{"id":30308,"date":"2021-06-29T18:32:24","date_gmt":"2021-06-29T18:32:24","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30308"},"modified":"2023-09-15T07:10:14","modified_gmt":"2023-09-15T07:10:14","slug":"thermal-annealing","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/heat-treatment-of-metals\/thermal-annealing\/","title":{"rendered":"Thermal Annealing"},"content":{"rendered":"
<\/a>Metals<\/strong> can be heat treated to alter the properties of strength<\/a>, ductility<\/a>, toughness<\/a>, hardness<\/a>, or resistance to corrosion. Several phenomena occur in metals and alloys at elevated temperatures. For example, recrystallization and the decomposition of austenite. These are effective in altering the mechanical characteristics when appropriate heat treatments or thermal processes are used. The use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.<\/p>\n The term thermal annealing<\/strong> refers to a heat treatment in which a material is exposed to an elevated temperature for an extended period and then slowly cooled. This process alters a material\u2019s physical and sometimes chemical properties to increase its ductility<\/a> and reduce its hardness<\/a>, making it more workable. In this process, atoms migrate in the crystal lattice<\/a>, and the number of dislocations<\/a> decreases, leading to a change in ductility and hardness. Metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends rather than breaking. Typically, annealing is carried out to relieve stresses, increase softness, ductility, and toughness, and\/or produce a specific microstructure.<\/p>\n Generally, in plain carbon steels, annealing produces a ferrite-pearlite micro-structure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages.<\/p>\n Any annealing cycle consists of three stages:<\/p>\n Time and annealing temperature are important parameters in these procedures. Especially the target temperature<\/strong> defines the annealing thermal cycle.<\/p>\n In practice, specific thermal cycles<\/strong> of an almost infinite variety are used to achieve the various goals of annealing. These cycles fall into several broad categories that can be classified according to the temperature at which the steel is heated and the cooling method used.<\/p>\n The body of the reactor vessel<\/a> is constructed of high-quality low-alloy carbon steel<\/strong>. To minimize corrosion, all surfaces that come into contact with reactor coolant are clad<\/strong> with a minimum of about 3 to 10 mm of austenitic stainless steel<\/strong>.<\/p>\n During the operation of a nuclear power plant<\/a>, the material of the reactor pressure vessel<\/a> and the material of other reactor internals are exposed to neutron radiation<\/a> (especially to fast neutrons >0.5MeV), which results in localized embrittlement<\/strong> of the steel and welds in the area of the reactor core. This phenomenon, known as irradiation embrittlement<\/strong>, results in:<\/p>\n Plant operators must monitor all these effects. Therefore nuclear regulators require that a reactor vessel material surveillance program be conducted in water-cooled power reactors.<\/p>\n Once a material of RPV is degraded by radiation embrittlement<\/a> (e.g., a significant increase in Charpy ductile\u2010brittle transition temperature or reduction of fracture toughness), thermal annealing<\/strong> of the RPV is the only way to recover the RPV material toughness properties.<\/p>\n According to 10 CFR 50.66 \u2013 Requirements for thermal annealing of the reactor pressure vessel:<\/p>\n \u201cFor those light water nuclear power reactors where neutron radiation has reduced the fracture toughness of the reactor vessel materials, thermal annealing may be applied to the reactor vessel to recover the material\u2019s fracture toughness. \u201c<\/em><\/p>\n \u00a0<\/em>Thermal annealing<\/strong> (\u201cdry \u201cmethod<\/strong>) of the reactor pressure vessel is a method by which the pressure vessel (with all reactor internals removed) is heated up to some temperature (usually between 420 \u2013 460 \u00b0C<\/strong>) by use of an external heat source (electrical heaters, hot air), held for a given period (e.g., 100 \u2013 200 hours<\/strong>) and then slowly cooled. The annealing equipment is usually a ring\u2010shaped furnace with heating elements on its external surface. The power output of installed heaters may reach up to 1 MWe. It was shown that the upper shelf recovered 100 % after the specially fabricated materials after 24 hours of annealing and more rapidly than transition temperature. Annealing for 168\u00a0 hours recovered 90 % of the transition temperature shift.<\/p>\n Wet Annealing<\/strong><\/p>\n There is also a possibility of the so\u2010called \u201cwet\u201d annealing<\/strong> method being applied in the USA and Belgium. The annealing at that temperature of ~340 \u00b0C was reached without external heating but by increasing the coolant temperature achieved by the energy of the circulating pumps of the primary circuit. This type of annealing provides only partial recovery for the material due to the limitation in maximum temperature.<\/p>\n Special Reference: Annealing and re-embrittlement of reactor pressure vessel materials. AMES report N.19; ISSN 1018-5593. European Communities, 2008.<\/p>\nThermal Annealing<\/h2>\n
\n
Annealing Thermal Cycles<\/h3>\n
\n
Reactor Pressure Vessel Annealing<\/h3>\n
\n
Other Processes<\/h2>\n
\n