Heat treating and inspection of metals Page: 55
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HEAT TREATING AND INSPECTION OF METALS 48
the principal stresses imposed in service, will be found very helpful for
the best interpretation of magnaflux test results. Magnaflux powder
collects along the lines of magnetic disturbances and thus defines the
defects as to location, character, and extent. Cracks, faulty welds,
seams, laps, cavities, inclusions, tool marks, and scratches, are among
the most common defects that may be detected. Most magnaflux indi-
cations of any consequence are unmistakable but many are often the
cause for speculation. The following is a description of these defects
as they appear under test:
(a) Open cracks.-A crack that is open to the surface is well defined
by a sharp line, usually jagged. Cracks resulting from heat
treatment or fatigue are regularly open to the surface.
(b) Hidden cracks.-A crack that is not open to the surface is traced
by a fuzzy line, the width increasing with its size and depth below
the surface. Hidden cracks are developed in the processes of
manufacture, such as forging, welding, and heat treating.
(c) Grinding cracks.-Grinding cracks appear in large numbers on
the surface of hardened steel. The edges of the cracks are raised,
giving the surface the appearance of being pulled up. Care must be
exercised in passing judgment on this type of crack. If the part is
to receive thrust in service, the crack may be of no consequence; how-
ever, if the part is subjected to torque, it may be well to reject it.
False indications, due to sharp changes in cross section, have a fuzzy
outline and show weak adherence of the inspection medium, which may
easily be blown off.
(d) Welds.-Faulty fusion lines and seams in welds cause the same
indications as cracks, and the medium follows the course of the joint.
Shrinkage cracks in welds show the same as other cracks. They are
usually quite short and irregular. Patch welds, often found in hol-
low steel propellers, give a very circular type of pattern or spot, which
often contains fine shrinkage cracks.
(e) Cavities and inclusions.-Cavities and nonmetallic inclusions,
of a globular or irregular form, show as spots and are fairly well de-
fined if near the surface. The powder will collect over the whole spot
rather than around the edges.
(f) Scratches.-Scratches and tool marks are indicated readily by
light lines of powder, which often have the appearance of cracks but
show little attraction.
(g) Sharp changes in sections, as at fillets, will possess sufficient
concentration of magnetic flux to give an appearance resembling a
crack, but experience will make differentiation possible.
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/57/: accessed April 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.