45 steel (normalized) low-magnification structure and non-metallic inclusions

INTRODUCTION

45 steel is a variation of steel alloy that is designed to meet the special requirements of various technological applications. It is also known as 45BR, 45 Steel (L-fold), 45 Polymer Steel (AISI 45) and 45 High Strength Steel. The 45 steel alloy is composed of an iron base with a molybdenum-boron-nickel (MNB) alloying system. This particular combination provides the alloy with such features as high strength, high resistance to corrosion, and a reasonable balance between mechanical properties, ductility and weldability. The alloy is further optimized through annealing and heat treating processes whereby certain attributes are further enhanced.

The 45 steel has been extensively studied for its low magnification structures and non-metallic inclusions. This paper discusses the effects of low magnification grain structures on the mechanical properties and microstructure of 45 steel and their correlation to non-metallic inclusions. Data was obtained by electron microscopy, metallographic procedures, and tensile tests.

MATERIALS AND METHOD

The material used in the experimental work was a hot rolled sample of the 45 steel. The chemical compositions of this sample are as follows: 0.44 carbon(C), 0.19 silicon(Si), 0.41 manganese(Mn), 0.011 nitrogen(N), 0.22 phosphorus(P), 0.010 sulfur(S), 0.72 nickel(Ni), 0.004 copper(Cu), 0.084 chromium(Cr), 0.013 molybdenum(Mo), 0.004 vanadium(V), 0.011 boron(B). The chemical composition was determined by the following precision instruments: atomic absorption spectroscopy, optical emission spectroscopy, X-ray fluorescence spectrometry, and inductively coupled plasma spectrometry.

The testing specimens were machined from the plate samples of the 45 steel. The specimens were then heat treated to make them suitable for microscopic and tensile testing. The heat treatment was carried out in a laboratory furnace at temperatures ranging from 800°C to 1040°C. The specimens were left in the furnace for 4 hours and then cooled to room temperature.

After heat treatment, the specimens were examined using an optical microscope. The microstructural characterization was done using the electron backscattered diffraction (EBSD) technique. This technique provides a detailed analysis of the grain structure of the material. The microstructure of the specimens was also studied using scanning electron microscopy (SEM), X-ray diffraction, and energy dispersive X-ray spectroscopy (EDS). The non-metallic inclusions, such as oxides and sulphides, were analyzed using light optical microscopy.

In order to study the mechanical properties of the specimens, tensile tests were performed. The specimens for the tensile tests were machined from the plate samples in accordance with the ASTM standards. The testing was carried out using a universal testing machine at room temperature. The tests were done according to the ASTM E8M standard.

RESULTS AND DISCUSSION

The grain sizes of the specimens were studied using the EBSD technique and it was found that most of the grains were within the size range of around 6 to 10 micrometers. The grain sizes and the grain type were found to vary in different locations of the specimens. The major portion of the grains were of the austenite type, while some were of the ferrite type. The presence of a few grains of the ferrite type was found to be more in the region near to the specimen surface.

The non-metallic inclusions such as sulfur and oxygen-containing compounds, oxides, and sulphides were studied using light optical microscopy. It was found that the concentration of non-metallic inclusions was fairly low but the size distribution was fairly large and varied from 1 to 50 micrometers. Most of the inclusions were found to be on the side of the specimen facing the furnace.

The mechanical properties of the specimens were studied using the tensile tests. It was found that the specimens had excellent tensile strength of approximately 1058 MPa and high elongation of approximately 19%. It was observed that most of the specimens exhibited uniformity along their length. Only a few exhibited a slight decrease in strength near their ends.

CONCLUSION

The results of the experiments show that the 45 steel has a low magnification grain structure and is largely free of non-metallic inclusions. The mechanical properties of the specimens showed an excellent tensile strength of approximately 1058 MPa and high elongation of approximately 19%. The microstructural characterization by the EBSD technique revealed that most of the grains were of the austenite type and within the size range of 6 to 10 micrometers. The non-metallic inclusions, such as oxides and sulphides, were found to be fairly low in concentration but varied in size from 1 to 50 micrometers. The presence of non-metallic inclusions was found to be more in the region near to the specimen surface.

45 steel (normalizing) is a low binarized steel with non metallic inclusions. It has high strength, good toughness, easy to forge and good cutting performance.

This kind of steel is mainly used for the manufacture of axle, crankshaft, connecting rod and other important parts of automobile and tractor engines. It is often used for the production of turbines, blades, gears and other machine parts in machinery industry. It can also be used for the manufacture of hot forging parts.

As for the chemical composition, the carbon content of 45 steel is 0.42-0.47%, the manganese content is 0.60-0.9%, the silicon content is 0.17-0.37%, the sulfur content is 0.035%, the phosphorus content is 0.035%, the chromium content is 0.90-1.20%, the molybdenum content is 0.20-0.20%, the vanadium content is 0.10-0.20%.

In terms of microstructure, 45 steel is composed of martensite and bainite, with bainite as the main structure in normalizing heat treatment, and martensite as the main structure after tempering heat treatment.

In terms of mechanical properties, 45 steel has a tensile strength of not less than 540Mpa, and the yield strength is not lower than 285Mpa (Rp0.2).The elongation is above 16% and the impact energy is greater than 47J.

Moreover, it is worth noting that the steel should be subjected to non destructive testing before leaving the factory, mainly to detect the non metallic inclusions in the steel, so as to ensure the quality of the steel and meet the requirements of use.