The microstructure of SAE 1050 steel (hot rolled and heat treated at 810°C) is studied using optical and scanning electron microscopes. For the microstructure evaluation, a metallurgical sample was prepared and replica was made.
The optical micrograph of the SAE 1050 steel shows a microstructure which consists of ferrite and pearlite. The ferrite microstructure appears coarse and granular with small particles (white color). The pearlite microstructure, which is recognized by its grayish tint, is composed of alternating lamellar structure of ferrite and cementite.
Under the scanning electron microscope (SEM), the cross-section of the SAE 1050 steel sample shows the presence of a micro-structure containing ferrite and pearlite. At the microstructure level, the ferrite appears as very fine grains compared to the ones visible from the optical microscope. The pearlite appears as alternate ferrite and cementite lamellar forms (grayish color).
The ferrite found from the microstructure of the SAE 1050 steel sample appears in two different types. First, there is alpha-ferrite which is soft and ductile. Alpha-ferrite is the main constituent of the SAE 1050 steel, and it has a lower carbon content than all other micro-alloyed ferrite. Secondly, there is lath-martensite which is harder and usually found in heat treated steels. It has a higher carbon content than alpha-ferrite and is formed during heat treatment.
The pearlite present in the microstructure of the SAE 1050 steel sample, is formed mainly from carbide. The carbide is formed from the chemical reaction of ferrite and cementite/iron carbide. During austenization, the carbon content of ferrite slowly diffuses into the cementite, in order to form the carbide. The carbide is a granular structure, and it is much harder and less ductile than ferrite and pearlite.
Finally, the SEM micrograph of the SAE 1050 steel reveals the presence of other secondary microstructures, such as bainite and spheroidite. The bainite is formed mainly by partial martensitic transformation of ferrite, and it is mostly composed of alpha- and lath-martensite. The spheroidite is a microstructure obtained during heat treating, and it is composed of fine colonies of ferrite and cementite.
In conclusion, the microstructure of SAE 1050 steel (hot rolled and heat treated at 810°C) consists mainly of ferrite and pearlite in an alternate lamellar arrangement, together with secondary microstructures such as bainite and spheroidite. The ferrite appears in two different phases, alpha-ferrite and lath-martensite, and the pearlite is composed of a granular structure of iron carbide. Such microstructure results in the guarantee of good mechanical properties.
