{"id":29268,"date":"2021-02-22T18:23:51","date_gmt":"2021-02-22T18:23:51","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=29268"},"modified":"2023-08-18T06:20:03","modified_gmt":"2023-08-18T06:20:03","slug":"austenitic-stainless-steel","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/stainless-steel\/austenitic-stainless-steel\/","title":{"rendered":"Austenitic Stainless Steel"},"content":{"rendered":"
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In metallurgy, stainless steel<\/strong>\u00a0is a steel alloy with at least 10.5% chromium with or without other alloying elements and a maximum of 1.2% carbon by mass. Stainless steels, also known as inox steels or inox from French inoxydable (inoxidizable), are steel alloys<\/a> that are very well known for their corrosion resistance<\/strong>, which increases with increasing chromium content.<\/p>\n

\"\"<\/a>Corrosion resistance may also be enhanced by nickel and molybdenum additions. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation<\/strong>. For passivation to occur and remain stable, the Fe-Cr alloy<\/strong> must have a minimum chromium content of about 10.5% by weight<\/strong>, above which passivity can occur and below is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties.<\/p>\n

Austenitic Stainless Steel<\/h2>\n

Austenitic stainless steels<\/strong> contain between 16 and 25% of chromium and can also contain nitrogen in solution, contributing to their relatively high corrosion resistance<\/strong>. Austenitic stainless steels<\/strong> are classified with AISI 200- or 300-series designations; the 300-series grades are chromium-nickel alloys, and the 200-series represent a set of compositions in which manganese and\/or nitrogen replace some of the nickel. Austenitic stainless steels have the best corrosion resistance of all stainless steels, and they have excellent cryogenic properties and good high-temperature strength.<\/p>\n

\"304<\/a>They possess a face-centered cubic<\/a> (fcc) microstructure that is non-magnetic and can be easily welded. This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements<\/strong> nickel, manganese, and nitrogen. Austenitic stainless steel is the largest family of stainless steel, making up two-thirds of all stainless steel production. Their yield strength is low (200 to 300MPa), which limits their use for structural and other load-bearing components. They cannot be hardened by heat treatment but have the useful property of being work hardened to high strength levels while retaining a useful level of ductility and toughness. Duplex stainless steels tend to be preferred in such situations because of their high strength and corrosion resistance. The best-known grade is AISI 304 stainless, which contains chromium (between 15% and 20%) and nickel (between 2% and 10.5%) metals as the main non-iron constituents. 304 stainless steel has excellent resistance to a wide range of atmospheric environments and many corrosive media. Cold forming usually characterizes these alloys as ductile, weldable, and hardenable.<\/p>\n

Stainless Steel – Type 304<\/h2>\n

Type 304 stainless steel<\/strong> (containing 18%-20% chromium and 8%-10.5% nickel) is the most common stainless steel. It is also known as \u201c18\/8<\/strong>\u201d stainless steel because of its composition, which includes 18% chromium and 8% nickel. This alloy resists most types of corrosion. It is austenitic stainless steel and has excellent cryogenic properties, high-temperature strength, and forming and welding properties. It is less electrically and thermally conductive than carbon steel<\/a> and is essentially non-magnetic.\"stainless<\/a><\/p>\n

Type 304L stainless steel<\/strong>, widely used in the nuclear industry, is an extra-low carbon version of the 304 steel alloy. This grade has slightly lower mechanical properties than the standard 304 grade but is still widely used thanks to its versatility. The lower carbon content in 304L minimizes deleterious or harmful carbide precipitation due to welding. 304L can, therefore, be used \u201cas welded\u201d in severe corrosion environments, and it eliminates the need for annealing. Grade 304 also has good oxidation resistance in intermittent service to 870 \u00b0C and continuous service to 925 \u00b0C.<\/p>\n

The body of the reactor vessel<\/a> is constructed of high-quality low-alloy carbon steel<\/strong>. All\u00a0surfaces that come into contact with reactor coolant are clad<\/strong> with a minimum of about 3 to 10 mm of austenitic stainless steel<\/strong> to minimize corrosion. Since grade 304L does not require post-weld annealing, it is extensively used in heavy gauge components.<\/p>\n

\"stainless<\/a><\/p>\n

Properties of Austenitic Stainless Steels<\/h2>\n

Material properties<\/strong> are intensive properties<\/strong>, which means they are independent of the amount<\/strong> of mass and may vary from place to place within the system at any moment. Materials science involves studying materials\u2019 structure and relating them to their properties (mechanical, electrical, etc.). Once materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and how it has been processed into its final form.<\/p>\n

Mechanical Properties of Austenitic Stainless Steels<\/h3>\n

Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial, and engineers must consider them.<\/p>\n

Strength of Austenitic Stainless Steels<\/h3>\n

In the mechanics of materials, the strength of a material<\/strong><\/a> is its ability to withstand an applied load without failure or plastic deformation. The strength\u00a0of materials<\/strong> considers the relationship between the external loads<\/strong> applied to a material and the resulting deformation<\/strong> or change in material dimensions. The strength\u00a0of a material<\/strong> is its ability to withstand this applied load without failure or plastic deformation.<\/p>\n

Ultimate Tensile Strength<\/h3>\n

The ultimate tensile strength of stainless steel \u2013 type 304 <\/strong>is 515 MPa.<\/p>\n

The ultimate tensile strength of stainless steel \u2013 type 304L <\/strong>is 485 MPa.<\/p>\n

\"Yield<\/a>The ultimate tensile strength<\/strong><\/a> is the maximum on the engineering stress-strain curve<\/a>. This corresponds to the maximum stress<\/strong> sustained by a structure in tension. Ultimate tensile strength is often shortened to \u201ctensile strength\u201d or \u201cthe ultimate.\u201d If this stress is applied and maintained, a fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after achieving the ultimate strength. It is an intensive property; therefore, its value does not depend on the size of the test specimen. However, it depends on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature<\/strong> of the test environment and material. Ultimate tensile strengths<\/strong> vary from 50 MPa for aluminum to as high as 3000 MPa for very high-strength steel.<\/p>\n

Yield Strength<\/h3>\n

The yield strength of stainless steel \u2013 type 304 <\/strong>is \u00a0205 MPa.<\/p>\n

The yield strength of stainless steel \u2013 type 304L <\/strong>is \u00a0170 MPa.<\/p>\n

The yield point<\/strong><\/a> is the point on a stress-strain curve<\/a> that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength<\/strong> or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is the point where nonlinear (elastic + plastic) deformation begins. Before the yield point, the material will deform elastically and return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behavior termed a yield point phenomenon. Yield strengths vary from 35 MPa for low-strength aluminum to greater than 1400 MPa for high-strength steel.<\/p>\n

Young\u2019s Modulus of Elasticity<\/h3>\n

Young\u2019s modulus of elasticity stainless steel \u2013 type 304 and 304L <\/strong>is 193 GPa.<\/p>\n

Young\u2019s modulus of elasticity<\/a> is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to limiting stress, a body will be able to recover its dimensions on the removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position, and all the atoms<\/a> are displaced the same amount and maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions, and no permanent deformation occurs. According to Hooke\u2019s law<\/a>, <\/strong>the stress is proportional to the strain (in the elastic region), and the slope is Young\u2019s modulus<\/strong>. Young\u2019s modulus is equal to the longitudinal stress divided by the strain.<\/p>\n

\"\"<\/a><\/p>\n

The hardness of Austenitic Stainless Steels<\/h2>\n

Brinell hardness of stainless steel \u2013 type 304 <\/strong>is approximately 201 MPa.<\/p>\n

\"Brinell<\/a>In materials science, hardness<\/strong><\/a> is the ability to withstand surface indentation<\/strong> (localized plastic deformation<\/strong>) and scratching<\/strong>. Hardness<\/strong> is probably the most poorly defined material property because it may indicate resistance to scratching, abrasion, indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.<\/p>\n

Brinell hardness test<\/strong><\/a> is one of the indentation hardness tests developed for hardness testing. In Brinell tests, a hard, spherical indenter<\/strong> is forced under a specific load into the surface of the metal to be tested. The typical test uses a 10 mm (0.39 in) diameter hardened steel ball<\/strong> as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. The load is maintained constant for a specified time (between 10 and 30 s). For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball<\/strong> is substituted for the steel ball.<\/p>\n

The test provides numerical results to quantify the hardness of a material, which is expressed by the Brinell hardness number<\/strong> \u2013 HB<\/strong>. The Brinell hardness number is designated by the most commonly used test standards (ASTM E10-14[2] and ISO 6506\u20131:2005) as HBW (H from hardness, B from Brinell, and W from the material of the indenter, tungsten (wolfram) carbide). In former standards, HB or HBS were used to refer to measurements made with steel indenters.<\/p>\n

The Brinell hardness number<\/strong> (HB) is the load divided by the surface area of the indentation. The diameter of the impression is measured with a microscope with a superimposed scale. The Brinell hardness number is computed from the equation:<\/p>\n

\"Brinell<\/a><\/p>\n

There are various test methods in common use (e.g., Brinell, Knoop<\/a>, Vickers<\/a>, and Rockwell<\/a>). Some tables correlate the hardness numbers from the different test methods where correlation is applicable. In all scales, a high hardness number represents a hard metal.<\/p>\n

Thermal Properties of Austenitic Stainless Steels<\/h2>\n

Thermal properties<\/strong>\u00a0of materials refer to the response of materials to changes in their\u00a0temperature<\/a> and the application of heat<\/a>. As a solid absorbs energy<\/a> in the form of heat, its temperature rises, and its dimensions increase. But different materials react<\/strong> to the application of heat differently<\/strong>.<\/p>\n

Heat capacity<\/a>, thermal expansion<\/a>, and thermal conductivity<\/a> are often critical in solids\u2019 practical use.<\/p>\n

Melting Point of Austenitic Stainless Steels<\/h3>\n

The melting point of stainless steel \u2013 type 304 <\/strong>steel is around 1450\u00b0C.<\/p>\n

In general,\u00a0melting<\/strong>\u00a0is a\u00a0phase change<\/strong>\u00a0of a substance from the solid to the liquid phase. The\u00a0melting point<\/strong><\/a>\u00a0of a substance is the temperature at which this phase change occurs. The\u00a0melting point\u00a0<\/strong>also defines a condition where the solid and liquid can exist in equilibrium.<\/p>\n

Thermal Conductivity of Austenitic Stainless Steels<\/h3>\n

The thermal conductivity of stainless steel \u2013 type 304 <\/strong>is 20 W\/(m. K).<\/p>\n

The heat transfer characteristics of solid material are measured by a property called the thermal conductivity<\/strong><\/a>, k (or \u03bb), measured in\u00a0W\/m.K<\/strong>. It measures a substance\u2019s ability to transfer heat through a material by conduction<\/a>. Note that\u00a0Fourier\u2019s law<\/strong><\/a> applies to all matter, regardless of its state (solid, liquid, or gas), therefore, it is also defined for liquids and gases.<\/p>\n

The\u00a0thermal conductivity<\/strong><\/a> of most liquids and solids varies with temperature, and for vapors, it also depends upon pressure. In general:<\/p>\n

\"thermal<\/a><\/p>\n

Most materials are nearly homogeneous. Therefore we can usually write k = k (T)<\/strong>. Similar definitions are associated with thermal conductivities in the y- and z-directions (ky, kz), but for an isotropic material, the thermal conductivity is independent of the direction of transfer, kx = ky = kz = k.<\/p>\n

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<\/span>References:<\/div>
Materials Science:\n

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
\nU.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
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\n<\/p><\/div><\/div><\/div>