Metal macrostructure defect inspection

Metal macrostructure defect inspection is also referred to as a macroscopic examination.It is with the naked eye or is not more than 10 times the magnifying glass to check the metal surface, fracture or macroscopic organization method and its defects.Macroscopic examination on the metal ingot casting, casting, forging, welding, rolling, heat treatment process, the commonly used inspection method is a kind of important.This test method is simple, rapid, can reflect the metal macro morphology and distribution characteristics of regional organizations and defects.Enable people to correct and comprehensive judge the quality of the metal material, in order to guide the production, the rational use of materials science.For the analysis of the optical microscopy and electron metallographic basic work.Macroscopic examination including macrostructure and defect inspection (including acid corrosion, sulphur print, tower turning and non-destructive methods such as damage) and fracture analysis, etc. TitaniumCN
1. The typical macroscopic defects
The typical macroscopic defects have segregation, porosity, shrinkage cavity, bubble, crack, low power inclusion, coarse grain ring, etc.
(1) the uneven phenomenon called segregation of alloy chemical composition segregation.Based on the characteristics of the range size and position of the segregation, generally can be divided into three kinds.The intracrystalline partial and segregation, regional segregation, gravity segregation.Coring and segregation as after solid solution alloy casting in the condensation process, due to the solid phase liquid ingredients in constant change, therefore, even within the same crystal, first part of the solidification after solidification of the part and its chemical composition is not the same.The intracrystalline chemicals called coring uneven phenomenon.
(2) the shrinkage and loose in the ingot head, middle, grain boundary and interdendritic, often have some of the macroscopic and microscopic shrinkage holes, collectively known as the shrinkage cavity.Large volume and concentration of shrinkage cavity called on shrinkage cavity;Small and dispersed shrinkage cavity called osteoporosis.Which appear in the grain boundary and interdendritic porosity is also called microporosity.Shrinkage cavity and loose irregular shape, the surface is not smooth, easily distinguished from the more pliable porosity.Ingot casting box segregation four years the national registered construction engineer examination in terms of construction project cost management of construction engineering construction project cost case analysis construction engineering technology and measuring shrinkage and loose the direct cause, is solidified body contraction of the liquid metal solidification occurs.Residual shrinkage cavity in the metal material of axial position of the transverse specimens acid leaching, easy to recognize.
(3) the hole according to the position of porosity in ingot casting, it can be divided into three classes of surface porosity, subcutaneous blowhole and internal porosity.Can be divided into the precipitation type according to the cause of porosity, porosity and reactive pores.
(4) the most complicated composition of crack or higher amount of impurities or have a small amount of non-equilibrium eutectic alloy, there are larger crack tendency.Especially large ingot casting.Under the condition of the cooling strength of continuous casting, the tendency of crack is bigger.Produced in the process of solidification crack is called hot crack.After solidification cracks occurring in the process of cooling is called cold crack.More hot cracking along the grain boundary extension, twists and turns and irregular, often for transgranular crack, cold crack in a straight line extension.
(5) low power inclusions
On the acid leaching at low sample, some naked eye can see fire-proof material, slag and other non-metallic inclusions, these relatively bulky inclusions are called macroscopic inclusions.Them on acid leaching test plate in the form of Mosaic, and maintain its inherent in all sorts of color, common are gray, beige and dark gray, etc.Some low power inclusions may fall off during production, and surface is empty.Empty characteristics different from the edge of the bubble is that it is not neat, spongy.
(6) streamline is the low melting point metal composition and banded structure segregation when rolling or extrusion stretch.At the same time, also in the process of rolling ingot casting grain in elongated into strips.After recrystallization heating process can restore the elongated grain into equiaxed grain, but due to the low melting point elements and the elongation of banded structure is formed by the distribution of stripe still exist.In steel on the longitudinal section after polishing and acid leaching, with the naked eye can see the line of the stripes.The macroscopic organization called fibrous tissue, also known as the streamline.
(7) coarse grain ring is one of the main defects of aluminium alloy extrusion products.Aluminum alloy extrusion in the subsequent heating of the outer coarse grains, according to the phenomenon of coarse grain rings.Its characterization features are: for single-hole extrusion bar, bulky zone, a symmetric circular.The porous squeeze rod eccentric crescent shaped.The presence of coarse grain ring could sharply reduce the mechanical properties.Coarse grain ring also has a "genetic" organization.With coarse grain ring blank after moulding, still keep original coarse grain area, causing uneven and affect the surface quality of molded parts.Even in the heat treatment process is easy to cause crack.Study shows that the formation of coarse grain ring is driven by extrusion deformation caused by uneven friends.
2. The organization and the defect is etching method
Macrostructure etching method is divided into hot acid etching method, cold acid etching method and three kinds of electrochemical corrosion method.Experiments using cold acid etching.Cold acid etching operation main points are as follows:
(1) according to the appropriate material selection type and a certain proportion of etching solution.
(2) on the surface of the sample inspection should finish in 7 above, acid corrosion tests should be surface wipe, remove the oil pollution.
(3) with the method of corrosion, pour acid slowly in the basin of land acid sample again, then acid brush divide evenly with a brush,
(4) etching, water rushs acid, and alkaline neutralization, then use hot water to rinse, blow dry immediately.
3. The fracture analysis of metal fracture according to the fracture methods can be divided into brittle fracture
And thus reduce the grain boundary strength.Transgranular fracture is due to the strength of the grain itself is not high, or along the grain in the process of crystallization of some crystal clip into the impurities.See the shape of the fracture physical objects.Ductile fracture rupture occurred plastic deformation before.Distorted the original grain (spin or broken), no longer keep the original size, shape, and can't study from these fracture metal internal organization.See the shape of the fracture physical objects.