Analysis Of Common Problems Caused By Material Selection On Forging Defects

The raw materials used for forging are ingots, rolled materials, extruded materials and forging blanks. Rolled materials, extruded materials and forging billets are semi-finished products processed by rolling, extruding and forging of ingots respectively. Under normal circumstances, the occurrence of internal defects or surface defects in ingots is sometimes inevitable. For example, internal composition and tissue segregation, etc. Various defects in raw materials will not only affect the forming of forgings, but also affect the final quality of forgings. Therefore, quality control of raw materials must not be ignored.
Forging defects caused by defects in raw materials usually include:

1. Surface cracks
   Surface cracks mostly occur on rolled bars and forged bars. They are generally linear in shape and consistent with the main deformation direction of rolling or forging. There are many reasons for this defect. For example, the subcutaneous bubbles in the steel ingot elongate along the deformation direction during rolling, and are exposed to the surface and develop deep inside. Another example is that during rolling, if the surface of the billet is scratched, stress concentration will occur during cooling, which may cause cracking along the scratches, etc. If this kind of crack is not removed before forging, it may expand during forging and cause cracks in the forging.
2. Fold
   The reason for the folding is that during the rolling process of the metal blank, due to incorrect sizing of the groove on the roll, or the burrs produced by the worn surface of the groove are involved in the rolling process, forming a certain inclination angle with the material surface. Creases. For steel, there are iron oxide inclusions in the creases and decarburization around them. If the folds are not removed before forging, it may cause the forging to fold or crack.
3. Scarring
   Scarring is a peelable film on a localized area of the surface of the rolled material.
   The formation of scab is due to the splashing of molten steel during casting and condensation on the surface of the steel ingot. During rolling, it is pressed into a thin film and adheres to the surface of the rolled material, which is scab. After forging, if the forging is pickled and cleaned, the film will peel off and become a surface defect of the forging.
   4.Layered fracture
   The characteristic of layered fracture is that its fracture or section is very similar to broken slate or bark. Layered fractures mostly occur in alloy steel (chromium-nickel steel, chromium-nickel tungsten steel, etc.), and are also found in carbon steel.
   This defect occurs due to defects such as non-metallic inclusions, dendrite segregation, and loose pores existing in the steel. During the forging and rolling processes, the steel is elongated along the rolling direction, making the steel lamellar. If there are too many impurities, the forging risks delamination and cracking. The more severe the lamellar fracture, the worse the plasticity and toughness of the steel, especially the very low transverse mechanical properties. Therefore, steel with obvious lamellar defects is unqualified.
4. Bright lines (bright areas)
   Bright lines are crystalline, shiny and reflective thin lines on the longitudinal fracture. Most of them run through the entire fracture, and most of them occur in the axial center.
   Bright lines are mainly caused by alloy segregation. Slight bright lines have little effect on mechanical properties, while severe bright lines will significantly reduce the plasticity and toughness of the material.
5. Non-metallic inclusions
   Non-metallic inclusions are mainly formed during the cooling process of smelting or casting molten steel due to chemical reactions between components or between metals, furnace gases and containers. In addition, during metal smelting and casting, inclusions can also be formed due to refractory materials falling into the molten steel. Such inclusions are collectively referred to as slag inclusions. On the cross-section of forgings, non-metallic inclusions can be distributed in the form of points, sheets, chains or masses. Serious inclusions can easily cause cracking of forgings or reduce the performance of the material.
6. Carbide segregation
   Carbide segregation often occurs in alloy steels with high carbon content. It is characterized by the accumulation of more carbides in local areas. It is mainly caused by the ledeburite eutectic carbides and secondary network carbides in the steel, which are not broken and evenly distributed during billet opening and rolling. Carbide segregation will reduce the forging deformation performance of steel and easily cause forging cracking. During heat treatment and quenching, forgings are prone to local overheating, overburning and quenching cracks. The edge of the finished knife is prone to chipping during use.
   8.Aluminum alloy oxide film
   The aluminum alloy oxide film is generally located on the web of the die forging and near the parting surface. It appears as a fine crack on the low-magnification tissue, and appears as a swirl pattern on the high-magnification tissue. The characteristics on the fracture can be divided into two categories: first, it is flat and flaky, with colors ranging from silver gray and light yellow to brown and dark brown. ; Second, they appear as small, dense and shiny dots.
   The aluminum alloy oxide film is formed when the exposed melt surface interacts with water vapor or other metal oxides in the atmosphere during the casting process. The oxide film is drawn into the interior of the liquid metal during the transfer casting process.
   The oxide film in forgings and die forgings has no obvious impact on the longitudinal mechanical properties, but has a greater impact on the mechanical properties in the height direction. It reduces the strength properties in the height direction, especially the elongation in the height direction, impact toughness and height direction resistance. Corrosion properties.
7. White spots
   The main characteristics of white spots are round or oval silver-white spots on the longitudinal fracture of the billet, and small cracks on the transverse fracture. The size of the white spots varies, ranging from 1 to 20 mm or longer in length.
   White spots are common in alloy steels such as nickel-chromium steel and nickel-chromium-molybdenum steel. They are also found in ordinary carbon steel and are defects hidden inside.
   White spots are produced under the combined action of hydrogen, structural stress during phase transformation, and thermal stress. They are more likely to occur when the steel contains more hydrogen and the cooling after hot pressure processing (or post-forging heat treatment) is too fast.
   Forgings made from steel with white spots are prone to cracking during heat treatment (quenching), and sometimes even fall off in pieces. White spots reduce the plasticity of steel and the strength of parts, and are stress concentration points. Like sharp cutters, they can easily turn into fatigue cracks and cause fatigue damage under the action of alternating loads. Therefore, there are absolutely no white spots allowed in forging raw materials.
8. Coarse crystal ring
   Coarse grain rings are often defects in aluminum alloy or magnesium alloy extruded rods.
   Aluminum and magnesium alloy extruded rods supplied after heat treatment often have coarse grain rings on the outer layer of their round cross-sections. The thickness of the coarse grain ring gradually increases from the beginning to the end during extrusion. If the lubrication conditions during extrusion are good, coarse grain rings can be reduced or avoided after heat treatment. On the contrary, the thickness of the ring will increase.
   The causes of coarse crystal rings are related to many factors. But the main factor is due to the friction between the metal and the extrusion barrel during the extrusion process. This friction causes the outer grains of the extruded rod cross-section to be much more broken than the grains in the center of the rod. However, due to the influence of the cylinder wall, the temperature in this area is low and the recrystallization is not complete during extrusion. During quenching and heating, the unrecrystallized grains recrystallize and grow up to engulf the recrystallized grains, thus forming coarse grains on the surface. ring.
   Blanks with coarse grain rings are prone to cracking during forging. If the coarse grain rings remain on the surface of the forging, the performance of the part will be reduced. For blanks with coarse grain ring defects, the coarse grain ring must be turned away before forging.
   11.Remnants of shrink tube
   The shrinkage residue is generally caused by the concentrated shrinkage holes produced in the riser part of the steel ingot that are not completely removed and remain inside the steel during billet opening and rolling.
   Dense inclusions, porosity or segregation generally appear in the area near the shrinkage tube remnants. Irregular wrinkled gaps appear in transverse low magnification. It is easy to cause cracking of forgings during forging or heat treatment.