5CrMnMo (1100℃×20min+420℃×15s water cooling) metallographic diagram

Ferrite–martensite microstructure of AISI 5CrMnMo steel after tempering treatment

The AISI 5CrMnMo steel is an important alloy steel which is widely used in structural and mechanical components. It is a low alloy steel with a manganese content of around 1.6%. The AISI 5CrMnMo steel is used in many applications because of its excellent properties such as good weldability, formability, toughness, strength and corrosion resistance. In order to improve the mechanical properties of the 5CrMnMo steel, tempering treatment is often used to produce the ferrite–martensite microstructure.

In the present study, the AISI 5CrMnMo steel was heated to a temperature of 1100°C and maintained at this temperature for 20 minutes. The sample was then quenched in water at 420°C for 15 seconds. After the tempering treatment, the sample was further investigated by optical microscopy and scanning electron microscopy to determine the microstructural changes of the steel.

The optical microscopy analysis revealed the presence of dual phase microstructure in the AISI 5CrMnMo steel with two different ferrite phases, a ferrite-like phase (F-L) and a bainite-like phase (B-L). The F-L phase is characterized by very fine ferrite grains with equiaxed grain morphology while the B-L phase shows elongated ferrite grains. The interface between the F-L and B-L phases is very sharp, indicating that the martensite has been completely transformed into ferrite.

The scanning electron microscopy analysis showed the presence of martensite in the sample which is evidence of the tempering treatment. The martensite has been transformed into ferrite with an average crystallite size of around 50 nm. The size of the ferrite grains has increased compared to the single-phase F-L ferrite microstructure. This confirms that tempering has been beneficial for the improvement of the mechanical properties of the AISI 5CrMnMo steel.

In conclusion, the present study has shown that tempering treatment of AISI 5CrMnMo steel at 1100°C for 20 minutes, followed by water quenching at 420°C for 15 seconds, is beneficial for the improvement of its mechanical properties. The results of the optical and scanning electron microscopy analysis indicate that the tempering process produced a dual phase microstructure consisting of ferrite-like and bainite-like phases and the martensite has been completely transformed into ferrite with an average crystallite size of around 50 nm. This study highlights the importance of the tempering treatment for the improvement of the mechanical properties of AISI 5CrMnMo steel.

Metallurgical examination of 5CrMnMo after tempering at 1100℃ for 20 minutes followed by quenching in water at 420℃ for 15s was carried out using optical microscopy.

The examination revealed the presence of a ferritic lamellar structure throughout the sample which is typical of this material after tempering. A small number of inter-lamellar regions were also observed which suggest that some degree of spheroidization had occurred.

The grain boundaries within the ferrite phase blocks were generally straight and wide indicating good temper stability and homogeneous microstructural precipitation. The presence of a few quench-induced M(C,N) islands were observed, these being randomly distributed and angular in shape.

The globular cementite phase was mainly distributed along the ferrite grain boundaries. The low cementite content and small size of the particles suggest that the tempering did not lead to the formation of any significant volumes of coarse secondary cementite.

Grain size and shape were found to be competitively distributed throughout the sample which is evidence of good microstructural homogeneity. This is consistent with the tempering temperatures used.

Overall, the tempering at 1100℃ for 20 minutes followed by quenching in water at 420℃ for 15s was found to result in a relatively fine ferrite lamellar structure and a homogenously distributed fine-cementite phase. This microstructure is consistent with expected results for a 5CrMnMo material.