1 INTRODUCTION

High-temperature nitriding, which determines the formation of diffusion layers in the metal volume without the formation of a compound layer (nitrides) at the surface, is among the challenging methods for increasing the strength properties of metals and alloys [1–5]. In this treatment, complex strengthening is realized, which can be solid-solution strengthening by nitrogen and precipitation strengthening by alloying element particles, which are distributed in a matrix and stabilize a structure, in particular, at high temperatures [2, 3, 6]. A high process temperature favors the diffusive mobility of elements and allows the nitriding time to be decreased. In particular in [7], through nitrogen saturation of 08Kh17T and 15Kh25T steel plates 0.5–1.5 mm thick was achieved in 1–4 h.

Nitriding performed under certain conditions and the subsequent heat treatment ensure a combination of a high strength and adequate plasticity along with a long fatigue life and high corrosion resistance of ferritic steels [7–9]. The efficiency of nitriding of small-thickness articles made from corrosion resistance ferritic (the Fe–Cr system) and austenitic (the Fe–Cr–Ni system) steels additionally alloyed with nitride-forming elements, such as titanium and vanadium, is discussed in [10–15]. In this case, no systematic studies of the effect of refractory nitride-forming alloying elements on the structure and mechanical properties of such steels subjected to high-temperature nitriding were performed.

The aim of the present study is to investigate the effect of high-temperature nitriding on the structure and phase compositions and mechanical properties of an Fe–Cr alloy additionally alloyed with vanadium and zirconium.

2 EXPERIMENTAL

As the material for investigation, we use a model alloy the composition of which includes (wt %) <0.04 C, 0.05 Si, 0.08 Mn, 0.01 P, 0.01 S, 21.8 Cr, 0.3 Ni, 0.31 Cu, 0.01 Ti, 1.6 V, 1.5 Zr, and 75.2 Fe (from here on, Fe–20Cr–1.5V–1.5Zr alloy). The alloy was melted in a vacuum induction furnace using a periclase crucible. The ingot was subjected to hot forging to a height of 5 mm and hot rolling to form a blank 1.5 mm thick. A sheet 0.5 mm thick was prepared by cold rolling of the blank. Samples were cut from the sheet by electric discharge sawing.

The effect of the following treatments was studied: recrystallization annealing at 800°C for 3.5 h, subsequent high-temperature nitriding at 1100°C for 4 h, high-temperature nitriding and tempering either at 700°C for 5 h, or 900°C for 5 h, or 900°C for 30 min, and high-temperature nitriding followed by double tempering at 900°C for 30 min and at 500°C for 5 h. Recrystallization annealing was performed in a vacuum and was followed by air cooling. Nitriding was performed in a pure nitrogen atmosphere using a sealed quartz tube; the subsequent tempering was carried out in a vacuum and followed by air cooling.

A structure was studied by transmission electron microscopy (TEM) using a JEM-2100 (JEOL) microscope and thin films.

The microhardness was measured using transverse sections of samples and a Micromet 5101 (Buehler) microhardness tester equipped with a digital video camera; the load was 1 N and the loading time was 10 s.

Phase composition was studied using a DRON 4-07 diffractometer, monochromatized CoKα radiation, and the Rietveld method. Theoretical calculations of the phase composition were performed using the ThermoCalc 3.1 software and the TCFE7 database.

Tensile test were carried out at room temperature and 700°C using Instron 5966 and Zwick testing machines, respectively. Samples 3.5 × 20 mm and 5 × 60 mm in rectangular gage sections were tested at room temperature and 700°C, respectively; in all case, the tension rate was 5 mm/min.

The fracture of the samples subjected to tensile tests was studied by scanning electron microscopy (SEM) using a JSM-6610LV (JEOL) scanning electron microscope, which was also used to perform electron microprobe analysis in the backscattered electron mode.

The weight fraction of nitrogen entering into the metal during nitriding was calculated as the specific weight increment of the samples subjected to nitriding. Weighing was carried out on a GR-202 analytical balance (weight determination error was ±0.0001 g).

3 RESULTS AND DISCUSSION

3.1 Effect of Nitriding on the Phase Composition

The weight fraction of nitrogen in the samples subjected to nitriding at 1100°C for 4 h, which was determined using the increment of sample weight, is 1.86 ± 0.04%; it is comparable with the weight fraction of nitrogen in steel 15Kh25T, which was obtained at the same time and temperatures of nitriding (1.70 ± 0.09%) [7].

According to X-ray diffraction (XRD) data, the structure of the Fe–20Cr–1.5V–1.5Zr alloy before nitriding corresponds to the α-Fe solid solution. A small amount of intermetallic phases, such as ZrFe2 and Zr6Fe23 with a total volume fraction of slightly higher than 2% was found (Table 1).

Table 1.   Phase composition of the Fe–20Cr–1.5V–1.5Zr alloy subjected to treatment under various conditions
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The structure of the nitrided alloy is represented by two α solid solutions (based on iron and chromium), γ solid solutions based on iron, and Cr2N and ZrN nitrides (see Table 1).

SEM analysis of the structure of nitrided alloy, which includes the mapping of chemical element distributions and local chemical analysis, allowed us to clarify the chemical composition (Figs. 1, 2). Numerous precipitates 0.5 to 1 μm in size, which were observed in the structure, are identified to be chromium-based solid solution, chromium nitrides, and Z phase (Cr,Fe)VN. In this case, chromium nitrides are enriched in vanadium and are described by the formula (Cr,V)2N [16]. Rare, relatively coarse, zirconium nitride precipitates (1–3 μm in size) enriched in chromium and vanadium also are observed in the structure.

Fig. 1.
figure 1

Microstructure (SEM) of the Fe–20Cr–1.5V–1.5Zr alloy subjected to nitriding at 1100°C for 4 h.

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Fig. 2.
figure 2

Microstructure (first image) and element distribution maps of the Fe–20Cr–1.5V–1.5Zr alloy subjected to nitriding at 1100°C for 4 h.

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The experimentally determined phase composition of the nitrided alloy was compared with the theoretical composition determined with ThermoCalc software. Figure 3 shows the calculated multicomponent phase diagram of the Fe–20Cr–1.5V–1.5Zr–N alloy. It follows from the diagram that, at an nitrogen weight fraction of 0.6% and more and at temperatures close to room temperature, the phase composition of the alloy is represented by two bcc solid solutions (iron- and chromium-based), zirconium ZrN and chromium Cr2N nitrides, and Z phase being (Cr,Fe)VN. In particular, according to the calculation, the alloy with an nitrogen weight fraction of 1.9% has the following phase composition: 75% α-Fe–Cr, 9% α-Cr, 1.7% ZrN, 11% Cr2N, and 3.3% (Cr,Fe)VN. The experimentally determined phase composition of the Fe–20Cr–1.5V–1.5Zr nitrided alloy does not completely correspond to the equilibrium phase composition. In particular, the total weight fraction of the α-Fe and α‑Cr phases and the weight fractions of Cr2N and ZrN phases are close to the calculated ones. At the same time, the presence of the γ-Fe phase was found experimentally, whereas, the (Cr,Fe)VN phase found by SEM was not identified by XRD.

Fig. 3.
figure 3

Portion of the phase diagram for the Fe–20Cr–1.5V–1.5Zr–N alloy.

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The partial discrepancy of the phase compositions can be related to the sufficiently high cooling rate of the alloy after nitriding (in air); because of this, the kinetics of phase transformations differs from the equilibrium kinetics. In particular, the accelerated heating favors the stabilization of the γ-Fe, (Cr,V)ZrN, and (Cr,Fe,V)2N phases at room temperature, which, according to calculations, are present in the alloy structure at the nitriding temperature (1100°C).

Tempering of nitrided samples at 700 and 900°C for 5 h leads to the decomposition of the γ-Fe solid solution and an increase in the volume fraction of Cr2N by 1.5 and 2.2 times, respectively (see Table 1). A decrease in time of tempering at 900°C from 5 h to 30 min decreases the intensity of Cr2N nitride precipitation, whereas the subsequent tempering at 550°C for 5 h does not change the phase composition of the alloy.

The fraction of zirconium nitrides having precipitated in the nitrided alloy is almost the same for all subsequent heat treatment conditions used in the work and is 2%.

3.2 Effect of Nitriding on Mechanical Properties

The microhardness of the thin-plate alloy subjected to recrystallization annealing is 180–200 HV and is uniformly distributed across the thickness. Nitriding leads to the substantial increase in the microhardness, which is likely to be related to both formation of nitride particles and saturation of solid solution with nitrogen. In this case, the microhardness near the surface is substantially higher than that in the center of sample; it is 750–820 and 550–570 HV, respectively (see Fig. 4) and is ~4.2 and ~2.3 times higher that the microhardness of the alloy before nitriding. Tempering performed after nitriding leads to the decrease in the microhardness by 1.3 to 2.4 times and its equalizing across the sample thickness. In this case, the microhardness of the samples subjected to double tempering (400 HV) is slightly higher than that observed after single tempering (320–340 HV).

Fig. 4.
figure 4

Variations of the microhardness across the sample section of the Fe–20Cr–1.5V–1.5Zr alloy subjected to (1) nitriding at 1100°C for 4 h, (2) nitriding followed by double tempering at 900°C for 30 min + 550°C for 5 h, and (3) nitriding followed by tempering at 700°C for 5 h, and (4) nitriding followed by tempering at 900°C for 5 h.

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After tempering, the strength properties of the alloy increase (Table 2). In this case, after tempering at 900°C for 5 h, the yield strength and ultimate tensile strength at room temperature increase by 1.2 and 1.4 times as compared to those observed for the initial recrystallized state, respectively; the relative elongation decreases by three times. The decrease in the tempering temperature to 700°C insignificantly affects the strength of the alloy at room temperature and markedly worsens the plastic properties. The maximum increase in the ultimate tensile strength (to 1000 MPa, i.e., by 1.9 times) at room temperature is ensured by 4‑h nitriding followed by double tempering; in this case, the relative elongation remains equal to 2.0%.

Table 2.   Mechanical properties of the Fe–20Cr–1.5V–1.5Zr alloy subjected to treatment under various conditions
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At a test temperature of 700°C, the ultimate tensile strength and yield strength of the nitrided alloy increases by 1.7 to 2.0 times in the case of both tempering at 700°C for 5 h and double tempering at 900°C for 30 min and 550°C for 5 h. In this case, the relative elongation in the case single tempering increases by 1.2 times; in the case of double tempering, it decreases by 1.3 times.

For comparison, Table 3 shows the mechanical properties of steel 08Kh17T and Fe–20Cr–5V and Fe–20Cr–1.5V–1.5Zr alloys tested at 700°C, which were subjected to the same treatment, namely, nitriding at 1000°C for 4 h and subsequent tempering at 700°C for 5 h [8]. It is noteworthy that the ultimate tensile strengths of steel 08Kh17T and Fe–20Cr–1.5V–1.5Zr alloy tested at 700°C are comparable and substantially higher than that of the Fe–20Cr–5V alloy despite the substantially higher weight fraction of nitrogen in the Fe–20Cr–1.5V–1.5Zr and Fe–20Cr–5V alloys. In this case, the relative elongation of the Fe–20Cr–1.5V–1.5Zr alloy is almost two times higher than that of 08Kh17T steel and Fe–20Cr–5V alloy.

Table 3.   Mechanical properties of the Fe–Cr-based alloys tested at 700°C in the initial state (numerator) and after nitriding followed by tempering at 700°C for 5 h (denominator)
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The fracture of the Fe–20Cr–1.5V–1.5Zr alloy in the recrystallized state, which was obtained in testing at room temperature, is completely ductile and characterized by dimples 12 μm in average size (Fig. 5). Nitriding followed by tempering at 700°C for 5 h leads to the formation of plane (at macrolevel) fracture. At a microlevel, the fracture is characterized by sufficiently developed relief; however, rare cleavage facets are observed. In the case of tests at 700°C, the fracture of the sample after both annealing, and nitriding followed by tempering at 700°C is completely ductile and is characterized by small dimples. The average size of dimples in the nitrided sample is 6 μm.

Fig. 5.
figure 5

Fracture of the Fe–20Cr–1.5V–1.5Zr alloy (a, b) in the recrystallized state and (c–f) after nitriding at 1100°C for 4 h and subsequent tempering at 700°C for 5 h: (a–d) testing at room temperature and (e, f) testing at 700°C; (a, c, e) general appearance of fracture and (b, d, f) microstructure of fracture.

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3.3 Effect of Nitriding on the Microstructure

It should be noted that, according to XRD data, the nitrided alloys subjected to tempering at 700°C and double tempering have similar phase compositions. Therefore, the differences in the mechanical properties at room temperature of the alloys in both states can be related to the differences in the microstructures and the phase morphologies.

TEM studies allowed us to find numerous nitride particles in the alloys in both states; the morphology of particles is different. After nitriding followed by tempering at 700°C for 5 h, the structure is characterized by the presence of thin extended nitride particles to 700 nm long and 8–100 nm wide (Figs. 6a, 6b). Triple junctions of grains are clearly observed in the structure; the dominant grain size is 2–5 μm. The structure of the nitrided alloy subjected to double tempering exhibits the presence of nitride particles mainly are oval and 50–250 nm in size (Figs. 6c, 6d). The grain size is substantially lower and is equal to 0.3–3.0 μm. We can conclude that grain refinement and the change in the nitride morphology from extended needlelike to oval favor an increase in the strength properties of the alloy.

Fig. 6.
figure 6

Microstructure of the Fe–20Cr–1.5V–1.5Zr alloy subjected to nitriding at 1100°C for 4 h followed by (a, b) tempering at 700°C for 5 h and (c, d) double tempering at 900°C for 30 min + 550°C for 5 h: (a, c, d) bright- and (b) dark-field images.

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4 CONCLUSIONS

Alloying of the base Fe–20Cr composition with zirconium and vanadium (1.5 wt % each) changes the nitrogen saturation kinetics and the type of structure during high-temperature nitriding (1000°C) but does not cause an additional increase in the strength at both room and high temperatures under the chosen treatment conditions. This fact can be related to the low volume fraction (2%) of precipitated zirconium nitrides. The maximum increase in the yield strength and the ultimate tensile strength (by 1.9 times) of the Fe–20Cr–1.5V–1.5Zr alloy at room temperature is ensured by nitriding for 4 h and double tempering (900°C for 30 min + 550°C for 5 h), whereas the maximum increase in the yield strength and the ultimate tensile strength (by 1.7–2.0 times) of the alloy at 700°C is ensured by nitriding followed by tempering at 700°C for 5 h. The use of other nitriding and subsequent heat treatment conditions, in particular, a decrease in the cooling rate after nitriding, is of interest.