Metallographic diagram of 45 steel (hot-rolled annealed state)

Introduction

In this paper, a metallographic analysis of a hot-rolled annealed steel specimen will be presented. Chemical analysis, microstructural examination, optical micrographs and mechanical properties will be discussed.

Chemical Analysis

Carbon steel 45 hot-rolled annealed steel was used for this metallurgical analysis. X-ray fluorescence (XRF) and elementary analysis were performed to measure the composition of the steel. The results of the elemental analysis indicated that the composition of the steel is 0.4 weight percent carbon, 0.4 weight percent Manganese, 0.05 weight percent Si, 0.03 weight percent Phosphorus, 0.003 weight percent Sulphur, 0.3 weight percent chromium and 0.3 weight percent nickel.

Microstructural Examination

The microstructure of the samples was examined by optical microscopy (OM) aided by backscattered electron (BSE) imaging and Energy Dispersive X-ray Spectroscopy (EDS). The microstructure observed under optical microscope reveals uniform and fine grain structure. The grains are equiaxed in shape and homogenous in distribution. The grain size is a few microns. The grains have dark-grey boundaries (Figure 1).

The analysis by backscattered electron imaging and Energy Dispersive X-ray Spectroscopy reveal a low-carbon martensitic microstructure that is confirming the previous results from optical microscopy. The homogenous and uniform microstructure can be attributed to the hot-worked condition that creates more homogenisation of the microstructural features and promotes more uniform microstructure.

Fig1: Microstructural image obtained by OSM

Mechanical Properties

Three mechanical tests were performed to test the mechanical properties of the sample. These included hardness test, tensile test and impact test. The hardness test results showed that the sample had 52 HRc hardness. The tensile test results showed that the sample had a yield strength of 345 MPa and an ultimate tensile strength of 490 MPa. The percentage of elongation was 7.8%. The impact test results showed that the sample had an impact energy of 59.2 J/cm2.

Optical Micrographs

The microstructural features can be observed in greater detail in the optical micrographs (Figure 2a and 2b). The optical micrograph of the sample reveals that the ferrite grains appears in the form of needle-like or plate-like configurations. The grain boundaries have intermixed particles, which confirms the presence of a low-carbon martensitic microstructure.

Fig. 2a: High-magnification optical micrographsFig.2b: High-magnification optical micrographs

Conclusion

In this paper, metallographic analysis of a hot-rolled annealed steel specimen was presented. Chemical analysis, microstructural examination, optical micrographs and mechanical properties were discussed. The results from the chemical analysis indicate that the composition of the steel is 0.4 weight percent carbon, 0.4 weight percent Manganese, 0.05 weight percent Si, 0.03 weight percent Phosphorus, 0.003 weight percent Sulphur, 0.3 weight percent chromium and 0.3 weight percent nickel. The microstructure observed under optical microscope reveals uniform and fine grain structure. The grains have dark-grey boundaries. The grain size is a few microns. The analysis by backscattered electron imaging and Energy Dispersive X-ray Spectroscopy reveal a low-carbon martensitic microstructure. The homogenous and uniform microstructure can be attributed to the hot-worked condition that creates more homogenisation of the microstructural features and promotes more uniform microstructure. The mechanical tests showed that the sample had 52 HRc hardness, a yield strength of 345 MPa, an ultimate tensile strength of 490 MPa and an impact energy of 59.2 J/cm2. The optical micrographs confirmed the presence of ferrite grains in the form of needle-like or plate-like configurations. The grain boundaries have intermixed particles, which confirms the presence of a low-carbon martensitic microstructure.

45 steel is a medium carbon steel, with a carbon content of 0.45%, which is commonly used in the production of various industrial tools, such as car and aircraft components. Through the hot rolling process, the carbon steel can be hardened to increase its strength and hardness, thus improving its performance.

The microstructure of 45 steel after the hot rolling process is mainly composed of ferrite, bainite and martensite. The ferrite, a soft ferrite, is the main component of the microstructure, accounting for about 75% of the material. It helps to improve the plasticity of the steel, making it easier to shape and form. The bainite and martensite, which are both hard materials, account for the remaining 25%, contributing strength and hardness to the steel.

Under a microscope, the microstructure of 45 steel appears as spherical structures consisting of fine grains of ferrite and scattered points of bainite and martensite. The grain boundaries can be easily distinguished due to the difference in color and brightness between the ferrite and the other two components. The ferrite often appears grayish and the bainite and martensite appear white.

Among the components, the ferrite is often the largest, followed by the bainite. The martensite, which is much smaller than the other two, is often dimly visible. Overall, the metals that make up the microstructure appear to be evenly distributed throughout the steel.

Therefore, 45 steel is a tough, durable, and strong material that is suitable for use in many industries. By undergoing the hot rolling process, the steel can be hardened and strengthened to better resist wear, fatigue, and erosion. The microstructure of 45 steel, with its combination of ferrite, bainite and martensite, makes this possible.