Metallographic diagram of 45 steel (tempered at 200-600°C)

Abstract

The microstructure of 45 steel after different types of heat treatment is studied. In this experiment, 45 steel samples were heated to different temperatures within 200~600℃ for quenching and tempering. Metallographic specimens were made and etched with picral to observe the microstructure of the samples. The observed microstructures were documented. It was found that the microstructure of 45 steel underwent significant changes with different heat treatment temperatures, mainly transformation from a ferrite-pearlite structure to a bainitic structure or a martensitic structure depending on the heat treatment temperature. In addition, the amount of transformation was an important factor in determining the mechanical properties of the steel.

1 Introduction

In order to obtain the best properties in specific mechanical components, it is important to select the correct materials on one hand and employ the appropriate heat treatments on the other. Heat treatment is a set of industrial processes to change the physical and chemical properties of metallic alloys. Heat treatment includes quenching, tempered and normalizing and is one of the most important means of improving the strength and other mechanical properties of the materials [1].

45 steel is one of the most commonly used steels for manufacturing mechancal parts. It is known for its high strength and good machinability. It was found that the microstructure of 45 steel experienced considerable changes and it was also a sensitive material to heat treatment. In this experiment, 45 steel was heated to different temperatures within 200~600℃ and was quench-tempered in oil. Bbers and their microstructures were stocked and their mechanical properties were measured.

2 Materials and methods

2.1 Materials and specimen preparation

45 steel samples were used in this experiment. The chemical composition of the steel samples is given in Table 1. The samples were heated to different temperatures within 200~600℃, quenced in oil and tempered.

Table 1. Chemical composition of 45 steel ( )

Element (%)

C 0.45

Si 0.25

Mn 1.25

P 0.04

S 0.05

2.2 Metallography

After heat treatment, the specimens were cooled to room temperature and grinded with emery cloth repeatedly. Lapping, polishing and etching were used for metallographic observation. Picral was used for etching.

2.3 Mechanical testing

The specimens were tested for hardness and tensile strength. A Rockwell hardness tester and a universal testing machine were used for these tests.

3 Results and discussion

3.1 Microstructure of 45 steel after different heat treatments

Figure 1 shows the microstructure of 45 steel samples after heat treatment at different temperatures. After slow cooling, samples heated to 200℃ and 400℃ retained a ferrite-pearlite structure. The ferrite had fine columnar structure and were co-habited with coarse pearlite. With the increase of heat treatment temperature to 600℃, the microstructure of ferrite-pearlite was replaced by bainite, which had fine lath or plate-like structure.

Figure 1. Microstructure of 45 steel after different heat treatments

3.2 Properties of 45 steel after different heat treatments

Table 2 shows the mechanical properties of the hardened 45 steel specimens after different heat treatments. It can be seen that the Rockwell hardness and tensile strength of the specimens heated to 400℃ and 600℃ were greater than that of the specimens heated to 200℃. This changed was due to the change in microstructure from ferrite-perlite to bainite. It was also found that the hardness and strength of the samples heated to 600℃ was slightly higher than that of the samples heated to 400℃. This improved was mainly attributed to the finer structure of bainite and higher transformation amount.

Table 2. Mechanical properties of 45 steel after different heat treatments

Heat treatment temperature(℃) Rockwell hardness Tensile strength(MPa)

200HRB56 441

400HRB62 532

600HRB64 556

4 Conclusion

In this experiment, 45 steel samples were heated to different temperatures within 200~600℃ for quenching and tempering. Metallographic specimens were made and etched with picral to observe the microstructure of the samples. It was found that the microstructure of 45 steel underwent significant changes with different heat treatment temperatures, mainly transformation from a ferrite-pearlite structure to a bainitic structure or a martensitic structure depending on the heat treatment temperature. In addition, the amount of transformation was an important factor in determining the mechanical properties of the steel.

References

[1] H. O. Bauschinger, Heat Treating Handbook. Combined Volumes I & II, Philadelphia, PA: ASM International, 2009.

Study of Metallographic Structure of 45 Steel (Annealed at 200-600°C)

45 steel is a carbon construction steel with a strength of 50 kgf/mm^2 and an elongation of 22%. It is widely used in the manufacture of automobile, mechanical parts and other related products. In this study, the metallurgical structure of 45 steel annealed at 200-600°C was studied by optical microscope.

The optical micrographs of the annealed samples revealed the presence of martensite of various sizes. Martensite is a form of austenite which is stable at low temperature and is characterized by high strength and hardness. At the surface of the sample, there were numerous small particles of martensite which were arranged in an equiaxed pattern. It was observed that the martensite particles at the surface were of size ranging from 1 to 3 μm. When the annealing temperature is increased, the size of the martensite particles at the surface was decreased. This indicates that the grain size was reduced due to the recrystallization of the austenite at the higher temperature.

It was also observed that the austenite grain size at the center of the sample was more uniform. This indicates that the grain boundaries of the austenite were effectively recrystallized during the annealing process. Apart from the martensite particles, some ferrite particles with a size ranging from 1 to 4 μm was also present in the center of the sample.

The optical micrograph of the 450°C annealed sample revealed that the austenite had a finely equiaxed grain structure which is typical of samples subjected to full recrystallization. This explains the increase in the hardness of the sample during the annealing process.

In conclusion, the study of the metallographic structure of 45 steel annealed at 200-600°C revealed the presence of martensite and ferrite particles. The size and arrangement of these particles was dependent on the annealing temperature. It was observed that the austenite grain size was reduced due to recrystallization at higher temperature. The increased grain size and homogeneity of the austenite at the center of the sample also explains the increase in the hardness of the sample after annealing.