particle inspection, the boundaries of the areas may create local leakage fields and attract magnetic particles to form
High Temperature Exposure.
a. Boundaries of Heat Treated Sections. Heat treating a part consists of heating it to a high temperature
and then cooling it under controlled conditions. The cooling may be relatively rapid or it may be done
quite decrease the hardness or the grain size of the metal by varying the temperature and the rate of
cooling. On a cold chisel the point is hardened to cut better and to hold an edge. The head of the
chisel, which is the end struck by the hammer, is kept softer than the cutting edge so that it won't
shatter and break. The edge of the hardened zone frequently creates a leakage field when the chisel is
inspected with magnetic particle inspection.
Delta Ferrite is brittle and has historically been considered a defect in applications
such as aircraft that are exposed to tensile and cyclic loading. While the presence of
delta ferrite does not indicate an actual defect, such a region would be a preferential
crack initiation area.
b. Delta Ferrite. Delta Ferrite is a ferromagnetic phase of steel that occurs at elevated temperatures. This
phase primarily occurs at normal temperatures because of rapid cooling after prolonged exposure to
high temperatures. A concentrated region of delta ferrite may cause non-relevant indications along the
regions boundary due to the magnetic disturbance caused by its presence.
Abrupt Changes of Section.
Where there are abrupt changes in section thickness of a magnetized part, the magnetic field may be said to expand
from the smaller section to the larger. Frequently this creates local poles due to magnetic field leakage or distortion. If
a part, as shown in Figure 3-70, is magnetized in a coil, poles are set up at each end and some leakage occurs at A and
B. Also, the change of section at C is quite abrupt and there may be a leakage across this corner as shown. These
leakage fields will attract magnetic particles, thereby creating an indication. The indications formed at A and B are
usually very easily interpreted; that at C may be more difficult to recognize as being non-relevant. If the indication is
continuous around the shaft, it should be suspected as being caused by the shape of the part rather than by a
discontinuity. The non-relevant indication at C will usually be "fuzzy" like an indication, which is produced by a defect
beneath the surface. If there is a crack or discontinuity in that area, it will usually produce an indication that is sharper
and it probably will not run completely around the part.
Figure 3-70. Local Poles Created by Shape of Part