Form of cementite in steels
Cementite is present in most steels, moreover:
– in hypoeutectoid steels - in the form of thin plates perlite;
– in hypereutectoid steels - in the form of thin pearlite plates, as well as in the form of thick plates and globules along the boundaries of austenite grains.
These forms cementite takes, when austenite turns into pearlite at temperature Ar1 and when it nucleates at the boundaries of austenite grains in hypereutectoid steels at temperature Acm. Exists, However, the ability to heat-treat steel so, to change the shape of cementite - to get cementite in the form of spherical particles.
Cementite is very hard, but also very fragile. When cementite takes the form of isolated spheres - globules - in a matrix, eg, ferrite or tempered martensite, then the mechanical properties of steel become close to those of the matrix phase:
– soft and well cut with ferrite matrix and
– strong and less fragile with martensitic matrix.
To obtain a spheroidized cementite structure, two processes of heat treatment of steel are mainly used.:
1) hardening on martensite and high vacation;
2) incomplete annealing.
Spheroidization of cementite: hardening and high tempering
In the first process, the steel is simply heated to a fully austenitic state and then quenched to martensite. Then steels of both types - hypereutectoid and hypereutectoid - are heated to a temperature just below point A1, held for about one hour and cooled. As a result, small spheres of cementite appear in the ferrite matrix.
Martensite at room temperature is not a stable phase - therefore it is not present at phase diagram - and at elevated temperatures it decomposes into small carbides (cementite) in a ferrite matrix. For hypereutectoid steels, cementite spherodization is obtained in a martensitic or bainitic matrix by heating hardened steel to a temperature between points A1 and acm in about one hour. In this case, martenisite decomposes into small carbides in the austenite matrix. Then, a martensite or bainite matrix is formed from austenite by quenching it at the required cooling rate. The volume fraction of spheroidized cementite in steel depends on the one-hour holding temperature - the maximum amount at a temperature slightly above A1 to almost zero at temperatures just below Acm.
Spheroidization by incomplete annealing
The second method is most effective for hypereutectoid steels.. It does not require quenching and therefore is used in industry to obtain spheroidized structures of cementite plus ferrite in those steels, which are delivered directly from the rolling mills.
This method uses the technique, which prevents the formation of pearlite when cooling austenite below temperature A1. The trick is, to obtain small particles of cementite in austenite and then cool it below temperature A1 low speed - usually a little slower, than just air cooling. This treatment forces the austenite to transform directly into a spheroidized cementite-ferrite structure., not the lamellar pearlite structure. This transformation is sometimes called a separate eutectoid transformation..
Small particles of cementite are naturally formed in austenite of hyperevectoid steel, when it heats up above A1 and below Acm. Experiments show, what happens then, when the original steel does not contain large particles of cementite - for example, contains perlite, bainite, martensite or already spheroidized ferrite-cementite structure. Besides, steel is not heated above about 800-840 ºС for, so that the separate eutectoid transformation could dominate the pearlite transformation.
This method will work with hypoeutectoid steels as well., but in this case it requires tighter control. The thing is, that when the hypoeutectoid steel is heated above A1, then the cementite plates in its pearlite completely transform into austenite very quickly. Since this method requires the presence of cementite when cooling the austenite, then hypoeutectoid steels cannot be kept for a long time at temperatures above the temperature A1 before cooling them onto a spheroidized structure. An efficient way to achieve spheroidization in pre-eutectodic steel is to cycle the steel by about 50 ºС above and below point A1 repeatedly.
Spheroidization of hypereutectoid bearing steel
On the picture 1 shows the spheroidized structure of hypereutectoid steel using the example of bearing American steel 52100 (analogue of steel ШХ15) , which was obtained by incomplete annealing, described above. The steel was originally pearlitic, and all the cementite was in perlite. The steel was then kept at a temperature 795 ºС during 30 minutes and cooled with the oven to 680 ºС with speed 370 ºС / hour to complete the separate eutectoid transformation. Then the steel was cooled to room temperature. Slow cooling to temperature 680 ºС caused austenite to transform into a mixture of ferrite + cementite by the mechanism of separate eutectoid pre-growth, not pearlite transformation. As a result, a completely spheroidized structure was formed with very small cementite particles in a ferrite matrix..
Figure - Electron micrograph of the spheroidized structure of American bearing steel 52100 (domestic analogue - steel ШХ15): fine spherical cementite particles in a ferrite matrix.
Initial magnification - 6000x.
Spheroidized structure for cutting
Examples of spheroidized steels relate mainly to hypereutectoid steels, such as bearing steels, which are supplied from the manufacturer with a spheroidized cementite structure in a ferrite matrix. This is done to facilitate machining of this steel by cutting. Besides, bearing and tool steels are often used for products, which require increased wear resistance. For this, these steels are thermally treated, to obtain a spheroidized structure by controlled cooling of steel with a fine pearlite structure, bainite or tempered martensite.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007