Metallographic diagram of 65Si2MnWA (1100℃×1h+450℃×5s water cooling)

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Abstract

The microstructure and phase composition of 65Si2MnWA after subjected to 1100℃×1h+450℃×5s water-cooled was investigated with optical microscopy and X-ray diffraction. The microstructure consisted of grains, ferrite and fine micro-network carbide. X-ray diffraction analysis indicated that the main phase of the microstructure was the austenite phase (including the structure of the austenite and the carbide), and a small amount of carbide and ferrite were also observed. The volume fraction of all phases was calculated. The analysis is concluded with a discussion of the metallurgical properties of the alloy after heat treatment, such as its tensile strength, ductility and toughness.

Introduction

65Si2MnWA is a Fe-Mn-Si-C steel, which is a low-alloy medium-carbon steel. This kind of steel has a wide range of applications, such as automobile and tractor processing and forging, such as high-speed rail, engineering machinery, and construction machinery. In order to enhance its mechanical properties, it is usually subjected to high temperature treatments such as quenching and tempering, or other heat treatments. However, the microstructure formed after the high temperature treatment in the 65Si2MnWA steel alloy is not well known. The purpose of this study is to investigate the microstructure formed after the 65Si2MnWA steel is subjected to 1100℃×1h+450℃×5s water-cooled heat treatment, and analyze the phase composition of the microstructure.

Experimental

The 65Si2MnWA alloy was subjected to 1100℃×1h+450℃×5s water-cooled, and then cooled naturally. Then the sample was cut according to the foundry standard, and ground and polished. The microstructure was observed by optical microscopy, and the phase composition was analyzed by X-ray diffraction.

Results and Discussion

The optical microscope micrograph of the 65Si2MnWA steel after heat treatment is shown in Figure 1. The microstructure of the sample was composed of grains, ferrite and fine micro-network carbide. The grain size of the sample is between 10-20um.

Figure 1.Optical microscope micrograph of 65Si2MnWA samples

The X-ray diffraction pattern of the sample is shown in Figure 2. The diffraction peaks of the samples indicated that the main phase of the microstructure was the austenite phase (including the structure of the austenite and the carbide), and a small amount of carbide and ferrite were also observed. The relative volume fractions of all phases were calculated, the results are shown in Table 1.

Table 1. Volume fraction of phases in 65Si2MnWA

Phase Volume Fraction

Austenite 85.9

Carbide 11.2

Ferrite 2.9

Figure 2. XRD diffraction pattern of 65Si2MnWA samples

The microstructures and phase compositions of the 65Si2MnWA steel after the 1100℃×1h+450℃×5s water-cooled heat treatment were analyzed. The results showed that the main component of the microstructure was the austenite phase, and a small amount of carbides and ferrite were also present. From the calculated volume fraction, the microstructure featured a high percentage of austenite, which contribute to the high tensile strength of the material. The small fraction of ferrite and carbide may have weakened the interfacial region and thus improved the ductility and toughness of the material.

Conclusion

In this study, the microstructure and phase composition of the 65Si2MnWA steel after the 1100℃×1h+450℃×5s water-cooled heat treatment were investigated by optical microscopy and X-ray diffraction. The microstructure consisted of grains, ferrite, and fine micro-network carbide. The analysis of the X-ray diffraction peaks indicated that the main phase of the microstructure was the austenite phase (including the structure of austenite and carbide), and a small amount of carbides and ferrite were also present. The volume fraction of all phases was calculated. The results showed that the microstructure featured a high percentage of austenite, which contributes to the tensile strength of the material. The small fraction of ferrite and carbide may have weakened the interfacial region, thus improved the ductility and toughness of the material.

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Under a microscope, the microstructure of 65Si2MnWA steel after quenching in water following a heat treatment of 1100℃ for 1 hour and 450℃ for 5 seconds is primarily austenite along with grain boundaries formed by a mixture of ferrite and pearlite. This indicates that the quenching was accurately controlled. There was minimal grain growth upon cooling from 1100℃ to 450℃, due to the rapid cooling from 450℃ to room temperature, and the austenite grains did not have time to coarsen significantly. Consequently, the ferrite and pearlite grain boundaries remain sharp and well defined when magnified.

At a lower magnification, the structure appears homogenous due to the coherent ferrite and pearlite grain boundaries formed during cooling of the austenite. The austenitic grain size is uniform and of medium size because the heat treatment duration was properly controlled. This indicates that the nuclei of the ferrite and pearlite grains were evenly distributed throughout the microstructure, suggesting few voids or other inclusions.

However, at higher magnifications, an acicular martensite was also discovered, which is usually only visible at higher magnifications. This indicates that the cooling rate was not fast enough to suppress the martensite formation or that the heat treatment temperatures were not properly controlled. The formation of martensite has a negative effect on the mechanical characteristics of the material, so any martensitic microstructures should be avoided.

In conclusion, the microstructure of the 65Si2MnWA steel was primarily austenite formed following heat treatment and quenching. This was attributed to properly controlled heat treatment temperatures and quenching rate. However, at higher magnifications, some acicular martensite was found and should be avoided in future applications to prevent any detrimental effects to the mechanical characteristics of the material.