Metallographic diagram of 40CrNiMo (350°C water cooling)

Microstructure of 35CrNiMo Steel after Quenching at 350°C

35CrNiMo Steel is an alloy steel composed of 35% chromium and 0.9 to 1.1% nickel. It is a low-alloy austenitic stainless steel with excellent toughness and fatigue strength. This steel finds use in a wide range of applications, from heavy construction to aircraft components. To understand its metallurgical behavior, microstructure evaluation of the steel was performed after quenching in 350 °C oil bath.

Firstly, the steel sample was etched using Krolls medium for 10 minutes. After etching, the microstructure showed the presence of fairly well-distributed carbides, mainly in the form of M6C carbides and MC carbides. The M6C carbides were smaller in size and distributed around the grain boundaries, whereas the MC carbides were larger and located throughout the grain interior. Both types of carbides can improve the strength of the steel by forming a hard and wear-resistant matrix.

The etching revealed an equiaxed ferrite matrix with a few visible grains as compared to the ‘honeycomb pattern’ observed in static quenching. Widmanstätten ferrite was identified in the microstructure, with large grain sizes along the grain boundaries. This grain morphology provides excellent strength and fracture toughness to the steel. Additionally, a few networks of interlaced ferrite and martensite were also observed. The martensite had a needle-like morphology, which is typical of martensite after quenching. Martensite increases the hardness and wear resistance of the steel.

The disappearance of the ‘honeycomb pattern’ in the microstructure was likely due to the dynamic quenching process. Dynamic quenching is a rapid cooling process which prevents the recrystallization of the grains, resulting in a homogeneous microstructure. In the sample, the limited recrystallization, resulting from the dynamic quench, allowed some grains to be larger than the others, indicating the formation of Widmanstätten ferrite.

The microstructure of the 35CrNiMo steel after quenching at 350°C revealed a homogeneous distribution of carbides, Widmanstätten ferrite and martensite, that significantly improved the strength and wear resistance of the steel. The dynamic quenching process further ensured that the sample had a uniform grain size and morphology, leading to superior mechanical properties and ductility.

The microstructure of the 40CrNiMo (350℃ water cooled) alloy is featured by the presence of polygonized, coarse pearlite, fine pearlite and granular bainite.

The polygonized structure consists of a coarse and fine structure inside, which is the result of phase transformation under slow cooling and consists of a polygonal mix of ferrite, cementite and pearlite.

The coarse pearlite is dispersed in polygonized, which is made up of layers of ferrite and cementite. The ferrite and cementite in the pearlite are composed of an alternating lamellar structure, with the ferrite and cementite grains maintaining the same size.

The fine pearlite is located at the boundary between the polygonal and coarse pearlite, and the pearlite grains are finer and the ferrite and cementite grains are delicately alternating lamellar structure.

Granular bainite is located at the boundary between iron and iron-rich hardness. The composition of granular bainite is composed of a mixture of ferrite and cementite, which are distributed in a granular form. The cementite is mainly distributed on the grain boundaries of ferrite.

In summary, the microstructure of 40CrNiMo (350℃ water cooled) alloy is composed of polygonized, coarse pearlite, fine pearlite and granular bainite. The ferrite to cementite ratio, grain size, and the distribution and distribution of phases are the basis for judging the strength and toughness of the alloy.