Defects, Metal

Defects, Metal

 

imperfections in the structure of metals and alloys. Metal defects impair the physicomechanical properties of such materials (for example, electrical conductivity, magnetic permeability, strength, density, and plasticity). A distinction is made between metal defects of the fine structure (on an atomic scale) and coarser defects—submicroscopic cracks that form along the boundaries of the blocks of a crystal and on its surface. Still coarser metal defects are the microscopic and macroscopic defects that are disturbances of continuity or homogeneity and that form in a metal as a result of the imperfection of processes or the poor technological nature of multicomponent alloys, which require particularly precise observance of conditions at each stage of their manufacture and processing.

The defects found in metal products and intermediate products are distinguished on the basis of size and location, as well as nature and origin. They may form during smelting of the metal and production of castings (nonmetallic and slag inclusions, contraction cavities, pores, gas porosity, and blisters); during pressure treatment (ply separation, forging defects, laps, fine cracks, blisters, and flakes); as a result of thermal, chemicothermal, electrochemical, and mechanical treatment (cracks, burns, and decarburization); and in the process of joining metals—during welding, soldering, riveting, and so on (poor penetration, poor soldering, cracks, and corrosion). In addition, defects in intermediate and finished products may occur during storage, shipment, and operation (corrosion damage).

Metal defects may be local (various faults of continuity, such as pores, cavities, cracks, ply separations, flakes, forging defects, and laps), distributed in limited zones (liquation conglomerations, incomplete hardening zones, zones of corrosive damage, and local cold hardening), or distributed throughout the entire volume or surface of a product (nonconformity of chemical composition, structure, or quality of machine processing).

Local defects, which are confined to a limited volume, may be point, line, plane, or three-dimensional. Depending on their location they are divided into external (surface and subsurface) and internal (deep-seated).

In an applied, technical sense, deviations from the normal quality that impair the operating characteristics of a metal or product and lead to a reduction in grade or to rejection of products should be considered defects. However, not every metal defect is a defect in the product; deviations from the normal quality of the metal that are not essential to the functioning of a particular product should not be considered defects for that product. Deviations from normal quality that are defects for products that operate under some conditions (such as fatigue loading) may be of no consequence under other conditions (such as static loading). The quality of a metal and of a product efficiently manufactured from it can be improved with complete elimination of the most dangerous defects (such as cracks, contraction cavities, ply separations, and flakes) and with a reduction to a certain minimum of the other defects that represent a lesser danger under the specific operating conditions of the particular product. High quality of the metal and of the products manufactured from it can be ensured by improving the production technology in order to preclude the possibility of the appearance of defects and by improving the methods of monitoring the quality of the metal in order to detect flaws and reject defective stock and intermediate and finished products. The quality of the metal is monitored by methods of chemical, spectral, X-ray structural, and metallographic analysis, which make it possible to detect deviations from the desired composition and structure. These methods generally require the taking of special samples of the metal and lead to damage to or destruction of the monitored products; therefore, they are used only for selective quality control. More reliable, thorough monitoring of metal defects that are disruptions of continuity or homogeneity is accomplished through the physical methods of nondestructive testing (flaw detection), which are based on the study of changes in the physical characteristics of the metal. For a final resolution of the question of whether the quality of the stock or product conforms to the standard, it is necessary to take into consideration not only the number, size, location, and character of the detected flaws but also the specific conditions of loading of the product and of its individual zones in use.

D. S. SHRAIBER

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Too much causes resisted compaction, increased stickiness, poor shakeout, gas-related defects, metal penetration, change in pH.