What is Normalizing ?
Normalization is a type of annealing process used to relieve stress in hardenable steels after cold work and to improve ductility and toughness properties. The steel is heated slightly above its upper critical temperature and held for sufficient time to allow new, smaller grains to form and high energy grain shapes to coalesce, also known as grain refinement.
It is also used for refining the steel structure. Here the steel is heated above the critical point and then it is cooled in air. It removes the internal stresses and increases the toughness. In this way the steel becomes suitable for commercial purposes. Normalization can also eliminate denritic segregation that may remain from the casting process. The steel is air cooled from the normalization temperature, yielding a microstructure that lends the desired toughness and ductility properties with a nominal tensile strength.
Normalizing of Steel is a heat-treating process that is often considered from both thermal and microstructural standpoints. In the thermal sense, normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the work is heated to a temperature about 55 °C (100 °F) above the upper critical line of the iron-iron carbide phase diagram. That is above Ac3 for hypoeutectoid steels and above Acm for hypereutectoid steels. To be properly classed as a normalizing treatment, the heating portion of the process must produce a homogeneous austenitic phase (face-centered cubic, or fcc, crystal structure) prior to cooling.
Normalizing is also frequently thought of in terms of microstructure.
The areas of the microstructure that contain about 0.8% C are pearlitic (lamellae of ferrite and iron carbide). The areas that are low in carbon are ferritic (body-centered cubic, or bcc, crystal structure). In hypereutectoid steels, proeutectoid iron carbide first forms along austenite grain boundaries. This transformation continues until the carbon level in the austenite reaches approximately 0.8%, at which time a eutectoid reaction begins as indicated by the formation of pearlite. Air-hardening steels are excluded from the class of normalized steels because they do not exhibit the normal pearlitic microstructure that characterizes normalized steels.
Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this ensures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility
Carbon steel is steel in which the main interstitial alloying constituent is carbon in the range of 0.12–2.0%. The American Iron and Steel Institute (AISI) defines carbon steel as the following: “Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.
The term “carbon steel” may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels. As the carbon percentage content rises, steel has the ability to become harder and stronger through heat treating, however it becomes less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.