What are the typical metallurgical defects
Metallurgical defects usually refer to various discontinuities or compositional imbalances that occur inside or on the surface of materials due to improper processes or operations during the smelting, solidification (such as casting ingots, continuous casting), and subsequent hot processing of metals. According to extensive production practice and standards, common metallurgical defects can be mainly divided into the following categories:
1. Uneven composition and organization
This type of defect originates from the selective crystallization and uneven diffusion of elements during the metal solidification process, ultimately leading to inconsistent steel properties.
Segregation: The phenomenon of uneven distribution of chemical components (such as carbon, sulfur, phosphorus, etc.) in steel. On the transverse acid leaching test piece of the ingot, it often exhibits "square segregation" or "point segregation" with varying shades of color.
Loose: Small pores formed inside the steel due to insufficient filling during solidification and shrinkage of the molten steel. Widely distributed is called "general looseness", while concentrated in the center is called "central looseness".
Banded structure: a layered structure formed by alternating stacking of different structures (such as ferrite and pearlite) parallel to the deformation direction in steel, often caused by dendritic segregation extending during pressure processing.
2. Purity and impurities
This type of defect mainly refers to the presence of foreign substances in the metal matrix, which are the origin of many fracture failures.
Non metallic inclusions: This is one of the most common defects, including oxides, sulfides, silicates, etc. formed during smelting and solidification processes. Sulfides with good plasticity will be elongated into strips during hot processing, while brittle oxides may become crack sources during subsequent processing.
Slag inclusion: specifically refers to the improper operation during continuous casting or casting, which causes the protective slag of the crystallizer, the peeling of refractory materials, or the inclusion of slag in the molten steel and retention in the steel.
Heterometallic inclusions: Refers to the mixing of metal fragments or particles that are completely different from the matrix composition, such as those caused by incomplete melting of alloy materials during smelting or the mixing of foreign metal substances.
3. Internal and surface cracks
These types of defects are interruptions in the continuity of metal and can seriously weaken the strength and toughness of the material.
Cracks: Depending on the formation temperature, there are hot cracks (formed in the later stage of solidification) and cold cracks (formed when cooled to near room temperature), which are related to excessive stress or enrichment of harmful elements.
White spot: mainly appearing in alloy structural steel, it is an extremely fine crack inside the steel. The macroscopic fracture surface shows silver white spots, mainly due to the combined effect of high hydrogen content and structural stress in the steel.
Layering: Two layers of structure parallel to the rolling surface and not bonded to each other appear in the steel matrix, often accompanied by iron oxide scales or segregated substances inside.
Folding (also known as "heavy skin"): A surface defect formed during hot rolling or forging, where protrusions, burrs, and other defects on the metal surface are pressed into the body but fail to fuse with the substrate, resulting in a tongue like or fish like shape.
4. Holes and shrinkage cavities
Shrinkage residue: Concentrated voids formed at the head of the ingot during solidification shrinkage, which may remain inside the steel if not completely removed during subsequent rolling.
Bubble: The gas that precipitates during the solidification of molten steel fails to escape, forming pores (internal and external bubbles) inside or on the surface of the steel, which may evolve into small cracks or layers after rolling.
Overview of Defect Classification
Defect Categories Common Defect Types Main Hazards/Manifestations
Uneven composition and organization, segregation, looseness, and banded structure lead to inconsistent mechanical properties in various parts of the steel
Purity and inclusion of non-metallic inclusions, slag inclusions, and non-metallic inclusions as crack sources significantly reduce fatigue life and plasticity
Internal and surface cracks, white spots, delamination, and folding directly damage the continuity of the matrix, which is the main cause of fracture failure
Residual shrinkage pores, bubbles, and air pockets reduce the material's density and effective load-bearing cross-sectional area
It should be noted that the above listed metallurgical defects occur during the smelting, casting, and hot processing (one-time processing) stages. In practical applications, defects generated by subsequent cold processing, heat treatment (such as overheating, overburning, decarburization), or service conditions should also be distinguished, as their causes and mechanisms are different.
