Gas carburizing technology

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Figure No.: Figure 1

Material: 38CrMoAlA

Process conditions: gas nitriding after conditioning (520°C for 20h, 560°C for 34h, slow cooling)

Etching method: 4% nitric acid alcohol solution etching

Organization Description:

The distribution of tissue from the surface nitrided layer to the heart. The outermost layer is the white bright layer ε phase (Fe2-3N), followed by white vein-like alloy nitrides, and the subsurface is the diffusion layer (until the dark area in the figure). The matrix is nitrogen-containing sorbite, and thicker white vein-like nitrides are distributed at 0.35mm. The light-colored area on the right side of the figure is the core structure, sorbite and a small amount of white ferrite distributed along the grain. The penetration layer depth is 0.65mm.

Nitriding is a chemical heat treatment process in which atomic nitrogen (active atoms or high-energy ions) penetrates into the metal surface. The active nitrogen atoms (ions) are absorbed by the hot metal on the surface and diffused inward to form a nitride layer. Specific processes include gas nitriding (hard nitriding), gas nitrocarburizing (soft oxidation), ion nitriding, etc.

The white bright compound is mainly composed of ε phase (Fe_N) and γ’ (Fe N). Single-phase compounds or multi-phase compounds can be obtained under different process conditions. When the ζ phase (FeN) appears, it is more brittle and should be removed during use. The vein nitrides appearing at 0.35mm in the figure are caused by overheating during nitriding.

Since the structure characteristics of nitrided parts are sensitive to the performance, it is necessary to evaluate and control the nitrided layer depth, brittleness, porosity and diffusion layer.

Figure No.: Figure 2, Figure 3
Material: 38CrMoAl
Process conditions: After the material is quenched and tempered, it is then treated with gas nitriding
Etching method: 4% nitric acid alcohol solution etching
Organization Description:
Figure 2, the surface is the white bright layer ε phase, the inward is the diffusion layer, the total depth of the nitride layer is 0.50~0.55mm, the matrix structure is pearlite and ferrite, and the ferrite is distributed in a network shape.
Figure 3 shows the high-magnification morphology of Figure 2. The white bright layer on the surface has peeling phenomenon (with depressions on the surface), and there are needle-like and vein-like nitride structures in the surface ferrite.
According to the micro-Vickers hardness measurement, the hardness of the nitrided layer near the surface is 850~710HV0.3, and the hardness of the center part is 237~263HV0.3.
Structural steel should be quenched and tempered before nitriding, but this sample did not achieve the quenching and tempering effect, and a large amount of white ferrite appeared. Since the increase of carbon content in steel will significantly reduce the diffusion rate of nitrogen in steel, the nitrogen concentration in the ferrite area in Figure 2 is obviously high, forming needle-like and vein-like nitrides, which increases brittleness and inhomogeneity.
With the development of the infiltration process, many steel types can now be nitrided and obtain corresponding effects.
For parts requiring corrosion resistance, any steel type can be used (low carbon steel is the best);
For parts that require improved fatigue strength, alloy steels containing chromium, molybdenum, vanadium and other elements are often used;
It is required that the surface has high hardness and fatigue strength, and the core has sufficient strength and toughness. Steel materials containing more aluminum and other alloying elements are often used, such as 38CMoAl, which is a commonly used nitrided steel. Aluminum is used to improve the nitrided layer. The main element of hardness and stable; chromium and molybdenum can improve the hardness of the nitrided layer, matrix hardenability and strength, and molybdenum can also prevent temper brittleness caused by nitriding.

Figure No.: Figure 4, Figure 5
Material: 38CrMoAlA
Process conditions: gas nitriding
Etching method:
Figure 4, 4% nitric acid alcohol solution etching;
Figure 5, copper sulfate (2g), hydrochloric acid (10ml), alcohol (100ml) solution etching
Organization Description:
Fig. 4, the white bright layer on the most surface is: phase compound, with vein-like diffusion inward, and the dark area on the subsurface is containing
Nitrogenous sorbite, the core is sorbite, the interface between the diffusion layer and the core is still clear, and the total nitrided layer is about 0.50mm.
Figure 5, the appearance of the same batch of samples as in Figure 4 after being etched by another reagent, the diffusion layer near the substrate junction is white, making the junction line very clear, and the total depth of the nitriding layer is measured to be 0.50mm. However, the effects of different steel types are different under the corrosion of this reagent. For some steel types, selenic acid is suitable, and sometimes the vacuum quenching method can be used (see Figure 6), but the hardness method should be used when judging the penetration depth.

Figure No.: Figure 6
Material: 38CrMoAlA
Process conditions: slow cooling after gas nitriding, reheating at 780°C, heat preservation at 1.5min/mm, and water quenching.
Etching method: 4% nitric acid alcohol solution etching
Organization Description:
Microstructure morphology of samples measured by vacuum quenching method for the depth of nitriding layer.
The outermost layer is a compound layer, and a white nitrogen-containing martensite band appears at the junction of the nitriding diffusion layer and the matrix structure, and the hardness is relatively high, so the prismatic Vickers hardness indentation is very small.
This method of measuring the depth of the nitriding layer is very intuitive, but the surface white layer and the morphology of each layer of the structure have changed, which brings difficulties to the evaluation of the structure.
The depth of the nitriding layer is about 0.50mm from the surface to the side near the center of the white nitrogen-containing martensite band. It is equal to the results in Fig. 4 and Fig. 5 .

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