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  Hardness is the resistance of a material to localized deformation. The term can apply to   deformation from indentation, scratching, cutting or bending . In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface. For  elastomers  and some polymers,  hardness is defined at the resistance to elastic deformation  of the surface. The lack of a fundamental definition indicates that  hardness is not a basic property  of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors. Hardness measurements are widely used for the  quality control of materials  because they are quick and considered to be nondestructive tests when the marks or indentations produced by the test are in low stress areas. There are a large variety of  methods used for determining  the hardness of a substance, as can be seen below: The  most common methods  are indicated below: 1.1. 

  Welding of Stainless Steels: General Concepts: Stainless steels, resumed here in a general way, are defined as steel alloys, where the  chromium content  ranges from  10.5% to 30%.  There are five distinct types of stainless steel: Ferritic stainless steel: Contains from  10.5 to 30% chromium , is low in carbon, with some alloys containing major amounts of molybdenum, columbium and titanium. Austenitic stainless steel: Contains from  16% to 26% chromium  and up to 35% nickel and have very low carbon content. Some of these steels are also alloyed with a minor amount of molybdenum, columbium and titanium. Martensitic alloy steel: Contains from  12% to 17% chromium  and up to 4% nickel and 0.1% to 1.0% carbon. Some alloys will also have minor additions of molybdenum, vanadium, columbium, aluminum and copper. Duplex stainless alloys: Contain  18% to 28% chromium , 2.5% to 7.5% nickel and low carbon contents. Some of the alloys will also have additions of nitrogen, molybdenum and copper.

Here are some rarely used  NDT – Few Other Methods 1.   Eddy Current Testing – ET: Eddy-current inspection uses  electromagnetic induction  to detect flaws in  conductive  materials. A circular coil carrying current is placed in proximity to the test specimen (electrically conductive). The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current. The  term eddy current  (also called Foucault currents) comes from analogous currents seen in   water  where localized areas of turbulence  known as eddies  give rise to persistent vortices. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current. A corresponding change in the phase and amplitude of the measured current. This is the basis of standard