Analysis of Defects Caused by Improper Forging

Forging low-multiplicity tissue defects

Large Grained:

If the initial forging temperature is too high and the degree of deformation is insufficient, or if the final forging temperature is too high, or if the final deformation amount falls into the critical deformation zone, large grain defects may appear in the low-magnification microstructure of steel forgings. Excessive deformation of aluminum alloys can result in texture formation; for high-temperature alloys, low deformation temperatures can lead to mixed deformation microstructure, all of which can cause coarse grain formation. Coarse grains reduce the plasticity and toughness of forgings, and significantly decrease their fatigue performance.

(2) Improper forging streamline distribution:

Improper forging die design,不合理forging method selection, or improper operation during forging, along with die wear, can lead to chaotic metal flow, such as streamline interruptions, backflow, and eddies on the low-magnification microstructure of forgings. This results in an unreasonable distribution of streamlines in the forging, which may cause early failure in use and affect the service life of the forging.

Forging Folding:

If there is excessive shape error in raw materials and billets, improper mold design, unreasonable arrangement of forming processes, poor lubrication, and improper forging operations, it may cause the oxidized surface metal from the metal deformation process to merge together and form folds. Folds align with the metal's streamline direction, and the ends of the folds typically present as small rounded corners. However, subsequent forging deformation can cause the folds to crack, resulting in pointed ends. Generally, there are severe oxidation and decarburization on both sides of the folds, and in some cases, there may also be an increase in carbon. Folds not only reduce the load-bearing area of the part but also, due to stress concentration at the fold during operation, often become fatigue sources.

(4) Trending:

The formation of穿流 is similar to that of folding, occurring when two metal streams or one metal stream carrying another merge together, yet the metal in the穿流 section remains an integral whole. 穿流 is also a form of improper streamline distribution. In the穿流 region, streamlines that were originally distributed at a certain angle converge, and the grain sizes within and outside the穿流 region often differ significantly. 穿流 can degrade the mechanical properties of the forging, particularly when there is a significant difference in grain sizes on either side of the穿流, resulting in more pronounced performance degradation.

Shear Bands

Alloys and high-temperature alloys are highly sensitive to quenching. During the die forging process, the area of the difficult-to-deform zone near the contact surface of the billet gradually expands. Intense shearing deformation occurs within this difficult-to-deform zone, forming shear bands. Fine grain areas with a wavy pattern appear on the transverse low-magnification microstructure of the forging. The shear bands cause the forging's microstructure to develop a strong directional characteristic, thereby reducing the forging's performance.

Forging high-multiple tissue defects

Grain Inhomogeneity:

Inhomogeneous deformation can lead to uneven grain breakage, or the degree of deformation in certain local areas may fall into the critical deformation zone, resulting in particularly coarse grains in some parts of the forging and finer grains in others, creating a defect of uneven grain size throughout the forging. Local work hardening during the forging process of heat-resistant steel and high-temperature alloys can also cause coarse grains, making these materials particularly sensitive to grain inhomogeneity. Inhomogeneous grain size significantly reduces the forging's endurance and fatigue properties.

(2) Strip Organization:

Forging deformation in a two-phase coexistence condition can produce lath-like structures. In the high-magnification microstructure of hypoeutectoid steel, ferrite and pearlite are distributed in a lath pattern. In austenitic and semi martensitic steels, a lath-like distribution of ferrite with austenite, ferrite with bainite, and ferrite with martensite can also appear in the forging. Lath-like structures can reduce the lateral plasticity of the forging, particularly the impact toughness. This often leads to cracking of the forging during use along ferrite bands or at the interfaces between the two phases.

(3) Carbon Depletion Layer Accumulation

Improper forging techniques can lead to defects such as the accumulation of decarburization layers in localized areas of the forging. For example, when drawing round billets, excessive hammering, over-compression, and flipping 90° to compress can create a double-dome shape. During further drawing, the metal of the double-dome shape flows outward, increasing width, while another part flows towards the center, thus forming a decarburization layer accumulation in the central area. The hardness of the decarburization layer accumulation area is lower than that of the normal tissue area, making the forging prone to cracking at this location during use.

(4) Carbide segregation level does not meet requirements:

If the original material has severe carbide segregation, combined with insufficient forging ratio or improper forging methods during the reforming process, uneven distribution of carbides may occur in the forging, presenting as large, concentrated carbide lumps or as defects with a network-like distribution of carbides. This type of defect is primarily found in Laves steel for dies and molds. Forgings with such defects are prone to localized overheating and cracking during heat treatment quenching. The cutting tools and molds made from them are susceptible to chipping and other issues during use.

(5) Casting Organizational Residue:

When the forging ratio is insufficient and the forging method is improper, cast as-cast microstructure may still occur within the forging. This type of microstructural defect primarily appears in forgings made from casting billets, mainly in the areas of the forging that are difficult to deform. This residual cast as-cast microstructure leads to a decrease in the performance of the forging, particularly evident in the reduction of impact toughness and fatigue resistance.