Research of heat stresses in components of blast furnace tuyere

Using computer simulating in Deform 2D it was researched the effect of insert thickness and air gap size, separating it from the internal cylinder, on temperature pattern and stresses in the insert and the internal cylinder. It was shown, that with the increase a clearance between the insert and the internal cylinder and with increase the insert thickness, weight average values of voltage decrease in it. They have been more evenly spread with increases of insert thickness. In tuyere design it was offered increasing it thickness to 13 mm for hardening.


Introduction
Tuyeres are the essential elements of blast furnace in determining its performance; tuyere`s breakdown requires a furnace shutdown for destroyed tuyere`s replacement.For this reason furnace downtime leads to significantly reduce of iron smelting and increasing coke consumption.Besides, 30% of all heat loss happened in tuyeres in blast furnace [1].That`s why the topical issue is tuyere hardening and reduction in heat losses through their surfaces.
At the present time the given issue is being solved by different ways: applying gasflame sprayed coatings on working surface of tuyere [2][3][4], refractoriness of refractory products from the side of the air passage [5] and others.For identifying the ways to improve the tuyere design it is necessary modelling its thermal state, influencing the origin of thermoelectromotive forces in it.
In the previous works it was established the substantial impact of insulation insert in air passage and air gap, separating it from the internal cylinder, on tuyere temperature pattern (fig. 1) [6][7][8][9].Estimation of thermal stresses is especially important for thermal insulation, the efficiency of which depends on its condition.Due to a large temperature change or the temperature variation rate along thickness the stresses arise in the insert and may lead to its premature failue during operation of the tuyere.Due to the temperature variation rate along thickness arising stresses can lead to deformation and the quality of welded joints in the internal cylinder.
Using modern software the purpose of work was researches of the influence of the insulation insert thickness in the air passage of tuyere and the size of air gap separating it from the internal cylinder, on the temperature pattern and the stresses in the insert and internal cylinder.

Research methodology and results
As we know, the cause of the occurrence of heat stresses in details is the presence of a temperature gradient in them [10].Due to the relatively low thermal conductivity of the most ceramic materials (less than 10 W/ m•K) [11] with one-direction thermal action, a temperature gradient appears inside the thermal insulation.At the same time, thermal insulation materials have a relatively high coefficient of thermal expansion (1-2•10 -5 К -1 ) [12].That leads to the fact that the more heated layers of the heat-insulating material are under compressive loads on the side of less heated layers and vice versa.In addition, in the absence of expansion space during heating, the heat insulating material is under additional compressive loads from the side of the internal cylinder.Arising under the temperature drop mechanical stresses can be calculated in accordance with Hooke's law: where σ0 is a stress in material, ΔТ is a change in the product temperature, α is the coefficient of linear thermal expansion of the material, E is the modulus of elasticity of the material.
We note that a slow change in temperature does not lead to the arising of significant mechanical stresses due to the phenomenon of stress relaxation [13].With time arising stresses decrease in accordance with the following dependence [14]: where σ is the actual stress in the material, σ0 is the stress in the material without relaxation (calculated by equation ( 1)), τ is the time since the beginning of the relaxation, E is the modulus of the material elasticity, К1 is the constant characterizing the stress relaxation rate, depending on the viscous properties of the material.
For ceramic materials the constant К1 varies in the range from 10 15 at room temperature to 10 7 -10 9 at a temperature of 1500-1700 0 С [15].
The insert thickness (hвст.) was 10 and 13 mm, and the air gap value (hвоз.) was 0,1 and 2 mm (tables 2 and 3).From tables 2 and 3 it can be seen that the weighted average insert temperature increase and the weighted average temperature of the internal cylinder decrease with an increase in the gap between the insert and the internal cylinder with the insert thickness of 10 mm.With an increase in the thickness of the insert with a gap of 1 mm, the weighted average temperature of the insert and internal cylinder decreases.As for the stresses, with increase in the gap with an insert thickness of 10 mm and with increase in the thickness of the insert with a gap of 1 mm, their weighted average values decrease, with an increase in the thickness of the insert, they are more evenly spread (fig.3-5).Similarly, in the internal cylinder the maximum stress values do not exceed the yield stress of cuprum M1, under tension the hard alloy is σТ=300-450 MPa.

Conclusion
The analysis of the temperature and stresses of the insert and internal cylinder of blast furnace tuyere with using computer simulating in Deform 2D, shows that increasing of the insert thickness to 13 mm increases its durability.

Fig. 2 .
Fig. 2. The geometry of tuyere for simulating in Deform 2D: 1air passage, 2 -water-cooled hole, 3 external cylinder, 4internal cylinder, 5 -the end of tyuere nose, 6 -the lateral surface of tyuere nose, 7tuyere nose from the side of the air passageThe elastic and thermal properties of used materials were set when simulating the temperature pattern and the stress in the insert and in the internal cylinder (table1).

Table 1 .
).The elastic and thermal properties of used materials

Table 2 .
Calculated values of temperature and stress in the insert

Table 3 .
Calculated values of temperature and stress in the internal cylinder