The dendrite structure and the crystallization zones of cast metal can be detected by the deep etching of macrosection in 15% water solution of ammonium persulphate (NH4)2S2O8 and to give the opinion about the cooling speed of the ingot or casting.
To defect the casting location the casting control is carried out:
the shrinkholes are cavities which borders in steel are concentrated with C, P, S, nonmetallic inclusions; shrinkholes are formed due to volume reduction of metal at crystallization and are situated in the central and mainly top part of the killed ingot;
the total porosity – pores which are uniformly distributed on the cross section of macrosection; due to the presence of gas blowholes and nonmetallic inclusions;
the intercrystalline cracking of the zigzag form (“spider”) is in the ingot central zone; meet at the accelerated casting and crystallization of poorly dead-melted steel with the increased hydrogen concentration;
the flakes are winding disk-shaped cracks on the shares to 100 mm in length; are formed at the rapid steel cooling containing hydrogen increased ductility temperature range from 200 0 to 20 0С; at the same time hydrogen released from the solid solution with iron and, passing from the atomic form into molecular, creates the large internal stresses leading to the cracks formation; the most flake susceptibility are rolled and hammered thick chrome-nickel steels there were also some carbonaceous became like rail-mounted.
The gas blowholes:
the subcortial (surface blowholes) are roundish cavities in quiet, strongly saturated with gases ( H2, O2, N3 ) steel, that failed to be allocated from the liquid metal during the crystallization;
cellular (cells) – the cavities which are elongated to center of the ingot filled with carbonic oxide СO, selected at the self-deoxidation of rimming and semi-killed steel in the process of casting (FeO + C = Fe + CO); settle down near the ingot surface at the beginning of the columnar zones; can be revealed when rolling, forming rags, cracks; rags can be caused and metal burning (in case of high steel heating: » 100¸150 0С below the solidus line).
density segregation is the chemical inhomogeneity on ingot height; it occurs in ingot when the density of the firm crystal and liquid phases considerably differ from each other; it is most strongly shown at the slowed-down cooling of the ingots containing heavy elements: Рv, W, etc.; isn’t eliminated with annealing;
zone segregation is non-uniform distribution of elements on ingot zones, forging or rod, for example, steel defect of type a forging square; isn’t eliminated with annealing;
dendritic segregation is the compositional inhomogeneity within crystallites (grains) and between them; is eliminated with long diffusive annealing at 1150¸ 1200 0С;
gas (freckle-type) segregation is enrichment of the gas blowholes walls (honeycombs) with the phosphorus; it promotes the rags formation.
All listed defects are estimated on 5 point scale according to GOST 10243-62: the point is higher, the defect is larger.