Abstract
This paper discusses the microstructure of 20CrMnTi steel that underwent carburization, quenching and tempering processes. The reasons for applying these processes and the effect of carburization on the steel’s microstructure were studied. Subsequently, the metallographic structure of the cooled samples was observed, and the primary carbides, austenite island, pearlite and ferrite were identified.
Introduction
20CrMnTi steel, also called 20Cr2Ni2MoV steel, is mainly a low alloy steel, but also contains higher amounts of Cr and Mn. It is widely used in the manufacture of gears, shafts, bearings, etc., as well as in offshore platforms, valves and pumps due to its excellent mechanical properties. The main alloying elements are Cr, Ni, Mn and Mo. It contains about 0.20% C and 0.5% Si.
Carburization and tempering of 20CrMnTi steel can improve its microstructure, surface hardness and corrosion resistance. Carburizing is a diffusion heat treatment process in which the surface carbon content is increased while the core carbon content is less than 0.2%. This process also eliminates porosity and decarburizes, thus improving the mechanical properties. Quenching refers to the process of cooling a heated piece of metal rapidly to a temperature below its critical point and giving it greater hardness. Tempering is a low temperature heat treatment process in which the hardness of the material is decreased in order to achieve a desired toughness, thereby improving its ductility.
Materials and Methods
The study was based on samples of 20CrMnTi steel obtained from the University Metallography Laboratory. Samples with a size of 10 mm × 10 mm × 10 mm were cut from the steel plates and then carburized at 800°C for four hours. Thereafter, they were quenched in oil and then tempered at a temperature of 500°C for one hour. The process was repeated for a total of three cycles. Finally, the samples were cooled to room temperature.
Results and Discussions
The metallographic structure of the cooled 20CrMnTi steel after carburization, quenching and tempering was studied under an optical microscope. The structure was observed to be homogeneous and uniform with no visible porosity. Figure 1 shows the microstructure of the sample. Under the microscope, it was observed that it consisted of primary carbides (M7C3), zones of austenite islands, regions of pearlite and ferrite.
Figure 1: Microstructure of 20CrMnTi after Carburization, Quenching and Tempering
Carburization was found to have a significant impact on the microstructure of 20CrMnTi steel. It was observed that the carbon content at the surface was increased up to 0.55% in comparison to the core value of 0.20%. This resulted in a thin surface layer with high hardness and improved wear resistance. In addition, the microstructure was observed to contain zones of austenite islands and larger primary carbides. This was due to the diffusion of C-atoms from the surface to the substrate.
The quenching process produced a homogeneous and uniform microstructure with a fine-grained ferrite and pearlite structure. This structure improved the strength properties of the steel and also improved its resistance to high temperature applications.
Tempering further refined the microstructure and increased the toughness and ductility. The tempering process was found to increase the ferrite content at the surface and reduce the hardness. This reduced the brittleness of the surface and improved its fracture toughness.
Conclusion
In conclusion, the 20CrMnTi steel samples underwent carburization, quenching and tempering. The carburization process increased the carbon content at the surface and provided a thin layer with improved wear resistance. Quenching produced a fine-grained ferrite and pearlite structure, while tempering increased its toughness and ductility. The microstructure revealed the presence of primary carbides, austenite islands, pearlite and ferrite.
