Why do cast steel parts produced using physical models sometimes contain burrs?

The raw materials used in forging are castings, rolled plates, extruded materials, and forged blanks. Rolled plates, extruded materials, and forged blanks are semi-finished products processed from castings through cold rolling, extrusion molding, and forging. Generally, internal or surface defects in castings are sometimes unavoidable. For example, internal structural composition and shrinkage porosity. Various defects in the raw materials will not only affect the forming of the steel castings but also affect their quality.

 

According to incomplete statistics, in the aerospace industry system, about half of the reasons for the damage to aerospace steel castings are due to inherent defects in the raw materials. Therefore, quality control of raw materials cannot be ignored.

 

Defects in steel castings caused by defects in raw materials generally include:

 

1. Surface cracks

 High-precision forging manufacturers have found that surface cracks are common in cold-rolled bars and forged bars, generally appearing as parallel lines, consistent with the main deformation direction of cold rolling or forging. There are many reasons for this type of defect, such as subcutaneous bubbles in the steel casting stretching along the deformation direction during cold rolling, while simultaneously exposing to the surface and developing into the deepest part of the internal structure. Also, if the surface of the billet is scratched during cold rolling, stress will be generated during cooling, which may lead to cracking along the scratches. If these cracks are not removed before forging, they may expand during forging, causing cracks in the steel casting.

 

2. Folds

 

Folds are caused when the metal billet is rolled, due to incorrect die grooves in the hot-rolled strip, or because burrs from the worn surface of the die groove are rolled in during cold rolling, forming a fold at a certain angle to the workpiece surface. In structural steel, the folds contain compound inclusions, and there is carburization around them. If the folds are not removed before forging, they may cause folds or cracks in the steel casting.

 

3. Scabs

 

Scabs are a layer of detachable film on a localized area of the rolled plate surface. The formation of scabs is mainly due to molten steel splashing and solidifying on the surface of the casting during casting, and then being rolled into a film during cold rolling, adhering to the surface of the rolled plate, which is the scab. After pickling and passivation, the plastic film on the forged steel casting may peel off, resulting in surface defects on the casting.

 

4. Lamellar Fracture Surface

 

The characteristic of a lamellar fracture surface is that the fracture surface or cross-section is very similar to broken bluestone slabs or tree roots. Lamellar fracture surfaces are common in carbon steel (chromium-molybdenum alloy steel, chromium-nickel-tungsten steel materials, etc.), and have also been observed in other carbon steels. This type of defect is mainly caused by non-metallic inclusions, twin crystal shrinkage, and porosity in the steel, which are elongated along the rolling direction during forging and rolling, making the steel appear layered. If there are too many impurities, there is a risk of layered cracking during forging. The more severe the lamellar fracture surface, the worse the plasticity and ductility of the steel, especially the transverse mechanical properties are lower. Therefore, steel with certain layered defects is unacceptable.

 

5. Bright Lines (Bright Zones)

 

Bright lines are fine lines with reflective properties that appear as dark crystals on the longitudinal fracture surface, mostly running through the fracture surface, and mostly occurring in the central part. Bright lines may be caused by alloy segregation. Slight bright lines do not affect mechanical properties, but severe bright lines will significantly reduce the plasticity and ductility of the material.