Causes of Cracks in Free Forgings and Preventive Measures

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:

1. Surface transverse cracks

• 1) Shallow transverse cracks appear on the surface of the billet during forging, which are formed due to the subcutaneous bubbles of the steel ingot exposed to the air and unable to weld, with a depth of over 10mm. Some metals with poor plasticity can also produce this defect when the relative feeding amount is too large.
• 2) The deep, transverse cracks on the surface of the billet during forging are caused by the improper pouring of the steel ingot. For example, if there are defects on the inner wall of the steel ingot mold, the phenomenon of hanging ingots may occur, which can cause cracking during cooling; High speed and high temperature pouring, slow forming of the outer skin of the steel ingot and swinging of the steel liquid film; Casting of steel ingots and ingot molds, etc.
• Surface transverse cracks often appear at the first heat during forging. Once discovered, large forgings can be cleaned with a flame to remove cracks and prevent them from expanding in future forging.

2. Internal transverse cracks

Internal defects can only be detected through magnetic particle testing and ultrasonic testing.

• 1) When heating cold steel ingots, the heating speed in the low-temperature zone is too fast, causing significant tensile stress at the center of the ingot and forming cracks.
• 2) High carbon steel and high alloy steel with poor plasticity may form cracks during forging due to relatively small feed size.

3. Surface longitudinal cracks

• 1) Surface longitudinal cracks appear during the first hot drawing or upsetting. The reason for this is that there are defects on the inner wall of the steel ingot mold, or the new steel ingot mold needs to be annealed well before use. Improper pouring operations, such as high-temperature and high-speed pouring, can cause the solidification skin to rupture; Improper cooling method or premature demolding of steel ingots after demolding; Excessive pressure during chamfering; There are longitudinal scratches and other defects during the rolling of steel ingots.
• 2) Longitudinal cracks on the side surface of the billet. When the blank is freely upset, uneven deformation is caused by the frictional force between the contact surface of the blank and the anvil. Therefore, the side of the billet is in a "bulging" shape. On the surface of the bulge, irregular longitudinal cracks are generated due to the effect of tensile stress. This type of defect mainly occurs on the billet of low-temperature upsetting.

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.

4. Internal longitudinal cracks

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 after forging.
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.

• (1) Exploding

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.

• (2) Self cracking

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.

• (3) Cracking

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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