Liquid Penetrant Inspection

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false indications frequently occur, you must be able to interpret  the  particle  indications  accurately. The factors that help you interpret the test results include  the  amount  of  magnetizing  current  applied,  the shape of the indication, the sharpness of the outline, the width of the pattern, and the height or buildup of the particles.  Although  these  characteristics  do  not  deter- mine  the  seriousness  of  the  fault,  they  do  serve  to identify the kind of defect. The indication of a crack is a sharp, well-defined pattern  of  magnetic  particles  having  a  definite  buildup. This indication is produced by a relatively low-magnet- izing current. Seams are revealed by a straight, sharp, fine indication. The buildup of particles is relatively weak, and the magnetizing current must be higher than that  required  to  detect  cracks.  Small  porosity  and rounded indentations or similar defects are difficult to detect  for  inexperienced  inspectors.  A  high-magnetizing current  continuously  applied  is  usually  required.  The particle  patterns  for  these  defects  are  fuzzy  in  outline and  have  a  medium  buildup. The  specifications  governing  the  job  determine whether  or  not  an  indicated  defect  is  to  be  chipped  or ground  out  and  repaired  by  welding.  Surface  cracks  are always  removed  and  repaired.  Indications  of  subsurface defects detected by magnetic particle inspection are evaluated  by  the  inspector.  When  the  indication  is  posi- tive, the standard policy is to grind or chip down to solid metal and make the repair. Unless the inspector can differentiate  accurately  between  true  and  false  indica- tions, the use of magnetic particle inspection should be restricted to the detection of surface defects, for which this  application  is  almost  foolproof. After the indicated defects have been repaired, you should reinspect the areas to ensure that the repair is sound.  The  final  step  in  magnetic  particle  inspection,  is to  demagnetize  the  workpiece.  This  is  especially  impor- tant when the workpiece is made of high-carbon steel. Demagnetization is essential when you use direct cur- rent to induce the magnetic field; however, it is not as necessary when alternating current was used in the test. In  fact,  the  usual  demagnetization  procedure  involves placing the workpiece in an ac coil or solenoid and slowly  withdrawing  it  while  current  passes  through  the coil. Demagnetization  can  be  accomplished  with  the portable unit if a special demagnetizer is not available. To demagnetize with the portable unit, form a coil of flexible  cable  around  the  workpiece.  Ensure  that the cable is plugged into the unit for the delivery of alternating current. Set the current regulator to deliver a current identical to that used for the inspection and turn on  the  unit.  Gradually  decrease  the  current  until  the ammeter indicates zero. On large pieces, it may be necessary to demagnetize a small portion of the work at a time. A check for the presence of a magnetic field may be made by using a small compass. A deviation of the needle from the normal position, when the compass is held near the workpiece, is an indication that a magnetic field is present. Also you can use an instrument called a field  indicator  to  check  for  the  presence  of  a  magnetic field.  This  instrument  usually  comes  with  the  magnetic particle  inspection  unite Liquid Penetrant Inspection Liquid penetrant methods are used to inspect metals for  surface  defects  that  are  similar  to  those  revealed  by magnetic  particle  inspection.  Unlike  magnetic  particle inspection, which can reveal subsurface defects, liquid penetrant  inspection  reveals  only  those  defects  that  are open to the surface. Four  groups  of  liquid  penetrants  are  presently  in use. Group I is a dye penetrant that is nonwater wash- able. Group II is a water washable dye penetrant. Group III and Group IV are fluorescent penetrants. Carefully follow the instructions given for each type of penetrant since there are some differences in the procedures and safety precautions required for the various penetrants. Before  using  a  liquid  penetrant  to  inspect  a  weld, remove all slag, rust, paint, and moisture from the surface. Except where a specific finish is required, it is not necessary to grind the weld surface as long as the weld surface meets applicable specifications. Ensure the weld contour blends into the base metal without under- cutting. When a specific finish is required, perform the liquid  penetrant  inspection  before  the  finish  is  made. This  enables  you  to  detect  defects  that  extend  beyond the final dimensions, but you must make a final liquid penetrant inspection after the specified finish has been given. Before using a liquid penetrant, clean the surface of the material very carefully, including the areas next to the inspection area. You can clean the surface by swab- bing it with a clean, lint-free cloth saturated in a non- volatile solvent or by dipping the entire piece into a solvent. After the surface has been cleaned, remove all traces of the cleaning material. It is extremely important to remove all dirt, grease, scale, lint, salts, or other 7-35

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