 1. 1 Foundry-Institute Seminar Metallurgical defects of cast steel Claudia Dommaschk TU Bergakademie Freiberg Foundry Institute, Germany
  2. 2. 2 Structure • Gas cavities • Oxide and slag inclusions, Nonmetallic inclusions • Shrinkage cavities • Hot tear • Primary grain boundary fracture • Defects caused by heat treatment
  3. 3. 3 Gas cavities Description and reasons: • Cavities in castings, especially in the upper parts of the castings • Formation during solidification because of degrease of gas solubility • often in combination with oxide and slag inclusions • formation of gas cavities depends on the concentration of oxygen, nitrogen and hydrogen • the inner surface of the cavities is smooth
  4. 4. 4 Gas cavities
  5. 5. 5 Gas cavities Prevention: • use of dry materials and ladles • use of clean charge • degasification of the melt • look at the mould sands (permeability of gas, vent…)
  6. 6. 6 Oxide and slag inclusions, nonmetallic inclusions Description and reasons: • Classification: endogenous and exogenous inclusions • endogenous inclusions are caused by the reaction products during the melting process (especially during deoxidation) • exogenous inclusion are caused by other materials in the melt (e.g. refractory lining) • thin fluid slag can precipitate at the grain boundaries  danger of formation of hot tears is higher • Classification of size: Macro inclusions > 20 μm Micro inclusions < 20 μm
  7. 7. 7 Slag inclusions GX3CrNiMoN17-13-5 GX2CrNiMo18-14-3 Oxide and slag inclusions, nonmetallic inclusions
  8. 8. 8 Prevention: • use of clean charge • optimization of gating and feeding system (lamellar flow) • decrease of the dissolved oxygen • decrease of the overheating temperature Oxide and slag inclusions, nonmetallic inclusions
  9. 9. 9 Example: G42CrMo4 • nonmetallic inclusions arise by reason of the reactions during the melting process Oxide and slag inclusions, nonmetallic inclusions
  10. 10. 10 Shrinkage cavities Description and reasons: • specific volume of melt is higher than the specific volume of solid  • contraction during solidification and cooling • feeding is necessary – if the feeding is not optimal  formation of shrinkage cavities • the shrinkage volume of cast steel is about 4-7 % • the inner surface is rough Liquid shrinkage Solidification shrinkage shrinkage RT TS TL TP Specific volume
  11. 11. 11 Shrinkage cavities GE 300 (GS 60)
  12. 12. 12 Shrinkage cavities Prevention: • use of optimal feeding system (calculation and simulation) • warranty of directional solidification • use of exothermic feeder sleeve • decrease of the pouring temperature
  13. 13. 13 Hot tear Description and reasons: • hot tears are intercrystalline discontinuity • cracks run along the grain boundaries • the risk of cracks at alloys with a high freezing range is higher than with a small freezing range • the reason are stresses during solidification because of hindered contraction (residual stress) • the main reason for formation of hot tears are the geometry of casting • if melt can flow into the crack - partial or completely annealed hot tears are possible
  14. 14. 14 Hot tear Influence of Carbon content on the inclination of hot tears Influence of Manganese and Sulphur content on the inclination of hot tears - Maximum of the hot tearing tendency by ~0.4 % C - Low tendency below 0.2 % - Sulphur is very dangerous - Manganese compensate
  15. 15. 15 Hot tear
  16. 16. 16 Hot tear Partial annealed hot tear
  17. 17. 17 Prevention: • design appropriate to casting, prevention of residual stresses, wide difference in the wall thickness and hot spots) • prevention of hot sand effects Hot tear
  18. 18. 18 • Caused by Al-N-precipitations • high content of Al and N and thick-walled castings Primary grain boundary fracture (“Rock candy or shell fracture”) G24Mn5 Al-N-precipitations
  19. 19. 19 Defects caused by heat treatment GS33NiCrMo • left: quenching and tempering not correct – ferrite, pearlite and bainite  lower ductility
  20. 20. 20 Defects caused by heat treatment G24Mn5 (thick-walled casting) • quenching and tempering not complete – ferrite, pearlite and bainite • different structure and lesser properties
  21. 21. 21 Defects caused by heat treatment G30Mn5 GS25 • Decarburization of the surface area caused by heat treatment without protective atmosphere  Chance of properties in the surface area
  22. 22. 22 Defects caused by heat treatment GX3CrNiMo20-18-7 • temperature of solution heat treatment to low and/or cooling rate not correct  • precipitation of delta-ferrite  • these components are brittle  • lower ductility
  23. 23. 23 Defects caused by heat treatment GX 120Mn13 • temperature of austenitizing to high  • coarse grain  bad mechanical properties
  24. 24. 24 Defects caused by heat treatment G105Cr4 = hypereutectoid cast steel • hardening crack • structure: coarse martensite and residual austenite • reason: temperature of austenitizing and cooling rate to high
  25. 25. 25 Defects caused by heat treatment • intercrystalline corrosion • heat treatment not correct  precipitation of Cr-carbides on the grain boundary  corrosion was possible GX 5CrNiMo19-11-2
  26. 26. 27