Pearlite transformation in hypoeutectoid (or hypereutectoid) steel, a common microstructure in steel
Release Time:
2016-07-04
Pearlite Transformation and Microstructure of Hypoeutectoid Steel The pearlite transformation in hypereutectoid steel is basically similar to that in eutectoid steel, but the precipitation of proeutectoid ferrite (or cementite) must be considered. (1) Hypereutectoid Transformation When the austenite of hypereutectoid composition is undercooled to the hypereutectoid region, it can decompose directly into a mechanical mixture of ferrite and cementite without precipitating the proeutectoid phase. The decomposition mechanism and the microstructure of the decomposition products are exactly the same as those of the pearlite transformation, but the amounts of ferrite and cementite are different from those in pearlite. The transformation product is called hypereutectoid structure, which is generally still called pearlite.
Pearlite transformation and microstructure of hypoeutectoid steel
The pearlite transformation in hypereutectoid steel is basically similar to that in eutectoid steel, but the precipitation of proeutectoid ferrite (or cementite) must be considered.
① Hypereutectoid transformation
After the austenite of hypereutectoid composition is undercooled to the hypereutectoid region, it can be decomposed directly into a mechanical mixture of ferrite and cementite without precipitating the proeutectoid phase. The decomposition mechanism and the microstructure of the decomposition products are exactly the same as those of the pearlite transformation, but the amounts of ferrite and cementite are different from those in pearlite.
The transformation product is called hypereutectoid structure, generally still called pearlite
② Precipitation of proeutectoid ferrite in hypoeutectoid steel
⑴ Morphology of proeutectoid ferrite
Proeutectoid ferrite has three different morphologies: network, blocky (or equiaxed), and lamellar (sometimes also called acicular)
Ferrite formation has no coherent relationship with austenite (blocky, network).
Ferrite growth has a coherent relationship with austenite (lamellar).
⑵ Formation of proeutectoid ferrite
The precipitation of proeutectoid ferrite is also a nucleation and growth process.
Nucleation of proeutectoid ferrite: Nuclei are mostly formed on austenite grain boundaries. The nucleus has a coherent interface with the austenite grain on one side, but a non-coherent interface with the austenite grain on the other side.
Growth of proeutectoid ferrite: After the nucleus is formed, the C% in the austenite near the ferrite will increase, forming a concentration gradient of C in the austenite, thus causing carbon diffusion. In order to maintain the C% balance at the phase interface, that is, to restore the high concentration of C in the interfacial austenite, low-C ferrite must continue to precipitate from the austenite, so that the ferrite nucleus continues to grow.
⑶ Growth mode of proeutectoid ferrite
When the transformation temperature is high: The activity of Fe atoms is stronger, and the non-coherent interface is easier to migrate, so the ferrite grows into a spherical cap shape on the side with no orientation relationship with the A grain:
If the C% of the original austenite is high, the ferrite will form a network;
While if the C% of the austenite is low, the ferrite will form a blocky shape;
In addition, if the A grain is large and the cooling rate is fast, the proeutectoid F may also precipitate in a network shape along the A grain boundary.
When the transformation temperature is low:
Fe atom diffusion is difficult, and the non-coherent interface is not easy to migrate. At this time, the ferrite will grow into the austenite grain with an orientation relationship through the coherent interface.
The coherent interface often has a certain orientation relationship with the matrix phase. In order to reduce the strain energy, the ferrite grows in a strip shape along a certain crystal plane of the austenite into the grain. The ferrites are parallel to each other, or at 60 or 90 degrees to each other. Widmanstätten structure is formed.
③ Formation of proeutectoid cementite in hypereutectoid steel
Hypereutectoid steel is heated to a temperature above Acm, and after holding to obtain uniform austenite, it is isothermally held or slowly cooled below Acm and above the extension line of GS, proeutectoid cementite will precipitate from the austenite.
Morphology of proeutectoid cementite: granular, network, and acicular (lamellar).
In the case of uniform austenite composition and coarse grains in hypereutectoid steel, the possibility of direct precipitation of granular cementite from undercooled austenite is very small, and it is generally network or acicular (lamellar).
If hypereutectoid steel has a network or acicular lamellar cementite structure, it will significantly increase the brittleness of the steel. Therefore, the network or acicular lamellar cementite should be eliminated.
In industry, the structure with (proeutectoid lamellar ferrite or proeutectoid acicular lamellar cementite + pearlite) is called Widmanstätten structure. The former is called α-Fe Widmanstätten structure, and the latter is called cementite Widmanstätten structure
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