and this increased vibration results in a stretching of the molecular bonds- which causes the material to expand. Themal expansion Thermal contraction 5.1 Linear expansion For a long rod the main thermal deformation occurs along the length of the rod where a is the linear coefficient of expansion for the material, and is the fractional change in length per degree change in temperature The term 'L' represents the initial length of the rod Over small temperature ranges, the thermal expansion is described by the coefficient of linear expansion 5.2 Thermal stress If the structure or members of the structure are constrained such that the thermal expansion can not occur, then a significant thermal stress may arise which can effect the structure substantially There are many cases where structures and materials are near or at their allowable stresses. In that case, if a thermal stress develops, the total stress may well exceed the allowable stress and cause the structure to fail This is the reason bridges are built with expansion ioints which allow the structure to expand and contract freely and thus avoid thermal stresses Additionally, this is why concrete sidewalks are built with spaces separating adjacent slabs, allowing expansions to avoid thermal stresses 6 Heat Treatment Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without hanging the product sha There are five basic heat treating processes: hardening (quenching), tempering, annealing, normalizing, and case hardening Although each of these processes bring about different results in metal, all of them involve three basic steps: heating soaking, and cooling Basic Steps Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific Once a metal part has been heated to the desired temperature, it must remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking The third step is cooling. Metals can be made to conform to specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth Heat Treatment of Ferrous metals Hardening( Quenching)淬火 A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine The hardening process increases the hardness and strength of metal, but also increases its britti Tempering回火 Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relie the internal stresses and reduce its brittleness Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool in still Temperatures used for tempering are normally much lower than the hardening temperatures Annealing i8 Metal is annealed by heating it to a prescribed temperature holding it at that temperature for the required time, and then cooling it slowly back to room temperature Annealing is used to relieve internal stresses, soften them, make them more ductile, and refine their grain structuresand this increased vibration results in a stretching of the molecular bonds - which causes the material to expand. Thermal expansion Thermal contraction 5.1 Linear expansion For a long rod the main thermal deformation occurs along the length of the rod, where α is the linear coefficient of expansion for the material, and is the fractional change in length per degree change in temperature. . The term 'L' represents the initial length of the rod.. Over small temperature ranges, the thermal expansion is described by the coefficient of linear expansion. 5.2 Thermal Stress If the structure or members of the structure are constrained such that the thermal expansion can not occur, then a significant thermal stress may arise which can effect the structure substantially. . There are many cases where structures and materials are near or at their allowable stresses. In that case, if a thermal stress develops, the total stress may well exceed the allowable stress and cause the structure to fail. This is the reason bridges are built with expansion joints which allow the structure to expand and contract freely and thus avoid thermal stresses. . Additionally, this is why concrete sidewalks are built with spaces separating adjacent slabs, allowing expansions to avoid thermal stresses. 6. Heat Treatment Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. There are five basic heat treating processes: hardening (quenching), tempering, annealing, normalizing, and case hardening. . Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. Basic Steps Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific temperatures. . Once a metal part has been heated to the desired temperature, it must remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking. . The third step is cooling. Metals can be made to conform to specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth. . Heat Treatment of Ferrous Metals Hardening ( Quenching ) 淬火 A ferrous metal is normally hardened by heating the metal to the required temperature and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine. . The hardening process increases the hardness and strength of metal, but also increases its brittleness. Tempering 回火 Severe internal stresses are set up during the rapid cooling of the metal. Steel is tempered after being hardened to relieve the internal stresses and reduce its brittleness. . Tempering consists of heating the metal to a specified temperature and then permitting the metal to cool in still air. . Temperatures used for tempering are normally much lower than the hardening temperatures. Annealing 退火 Metal is annealed by heating it to a prescribed temperature, holding it at that temperature for the required time, and then cooling it slowly back to room temperature. Annealing is used to relieve internal stresses, soften them, make them more ductile, and refine their grain structures