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The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope.Metallic specimen cross sections cut from a component or sample are macroscopically examined by light illumination in order to reveal various important macro structural features such as Flow lines in wrought products, Solidification structures in cast products, Weld characteristics, including depth of penetration, fusion-zone size and number of passes, size of heat-affected zone, and type & density of weld imperfections, general size and distribution of large inclusions & stringers, fabrication imperfections such as laps, cold welds, folds and seams in wrought products, Gas and shrinkage porosity in cast products, depth & uniformity of a hardened layer in a case-hardened products.
Traditionally, the structure revealed by the microscope is called the microstructure.The very coarse sizes are starting from 1 (very coarse) encountered in large ingots or in steels held near their melting points for long times.The finest sizes can be up to 10 (very fine) are encountered in steels given special treatments that inhibit grain growth.ASTM International adapted a method for measuring grain size in 1947 that is widely used for characterizing grain size in steels.In order to establish a scale for grain size, ASTM E112 shows charts with outline grain structures at various dimensions.Grain size is determined from a polished and etched sample using optical microscopy at a magnification of 100X The depth of the layer affected by the surface hardening treatment is commonly referred to as the case depth.
Surface heat treatment involves heating a steel part very rapidly above the AC3 temperature, resulting in a thin layer of austenite on the surface.
A problem with the tensile test is that the metal is destroyed by the test.
Another test that characterizes the strength of a metal but does not destroy the metal is the hardness test.
More sophisticated and expensive techniques involve heating the surface by laser beams or by electron beams.
Test methods for evaluation of case depth by means of chemical, mechanical or visual are available and should be selected on the basis of specific requirements.
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