ISIJ International, Vol. 61 (2021), No. 1

© 2021 ISIJ71

ISIJ International, Vol. 61 (2021), No. 1, pp. 71–78

https://doi.org/10.2355/isijinternational.ISIJINT-2020-138

* Corresponding author: E-mail: jiaokexin_ustb@126.com.

© 2021 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license (https://creativecommons.org/licenses/by-nc-nd/4.0/).CCBYNCND

1. Introduction

The tuyere, generally made of pure copper, is an impor-tant part of the blast furnace. It is installed between the blast furnace hearth and the bosh1,2) and works in a complex environment with high-temperature gas and radiation.3–5) High-temperature gas and coal powder above 1 000°C are sprayed inside the blast furnace through the tuyere, while high-temperature slag, iron and coke around 2 000°C are distributed outside the tuyere.6–8) With the impact of high-speed gas around the tuyere, the tuyere will also be eroded by slag and iron at about 2 000°C, and the high-temperature coke will also produce micro-cutting effect on the tuyere.9,10) Therefore, in order to ensure the normal operation of the tuyere, it needs to adopt high-speed water cooling. Generally speaking, the service life of the tuyere in small and medium-sized blast furnace can reach more than 1 year, nevertheless, the tuyere in the super-large blast furnace is only 3–4 months, or even shorter. Sometimes, the tuyere will break and leak after several days. Tuyere damage and leakage will seriously affect the normal smelting of blast furnace, shorten the service life of the hearth, and bring serious economic losses. In consequence, it is extremely important to solve the problem of tuyere damage in the blast furnace and prolong the service life of tuyere.

Melting Erosion Failure Mechanism of Tuyere in Blast Furnace

Tianlu GAO,1,2) Kexin JIAO,1,2)* Jianliang ZHANG1,2) and Hengbao MA1,2)

1) School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083 China.2) State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083 China.

(Received on March 13, 2020; accepted on July 3, 2020; J-STAGE Advance published date: September 9, 2020)

In this paper, the common damage types of tuyere were sampled and analyzed. Specifically, the element content in tuyere was measured by Inductively Coupled Plasma Source Mass Spectrometer, Nitrogen-Hydrogen-Oxygen Analyzer, and Carbon-Sulfur analyzer. Then, Scanning Electron Microscope was used to analyze the microstructure of tuyere damage, and the element distribution of the damaged area was observed by Energy Dispersive Spectrometer. Finally, a metallographical analysis of the damaged location was carried out by an optical microscope. On account of those above analyses, the following results were obtained: firstly, the tuyere damage was mainly caused by erosion. After that, the grains at the hot surface and melting area of the tuyere were large, while those in the middle region were small. The content of the impurity element in tuyere nose increased, and the content of copper decreased. Moreover, there were two interfaces of slag-copper and iron-copper in the damaged area, and the Cu–Fe alloy was formed. At last, the failure mechanism of blast furnace tuyere erosion was explained in the paper.

KEY WORDS: blast furnace; tuyere damage; Cu–Fe alloy; erosion.

The damaged part of the tuyere is mostly concentrated in the front of the tuyere, namely the head area of the tuyere. In order to solve the problem of tuyere damage, researchers have conducted a series of explorations on tuyere improve-ment, including the improvement of tuyere cooling struc-ture, the control of cooling water speed, and the addition of abrasion-resistant coatings. Although the service life of the tuyere has been increased to a certain extent, the damage of the tuyere is still relatively frequent.

The phenomena of damaged tuyere have analyzed by many researchers. A. Pathak et al.11) believed that the failure of tuyere was caused by a variety of factors such as the spat-ter of molten slag and iron, the high-temperature corrosive gas, and the wear and tear by falling burden. They used thermal spraying technology to spray an alloy wear-resistant layer coating on the external surface of the tuyere, which can prolong the service life of the tuyere. And other researchers have used ceramic materials at the tuyere head to apply a slurry coating on the inner surface of tuyere.12,13)

Chai et al.14) found a layer of limescale in the water pas-sage through the dissection of a fractured tuyere, and studied the heat transfer efficiency of the tuyere with different lim-escale thickness by numerical simulation. Results show that the existence of limescale in the water channel will greatly reduce the heat transfer efficiency. As the limescale thick-ness reaches a certain limit, the heat of the tuyere cannot be exported in time, thus causing the tuyere damage.

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Chen et al.15) conducted numerical simulation studies of different parameters of the tuyere and believed that the high temperature of the tuyere head has little effect on the overall temperature of the tuyere. However, there will be a large temperature difference at the front end of the tuyere, which will generate a large thermal stress and cause damage to the tuyere. In particular, the thick wear-resistant layer at the tuyere head will produce a heavy temperature concentra-tion and a large thermal stress, which leads to the premature shedding of the wear-resistant layer, and finally resulting in the tuyere failure.

Natasa et al.16) considered that the damage of the tuyere is a complicated process. They used an electron microscope to analyze the microstructure and conducted an experiment to simulate the tuyere working environment. The damaged tuyere can be divided into two stages: firstly, the corrosive compounds such as chlorine compounds gave rise to local corrosion of the tuyere and made a severe impact on local area cooling. Secondly, the corrosion zone was subject to high-temperature slag and iron splashing, which reduced the thermal conductivity and finally resulted in local overheat-ing and melting. The two stages were repeated and the waste went into the furnace with the water.

Wang and others17,18) claimed that the damage of tuyere was closely related to the zinc accumulation in the blast fur-nace. The zinc vapor adhered to the surface of the tuyere and formed a copper-zinc alloy at high temperature. In addition, zinc will continue to diffuse into the copper at a high tem-perature. The Cu–Zn alloy had a lower thermal conductivity and was easy to cause damage to the tuyere.

The tuyere damage of large blast furnace in China is particularly serious, and the average service life of tuyere is mostly 3–4 months, but there is relatively few research on the tuyere damage mechanism in China. Moreover, some tuyere erosion damage occurs after several days of use, so it is necessary to explain the mechanism of tuyere erosion damage, to put forward corresponding solutions to extend the service life of the tuyere. In this paper, the element and microstructure analysis of tuyere samples were carried out. Inductively Coupled Plasma Source Mass Spectrometer (ICP-MS), Nitrogen-Hydrogen-Oxygen (NHO) Analyzer, and Carbon-Sulfur (C-S) analyzer were used to analyze the tuyere elements. Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), and metallograph were used to analyze the structure of the damaged part of the tuyere. Finally, the mechanism of tuyere melting erosion was described.

2. Experiment

The average life of tuyere was calculated in a large blast furnace in China. According to the statistics, the average life of the tuyere in the blast furnace is about 3–4 months (Fig. 1 shows the average life of tuyere in the blast furnace over the years). Obviously, the tuyere life from 2015 to 2019 is significantly shortened, with an average life of fewer than 90 days (some production data of the blast furnace are given in Table 1). Furthermore, the service life of the recently damaged tuyere is about 60 days. As shown in Fig. 2, the damage of tuyere is mainly caused by melting erosion in the blast furnace. The tuyere damage is concentrated in the

tuyere head area, but there is no damage in the central part of a tuyere. It was found that the head of a majority of tuyere was covered with slag, and the average thickness of the slag layer was thinner but thicker in local areas. The local slag layer is closely combined with tuyere, and it is difficult to fall off while the thinner slag layer is easy to fall off. In addi-tion, there also can be seen the large block of iron bonded in the tuyere head.

After the slag is cleaned, the front of the tuyere shows the characteristics of unevenness, and the transition point is smooth without a sharp edge, which is caused by typical melting erosion. Therefore, the damage characteristic of the tuyere is defined as erosion damage.

The sampling method for the tuyere that covered by slag and iron is as follows: the damaged part of the tuyere is firstly cut by grinding wheel, and linear cutting is used to acquire the local damage characteristics. Additionally, the sample is further processed for various experimental characterization.

1) In Fig. 2(b), area 2# is the undamaged area, and area 1# is the head damaged area. Drill chip (5 g) was taken from the tuyere head and the undamaged area, which was analyzed by ICP-MS analysis, C-S analysis, and NHO analysis respec-tively to obtain the content of each component in copper.

2) The samples were respectively polished with 60–5 000 purpose sandpaper and then polished with 0.5 μm polishing paste on the polishing cloth to obtain SEM samples.

3) Conduct metallographic erosion on the polished sam-ples: Tuyere erosion for about 1 min by using FeCl3 acid solution, then wash and dry with deionized water as well as alcohol for optical microscope observation.

3. Results and Discussion

3.1. Composition and Metallographic AnalysisThe samples were taken from the head area and undam-

Fig. 1. Average life of tuyere over the years (15 days margin of error).

Table 1. Production and operation data in 2020.03.

Project Units Value Project Units Value

Inner volume m3 5 800 Fuel rate kg/t 501.9

Productivity t/d 12 618 O2 enrichment rate % 7.7

Smelting intensity t/d/m3 1.0 O2 enrichment Nm3/h 38 828

Coke rate kg/t 343.5 Blast kinetic energy kg·m/s 15 736

Coal rate kg/t 158.4 Blast temperature °C 1 190

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aged area of the tuyere to analyze the elemental composition and make a quantitative analysis of the contents of various elements in copper. The results are shown in Table 2. Results indicated that the copper content in the head area of the tuyere was 99.931% and was 99.962% in the undamaged part. The copper content in the head area was lower than that in the undamaged area. The element content of tuyere was generally higher than 99.95%, which can be considered to meet the use of standards. Elements Si, Ni, Fe, As and O increased in the head area of the tuyere. The element con-tent at the undamaged area of the tuyere accorded with the production standard. However, the head area of the tuyere was in a complex environment of high-temperature slag and iron, the copper content decreased slightly, and the impu-rity content increased to a certain extent. In particular, the content of iron and nickel increased obviously, which was caused by the diffusion of impurities in the slag and iron at the front of the tuyere to copper at high temperature.

Figures 3(a), 3(b) is the metallographic diagram of the vicinity of the melting erosion site, and the sample was taken from area 1# in Fig. 2(b). Figures 3(c), 3(d) is a metal-lographic diagram of the undamaged part and the sampling location is in the 2# area of Fig. 2(b). The grains near the melting site become larger, the structure and compactness of the grains were destroyed, and the liquid iron penetrated into the copper. Iron is banded in copper and expended in a larger area, with circular iron droplets on either side, which may be cross-sections of banded iron. The tuyere grains are larger in the melting zone and smaller in the tuyere grains

away from the melting zone. There were obvious boundaries between the grains, and the small grains were embedded in the large grains, which was caused by the gradual growth of the grains at high temperature.

3.2. Microscopic Morphology and Phase AnalysisFigure 4 shows the melting erosion damage of tuyere, in

which four distinct boundaries can be seen. The upper part is the slag including (FeO) area, the middle part is the iron area with a small amount of copper, and the lower part is the copper area. The reason for the stratification may be that the copper was not completely covered with a layer of slag, and the high-temperature iron was bonded to the slag layer before the slag falls, and the slag layer was covered before the iron falls; thus the sandwich layer structure formed in a failure area. The element distribution is shown in Fig. 4(b), and the element content of the whole region is shown in Table 3. The slag area of the first layer was mainly composed of Ca, Si, Mg and Fe elements. The second layer was dominated by iron, and the third layer was a mixture of damaged copper, slag and iron. In the presence of slag, copper and slag could not form an alloy, but the copper was partially melted at high temperature and entered in the slag. Therefore, there was a clear boundary between the slag and copper. The iron and the copper would form a Cu–Fe alloy while there was no slag, so there was no apparent boundary between the iron and copper. Along the middle line of Fig. 4, EDS analysis was conducted to obtain the content trend diagram of Cu and Fe, as shown in Fig. 5. Along with the

Fig. 2. Tuyere damage location and slag, iron on tuyere’s surface. (Online version in color.)

Table 2. Element content in different parts of tuyere, %.

Element Cu Sn Sb Pb Bi Si Ni Fe

Head area 99.93135 0.00006 0.00005 0.00016 0.0008 0.016 0.0100 0.032

Undamaged area 99.96161 0.00007 0.00006 0.00016 0.0008 0.010 0.0005 0.016

Element Zn As P S H O N

Head area 0.0012 0.00016 0.0036 0.0011 0.00001 0.00245 0.00106

Undamaged area 0.0010 0.00006 0.0054 0.0011 0.00003 0.00209 0.00112

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line 1, the copper and iron contents could distinguish five areas in the damaged area and also reflected the process of Cu losing, which formed the copper moved to the slag area.

It can be proved by the above analysis that it exists the bond of high-temperature slag and iron at the front of the tuyere when it works. The high-temperature slag and iron

were enough to soften and melt the copper. Finally, the mol-ten copper entered into the slag or formed the Cu–Fe alloy and caused the loss of tuyere. There was sufficient evidence to show that lots of slag and iron surrounded near the tuyere, and the copper could be melted by the high-temperature slag and iron. The melting process occurred repeatedly, and the molten copper entered the blast furnace with slag and iron, which resulted the damage of tuyere.

Figure 6 shows the two-phase interface of slag and iron, and the area is in the 1# area in Fig. 4. There were free metal droplets that can be found near the interface of slag and iron, and a few holes of different sizes can be found in the slag

Fig. 3. Metallographic diagram of different parts. (a), (b) Near the melting area; (c), (d) Far from the melting area. (Online version in color.)

Fig. 4. Interface characteristics of damaged tuyere; (a) SEM for melting erosion area of tuyere, (b) Element distribution of damage location. (Online version in color.)

Table 3. Element content of the whole area in Fig. 4.

Elements Na Mg Al Si K Ca Fe Cu Zn

At% 5.04 0.90 1.46 4.27 0.17 2.52 34.95 50.34 0.36

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including (FeO) area. Round holes in the mixed area may be the small pits formed by metal particles falling off during polishing. According to the EDS surface scanning results, Ca, Al and Si were densely distributed in the slag area, and the concentrated distribution of Si in holes may be generated by the polish used in sample preparation. Fe element was found in the whole sample, but the boundary between the slag region and the iron region was still visible, and circular iron droplet dispersion could be seen in the slag region.

There was also a small amount of dispersed little droplets copper in the area. When the copper was melted at high temperature, some of the copper melt remained in the iron to form an alloy, while the other part went into the slag and finally was carried away by the blast furnace slag. The metal droplets floating near the interface in Fig. 6 were dotted by the EDS, and the analysis results were shown in Fig. 7. The positions of p1 and p3 are mainly Fe and C. The positions of p2 and p4 are mainly Cu, Fe, and C. The content of copper at position p2 is due to the mixing of copper and iron in the slag. The copper at the p4 position is formed into copper-iron alloys at high temperatures.

In order to study the diffusion of copper to slag, the inter-face between copper and slag was observed. The 2# area

was selected in Fig. 4 for observation, and it was known that the slag area contained holes, cracks, and molten cop-per in Fig. 8(a). The boundary between slag and copper was relatively round, which was caused by the uneven melting of copper. This phenomenon indicated that the temperature of slag has reached the lowest temperature of copper melting. There were copper particles of different sizes distributed in the slag, and some copper was missing. The loss of copper showed that the copper in the slag is not firm. Besides, it was more susceptible to external forces and fell off. Crack propagation in slag may be the main factor that causes the slag layer to fall off easily under external force or thermal stress. After the slag layer falls off from the tuyere’s surface, the new copper will be re-exposed to the high temperature. Under the action of the molten slag and iron at the high temperature, the copper will melt and then fall off again. The above process occurs repeatedly and causes the erosion damage of copper eventually.

It can be seen that a part of iron coexists with slag in the area was showed in Fig. 8(b), and the element content of the whole area in Fig. 8 was shown in Table 4. The distribution areas of Ca, Si, Fe, and O elements were consistent with each other, and there were obvious boundaries with copper. The proportion of Fe element in this area is 27.62% and consists of (FeO) and iron. About 11.82% are calculated to be (FeO) and 15.8% are iron. And as can be seen from the distribution of elements in Fig. 8, there were two forms of

Fig. 5. Distribution of copper and iron in different regions of Line 1. (Online version in color.)

Fig. 6. Phase distribution at the interface of iron and slag at the damaged location; (a) SEM of slag and iron interface with different phase; (b) Element distribution of slag and iron interface. (Online version in color.)

Fig. 7. Main element content in P1, P2, P3, P4. (Online version in color.)

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iron in the damaged position: one is the small iron droplet, and the other is the (FeO) dispersed in the slag. There was a distinct interface between the two forms of iron, and there was no distribution of oxygen element on the aggregated iron droplets, and the distribution of Fe in the slag did not totally matches the distribution of O. Therefore, it is believed that Fe in the slag is in the form of iron droplet and (FeO).

3.3. Interface Analysis of Cu–Fe AlloyIn general, the highest temperature of the slag in the

tuyere area can reach 1 500–2 000°C. When the slag falls to the surface of copper, a small liquid pool will form at the interface between the copper and slag due to the high temperature. The high temperature will lead to the melting of copper, and the molten copper, mixed with iron, will form the alloy shown in Fig. 9(a). Figure 9(a) was selected from the 3# area in Fig. 4(a). In the figure, the copper and iron did not mix utterly with each other, but the copper dispersed in iron. The relatively clear veins of copper also can be seen in Fig. 9(b). Two factors contributed to the formation of the heterogeneous Cu–Fe alloy. Firstly, there was no mechanical stirring, while the molten copper and iron are connected, which led to an uneven mix to the cop-per and iron. Secondly, the thermal conductivity of copper was very high, and the cooling water with a high rate cooled the copper; both of the iron and copper solidified fast. With the influence of the two factors, the diffusion rate between copper and iron decreased quickly, and the nonuniformly mixed copper and iron alloy was formed.

The phase diagram of Cu–Fe alloy can be used to analyze the crystallization temperature of the alloy with different

copper and iron contents. According to the phase diagram, the melting temperature of pure copper was 1 083°C, while the Cu–Fe alloy was 1 094°C; that is, the Cu–Fe alloy will not melt when the temperature is lower than 1 094°C.19) The temperature difference between the pure copper and the Cu–Fe alloy determined the melting order of the two metal, and it is copper melted firstly when the high-temperature slag was bonded again. Besides, when there was less iron content in the alloy copper, the iron solid solution would also be formed at a low temperature, and the iron element would diffuse into the copper. At the same time, the differ-ence of Fe content between the front end of the tuyere and the middle region in Table 2 can prove the diffusion of Fe.

3.4. Analysis of Melting Mechanism of Tuyere Small Sleeve

To estimate the surface temperature of tuyere with dif-ferent materials covered, the one-dimensional heat transfer theory was used to calculate. The heat conduction of tuyere from high-temperature area to low-temperature area can be approximately regarded as the heat conduction of cool-ing water. The heat transferred by cooling water can be calculated by the formula (1), and the conduction of heat between materials with different thermal conductivity can be calculated by the formula (2) and (3).20,21)

qcM t

Si �

� ................................. (1)

qT T

LiH C

W

i

i

��

�1� �

�.............................. (2)

q

T T

Li

H C

i

��

� .............................. (3)

qi: Heat flow intensity, J/(m2·s); c: Specific heat capacity of water, J/(kg·K);

Fig. 8. Interfacial phase characteristics of copper and slag at the damaged location; (a) SEM for interfacial phase of cop-per and slag; (b) Element distribution of copper-slag. (Online version in color.)

Table 4. Element content of the whole area in Fig. 8.

Elements O Si Ca Fe Cu

At% 23.50 4.43 2.82 27.62 41.63

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M: Quantity of cooling water, kg; Δt: Coolingrange, K; S: Unit area, m2; TH: Hot surface temperature, K; TC: Temperature of cold surface, K; Li: Thickness, m; λi: Thermal conductivity, W/(m2·K); αw: Convective heat transfer coefficient, W/(m2∙K).Assuming that the tuyere works regularly, the cold sur-

face temperature of tuyere is 90°C,15) the total remaining thickness of the tuyere head is 0.02 m, and the thickness of the slag layer and iron layer is 0.001 m respectively (SEM results). Table 5 is the parameters used to calculate the tuyere temperature.22–24) The results are shown in Fig. 10. The surface of tuyere was at a low temperature when the tuyere has no slag and iron. However, the temperature rises when the slag and iron cover on the surface of the tuyere. When the surface of tuyere is not covered with slag or iron, the tuyere is under normal operating temperature. However, when there is a slag layer on the surface of the tuyere, its surface temperature rises rapidly and exceeds the normal operating temperature of the tuyere. Therefore, the existence of slag in the head of the tuyere has a significant influence on the temperature field of the tuyere and affects the heat transfer efficiency of tuyere.

These studies show that tuyere damage is due to the high temperature of slag and iron, external impact and other com-plex factors. The mechanism diagram of a damaged tuyere is shown in Fig. 11. Firstly, the high-temperature slag bonds in the tuyere, and the temperature of the slag is much higher

than the melting temperature of copper, which caused the smelting of copper.

Secondly, the contact between slag, iron and copper are divided into two types: one is the contact between slag and copper, the other is the contact between iron and cop-per, and the two types of contact leads to different damage mechanisms. The slag with high temperature will lead to the melting of the copper, and the molten copper will spread into the slag and form small droplets in the slag. Slag on the tuyere has the characteristic of loose texture, holes and cracks, and it is easy to fall off due to the high temperature of coke particles in tuyere. Another type of damage is that Cu–Fe alloys are formed when hot molten iron comes into contact with copper. The thermal conductivity of copper and iron alloy is different from that of copper, which is shown in Fig. 12. When heat cannot be transferred in time, the temperature of the tuyere head will rise quickly. As the

Fig. 9. Phase analysis of Cu–Fe alloy in the damaged area. (Online version in color.)

Table 5. Parameters used to calculate the tuyere temperature.

Project Units Value Project Units Value

Water temperature difference °C 5 Thermal coefficient of copper W/(m∙K) 350

Internal surface of tuyere m2 0.7 Thermal coefficient of slag W/(m∙K) 1.66

Cold side temperature °C 90 Thermal coefficient of iron W/(m∙K) 40

Specific heat capacity of water J/(kg·°C) 4 200 Aggregate thickness m 0.02

Waterflow of tuyere t/h 45 Thickness of iron or slag m 0.001

Fig. 10. The surface temperature of tuyere covered by slag or iron (0.001 m thick).

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temperature rising, the copper is soften firstly, which causes the parts to separate from the copper side, eventually remov-ing some of the copper and causing the tuyere to break. The copper is softened by the rising temperature, which finally smelts the copper around the interface of copper and slag. The slag separates from the copper side, carrying away a part of the molten copper to make the tuyere break.

Finally, after the slag and iron fall off from the surface of the tuyere, the new copper exposed, and then the high-temperature slag and iron covered the tuyere again. After three processes of repeated erosion, the thickness of the tuyere reduced gradually. The tuyere failed because of a leak. The slag iron bonded to the tuyere is small, and the bonding position is random, therefore the tuyere eventually forms a partial-uneven erosion damage.

4. Conclusions

(1) Evidence shows that, in the damaged tuyere of 5 800 m3 blast furnace, the uneven pit erosion at the front of the tuyere was caused by erosion rather than wear.

(2) The front of the tuyere works in the environment of high-temperature gas, slag and iron for a long time. The diffusion of impurity elements to copper at a high tempera-ture is one of the factors leading to the decrease of copper content in tuyere head.

(3) Cu–Fe alloy and copper droplets were found in the damaged position of the tuyere, which indicated that there were two ways of the tuyere melting erosion, one was slag erosion and the other was iron erosion.

(4) High-temperature slag and iron repeatedly bonded and fall off in the front of the tuyere, so that the copper continued to melt into the slag and iron, and eventually made the tuyere damage.

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Fig. 11. The mechanism of tuyere damage. (Online version in color.)

Fig. 12. The ratio of the thermal conductivity of copper to iron.