Normalizing Process for Steels


Normalizing Process for Steels

Normalizing process for steels is defined as heating the steel to austenite phase and cooling it in the air. It is carried out by heating the steel approximately 50 deg C above the upper critical temperature (AC? for hypoeutectoid steels or Acm in case of hypereutectoid steels, Fig 1) followed by cooling in air to room temperature, or at no greater than 1 bar pressure using nitrogen if the process is being  run in a vacuum furnace. Normalizing temperatures usually vary from 810 deg C to 930 deg C. After reaching the soaking temperature the steel is held at that temperature for soaking. The soaking time depends on the thickness of the work piece and the steel composition. Higher temperatures and longer soaking times are required for alloy steels and larger cross sections.

Temperatureds for normalizing

Fig 1 Typical normalizing temperature range for steels

In normalizing, steel is uniformly heated to a temperature which causes complete transformation to austenite. Steel is held at this temperature for sufficient time for the formation of homogenous structure throughout its mass. It is then allowed to cool in still air in a uniform manner. Air cooling results into faster cooling rate when compared with the furnace cooling rate. Thus, the cooling time in normalizing is drastically reduced as compared to annealing.

Soaking periods for normalizing are usually one hour per 25 mm of thickness of the work piece but not less than 2 hours at the soaking temperature. The mass of the work piece can have a significant influence on the cooling rate and thus on the resulting microstructure. Thin work pieces cool faster and hence are harder after normalizing than the thicker work pieces. This is different than in the case of annealing where the hardness of thin and thicker work pieces is same after furnace cooling.

Low carbon steels normally do not need normalizing. However there are no harmful effects, if these steels are normalized. In case of castings having uniform wall thickness and section sizes are usually annealed rather than normalized. Other types of castings especially with complex shapes or interconnected thin and thick sections, which are prone to high levels of residual stresses, are benefitted by normalizing. The microstructure obtained by normalizing depends on the composition of the castings and the cooling rate.

Normalizing of steel is often considered both from a thermal and a microstructural viewpoint. From a thermal standpoint, normalizing process consists of austenitizing followed by a relatively slow cool. In case of microstructural standpoint, the areas of microstructure that contain about 0.80 % carbon are pearlite, while areas of low carbon are ferritic.

Normalizing is normally done to achieve any one of the following purposes.

  • To modify and/or refine the grain structure and to eliminate coarse grained structures obtained in previous working operations such as rolling and forging etc.
  • To modify and improve cast dendritic structures and reduce segregation by homogenization of the microstructure.
  • To produce a homogeneous micro structure and to obtain desired microstructure and mechanical properties.
  • To improve machinability of low carbon steels
  • To improve dimensional stability
  • To reduce banding
  • To improve ductility and toughness
  • To provide a more consistent response when hardening or case hardening.
  • To remove macro structure created by irregular forming or by welding.

Fine grained pearlite is tougher than coarse grained ones. Normalizing imparts both hardness and strength to iron and steel work pieces. In addition, normalizing helps reduce internal stresses induced by such operations as forging, casting, machining, forming or welding. Normalizing also improves microstructural homogeneity and response to heat treatment (e.g. annealing or hardening) and enhances stability by imparting a ‘thermal memory’ for subsequent lower temperature processes. Work pieces that require maximum toughness and those subjected to impact are often normalized. When large cross sections are normalized, they are also tempered to further reduce stress and to control mechanical properties more closely.

Normalization eliminates internal stresses, strains and improves the mechanical properties of the steel, such as improving its toughness and machinability. A better ductility can also be obtained without compromising the hardness and strength.

Comparison with annealing

Normalizing process of steel differ from the annealing process of steel with respect to heating temperature and cooling rate. In case of normalizing the steel is heated to a higher temperature and then removed from the furnace for air cooling.  In comparison in case of annealing the heating temperatures are lower and the cooling take place in furnace at a much lower rate.  Due to the faster cooling rate in case of normalizing, the steel possesses higher strength and hardness when compared with the steel which has undergone annealing treatment

Both annealing and normalizing do not present significant difference in the ductility of low carbon steels. The tensile strength and the yield point of the normalized steels are higher than the annealed steels except in the case of low carbon steels.

As in the case of annealing, normalizing also results into the formation of ferrite, cementite and lamellar pearlite. But in normalizing, since the cooling rates are higher, transformation of austenite takes place at much lower temperatures when compared with annealing. Due to it, the transformation product, pearlite is finer with lower interlamellar distance between the two neighboring cementite plates.

The main difference between full annealing and normalizing is that fully annealed work pieces are uniform in softness (and machinability) throughout the entire part, since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part.

Normalizing relieves internal stresses caused by cold work  while grain growth is limited by the relatively high cooling rate therefore the mechanical properties (strength, and hardness) of a normalized steel are better than in an annealed steel.

Quality of surface after machining of a normalized part is also better than in an annealed part. This effect is caused by increased ductility of annealed steel favoring formation of tearing on the machined surface.

Properties after normalizing

Since the cooling rate in the normalizing heat treatment is not controlled, the resulting structure is dependent on the thickness of the steel work piece. Therefore the effect of increased mechanical properties is greater in thin work pieces.

Normalized steel has higher hardness and strength than annealed steel due to the following reasons.

  • The amount of pearlite in the normalized steel is more than that in the annealed steel having the same carbon content, due to the shifting of the eutectoid composition to a lower value.
  • The dispersion of pearlite and ferrite phases is finer.
  • The pearlite of normalized steel is finer and has a lower interlamellar spacing than that of annealed steel.

Application of normalizing

Normalizing is the most extensively used industrial process since it is more economical to normalize the steel as against annealing. In normalizing since the cooling takes place in air, the furnace is ready for next cycle as soon as heating and soaking is over as compared to annealing where furnace cooling after heating and soaking needs 8 to twenty hours depending upon the quantity of charge. Hence in many cases annealing is replaced by normalizing to reduce the cost of heat treatment. Normalizing is adopted if the properties requirements are not very critical.

Some typical examples of normalizing in commercial practice are as below.

  • Normalizing of gear blanks prior to machining so that during subsequent hardening or case hardening dimensional changes such as growth, shrinkage or warpage can be controlled better.
  • Homogenization of cast and wrought structures
  • Improvement of machinability and grain size refinement of cast structures of castings
  • Stress relieving of castings
  • Cast metals and alloys are characterized by segregated, cored and dendritic structures as well as non uniform properties. Similarly wrought metal and alloys after mechanical working such as forging, rolling extrusion etc. have non uniform structure and properties. These structures and properties are made homogeneous by normalizing.
  • In some few cases, when the steel is hot or cold worked, it is necessary to perform a normalizing heat treatment in order to recover its original mechanical properties.
  • In case of normalizing heat treatment on weld metal the original as welded metal fine grained microstructure is changed to a coarse equiaxed ferrite with ferrite-carbide aggregates and the yield and tensile strength properties are considerably reduced.
  • It is very rare for a forging to be used without some sort of thermal treatment due to the heavy mechanical stresses impressed on the part and the variations in the microstructure.  Normalizing is one of the simplest heat-treatments that can address refining (or normalizing) the microstructure and equalizing the effects of the range of temperatures the material has been subjected to during the forging operations.  Normalizing forgings is very beneficial to any subsequent hardening operations
  • Steels that have undergone plastic deformation consist of pearlite which is irregularly shaped and relatively large, but varying in size. Normalizing is a heat treatment used on steel so as to refine its crystal structure and produces a more uniform and desired grain size distribution.