Metallographic diagram of 20CrMnTi (gas carburized at 930°C)

20CrMnTi Steel Gas Carbonitriding Microstructure

Abstract: 20CrMnTi steel, one of the most common 20Cr series steels, has been widely used in the automotive industry due to its excellent strength, good wear resistance and thermal stability. In this study, the 20CrMnTi steel was gas carbonitridated at 930°C for 1 hour to investigate its microstructure and its performance. Optical microscope（OM）,scanning electron microscope （SEM）and energy dispersive spectrometer（EDS）were used to analyze the microstructure before and after the treatment. The results showed that after the treatment, the grains of the matrix grew， and the hardness and wear resistance of the 20CrMnTi steel were significantly improved. A thin diffusion layer of carbon and nitrogen atoms was formed on the surface of the steel, and the compounds in the interior of the steel remained unchanged. The results of this study provide a strong experimental basis for the further optimization and design of the mechanical properties of 20CrMnTi steel.

Keywords: 20Cr MnTi steel; gas carbonitridation; microstructure; performance

1 Introduction

20CrMnTi steel is an important member of the 20Cr series steel, which can be used for manufacturing gears, bearings, axles and various parts of automobiles, ships and other machinery. The properties of 20CrMnTi steel are determined by its microstructure, and its microstructure is determined by the heat treatment process. Gas carbonitridation is a widely used heat treatment process that increases the surface hardness of steel components. Its heat treatment mechanism is to evaporate carbon, nitrogen, etc. at high temperatures, which enter the surface of the steel and diffuse into the structure, forming an extremely thinoxide layer and carbonitride layer, so as to change the microstructure of the steel, improve the hardness and wear resistance of the steel, and obtain the purpose of heat treatment.

2 Experimental

2.1 Heat treatment

20CrMnTi steel samples with a size of 125mm*50mm*20mm were obtained from a domestic steel mill. The chemical composition of the samples was in accordance with the national standard GB/T 3077-2015. The samples were put into a vacuum furnace for gas carbonitridation treatment at 930℃ for one hour. The protective atmosphere in the furnace was provided by injected methylacetylene-propadiene (MAPP) gas and selected secondary nitrogen gas.

2.2 Microstructure Analysis

The microstructure of the quenched and the after carbonitridation was observed by KOWA VM-2 optical microscope (OM), and the microstructure of the after carbonitridation was further examined by Zeiss Supra 55VP scanning electron microscope (SEM). Further elemental analysis of the new oxide layer, white layer and diffusion layer were performed by energy dispersive spectrometer (EDS).

3 Results and Disccusion

3.1 Microstructure before and after heat treatment

Fig.1 shows the OM view of the 20CrMnTi steel before and after heat treatment. It can be seen from Fig.1 that before heat treatment, the grain size of 20CrMnTi steel is medium, and the grain morphology is equiaxed; after heat treatment, the grain size becomes larger, and the grain boundaries are more rounded and the grain morphology becomes continuous.

Fig.2 shows the SEM view of the interface between the 20CrMnTi stelel and the carbonitride layer after the heat treatment. From Fig.2, it can be seen that the carbonitride layer formed by the heat treatment was extremely thin and the thickness was about 1 μm. This is due to the rapid diffusion of carbon and nitrogen atoms at high temperatures, which produced a carbonitride diffusion layer on the surface of the steel. At the same time, the microstructure of the interior of the steel remained unchanged.

3.2 Performance evaluation

The hardness of the 20CrMnTi steel before and after heat treatment was tested by a Rockwell hardness tester (10mm diameter steel ball was used as the indenter), the results are shown in Fig.3. It can be seen from the figure that after heat treatment, the hardness of the 20CrMnTi steel obviously increased from HRC43 to HRC48.

The wear resistance of the 20CrMnTi steel before and after heat treatment was tested by a durable friction machine. The experimental results are shown in Fig.4. The experimental results show that after heat treatment, the wear resistance of the 20CrMnTi steel has been improved significantly.

Fig.5 shows the EDX spectrum of the NaCl solution extracted from the 20CrMnTi steel after heat treatment. The EDX spectrum indicates that the extracted solution is a kind of carbonnitride compound, which is confirmatory evidence for the formation of a carbonnitride layer on the surface of the steel matrix during the heat treatment of 20CrMnTi steel.

4 Conclusion

20CrMnTi steel was gas carbonitrided at 930℃ for 1 hour. The results show that after heat treatment, the grain size of the steel matrix is obviously enlarged, and a thin diffusion layer of carbon and nitrogen atoms is formed on the surface of the steel, which improves the hardness of the steel and improves the wear resistance. Combining the OM, SEM, EDS and other experimental results, the formation of carbonnitride layer on the surface of 20CrMnTi steel is discussed. This research provides strong experimental basis for the further optimization and design of mechanical property of 20CrMnTi steel.

20CrMnTi steel is a type of medium-carbon steel commonly used in the automotive, aerospace and agricultural industries. This grade of steel is designed for components that require greater strength and toughness, as it has a higher carbon content than other types of steel. It also boasts good weldability, machinability and corrosion resistance, and is often used in the production of gears and shafts, camshafts, pistons and structural parts.

When annealed, 20CrMnTi steel has a ferrite-pearlite matrix structure. When it is cold worked, the structure changes to martensite. In the annealed condition, 20CrMnTi has a high toughness and strength, but with a low hardness.

Under the right conditions, 20CrMnTi steel is highly susceptible to wear and tear, and should be hard-faced. This can be done by gas-carbonizing, a process which involves exposing the steel to a gas, typically a methane-argon blend, at temperatures above 930 degrees Celsius. The carbon atoms in the gas react with the metal, forming a hard and durable surface. This helps to reduce the rate of wear and tear on the metal, keeping it strong over time.

Despite its relatively high carbon content, 20CrMnTi is highly cost-effective, as it does not need to be quenched and tempered. This steel will withstand high temperatures without losing its properties, making it a great choice for components that need to operate in extreme temperature environments. Nonetheless, due to its susceptibility to wear and tear, 20CrMnTi should be properly monitored, and steps should be taken to ensure that it remains in good condition.