Free forging is a type of metal component
manufactured through forging technology, and cracks are one of the possible
defects that may occur during the manufacturing process. Various factors mainly
influence the occurrence of cracks. The following are some possible causes of
cracks in free forging and corresponding preventive measures:
Internal defects can only be detected
through magnetic particle testing and ultrasonic testing.
Once surface longitudinal cracks are
discovered during forging, they should be immediately removed by blowing oxygen
to prevent the cracks from expanding in future forging.
There are three types of longitudinal
cracks inside forgings
1) Longitudinal cracks appear near the
center of the riser end of the billet. This is because during the
solidification of the steel ingot, the shrinkage cavity is not concentrated in
the riser part, or the cutting amount at the riser end during forging is too
small, resulting in a secondary shrinkage cavity or residual shrinkage cavity
near the riser end of the billet, which causes internal longitudinal cracks
2) Hollow longitudinal cracks appear inside the billet. This is due to the tensile stress on the central part of the metal during the drawing of the round section billet with a flat anvil Or due to incomplete heating of the billet, low internal temperature, and internal longitudinal cracking during elongation, Excessive pressure during chamfering may cause longitudinal cracks.
3) There are longitudinal cracks inside the billet. This is due to excessive feeding during elongation of low plasticity high alloy steel or repeated elongation in the same area. This type of internal longitudinal cross crack often occurs in high alloy steel.
Explosion is a crack formed on the surface
or inside of a billet during heating before forging or after cooling or heat
treatment of the forging. The reason for the explosion is that the billet has a
large residual stress, which is caused by rapid heating or inappropriate
cooling without being eliminated.
Self-cracking of forgings generally occurs
after forging or heat treatment, or after a long period after forging. The
reason for the occurrence is that small cracks have formed in the billet during
the forging process, which is exacerbated by cooling or heat treatment, or
there is significant residual stress inside the forging.
Cracking refers to shallow cracks resembling turtle shells that appear on the surface of forgings during forging. The reason for this is that there is a higher content of Cu and Sn in the steel. Or, if the slag is not completely removed after heating the copper material in the heating furnace, the melted copper may penetrate the grain boundaries of the steel material. Alternatively, shallow cracks and cracks on the surface of the steel material should be promptly removed due to factors such as the high initial forging temperature of the billet or excessive hammering during forging. Once removed, they should not affect the continuation of forging.
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