Normalizing is also heat treatment process that mainly considered for thermal and micro structure properties. It is used to relieve internal stresses, refine grain size and improve mechanical properties. In this article we learn difference between two processes annealing and normalizing. Before you see difference you should know definition and process of normalization.
What is Normalizing?
Normalizing is heat treatment process done by heating material to temperature above its critical temperature, and then material is soaked at that temperature until transformation occurs. Finally, this heated material is taken out from oven and cooled outside the oven at room temperature.
This treatment increase grain size and improves stability of microstructure. Normalized materials are softer but do not produce uniform material properties of annealed materials. Read also Engineering Materials – Desirable Bearing Materials Properties
Normalizing is process in which steel is heated to about 100 (55 ) above critical temperature. This normalizing treatment is same to annealing treatment. There is different method for cooling and annealing involves slow controlled cooling if heater providing slow cooling rate.
Other purpose of Normalizing is:-
Normalizing defined as heating steel to austenite phase and cooling it in air. Normalizing is normally done to achieve any one of following purposes.
- To get rid of coarse grained structures obtained in previous working operation such as rolling and forging etc.
- To modify and improve cast dendritic structures and reduce separation by homogenizing microstructure.
- To get desired microstructure and mechanical properties
- To develop machinability of low carbon steels
- To improve dimensional stability
- To give more constant response when hardening or case hardening.
- To improve ductility and toughness
In normalizing steel is equally heated to temperature which causes complete transformation to austenite. Fine grained pearlite is tougher than coarse grained ones. Normalizing pass both hardness and strength to iron as well as steel work pieces
In addition, normalizing helps to reduce internal stresses induced by such operation like forging, casting machining, forming or welding. Work pieces that need maximum toughness that impact are often normalized. When large cross sections are normalized, they are also tempered to reduce stress and to control mechanical properties more closely.
Normalization reduces internal stresses, strains and improves mechanical properties of steel, such as improving its toughness and machinability. Also, ductility can get without give and take hardness and strength.
Properties after Normalizing
Normalizing also effect in shape of ferrite, cementite and lamellar pearlite like annealing. But in normalizing due to high cooling rate, transformation of austenite takes place at lower temperatures than in annealing.
In normalizing heat treatment cooling rate is not controlled and structure is dependent on thickness of steel work piece. Therefore effect of mechanical properties is greater in thin work pieces. Normalized steel will have higher hardness and strength than annealed steel due to following reasons.
- In normalized steel amount of pearlite is more than in annealed steel having same carbon content, due to shifting of eutectoid composition to lower value.
- Spreading of pearlite and ferrite phases is more
- Pearlite of normalized steel is better and has lower interlamellar spacing than annealed steel.
Application of normalizing
Normalizing is mostly used in industrial process as it is more economical to normalize steel as against annealing. In normalizing, cooling takes place in air and furnace is ready for next cycle, but in annealing, cooling takes place in furnace, which may take about 8 to 20 hours depending upon quantity of charge.
Hence, annealing is replaced by normalizing to reduce cost of heat treatment in many cases. Normalizing can be taking only if properties needs are not very critical. Some typical examples of normalizing in commercial practice are as below.
- Stress relieving of castings
- Improvement of machinability and grain size refinement of cast structures of castings
- Cast metals and alloys are described by segregated, cored and dendritic structures as well as non-uniform properties. Similarly wrought alloys after mechanical working such as forging, rolling, extrusion etc are non-uniform in structure and properties. These structures and properties are made by normalizing homogeneous.
- In some cases, when steel is hot or cold worked, it is necessary to perform normalizing heat treatment in order to recover its original mechanical properties.
- Steels that have undergone plastic deformation consist of pearlite which is irregularly shaped and large, but varying in size. Normalizing is heat treatment used on steel so as to refine its crystal structure and produces more uniform and desired grain size distribution.
Comparison between normalizing and Annealing
Normalizing and Annealing are two commonly used for heating and cooling operation. You can notice two differences between two processes. Both methods follow similar procedure at beginning of process, but there is difference in last cooling step.
Key difference between annealing and normalizing is that, in annealing cooling process is done in oven while in normalizing it is done in air. However, both methods are equally important and they modify microstructure of material in different ways.
Normalizing process is similar to that of annealing, but latter part is different. In annealing cooling process is done in oven. But, in normalizing it is cooled in air. Normalizing process of steel differ from annealing process of steel with respect of heating temperature and cooling rate. In case of normalizing steel is heated at high temperature and then remove from furnace for air cooling.
In comparison of annealing heating temperatures are lower and cooling take place at lower rate in furnace. Due to faster cooling rate in case of normalizing, steel give higher strength and hardness when compare with steel which has undergone annealing treatment. Both annealing and normalizing do not present much difference in ductility of low carbon steels.
In case of low carbon steel, strength and yield point of normalized steels are higher than annealed steels. Main difference between full annealing and normalizing is that work pieces are in softness throughout entire part, since this entire part is exposed to controlled furnace cooling. In case of normalized part, it depends on part geometry and cooling is non-uniform across the part.
Normalizing Process for Steels
- Normalization is kind of heat treatment that relieves stress on steel and this improves ductility and toughness in steels that may harden after cooling process.
- For normalization steel is warmed to temperature above its upper critical point. Generally, this is more than 50 C of that temperature. This steel hold at this temperature for smaller metal grains to form and high energy grain shapes will come together
- On other hand, temperature is equalize throughout section and for that time should be sufficient.
- Normalizing structure is similar to that of annealing. But air cooling is faster than furnace cooling. Thus normalized steels possess fine grains than annealed steels.
- Normalized structure of low-alloy steel consists of ferrite and sorbite
- Normalizing is performed after forging and casting to put steel in best condition for hardening or machining.