Nov. 19, 2020
There are many kinds of defects in forgings, and there are also many kinds of reasons for such defects, such as bad forging process, raw materials, unreasonable mold design and so on.Especially for precision forging with little or no cutting, it is difficult to achieve complete control.Several common forging defects are introduced in this paper.
1. Shrinkage cavity: when casting steel ingot before forging, the metal solidification volume shrinks as the steel ingot cools, forming a large hole, which is commonly seen at the head (riser end) of the ingot.
Shrinkage cavity characteristics: generally located in the center of the cross section, and has a large volume and axial extension length.
2.Loose: the gas in the solidified metal liquid of the steel ingot has no time to be removed and the metal is cooled and shrunk, forming its internal holes and non-compactness. In forging, due to insufficient forging ratio, the metal structure is not soft and exists in the forging.
Loose characteristic: ingot center and head appear mostly, single size is small, but usually show regional dispersion distribution.
3. Inclusions: there are non-metallic inclusions and metallic inclusions
(1) Nonmetallic inclusion: it is the reactant produced by deoxidizer in steel, alloying element lamp and gas, generally small in size, floating in the ingot and finally squeezed to the center area and head of the ingot which solidifies at the latest;By smelting, pouring into the process of refractory or impurities, large size, often mixed with the lower ingot.
(2) Metal inclusions: defects caused by excessive or large size of iron alloy applied during smelting, splashing metal during pouring, or undissolved metal input into the mold.
4. Cracks: There are many kinds of cracks with different causes
(1) Intergranular crack, mostly in austenitic steel stainless steel forgings;
(2) In the ingot center of high alloy steel, the crack is distributed along the intercrystal, showing a curved line, the size is larger than the inclusion, and there is a certain directivity
(3) Improper forging or heat treatment, large temperature difference between the inside and outside of the workpiece, and drastic change of section size will lead to thermal cracks, which often occur at the change of section at the center of the forging;
(4) The group of large tissues and brittle cracking caused by overheat and overburn mostly start from the surface of the workpiece.
(5) The folding formed by pressing the oxide scale or protruding part of the ingot surface into the steel during forging is also one of the cracks formed by improper deformation;
(6) The common white spot of alloy steel is essentially a microcrack caused by hydrogen embrittlement. Its single size is large, its distribution is wide, its forging section changes greatly, and it is easy to form white spot after forging cooling.
(7) After quenching, if the tempering is not timely or improper, the residual stress of heat treatment is still large, which is prone to crack and seriously leads to spontaneous cracking.
