Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. There is a number of phenomena that 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. In fact, the use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.
The term thermal annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. This process alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. In this process, atoms migrate in the crystal lattice and the number of dislocations 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.
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.
Any annealing cycle consists of three stages:
- heating to the desired temperature,
- holding or “soaking” at that temperature,
- cooling, usually to room temperature.
Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle.
Annealing Thermal Cycles
In practice, specific thermal cycles 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 to which the steel is heated and the method of cooling used.
- Process Annealing. Process annealing is a specific heat treatment that restores some of the ductility to a product being cold-worked so it can be cold-worked further without breaking. It is commonly used during fabrication procedures that require extensive plastic deformation, to allow a continuation of deformation without fracture or excessive energy consumption. The temperature range for process annealing ranges from 260 °C to 760 °C, depending on the alloy in question. This process is mainly suited for low-carbon steel. This is mainly carried out on cold-rolled steel like wire-drawn steel, centrifugally cast ductile iron pipe etc.
- Stress-relief Annealing. Stress-relief annealing is used to relieve stresses from cold working. In contrast to process annealing, this heat treatment is usually performed after the product has been made. Care must be taken to ensure uniform cooling, particularly when a component is composed of variable section sizes. If the rate of cooling is not constant and uniform, new residual stresses can result that are equal to or greater than those that the heat-treating process was intended to relieve. The annealing temperature is typically a relatively low one such that effects resulting from cold working and other heat treatments are not affected. Stress-relief heat treating can reduce distortion and high stresses from welding that can affect service performance.
- Recrystallization Annealing. Recrystallization annealing of cold-worked steels (carbon content up to 0.5%) can produce a new grain structure without inducing a phase change. Metal is heated to a temperature at which the hardening caused by the previous cold-working is removed. During recrystallization, the internal bonds between the atoms change, the crystal lattice does not change. Annealing temperatures are between 550-700 ° C and the endurance is about 1 hour or more, cooling in air. It is used as an inter-operational annealing in cold forming, especially for low-carbon parts.
- Full Annealing. Full annealing produces a microstructure that is softer and more amenable to other processing such as forming or machining. The temperatures for full annealing are typically 50 °C above the upper critical temperature (A3) for hypoeutectic steels and the lower critical temperature (A1) for hypereutectoid steels. It is referred to as full annealing because it achieves full austenitization of hypoeutectoid steels. The alloy is then furnace cooled. That means, the heat-treating furnace is turned off, and both furnace and steel cool to room temperature at the same rate, which takes several hours. The cooling rate of the steel has to be sufficiently slow so as to not let the austenite transform into bainite or martensite, but rather have it completely transform to pearlite and ferrite or cementite. A full anneal typically results in the second most ductile state a metal can assume for metal alloy. The metal attain relatively low levels of hardness, yield strength and ultimate strength with high plasticity and toughness. Full annealing is often used in low- and medium-carbon steels that will be machined or will experience extensive plastic deformation during a forming operation. Stainless and high-alloy steels may be austenitized (fully annealed) and quenched to minimize the presence of grain boundary carbides or to improve the ferrite distribution.
- Normalizing. Normalization is an annealing process applied to ferrous alloys to refine grain size, make its structure more uniform, make it more responsive to hardening, and to improve machinability. Normalizing is performed on steels that have been plastically deformed by, for example, a rolling operation. This cold worked steels consist of grains of pearlite, which are irregularly shaped and relatively large and vary substantially in size. Normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the temperatures for normalizing are approximately 55 °C above the upper critical line. Normalization temperature is higher than temperature for full annealing, on the other hand the cooling more intense. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes. The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment.
Reactor Pressure Vessel Annealing
The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel in order to minimize corrosion.
During the operation of a nuclear power plant, the material of the reactor pressure vessel and the material of other reactor internals are exposed to neutron radiation (especially to fast neutrons >0.5MeV), which results in localized embrittlement of the steel and welds in the area of the reactor core. This phenomenon, known as irradiation embrittlment, results in:
- Steadily increase in DBTT. It is not likely that the DBTT will approach the normal operating temperature of the steel. However, there is a possibility that when the reactor is being shut down or during an abnormal cooldown, the temperature may fall below the DBTT value while the internal pressure is still high.
- Drop in the upper shelf fracture energy. Radiation effects are also manifested by a drop in the upper shelf fracture energy and decrease in fracture toughness.
All these effects must be monitored by plant operators. Therefore nuclear regulators require that a reactor vessel material surveillance program be conducted in watercooled power reactors.
Once a material of RPV is degraded by radiation embrittlement (e.g. significant increase in Charpy ductile‐brittle transition temperature or reduction of fracture toughness), thermal annealing of the RPV is the only way to recover the RPV material toughness properties.
According to 10 CFR 50.66 – Requirements for thermal annealing of the reactor pressure vessel:
„For those light water nuclear power reactors where neutron radiation has reduced the fracture toughness of the reactor vessel materials, a thermal annealing may be applied to the reactor vessel to recover the fracture toughness of the material.“
Thermal annealing („dry“ method) of 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 – 460 °C) by use of an external heat source (electrical heaters, hot air), held for a given period (e.g. 100 – 200 hours) and then slowly cooled. The annealing equipment is usually a ring‐shaped furnace with heating elements on its external surface. The power output of installed heaters may reach up to 1 MWe. It was shown that for the specially fabricated materials the upper shelf recovered 100 % after 24 hours annealing and more rapidly than transition temperature. Annealing for 168 hours recovered 90 % of transition temperature shift.
There is also a possibility of the so‐called “wet” annealing method which was applied in USA and Belgium. The annealing at that temperature ~340 °C 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.
Special Reference: Annealing and re-embrittlement of reactor pressure vessel materials. AMES report N.19; ISSN 1018-5593. European Communities, 2008.
- Annealing. The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. In this process, 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; it is also easier to sand, grind, or cut annealed metal.
- Quenching. The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.
- Tempering. The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
- Aging. Age hardening, also called precipitation hardening or particle hardening, is a heat treatment technique based on the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix to enhance The strength and hardness of some metal alloys. Precipitation hardening is used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.