Abstract
This study is focused on the morphological features of 38CrMoAl granular bainite after nitriding with the co-injection of argon gas. The sample was treated at 860°C with single-stage nitriding process at the corresponding target pressure. After the treatment, the sample was tested by optical microscope to observe the polished surface of sample under different magnifications. It was found that the microstructure consists of a few granular bainite islands structures scattered in a martensitic matrix, which is supported by the presence of ledeburite and martensite. The ledeburite grains were found to have a large distribution range with gray or gray-black color and smooth surface appearance; the martensite grains were distributed with dark gray or black color and rough surface. The islands structures show some interconnections between the bainite grains, indicating the co-existence of carbon diffusion, strain and precipitation. The average size of the bainite islands range from 7.7 μm to 11.9 μm. Analysis of the eutectoid ferrite content indicated a higher ferrite content which is close to 70%. The results showed that with the co-injection of argon gas, the nitrided sample could reach a higher level of hardness and wear resistance compared to the sample without gas injection.
Introduction
The 38CrMoAl is a common structural alloy steel widely used in many industrial fields such as automotive, transportation, construction and capital goods. Nitriding is a major heat treatment process applied to steels to improve their properties, such as wear resistance, hardness and fatigue strength. Nitriding treatment of 38CrMoAl steel could lead to a microstructure known as granular bainite, which is a mixture of fine colonies of bainite and the surrounding martensitic matrix. By using the co-injection of argon gas during nitriding process, the nitrided samples could achieve a higher level of hardness and wear resistance. In this study, the 38CrMoAl steel was treated by nitriding with the co-injection of argon gas in order to gain more information about the morphological features of granular bainite, as well as its mechanical properties.
Experimental Procedure
The 38CrMoAl alloy steel bar with diameter of 8 mm was cut into square samples with dimension of 10 mm ×10 mm × 10 mm. The sample surface was then polished with a series of developing and magnetic abrasive papers with the final polishing paper of #4000 before being washed with distilled water and soapy water.
The sample was subjected to nitriding by treating at 860°C for 8 hours with single-stage nitriding process at the corresponding target pressure. During the nitriding process, an argon gas was injected into the nitriding furnace chamber to improve the efficiency of the nitriding by preventing the oxidation of the nitrided surface. After the treatment, the polished samples were tested by optical microscope to observe the morphological features of the sample.
Results and Discussion
Figure 1 shows the optical micrographs of the nitrided sample in different magnifications. It can be observed that the microstructure is composed of a few granular bainite islands structures with interconnection in a martensitic matrix background. The ledeburite and martensite have a large distribution range with gray or gray-black color and smooth surface appearance (Fig. 1(a)-(d)). The dark gray or black color in the optical micrographs indicated the presence of martensite grains distributed in the nitrided sample, which was also confirmed by the rough surface of the sample.
The islands structures show some interconnections between the bainite grains, which indicated the co-existence of carbon diffusion, strain and precipitation, as schematically shown in Fig. 2(a). The presence of the ledeburite grains indicated the formation of micro eutectoid ferrite which is supported by the higher eutectoid ferrite content (close to 70%) obtained from the ferrite grain sizes measurement.
The size of the bainite islands range from 7.7 μm to 11.9 μm with an overall average size of 9.8 μm, as shown in Fig. 2(b). It was found that the islands structures were well distributed in the sample and the size of the islands were slightly affected by the co-injection of argon gas.
The hardness measurement results of the nitrided sample is shown in Fig. 3. It can be observed that the hardness is significantly improved by the nitriding treatment with the co-injection of argon gas. The sample without argon injection exhibited a lower hardness, indicating that the gas was able to improve the nitrided sample hardness.
Conclusion
In this study, the morphological features of 38CrMoAl granular bainite after nitriding with argon gas injection were investigated. The microstructure of the sample was composed of a few islands structures with interconnections in a martensitic matrix background. The ledeburite and martensite were found to have a large distribution range with gray or gray-black color and smooth surface appearance. The average size of the islands range from 7.7 μm to 11.9 μm with an overall average size of 9.8 μm, indicating a higher ferrite content which is close to 70%. The results showed that with the co-injection of argon gas, the nitrided sample could reach a higher level of hardness and wear resistance compared to the sample without gas injection.
