SURFACE INSPECTIONS

SURFACE INSPECTIONS

1. LIQUID PENETRANT

   • PROCESS (6 Basic Steps)
     • Clean Surface
       • Penetrant Application
         • Dwell (5 -10 mins)
           • Remove Excess
             • Developer (7 mins)
               • Inspect
   • THEORY
     • Surface Tension
       • Forces acting on the molecules in a liquid. Surface has an energy is described in terms of tension force.  This property makes liquid try to reduce surface their surface area.
     • Capilliary Action
       • Rate at which liquid flows and penetrates is influenced by VISCOSITY
       • Penetrant should be nonvolitile; could decrease ability to spread or dry up
     • Wetting
       • Surfactant (detergent) increases wettability by reducing contact angle
   • PROPERTIES (Classification of Methods)
     • Type of Dye
       • Fluorescent
       • Nonfluorescent or Visible
     • Method of Excess Penetrant removal
       • Lipophilic / hydrophilic; Higher sensitivity of fatigue cracks Post Emulsified (PE); Requires an emulsifying agent to be applied before excess penetrant can be removed by water wash.  Immersion or Spray. Post Removers (PR); uses a dilute detergent and limited solvent action allows for closer control of removal.
       • Direct water wash
       • Addition of emulsifying agent (Lipophilic remover)
       • Addition of Detergent (Hydrophilic remover)
       • Use of volatile solvent remover
         • These removers dry quickly prior to application of developer. This feature makes them useful for spot / localized testing
     • Form of Developer
       • Dry Powder
         • Light free flowing applied by powder gun. Only used with fluorescent penetrants. contamination with penetrant can appear as indications (specks)
       • Nonaqueous
         • Developers consists of particles suspended in a volatile organic solvent. Must be thoroughly agitated for uniform suspension. Applied by gun or aerosol and dries on white coating.
       • Water-soluble
         • Supplied as powders that are made up in water at recommended concentration. Including water suspendable can be applied by spray or submersion.
       • Water-suspendable
   • PROCEDURES
     • Qualifying procedures once established can be done on a Comparator Block. Section for comparing new and the standard.
     • Major Factors
       • Types of discontinuities sought
       • Surface condition
         • Layers of paint could cover discontinuities. Extent of mechanical means should be limited to wire brushing. Chemical solvents (acid or caustic cleaner) or Vacuum degreasing should be used to clean oils and greases.
       • Facilities and location of test
     • Operating Temperatures
       • 12 to 52C (60 to 125F) Lower temps (sluggish) Higher (increased evaporation)
     • Application of Penetrant
     • Dwell Times
     • Excess Penetrant Removal
     • Applying Developer
   • IINTERPRETATION
     • Nonrelevant Indications
       • Caused by feature of design or material
     • Relevant Indications
       • Continuous Line
         • Cracks, lack of fusion, incomplete penetration, forging laps, scratches, or die marks. Cracks; appear as jagged lines Forge Laps; smooth wavy Seams; more straight
       • Intermittent Line
         • Same as "Continuous" Will show on surfaces that has been worked. Large cracks
       • Rounded
         • Porosity, gas holes, pin holes. Deep cracks due to spreading.
       • Small Dots
       • Diffused or Weak
     • False Indications
       • Marks from handling tool Fingerprints Lint & dirt Patch from contact with another article

2. MAGNETIC PARTCLE

   • THEORY
     • Magnetism
       • Field represented by lines of force emanating from ends of magnet
         • Lines of force are continuous so that a line emerging from N must curve and reenter at the S
     • Flux Leakage
       • Local field across a crack. if fields run parallel to crack no poles will form.
     • Magnetic Induction
       • Flux Density
         • B - Strength of Induced Flux
           • tesla
         • H - Magnetized Field Strength
           • Amperes per metre
         • μ - Permeability
       • Saturation
         • As field strength becomes large, Flux reaches Maximum. Therefore material cannot be magnetized any more
       • B-H Curve
         • Residual magnetism
           • Created when material with high retentivity is saturated. Reversing field to bring induced flux to zero is "Coercive Force"
       • Electric Current
         • Right Hand Rule
           • Fingers point towards direction of magnetic field Thumb points to Direction of conventional current. And "Anode"
         • Ampere Turns
           • Net field (mag field) has a direction along axis of the coil. Strength of mag field is due to increased electric current by winding of wire into coil.
   • METHODS
     • Magnetic Flow, Current Induction, Current Flow
       • Mag Flow: Place part across poles of a permanent / electro magnet Current Induction: Place wire or coil carrying a current near a part Current Flow: Pass an electric current directly through the part
     • Direction of Field
       • Optimal angle of flux is 90 deg. Field is transverse between prods.
   • EQUIPMENT
     • Prods
       • Arc Strikes Contamination by prod strike.Removed by light grinding. Can start a fracture or a fatigue crack.Power on after Prod is in place, and Power off in reverse.
     • Yokes
       • AC cannot detect subsurface discontinuities
       • AC vs DC (Current Flow - Prods)
         • FWDC (Full wave rectified): Very High currents (Direct Current) HWDC (Half wave rectified): Pulsating effect which enhances mobility of particles. Used in portable DC systems.
       • AC vs DC (Magnetic Flow - Yokes)
     • Magnetic Particles
       • Dry (Up to 300C)
       • Wet (more sensitive; smaller)
   • PROCEDURES
     • Prepare test surface
       • Magnetize test piece
         • Prods: Prod spacing ~ 1 inch 90 to 125 AmpsYoke:At least two tests with flux fields at 90deg to each other
         • Apply indicating medium
           • Attraction of particles to discontinuities relies on residual magnetism
           • Examine the surface and interpret
             • Crack open to surface or Deep cracks: Heavy buildup of tightly held powder Subsurface crack: Less buildup and less tightly held powder
             • Nonrelevant Indications; e.g. Austenitic SS weld to Carbon Plate:  (Austenite is nonmagnetic) Permeability change: Work hardened regions Excessive magnetizing field and Residual magnetism Nonrelevant not due to material changes:Heat treatment, Composition, Work harden, Microstructure
             • Criteria for evaluation is same as RT
             • Demagnetize
               • Residual Mag removal may be specified for situations: -Part is to be plated or painted -Shafts or rotating components where particles can lead to failure -Parts to be machined -Followed by welding where it can cause arc blow -Stringent postcleaning requires it.
               • Postclean
     • Calibration
       • 4.5 kg (10lbs) 18 kg (40lbs)
       • AC; 10lb
       • DC; 40lb

3. OTHER METHODS

   • Flux Leakage
     • Magnetic Tape
     • Probe Coil / Hall Detector
       • Mass testing of similar components. Probe Coil; Electric current passing over flux leakage field.Hall;Measures electrical potential created at right angles to an electric current.
   • Eddy Current
     • Coil carrying an AC; magnetic field iduces a small current (eddy) Eddy itself creates a magnetic field that opposes original field and this can detect change of inductive reactance of energizing coil. Flaw will disturb reactance. Smooth surface only and on-line products i.e. tubes. Nonferrous; 6 mm depth of penetration Ferrous; 0.25 mm
   • Potential Drop
     • Accurate for measure of depth and shape of a fatigue crack growing from surface. Two current probes with two other probes (that measure potential diff).
   • Leak Test
     • Pressure by low pressure air with a tracer such as helium or bubble solution. Vacuum box (Less sensitive) Soap solution. Liquid penetrant on one side. 3x the dwell for this method.