There are many defects in forgings, and there are many reasons for these defects, such as unreasonable forging process, raw materials, and mold design.
Especially for precision forgings with little cutting, it is difficult to achieve complete control. Several common forging defects are introduced.
Shrinkage:
When pouring the steel ingot in the early stage of forging, the metal solidification volume shrinks when the steel ingot cools, forming large holes, which are common in the head of the steel ingot (riser end).
Shrinkage characteristics: Generally located in the center of the cross section, with large volume and large axial extension length.
Looseness:
When the steel ingot solidifies, the gas in the molten metal has no time to be discharged, and the metal cools and shrinks, forming internal cavities without being dense. During forging, due to insufficient forging ratio, the metal structure is soft, and there is metal structure in the forging. Loose characteristics: There are more in the center and head of the ingot, and the single size is small, but it is often distributed in a regional dispersion.
Inclusions:
There are non-metallic inclusions and metal inclusions.
(1) Non-metallic inclusions: Deoxidizers in steel, reactants produced by alloying elements and gases, generally small in size, floating in the ingot, and then squeezed in the center and head of the ingot until they are newly solidified; refractory materials or impurities mixed in during smelting and pouring are large in size and often mixed in the lower part of the ingot.
(2) Metallic inclusions: defects caused by excessive or large ferroalloys added during smelting, metal splashing during pouring, or unresolvable irregular metal entering the mold.
Cracks:
There are many types of cracks with different causes.
(1) Intergranular cracks, more common in austenitic steel and stainless steel forgings;
(2) In the center of high-alloy steel ingots, cracks are distributed along the intergranular space, are curved, are larger in size than inclusions, and have a certain directionality.
(3) Improper forging or heat treatment, excessive temperature difference between the inside and outside of the workpiece, and drastic changes in cross-sectional dimensions can all lead to thermal cracks, which often appear at the cross-sectional changes of the forging core;
(4) Large microstructures and brittle cracks caused by overheating and overburning often start from the surface of the workpiece.
(5) The wrinkles formed by the oxide scale or protruding parts on the surface of the ingot pressed into the steel during forging are also one of the cracks formed by improper deformation.
(6) The white spots of ordinary alloy steel are essentially microcracks caused by hydrogen embrittlement. The individual size is large, the distribution is wide, the forging section changes greatly, and the white spots are easy to form quickly after cooling after forging.
(7) After quenching, if the tempering is not timely or improper, the residual stress of the heat treatment is still large, which is easy to crack and may cause self-explosion in severe cases.
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