Nitrogen diffusion along the layer boundaries after nitriding of multilayer materials

Introduction

Currently various mechanical wood treatments by milling are widely used. The main unit of a router is the cutter. There are numerous cutters of different designs and geometries, but, in each case, they should be characterized by high strength and wear resistance, which can be achieved by chemical heat treatment [1]. The multilayer materials used after nitriding for manufacturing cutters can enable to enhance tool durability, preserve tool geometry, increase tool life, and improve processing performance due to a significantly hardened layer. Fig. 1 shows an example of such a cutter.

Additionally, multilayer materials can be used to manu­facture gear wheels that also operate under wear (Fig. 2).

This structure can be obtained if steels with different crystalline structure are included in the initial composition [2 – 4]. The developed technological route (Fig. 3) enables to produce sheet billets, 2 to 10 mm thick [5].

The material microstructure has a multilayer laminar structure with the layers, 100 to 0.8 µm thick. At the same time, the layers are characterized by crystallographic disorientation from 15 to 20°, which corresponds to the large-angle grain boundaries in the initial steels (Fig. 4) [6; 7].

The works [8; 9] show that with an appropriate choice of steels included in the initial composite billet, the multilayer structure is preserved up to the temperature of 1000 °C, which corresponds to the temperature of hot pack rolling. In this case, the cross-section of the sheet billets has a structural orientation ready for chemical heat treatment (Fig. 5).

Preliminary evaluation of nitriding the multilayer composition that includes AISI304 and AISI430 steels showed that the nitriding depth in the multilayer material is greater than its depth in AISI304 steel which is hard to nitride. The reason behind the increased thickness of the nitrided layer in multilayer materials may be the accelerated diffusion of nitrogen along the layer boundaries with subsequent saturation of layer volumes from their boundaries as from the diffusing element sources [10].

To thoroughly investigate the resulting effect, we used model compositions of multilayer materials containing steels of different compositions (grades). The following study objects were selected for nitriding: composites consisting of 1008 and AISI304 steels, as well as W108 and AISI304 steels. After hot pack rolling, we obtained 10-mm thick samples with 100 layers, the single layer being 100 μm thick.

The samples of these compositions were nitrided, the nitriding surfaces being in all cases perpendicular to the rolling directions used to produce the multilayer samples. Nitriding was performed in the gas atmosphere containing 20 – 40 % ammonia dissociation products. We used two nitriding conditions with the following temperature and time parameters: 540 °C for 45 h and 580 °C for 25 h. To study the structure of the resulting nitrided layers and to determine their geometric characteristics, we prepared sections with the surfaces perpendicular to the rolling direction and parallel to the direction of nitrogen diffusion penetration that occurred in the course of nitriding (Fig. 6).

Fig. 7 shows the general view of diffusion profiles formed during nitriding for AISI304 and 1008 (a), AISI304 and W108 (b) compositions obtained after nitriding at 540 °C for 45 h.

The micrographs show the concentration profile of diffusing nitrogen in AISI304 steel with a considerable nitrogen penetration depth (and a long nitrogen diffusion path, respectively) along the layer boundaries of the multilayer material. Inside the AISI304 component layer, for both cases, the nitrogen penetration distance shrinks with increasing distance from the interlayer boundary. The smallest depth of nitrogen penetration is about 100 μm from the outer surface of the samples and midway between the layers of neighboring components (midway between the layers of 1008 steel (a) and W108 steel (b)) inside the layer of AISI304 steel. This indicates that the source of nitrogen penetration into the volume of AISI304 steel during nitriding is the interface between the layers of the material. The diffusion along these boundaries occurs faster than that through the layer of AISI304 steel from the outer surface.

Nitrogen penetration into the layers of 1008 and W108 steels at this depth from the nitriding surface was not detected. This is proved by the results of electron microscopy and X-ray spectral analysis. There is practically no volume diffusion of nitrogen in 1008 and W108 steels due to low solubility of nitrogen in α-iron (maximum 0.1 wt. %), which is the main pathway of nitrogen diffusion in the ferrite phase [11]. The second reason inhibiting nitrogen movement in 1008 steel is the surface nitride formation consisting of nitrides (Fe₂N), which is also proved by the results of electron microscopy (Fig. 8).

Thus, it can be concluded that the nitriding process in multilayer materials of this type occurs due to the fast nitrogen diffusion along the layer boundaries [12; 13].

It can be assumed that simultaneously nitrogen atoms migrate inside the AISI304 steel perpendicular to the interlayer boundary, which can be considered as an extended source of nitrogen diffusion. This steel is an austenitic grade steel and the solubility of nitrogen in austenite (γ-iron) is about 2.8 wt. %, therefore, diffusion saturation of AISI304 layers with nitrogen is possible. According to literature data [14], the diffusion coefficient of nitrogen in γ-iron at temperatures ranging from 500 to 600 °C is determined by the following equation

We used the Fisher model for calculating diffusion along grain boundaries in metallic samples to determine the diffusion permeability of layer boundaries of the multilayer material [15 – 18]. According to this model, the product of the diffusion coefficient D_b along the grain boundary (layer boundaries in this case) and the boundary thickness δ, that is value of sδ D_b, can be calculated by the formula [19]

where θ is the angle at the top of the component concentration profile (Fig. 7), which passes into the phase (layer) volume from the grain (layer) boundary; s is the ratio of boundary enrichment with atoms of the diffusing component, which can be estimated based on the dependence proposed in [20]:

where х₀ is the volumetric concentration of the impurity in mole fractions.

If we assume that the enrichment of layer boundaries is mainly determined by the ferrite phase, since in accordance with the phase diagram it has the smallest Fe – N concentration of nitrogen, according to formula (3), the value of enrichment of layer boundaries shall equal s = 5·10³.

It should be noted that the formula (2) is suitable for describing diffusion along the layer boundary when the diffusive removal of the component (nitrogen in this case) from the boundary into the bulk is one-sided – volume diffusion occurs only towards the layers of AISI304 steel. The experimental data shows that there is no diffusion of nitrogen toward the layers of 1008 steel (Fig. 10, a) or W108 steel (Fig. 10, b).

The values of angles θ required to calculate the diffusion coefficient D_b along the layer boundaries were determined by analyzing micrographs of cross-sections of multilayer samples of both compositions after nitriding using two processing modes.

For the 1008 + AISI304 composition, Fig. 8, a shows the method for measuring these angles when nitriding is performed at 540 °C for 45 h and Fig. 10, b – when the operating parameters are 580 °C and 25 h. For the W108 + AISI304 composition, the similar procedure is shown in Fig. 11.

The Table presents the diffusion coefficients of nitrogen atoms along the layer boundaries for 1008 + AISI304 and W108 + AISI304 multilayer compositions obtained by analyzing experimental data as nitriding of these materials samples was investigated. The calculation assumes that the layer boundaries thickness δ is 10\(^–\)⁹ m.

Conclusions

The experimental study of nitriding the samples of multilayer metallic materials with alternating layers of two different steel grades revealed that the main mechanism behind the process is mass transfer (diffusion) of nitrogen atoms along the boundaries of the material layers.

The analysis of experimental data obtained while investigating cross sections of surface layers of two compositions of multilayer materials after nitriding using two modes enabled us to obtain the estimated values of nitrogen diffusion coefficients D_b along layer boundaries. The D_b values were 10⁴ times higher than the volume diffusion coefficient of nitrogen in AISI304 steel under the same conditions.

The study showed that the nitriding depth in both multilayer compositions increased due to fast diffusion penetration of nitrogen atoms along the layer boundaries of multilayer materials.