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Study on Metallographic Structure and Mechanism of Vacuum Nitriding

Summary

Based on the Fe-N phase diagram, nitride crystal structure and phase transformation equilibrium state, various interstitial compounds formed in the vacuum nitriding process, the transformation structure of the γ phase and the structure of the precipitated structure in the α-Fe phase were studied and analyzed. characteristics, and analyzed the growth mechanism and influencing factors of various phases.

Foreword

The process of steel parts being held in a medium that can release active nitrogen atoms at a certain temperature for a certain period of time, and in a vacuum environment, the process of infiltrating nitrogen atoms on the surface is called vacuum nitriding or nitriding of steel. Since it was applied to steel in 1923, the vacuum nitriding process is one of the widely used technologies for surface strengthening treatment of parts. Compared with the commonly used carburizing process, the heat treatment temperature of this process is low and the distortion is small, which can significantly improve the surface hardness, wear resistance, corrosion resistance, fatigue resistance and seizure resistance of parts, and effectively prolong the service life of parts . Due to these advantages, vacuum nitriding treatment is widely used in the production of mechanical parts such as automobiles, ships, and aviation.

With the development of industrial technology, the requirements for surface strengthening of workpieces are getting higher and higher, and there are more and more types of vacuum nitriding processes. However, because the steel contains aluminum elements in different degrees, the non-metallic inclusions formed are overheating sensitive, which increases the brittleness of the surface of the nitrided layer, and is prone to peeling or cracking. At the same time, in order to control the thickness of the nitrided layer and the thickness of the nitrided layer It is of great significance to study the growth mechanism of the metallographic structure of the surface nitride layer to realize the regulation of different process parameters and to make up for the lack of a large amount of data on the growth mechanism of the abnormal nitride structure in the industry.

Mechanism of Nitriding Phase Transition

Usually, gas, molten salt or particles are used as the nitriding agent. The surface nitriding or carbonitriding of the workpiece is generally carried out at 480-590 °C, and the hardness reaches 68-72 HRC. With the development of technology, a low-temperature nitriding process has been developed. At the same time, in order to increase the thickness of the nitriding layer and improve the nitriding efficiency, low-temperature nitriding, cyclic nitriding, and active screen ion low-temperature nitriding have been studied. Eutectoid phase transformation occurs at 592 °C, γ-Fe(N)→α-Fe(N)+γ′-Fe(N) has austenite; eutectoid phase transformation occurs at 650 °C, ε→α-Fe (N)+γ′-Fe4N will form different phase structures under different temperature, different nitrogen concentration and other parameters during the nitriding process.

The vacuum nitriding process and phase transition sequence are briefly summarized. When the nitriding layer is formed, the nitrogen-containing ferrite (α-Fe(N)) does not reach saturation, and thickens with the prolongation of nitriding time; ] atoms infiltrate and diffuse, N atoms reach saturation and phase transformation occurs, forming nitrogen-containing austenite (γ-Fe(N)), and the phase transformation of austenite, α-Fe(N)→γ-Fe(N )→γ′-Fe4N; when the γ′ phase reaches the supersaturated state, the ε phase (Fe2-3N) with higher N content is formed. The final permeation layer is mainly composed of compounds composed of γ′ phase and ε phase and a diffusion layer composed of nitrogen-containing ferrite. In the permeation layer during austenite vacuum nitriding, there will also be a composition of γ-Fe(N) The transition layer between the compound layer and the diffusion layer. The mechanisms of carbonitriding and nitriding are basically similar, but due to the addition of carbon, the phases formed on the surface are more complex. At present, there is no clear ferrite carbonitriding for the Fe-N-C ternary phase diagram. and the temperature division point of austenitic carbonitriding.

Characteristic Analysis of Nitriding Structure

Different phases in the vacuum nitriding process are composed of different crystal structures as the matrix, and different crystal orientations will reflect different mechanical properties. Therefore, parts vacuum nitrided by different process parameters will show various advantages and disadvantages. Regulating the phase structure of the surface layer of parts is an important means to improve the surface performance of parts. At the same time, studying the structure and morphology of various phases is also an important way to determine the structure and innovate the process.

In conclusion

The metallographic structure characteristics and growth mechanism of various phases in the vacuum nitriding process are analyzed through a large amount of data research. These data can be used as the basis for judging the phase structure formed in the vacuum nitriding process, and its growth mechanism is studied in depth.

a.Vacuum nitriding metallographic structure, mainly formed into three types, interstitial compound (ε phase, γ′ phase, ζ phase), transformation structure of γ phase, precipitation structure in α-Fe phase (acicular, granular, Veined or reticulated, etc.). Taking the γ′ phase structure as the main body, analyze the diversity and complexity of the nitride structure; the nitrogen concentration of the ζ phase has a small change, which is not easy to determine in the test.

b.The transformation structure of the γ phase is mostly an abnormal structure formed in the nitride, which is nitrogen-containing troostite, nitrogen-containing martensite plus retained austenite, and also has a shape similar to bainite, mostly formed by the nitrogen of the nitriding process Due to insufficient potential, the surface compounds cannot be precipitated stably and uniformly; the discovery of this abnormal structure will also provide a test basis for the failure of surface nitrided parts.

c.The precipitation structure in the α-Fe phase is the nitride in the diffusion layer, which is produced in the cooling process and has a relatively complex shape. Some precipitates, such as veins and laths, are arranged in an orderly manner along the crystal plane. It reflects the preferred orientation of the organization and can provide parameters for the design of texture control schemes.