Effects of Carbon Content Change on Growth of Compound Layer in Surface of Nitrided Steel

When steel is nitrided, a compound layer mainly composed of iron nitrides, ε-Fe2~3N and the γ’-Fe4N phase, is formed on the steel surface. It is an extremely important industrial issue to clarify factors governing the formation of the compound layer during nitriding and to establish unified views on the mechanism of compound layer formation. Therefore, in order to clarify the effect of change in carbon concentration on the growth of the ε phase and the γ’ phase in the compound layer on nitrided steel, we evaluated the change over time in the concentration of the alloy elements in the surface layer, and the phases of the compound layer on nitrided steels containing various amount of carbon in the matrix. The results were that the change over time in the carbon concentration in the compound layer was mainly responsible for the change over time in the phases of the compound layer. Furthermore, it was discovered that the change over time in the carbon concentration distribution occurred because both increasing of carbon from the matrix to the compound layer, and decreasing of carbon from the surface of compound layer to the atmosphere. That caused the gradient change of chemical potential of carbon in the through-thickness direction of compound layer, and the phases of the compound layer were changed with the treatment time.

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Modeling the Precipitation Kinetics of Cementite in Bainite in 0.17% Carbon Steel

Materials Science Forum ◽

10.4028/www.scientific.net/msf.898.832 ◽

2017 ◽

Vol 898 ◽

pp. 832-839 ◽

Cited By ~ 1

Author(s):

Yong Yang ◽

Bin Wang ◽

Zhao Dong Wang ◽

Yan Mei Li ◽

Guo Dong Wang ◽

...

Keyword(s):

Carbon Steel ◽

Carbon Concentration ◽

Unit Volume ◽

Treatment Time ◽

Growth Period ◽

Carbon Diffusion ◽

Isothermal Treatment ◽

Precipitation Process ◽

Ultra Fast Cooling ◽

The Matrix

Nanoscale cementite can be obtained in 0.17% carbon steel during isothermal treatment at 500oC after ultra fast cooling (UFC) and thermo-mechanical treatment. The precipitation strengthening contribution to yield strength was more than 250 MPa, when the heat treatment time was less than 20 min. The carbon diffusion is impacted by Mn and Si, which are redistributed during the precipitation process. All the effects induced by substitutional elements can be manifested through the restricted carbon diffusion, which is equal to the carbon diffusion multiplied by adjustable parameters. Based on this, a kinetic model has been adapted to simulate the precipitation behaviors of cementite involving the evolution of the number density per unit volume, radius of cementite over time, and the evolution of carbon concentration in matrix. An excellent agreement in mean radius of particles between the predictions of the model and experimental observations was obtained. It was found that the nucleation period of cementite was very short and did not exceed 0.2 s, and there was an overlap between the nucleation period and the growth period, and the coarsening period began at about 1s. In the growth stage, the carbon concentration in the matrix dropped rapidly and the mean radius of particles increased quickly. In the coarsening stage, the carbon concentration remained unchanged and the number of particles per unit volume fell sharply.

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