Normalizing
Normalizing , normalizing or return refining is a heat treatment method for steel . Normalizing consists of austenitizing and subsequent cooling in still air. In general, normalizing should remove irregularities in the structure and achieve a fine-grained , uniform structure with reproducible strength and deformability properties. According to DIN 8580, annealing is one of the manufacturing processes by changing the material properties.
Procedure
Sub-eutectoid steels (<0.8% carbon content) are first slowly heated to approx. 600 ° C. Then the steel is heated more quickly to ~ 30 to 50 K above the upper transformation point A c3 . Hypereutectoid steels are heated just above the lower transformation point of A c1 in order to prevent the formation of shell cementite.
The workpiece is kept at this temperature until the structure is completely austenitized . The temperature should remain below 1000 ° C, as prolonged exposure to this temperature leads to coarse grain.
The workpiece is then quickly cooled down to A R1 in order to prevent further grain growth. This is followed by cooling in still air. Alloy steels must cool down slowly so that they do not harden.
When it cools down, it is converted back to a normal structure (in the sense of the iron-carbon diagram ), which is largely independent of the initial state . The resulting structure depends on the carbon content : In hypoeutectoid steels, ferrite and pearlite are produced , in hypereutectoid steels pearlite and cementite . The cooling in a still atmosphere ensures further grain refinement , i.e. H. Formation of a fine structure. If the cooling leads to bainite or martensite , the process is called "air hardening".
Applications
Normalizing is of great importance for steel castings , welded constructions or forged parts . The uncontrolled cooling creates unequal grain sizes and shapes. Slow cooling in the air can lead to undesirable grain growth.
High-strength steels are rolled in a normalizing manner . The last rolling pass is carried out in the austenite area. The material is then cooled down as in normalizing.
On the other hand, normalizing is not possible for non-transformable, ferritic or austenitic steels, as they do not carry out any α-γ-α transformation.