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NDT.ppt
- 1. 1 Non-destructive Testing Egyptian Welding Academy Welding Consultancy and training services
- 2. 2 What faults can inspection find? Physical features Surface and buried flaws, dimensional deviations Can often be discovered by inspection Material property features Defective strength, ductility or corrosion performance Caused by defective composition or processing Rarely discovered by inspection Management requires careful process control or destructive testing
- 3. 3 Physical flaws Flaw, discontinuity or imperfection Deviation from perfection Can include atomic scale and large flaws Non-conformity and defect Flaw which fails to meet prescribed standard Nonconformity may not be discovered, or may be allowed after critical engineering assessment Defects cause rejection, material is repaired or scrapped.
- 4. 4 Seriousness of flaws Planar flaws May initiate catastrophic brittle or fatigue failure Often difficult to find Cracks, incomplete fusion, inadequate penetration Volumetric flaws Loss of cross section Easier to find Pores, blowholes, inclusions, many surface irregularities Critical flaws Non-critical flaws
- 5. 5 Cracks Metallurgical phenomena Presence may indicate low ductility Variety of locations Weld, HAZ, base metal Transverse, longitudinal Often difficult to find Inspection does not guarantee freedom from cracking Serious stress raisers Not tolerated Longitudinal crack Crater crack
- 6. 6 Inadequate penetration Loss of cross section Sharp edge causes stress concentration Joint geometry or welding parameters error May cause fatigue or brittle failure Not tolerated in fatigue applications
- 7. 7 Incomplete fusion Low arc energy, high quench severity, poor joint design Planar flaws with a sharp edge If small and intermittent, not very significant Low effect on toughness, fatigue If large and continuous as serious as inadequate penetration Some standards allow significant levels
- 8. 8 Slag inclusions Due to improper technique Volumetric defect easily found by radiography Only occurs using processes with a flux Up to 4% may be tolerated without loss of impact toughness Ductility is only affected in high strength steels Some is tolerated by all standards
- 9. 9 Porosity Caused by evolution of gas (usually hydrogen, oxygen or nitrogen) during solidification Contamination of base material, shielding gas or electrodes No effect on strength unless >3% by volume No effect on ductility Only affects fatigue in butt joints if it breaks the surface Easily found by radiography Far less is tolerated by standards than is structurally significant
- 10. 10 Surface defects Underfill and undersized welds Excessive weld metal Convexity, distortion and waste Undercut Overlap Warping or distortion Misalignment Arc strikes and spatter
- 11. 11 Weld profile defects Incomplete penetration Insufficient throat Undercut Overlap Insufficient leg length Excessive size
- 12. 12 All inspection requires An identified area to test Uniquely identified Properly prepared and accessible A defined test procedure National standard, work instruction An acceptance standard Permitted level of imperfections A competent and unbiased inspector, with appropriate equipment
- 13. 13 Accreditation National Association of Testing Authorities Accreditation of Laboratories in Australia Not individuals Accreditation and audit by peer review Reports can be endorsed with NATA logo
- 14. 14 NATA requirements Written test methods and procedures Adequate staff and technical control Appropriate and calibrated equipment Control of test reports and laboratory or field records
- 15. 15 Level of inspection Decided by design authority and owner Random or 100%? Criticality of area to be inspected Chance of a flaw existing Cost of inspection May be better to target inspection
- 16. 16 Random inspection NOT for critical nonconformities As specified by code / contract Must be random (not just the most convenient) Logically defined batch Statistical sampling techniques AS 1199 and AS 1399
- 17. 17 Targeted inspection May be better to target inspection to: where defects are most likely to occur, or would have the most serious implications on performance
- 18. 18 Visual inspection of welds
- 19. 19 Visual scanning The finished component is viewed from a distance to see if all fabrication and welding has been done, and there are no gross discrepancies 100% is required
- 20. 20 Final visual inspection (VI) Finished welds are viewed from a close distance (300mm) with adequate light to determine if there are surface defects Primary evaluation method of underestimated importance Detects critical flaws Prerequisite to other tests Method often not detailed, but see AS3978 and BS5289 (superseded) Aided possibly with a mirror, magnifier, video camera or borescope
- 21. 21 Limitations of visual inspection May miss significant defects Buried defects Small or narrow defects (cracks) Surface colour variation (scale, heat tint) masks defects
- 23. 23 Liquid penetrant inspection Dye penetrant inspection (DPI) Bleed-out of penetrant against a contrasting developer Can reveal leaks through a vessel wall Reveals much finer flaws than visual inspection For example, fine cracks 0.2mm long
- 24. 24 Features of DPI Finds surface defects in non-porous materials Non-magnetic materials can be examined Light, portable equipment Aerosol cans, rags, paper towels Properly done it takes time Test surface has to be smooth, uncontaminated, and undeformed Machined or ground surfaces may have defects smeared over
- 25. 25 Procedure Follow the supplier's directions Standards specify test methods, eg ASTM E165, ASME Section V, ISO 3879, AS 2062 All materials (penetrant, remover and developer) should be from one supplier as a kit Parts must be clean and dry and at the correct temperature Precleaning requires removal of all oil, grease, dirt, paint, slag, spatter Dry after cleaning by warming
- 26. 26 Penetrant Liquid with a high surface tension is applied and wets the surface Soaking into surface flaws Coloured with a dye for visibility Fluorescent dyes for use with UV illumination in a darkened cubicle Applied by dipping, flooding, brushing or spraying Wait the specified dwell time
- 27. 27 Removing surplus penetrant Water washable penetrants are flushed with water Post emulsifiable penetrants require the emulsifier to be applied by dipping, flooding or spraying Solvent removable penetrants are first wiped up with dry rags then with a rag dampened with the solvent
- 28. 28 Developers Blot up dye from flaws revealing their location Absorbent material with a colour which contrasts the penetrant Developers may be suspended in water or other liquid, or may be dry powder Applied by dipping, immersing, flooding, dusting or spraying Wait for a minimum dwell time of 7 minutes, or as specified
- 29. 29 Defects found by DPI Most are cracks, which can be exceedingly fine Also finds inadequate penetration, incomplete fusion, and pores that are open to the surface Does not find buried flaws Overlap and undercut best seen by visual inspection
- 30. 30 Limitations of DPI Rough surfaces and those with scale can give false indications Coatings, smeared metal and contamination may hide defects Materials may emit hazardous or toxic vapours The process takes considerable time and effort Cleaning is essential before testing and may be required after testing
- 31. 31 Magnetic Particle Inspection Detecting distorted magnetic fields around a flaw Flux leakage Part must be magnetised Magnetic particles are applied while part is magnetised Accumulations of magnetic particles may indicate a flaw
- 32. 32 Application of MPI Technique applied after & during welding Root run, backgouged second side and during repair All welding supervisors and inspectors should be familiar with it Final inspections should be by authorised personnel
- 33. 33 Features of MPI Part must be uniformly ferromagnetic (ferritic steel) Cannot be used for non magnetic materials: austenitic steels, non-ferrous alloys Dissimilar joints can show indications at the interface Finds planar flaws close to surface Sub-surface rounded flaws are indistinct Quicker than dye penetrant inspection Not as influenced by contamination as DPI
- 34. 34 Magnetisation May magnetise whole component or only part of it Passing current through the part Making the part the core of an electromagnet Applying an electromagnet Applying a permanent magnet
- 35. 35 Magnetisation by current flow Current Current passing longitudinally in shaft generates a circumferential field Longitudinal Flaw Amps
- 36. 36 Magnetisation in solenoid core Current Longitudinal magnetic field created by inserting shaft in a coil, And passing an electric current through the coil. N Amps
- 39. 39 MPI Media Dry powders Rounded, mobile particles Can be used at high temperatures Easily removed Inks (powders suspended in liquid) Finer particles than dry powders - more sensitive Coloured for visibility Black light version
- 40. 40 Test method ASTM E709, ASME V, AS1171 describe method Written procedures required Technique is to ensure particles are flowed gently across the surface while the magnetic field is applied Remanent magnetism can sometimes be used Confirm all indications
- 41. 41 False Indications Joints with dissimilar magnetic properties Rough surfaces Local cold work Residual magnetism from some other source