With the macro-analysis reveal defects that violate the metal continuity:
- The shrinkholes, gas blowholes, caverns, cracks in cast metal (slabs, moulding);
- The cracks occurring during heat treatment or pressure treatment to a rolled or hammered metal;
- The shrinkholes, gas blowholes arising during welding the weld seams;
The etching methods depend on the metal type. The macro-analysis allows you to determine in steels the following:
- The depth of the hardening zone, having the high hardness in comparison with the soft core.
- The depth of the cemented layer.
The defects at the macrotemplate analysis:
- The segregation square is the contour of the raised echtability by means of the zone and coring segregation on borders between zones column and the equiaxed crystals. The segregation decreases at reduction of ingots mass, at decrease in content of sulfur, phosphorus, and also the temperature fall of casting in molds.
- The dot heterogeneity is the small points which are poisoned on all area of a macrosection except for a regional zone. The dendritic segregation decreases at decrease in content in steel of sulfur, phosphorus, oxygen, nitrogen.
- The freckle-type segregation is the black spots on the template (the general freckle-type segregation) or near the surface (the regional freckle-type segregation). Here the congestion of nonmetallic inclusions is found. It is formed at high gas content in the metal, resulting in occurrence of blowholes in the metal cavity which falls the segregation.
The preventive measures of this refused material:
- the gas content reduction;
- the optimal metal deoxidation;
- the dried charge use;
- the not overheated metal release.
- The pipe segregation is the strips or spots in the center of bars. In the defect zone are the cluster of sulfides and oxides. For the purpose of prevention of this type of defects it is necessary to cut off the head trimming in top scrap.
- The central porosity is the group of small pores in the center of bars. At these zones there are segregated materials and nonmetallic inclusions. Due to the presence of inclusions or the low degree of compression is not completely welded. The prevention method is the casting steel into molds with a large taper with a smaller diameter with respect to height, as well as insulation and heating lucrative part of the ingot.
- The rimholes. They are formed due to the release of gases (CO, N2, H2) in the crystallization period is a measure to combat the full metal deoxidation, well dried charge, well dried firebricks.
- The intercrystalline cracks. The winding, spider like strips extending from the side of the rod axis. They are formed due to separation at the boundaries of crystal films of oxides and sulphides. The preventive measures are:
- the sulfur and oxygen content reduction.
- the optimal deoxidation;
- The forging cracks. They are located in the axial zone and have the form of the cross, go diagonally or in a branched fractures extending from the rod center. The fracture surfaces are oxidized coarse bundles produced due to high stress during forging, when the metal has a high strength and low ductility.
- The pigeon hole. It occurs due to the cold cracks formation when planting ingots or billets to the furnace with the high temperature.
- The flakes are zigzag cracks, formed due to the presence of hydrogen in the steel.
For the research of a microstructure the special sample called microsection is prepared. The sample for the micro research has to be cut out from the place giving the characteristic of structure of all studied product. Most often make samples of the small sizes in the form of cubes 15 x 15 either 20 x 20 mm, or cylinders with a diameter of 10 – 15 mm.
Such samples as a wire, thin sheet, roll and others should be placed in a metal tube and fill with low-melting alloys. The cutting made a saw, cutter, abrasive wheels, but do not put the heating of the metal.
