|Regular SHS Synthesis from elements||Metallurgical SHS Synthesis from alloys|
|Gas-free synthesis||Hf + C HfC||FeB + Ti TiB + Fe|
|Gas-absorbing synthesis||B + N BN||FeTi + N TiN + Fe|
|Gas-yielding synthesis||Mo + S MoS||CrN + Ti TiN + Cr|
1. SH-Synthesis Of Ferrosilicon Nitride
In the 1970s, a new refractory material ferrosilicon nitride (Si N -Fe) came into use in metallurgy. It was produced from Fe-Si powder, nitrided in a high-temperature resistance furnace. The main component of the alloy was silicon nitride Si N , the concentration of which reached 70–80%. Before that, silicon nitride had only been used as a ceramic material. The new material was developed for non-shape refractory mixtures as a hardening additive. Later, Si N -Fe was adopted by for steelmaking. Its main advantage was a high nitrogen content (25–30 % N), which significantly reduced the consumption of alloying material.
During the synthesis of nitrides, the actual process temperature is lower than the calculated temperature due to the incomplete transformation in combustion. This is a result of melting of the furnace charge components and loss of permeability, or the low temperature of nitride dissociation. Both phenomena occur in response to nitriding of silicon. The adiabatic temperature of Fe-Si combustion in nitrogen is lower than the temperature of Si combustion due to the presence of Fe. The reason for the lower heat generation is the fact that Fe and Si are bound to thermally stable silicide. The adiabatic temperature of combustion is high for the alloys with different Si content (Table 3). Consequently, there is a favorable environment for SHS in the (Fe-Si) - N system.
|Si content in ferrosilicon, %||Si N content in combustion product, %||T , C|
Ferrosilicon dust FS90 (89.9 % Si), FS75 (79.4 % Si), FS65 (68.1 % Si), and FS45 (48.25 % Si) was used for nitriding. The cyclone dust is a dusty fraction of ferrosilicon powder, which is formed during crushing and grading and accumulated in the dust removal system . The pattern of Fe-Si combustion in nitrogen happened to be similar to the combustion of metallic silicon. The melting point of alloys in the Fe-Si system was lower than the melting point of Si. In alloys with Si content from 40 to 80%, the liquid phase appeared already at temperatures above 1,210 . Hence the processes, associated with melting of the initial material during Fe-Si combustion, were more pronounced.
Figure 2 shows the influence of Si content in ferrosilicon on the combustion rate, the nitriding degree of the alloy, and the maximum temperature in the reaction wave. The data were obtained in laboratory conditions using, dust samples with particle size of 0.08 mm and less. As the Si content in the initial silicon alloy increases, the intensity of its interaction with nitrogen rises, which manifests itself in higher rate and higher temperature of combustion. The concentration of nitrogen in the combustion products also increases.
The examination of the microstructure of burnt samples confirmed that molten ferrosilicon particles coagulate in the combustion wave. As a result, the reaction surface decreases, leading to incomplete conversion of silicon to nitride. The high temperature in the combustion wave enhances active melting of the initial particles. The X-ray phase analysis of the products of ferrosilicon combustion in nitrogen showed that β- Si N remains the main phase in the entire range of initial parameters. Only insignificant amounts of α- Si N were detected.
However, the furnace synthesis demonstrated a combination these two phases . This can be explained by the fact that α-Si N remains stable only up to 1,400 , and at a higher temperature it irreversibly transforms into the β-phase. The temperature of ferrosilicon combustion in nitrogen is over 1,750 ; therefore, the formation of α-Si N becomes unlikely. The phase composition of the ferrosilicon nitriding products appears to be most influenced by the degree of Si Si N conversion. At the maximum degree of conversion, the product is two-phase: β-Si N + Fe. Unreacted silicon occurs as iron silicide. At that, the volume fraction of nitride exceeds 90%. This is due to the high concentration of silicon in the initial ferrosilicon and a large difference in the densities of Si N and Fe. The energy-dispersive analysis by a scanning electron microscope showed that iron is distributed in the matter in the form of separate `islets' islands 200 μm or less in size. Such metallic inclusions are formed due to the fusion of molten iron and iron silicide, which are yielded during the nitride formation in combustion. The iron-containing inclusions are evenly distributed in the volume of combustion products. Figure 3 shows the microstructure of ferrosilicon nitride, and Table 4 presents the results of microanalysis.
|Phases||Fe-Si, Si N||Si N||Fe-Si, Si N||Si N||Si, Si N||Si N||Fe-Si, Si N||Si N||Si, Si N|
Thus, the metallurgical SHS method allows synthesizing thermostable β-Si N -based materials using ferrosilicon as a raw material. The β-phase of nitride is quite efficient in non-shape refractory mixtures and can serves as a component of doping media. In practice, it is optimal to use FS75 and FS90 alloys for manufacturing refractory materials, and high-purity alloys FS65 and FS75, for doping steel.
2. SH-Synthesis Of Nitrided Ferrovanadium
Industrial alloys (FeV80 (78.8 % V), FeV60 (59.2 % V), FeV50 (52.4% V), and FeV40 (41.6 % V) according to GOST 27130-94) and model alloys were used for nitriding of ferrovanadium in combustion. The former ones were obtained from industrial screenings (sub-standard fines of 5 mm or less), and the latter ones were made in a vacuum furnace by alloying powdered electrolytic vanadium (VEL-1, purity 99.8% V, Technical Specification TU 48-05-33-71) and high-purity carbonyl iron (OSCh 13-2, purity 99.98% Fe, Technical Specification TU 6-09-05808009-262-92). The alloys were melted twice to homogenize their composition. Alloys with calculated vanadium content of 80.0, 70.0, 60.0, 55.0, 48.0, 40.0, and 35.0 % V were obtained. The X-ray phase data showed that they were single-phase. The alloys with 60.0–80.0% V were α-V based solid solutions. The alloys with 35.0–55.0% V were σ-intermetallides.
The combustion rates of industrial and modelled alloys were similar (Figure 5a). Ferrovanadium can be nitrided in combustion if the V content is 40% or more. Similar to V and Fe powder mixtures, the combustion rate of the alloys decreases significantly as the V content falls from 80 to 60 %. When passing from 60% V to 55% V, the combustion rate increases sharply and then decreases again. At that, the nitrogen content in the combustion products decreases (Figure 5b).
When nitriding ferrovanadium, the combustion temperature (T ) was measured with W-Re thermocouples BP5/BP20. Depending on the process conditions, maximum T varied from 1,780 to 2,060 for FeV80; from 1,630 to 1,830 for FeV60; from 1,480 to 1,560 for FeV50; and from 1,420 to 1,490 for FeV40. Thus, the more nitrogen is absorbed by the alloy, the higher combustion temperature is observed.
The layer-by-layer metallographic examination and the X-ray phase analysis of the samples, quenched to stop combustion, showed that nitriding of σ-Fe-V is activated due to the transformation of the intermetallide into a solid solution upon reaching the phase transition temperature ( 1200 ). Such activation causes the increase in the combustion rate for alloys with 35–55% V (Figure 4).
Bанадий лигатуpа – Vanadium master alloy
Bанадий BЭЛ - Vanadium VEL
Пpомышленные сплавы – Industrial alloys
Модельные спpавы – Modelled alloys
Cмесь - Mixture
V электpолитический – V electrolytic
V лигатуpа – V master alloy
FeV модельный – Modelled FeV
FeV пpомышленный - Industrial FeV
Cмеси - Mixtures
The layer-by-layer X-ray phase analysis of the combustion zone of quenched samples confirmed the same. The initial powder was in the intermetallic σ-phase. In the area immediately adjacent to the combustion front, we detected a layer of particles, which was radiographically manifested as an α-solid solution. Along with the solid solution, the deeper layers contained δ-VN and α-Fe, the number of which increased rapidly. The final product was two-phase (δ-VN + α-Fe). The particles are heated by to the heat released in the nitride formation, and, upon reaching the melting temperature of the V-Fe-N eutectic, the liquid phase occurs. Further, a solid-liquid particle-droplet is formed, consisting of molten iron and solid vanadium nitride. Such semi-liquid particles combine into a solid-liquid layer, which is parallel to the combustion front. At the next moment, solid-liquid layer shrinks under the surface tension forces. Having fused, the mass crystallizes to form a dense material with a composite microstructure.
Two types of SHS-obtained ferrovanadium nitride FERVANIT were developed for industrial application: melted and sintered. The first one, in lumps, is intended for alloying of steel in a ladle or in a furnace (Figure 5). The second one should be used as a filler for cored wire to modify nitrogen content before casting. Table 5 gives the specifications of FERVANIT; the specification of Nitronvan Vanadium alloy is given for comparison. Melted ferrovanadium nitride has been tested in smelting of high-strength low-alloy steels, as well as rail and high-speed steel. Nitrogen recovery amounted to 86–98%; vanadium recovery, more than 95%.
3. SH-Synthesis Of Nitrided Ferrochrome
Nitrided Fe-Cr alloy is used in smelting of stainless and other grades of chromium steel, and nitrided Cr is used in production of Cr-Ni superalloys. Aluminum thermal treated Fe-Cr powder of grade PFN was chosen for experimental nitriding. This kind of ferrochrome (75.6 % Cr) melts at 1550–1670 . In the range of nitrogen pressures from 2.0 to 10.0 MPa, the combustion temperature was 1220–1300 at T 1680 . Therefore, nitriding took place by the solid-phase mechanism. The finer the ferrochrome powder, the more rapidly it is nitrided and the greater amount of nitrogen is recovered.
Larger fraction of alloy powder can be nitrided in combustion using either of two techniques: first, increase the temperature of the initial charge (Figure 6); second, use forced injection of the co-current inert and/or reacting gas into the combustion zone.
Combustion in the presence of forced gas filtration has been previously studied by the example of chromium nitriding . In the co-current nitrogen flow ferrochrome burns at a high gas flow rate (Figure 7). The combustion rate of both Fe-Cr and Cr increases as the nitrogen consumption rises. At that, the nitriding degree of ferrochromium (4.7–7.5 % N) is less than its nitriding degree under the conditions of natural filtration (8.8–14.2 % N). The can be explained by the absence of a post-reaction `maturing' stage in case of forced nitrogen filtration. Quenching nitrided products with incoming gas flow fixes the amount of nitrogen that was absorbed directly in the combustion zone.
The metallographic analysis of combustion products did not reveal any traces of melting. The solid-phase mechanism facilitates a high nitriding degree of Fe-Cr. However, the maximum degree of nitriding could not be attained in the nitriding of ferrochrome. The limiting nitrogen concentration is 16.8 % N for alloys with 75.6 % Cr, while the actual nitrogen content in the ferrochrome was 13.0 % N. Thus, the maximum nitriding degree is 77 % of the calculated value. The main phases in the ferrochrome combustion products and the furnace alloy were CrN, Cr N, (CrFe) N, and Fe. Four grades of Cr-based master alloys were developed for industrial application in steelmaking (Table 6).
The reduced nitrogen content in fused nitrided ferrochrome/chromium, in contrast to sintered alloys, can be explained by the fact that nitrogen occurs in the material in the form of lower nitride Cr N as a result of partial dissociation of chromium mononitride CrN. The microstructure and the microanalysis of nitrided ferrochrome are shown in Figures 7 and 8. The microanalysis results are interpreted in Table 7.
Figure 9 shows a batch of sintered nitrided ferrochrome FHN10.
New nitrogen-bearing master alloys of Cr, V, Si, Mn, etc. has already found use in steelmaking in Russia and abroad. Steel grades 12H18AG18, 35H2AF, 55H21G9AN4, 16G2AF, 110G13HPAL and nickel-chromium alloy with 0.5 % N were produced with the proposed method.
The laboratory studies helped to create the industrial SHS technology for production of nitrided ferroalloys, master alloys, and other materials based on oxygen-free compounds (Table 7). The technological process includes the following stages: preparation of the exothermic charge, synthesis of composite alloys by combustion, and processing of synthesis products. Synthesis by combustion is carried out in industrial vertical-type reactors with reaction space of 0.15 m . NTPF Etalon Ltd (Magnitogorsk, Russia) produces SHS materials for metallurgy. Etalon's facilities include crushing and grinding units, crucible dryers, SH-synthesis equipment, control rooms, quality control laboratories, chemical laboratories, and warehouses of raw materials and finished products. The SH-synthesis shop operates 40 SHS reactors and can produce up to 10 tons of nitrided products per day. The SHS technology in metallurgy has completely supplanted the vacuum-thermal technology of the nitrided master alloy production. The production plan of the new shop focuses on specialized compositions of refractory materials and doping materials for steelmaking (Table 8).
In conclusion, a scientifically grounded industrial SHS technology was created for producing nitrided ferroalloys and composite materials based on oxygen-free compounds in steelmaking and blast-furnace ironmaking. The principal problem of creating a large-scale SHS-based production facility has been solved. A new approach to the practical implementation of SHS method was developed. The possibility of using synthesis products in metallurgy was demonstrated by involving ferroalloys in the research. The metallurgical SHS process, based on various alloys as raw materials, including dusty wastes of the ferroalloys production, extends the boundaries of synthesis by combustion. The problem of micro-alloying of steel with nitrogen, boron and titanium in smelting of high-quality steels was solved by creating a new class of master alloys—composite alloys based on nitrides, borides and silicide of titanium, vanadium, chromium, and other transition metals. Specialized SHS reactors with reaction space of 0.06, 0.15, and 0.3 m were designed for mass production of refractory inorganic compounds. Scientific and technical production company Etalon established the industrial-scale SHS production of composite materials based on oxygen-free compounds in Magnitogorsk, Russia. The capacity of new production facility is 5 thousand tons of SHS products per year.