Low Microstructure and Nonmetallic Inclusions of 35CrMo Steel

Low alloy steels, such as the 35CrMo, are sensitive to heat treatments and weldability. Therefore, it is important to pay attention to their microstructure and non-metallic inclusions.

Microstructures

In general, low alloy steels are especially sensitive to heat treatments due to their relatively low amounts of alloying elements. Quenching and tempering is an effective way to improve the mechanical properties of these materials. In quenching operations, large grains are created that require tempering afterward to reduce residual stresses and adjust the microstructure to a suitable one. In fact, it is essential to combine suitable steel grades with an appropriate heat treatment cycle for an optimal mechanical performance of these steels.

The low alloy steel 35CrMo is a common steel used in engineering and automotive components. This alloy has a Martensitic microstructure that is formed by austenitizing and quenching during heat treatments. The Martensite has a lath, banded, or globular-like morphology that offers a suitable combination of mechanical properties. This combination of mechanical properties is related to the alignment and size of these microstructural features, which depend upon the heat treatment parameters.

Non-Metallic Inclusions

Non-metallic inclusions can also play a crucial role in the performance of materials like the 35CrMo. These inclusions are formed when non-metallic elements, such as oxides and sulfides, are present in the steel during the manufacturing process. During the melting process, many elements can dissolve in the steel and might act as potential nucleation sites for further inclusions. Moreover, they can also act as creep sites, benefiting particular applications.

Non-metallic inclusions have destructing effects specialized in some cases. When they reach a critical size they become harmful, particularly in stress-bearing areas. Moreover, they will also interact with grain boundaries and microscopy defects, leading to a decrease in uniformity and strength. Additionally, coarse nickel sulfides and manganese sulfides present in non-metallic Inclusions further reduce ductility and reduce toughness.

Conclusion

In conclusion, the low alloy steel 35CrMo is an important and popular part of engineering and automotive components. Its combination of mechanical properties is obtained from a Martensitic microstructure created by suitable heat treatments. However, it is important to take into consideration the effect of non-metallic inclusions. These non-metallic inclusions can degrade its mechanical properties when they reach a critical size. It is thus essential to control their presence in this steel to ensure quality performance.

35CrMo steel is a Chinese alloy steel grade for gas cylinders and household gas containers. It is one of the most commonly used steel grades in the world.

At the lower magnification of 35CrMo steel, pearlite microstructure is observed. Pearlite is a two-phase mixture of ferrite and cementite, where ferrite is the main component. Ferrite has ferromagnetic properties, and its structures are soft magnetic materials with high ductility, while cementite has a nonmagnetic property and a hard structure.

The non-metallic inclusions in 35CrMo steel may include oxides, sulfides, phosphates, silicates and carbides. They are mainly spherical or irregularly dispersed in the ferrite matrix. These inclusions act as barriers or obstacles to deformation, eventually leading to the development of strain-induced dislocation networks, which can eventually lead to the formation of internal cracks, reduce plasticity and yield strength, and even cause rupture failure.

In addition, non-metallic inclusions also have an effect on hydrogen embrittlement, fatigue crack initiation and growing mechanism. Abrasive particles will increase the abrasive wear of the material, reduce the hardness and adhesion of the surface layer, increase the coefficient of friction and reduce the bearing capacity.

Therefore, the non-metallic inclusions in 35CrMo steel need to be dexterously handled and monitored. Control mainly includes heat treatment process, proper electrochemical control, steel selection standard and non-metallic inclusion control measures, etc., to ensure the optimal combination performance of 35CrMo steel and its components.