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Low temperature carburizing of 9Cr18 stainless steel surface

1 Introduction

9Cr18 martensitic stainless steel is the most widely used stainless steel in the bearing field. It has good corrosion resistance and its quenching hardness (58-60HRC) is lower than that of conventional bearing steel (60-64HRC). Bearing failure mainly occurs in the working surface and sub-surface layer. It is an effective way to improve the service life of the bearing by using suitable technology to strengthen the surface of 9Cr18 stainless steel.

Studies in recent years have found that when stainless steel is subjected to vacuum nitriding or carburizing treatment at temperatures below 500 °C, a solid solution and diffusion layer containing nitrogen or carbon can be obtained. Nitrogen or carbon atoms play a role in solid solution strengthening, not only The surface hardness of stainless steel is improved, and the corrosion resistance is enhanced.

The low temperature carburizing process flow is shown in Figure 1, and the process parameters are shown in the table. The activation process is to increase the surface activity of the material and eliminate the oxide film. The chlorine-containing mixed gas is selected as the pretreatment atmosphere for the low-temperature carburizing. , In this way, in the pretreatment process, the oxide film is eliminated many times, which can prevent the regeneration of the oxide film in the early stage of low temperature carburization, and the effect is better than one treatment.

2.1 Micromorphology of carburized layer

Figure 2 shows the SEM microscopic morphology of the section of 9Cr18 martensitic stainless steel after low temperature carburizing treatment. It can be seen from the figure that an obvious white bright layer is produced on the surface of the sample, and the structure of the infiltrated layer is dense. After being corroded by picric acid alcohol solution, an obvious infiltration layer can be seen, and there are no holes, cracks and other defects in the infiltrated layer. The thickness is 10um~15um.

2.2 Hardness analysis of seepage layer

The Vickers hardness distribution of G95Cr18 martensitic stainless steel from surface to matrix after low temperature carburizing treatment is shown in Figure 3. It can be seen from the figure that the surface hardness of the sample is 1700HV, the hardness at 5um from the surface is 1610HV, and the hardness at 10um from the surface is 1420HV. It can be seen that the hardness of the infiltrated layer is distributed in a gradient, from the most surface → the transition zone → the hardness of the core matrix. also has a gradient descent. The surface hardness is much higher than that of the matrix, which is due to the formation of high-hard carbides on the surface of G95Cr18 stainless steel during the low-temperature carburizing process.

2.3 Distribution of carbon elements on the surface of the infiltrated layer

After the low-temperature carburizing treatment, it is known from the distribution of carbon elements from the surface of the sample to the core that the proportion of carbon elements gradually decreases. Therefore, after the low-temperature carburizing treatment, the surface is enriched with carbon elements, and a large amount of carbon accumulates in the matrix. The surface, combined with chromium and iron to form carbides, improve the surface strength.

2.4 Changes in matrix structure and hardness before and after low temperature carburizing treatment

The metallographic diagram of the matrix structure before and after the low temperature carburizing treatment shows that due to the low treatment temperature, the matrix structure of 9Cr18 martensitic stainless steel does not change significantly under a 500-fold optical microscope. The hardness test value of the matrix structure before and after treatment shows that after the low temperature carburizing treatment, the hardness of the matrix structure of the sample decreases slightly (60.3HRC to 57.1HRC). After vacuum tempering, the precipitation of carbides in martensite increases, and the lattice distortion decreases. Therefore, the hardness of the matrix decreases slightly.

2.5 Influence of low temperature carburizing treatment on straightness and surface roughness

The change of the straightness of the sample before and after the low temperature carburizing treatment shows that after the low temperature carburizing treatment, the straightness of the sample increases slightly (0.0018mm~0.0025mm), mainly because the heat preservation at 440 ℃ for 30h is equivalent to high temperature tempering , the matrix structure undergoes phase transformation and stress release. The change of the surface roughness of the sample before and after the low-temperature carburizing treatment shows that after the low-temperature carburizing treatment, the surface roughness Ra of the sample increases from 0.18 to 0.30, which is because the carbon reacts with the test surface during the low-temperature carburizing process. , a new phase of carbide is formed, which affects the surface roughness.

3.Conclusion

(1) The surface hardness of G95Cr18 martensitic stainless steel is significantly improved after low temperature carburizing treatment under conventional quenching and tempering conditions.

(2) The low temperature carburizing treatment had little effect on the matrix structure, and the structure and hardness did not change significantly after treatment.

(3) The bonding force between the infiltrating layer and the substrate is good, no obvious peeling occurs, and the comprehensive performance is good.

(4) Low temperature carburizing treatment has little effect on the straightness and surface roughness of the sample.

Selection of carburizing equipment: The RVC series vacuum carburizing furnace produced by SIMUWU is a high-quality product for the vacuum carburizing process of tools and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum carburizing process.