Intergranular cracking makes its presence known by the emergence of long, vertical cracks,
reaching from the top of a shell, down to the bottom. These cracks, caused by the residual
tensile stresses within the material, may be completely invisible and yet the part may be
prone to cracking perhaps later on, already in service.
To test a drawn cup for the existence of intergranular cracking tendency, a corrosive liquid
can be used. Remaining in the cup for a certain period of time, the corrosive liquid will
eat through the cracks and these will open up like a flower.
Where the cups were stress relieved for removal of residual stresses right after forming,
no cracks will be present.
Intergranular cracking is caused by the displacement of material during the drawing
process. Where a drawn, diametrally restricted part is ejected from the die, the springback
of the material has a tendency to force its structure back, toward its basic shape, in this case
to a round blank. By doing so, the diameter of the cup bulges, giving rise to residual tensile
stresses at the same time.
With box-type drawn objects, the cracking is called compression cracking, since it
occurs at the corners, where the material is in compression. The shape and location of these
cracks resembles those of a round shell, which is logical, considering the fact the rounded
corners of the box do resemble a quarter of the shell, each.
Compression cracking is insidious in that, the part may look absolutely perfect and on
impact, or perhaps with time the cracks develop and propagate. As with round shells, stress
relieving after forming removes the residual stresses from the material and along with
them, it removes the material’s tendency to cracking.
reaching from the top of a shell, down to the bottom. These cracks, caused by the residual
tensile stresses within the material, may be completely invisible and yet the part may be
prone to cracking perhaps later on, already in service.
To test a drawn cup for the existence of intergranular cracking tendency, a corrosive liquid
can be used. Remaining in the cup for a certain period of time, the corrosive liquid will
eat through the cracks and these will open up like a flower.
Where the cups were stress relieved for removal of residual stresses right after forming,
no cracks will be present.
Intergranular cracking is caused by the displacement of material during the drawing
process. Where a drawn, diametrally restricted part is ejected from the die, the springback
of the material has a tendency to force its structure back, toward its basic shape, in this case
to a round blank. By doing so, the diameter of the cup bulges, giving rise to residual tensile
stresses at the same time.
With box-type drawn objects, the cracking is called compression cracking, since it
occurs at the corners, where the material is in compression. The shape and location of these
cracks resembles those of a round shell, which is logical, considering the fact the rounded
corners of the box do resemble a quarter of the shell, each.
Compression cracking is insidious in that, the part may look absolutely perfect and on
impact, or perhaps with time the cracks develop and propagate. As with round shells, stress
relieving after forming removes the residual stresses from the material and along with
them, it removes the material’s tendency to cracking.
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