thermal and mechanical stress in refractory.pptx

Thermal and mechanical stress in refractory

1. Seminar and Technical writing (CR4900) Thermal and mechanical stress in refractory Presented to: Prof. Debasish Sarkar Presented by: Sumit Kumar Roll No.: 120CR0398 Department of Ceramic Engineering, NIT Rourkela, Odisha – 769008 Date: 30/04/2024

2. What is Refractory? Refractories are non-metallic inorganic materials suitable for use at high temperatures in furnace construction. These are heat-resistant materials that constitute the linings for high-temperature furnaces and reactors and other processing units. As per ASTM C71 specification, refractories are defined as “ Nonmetallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1000°F (~811°K or 538°C)” Different kinds of classification of Refractory Refractories are resistant to thermal stress and other thermal energy–related physical phenomena, in addition to withstanding mechanical loads and shocks, resisting abrasion and wear of the frictional forces and corrosion by chemical agents.

3. Types of stresses in refractories during their period of service Types of Stresses Chemical stresses: Chemical stresses provide resistance to chemical attack by slags, liquid materials, gases, and flue dust. Key properties include chemical composition, mineralogical composition, pore size distribution, and gas permeability. Mechanical stresses: Mechanical stresses significantly influence refractories' strength, with key properties including cold modulus, deformation modulus, crushing strength, abrasion resistance, porosity, and density being crucial. Thermal stresses: It arises due to temperature differences on hot and cold side of refractories, requiring properties like pyrometric cone equivalent, refractoriness, hot modulus of rupture, thermal expansion, and thermal shock resistance. Thermo-technical stresses: Includes thermal conductivity, specific heat, bulk density, melting point, thermal capacity, and temperature conductivity, are crucial for heat flux and stored heat calculations.

4. The relationship between stresses and the important properties of the refractory bricks during the service  Thermal stresses – Refractory materials undergo thermal expansion in high temperatures, with varying degrees of change due to proximity to heat sources or insulation differences. For example, in a furnace lining, the inner layers might experience much higher temperatures than the outer layers. This non-uniform expansion results in internal stresses within the material. Mechanical stresses – Mechanical stress in refractories refers to the external forces or pressures that can cause deformation, damage, or failure of refractory materials structures. They determine the strength of the refractories under different service conditions. The relationship between stresses and refractory bricks

5. Thermal stress:- Thermal stress in refractory materials occurs when there is a significant difference in temperature across different parts of the material, causing it to expand or contract unevenly. This can lead to the development of cracks, fractures, or even failure of the refractory structure.  Refractory materials undergo thermal expansion in high temperatures, with varying degrees of change due to proximity to heat sources or insulation differences.. For example, in a furnace lining, the inner layers might experience much higher temperatures than the outer layers. This non-uniform expansion results in internal stresses within the material. Thermal stress in refractory materials, such as spalling, can cause surface layers to flake off due to heating and cooling cycles, reducing the lining's lifespan and potentially compromising its equipment integrity. Another example is thermal shock, which occurs when a refractory material is rapidly heated or cooled. if a hot metal is suddenly placed on a cooler refractory surface, or if cold water is sprayed onto a hot refractory lining, the rapid temperature change can cause intense thermal stress, leading to cracking or even immediate failure of the material.

6. Thermal Expansion :- Differential thermal expansion occurs when different parts of a refractory experience varying temperature changes, leading to internal stresses. The strain due to differential thermal expansion can be calculated using the equation: ΔL/L= 𝛼⋅ Δ 𝑇 Where: α is the coefficient of thermal expansion. Δ T is the temperature change. Example: In a refractory lining, if the inner layers are exposed to higher temperatures than the outer layers, the resulting differential expansion can induce stress, potentially causing cracking or spalling

7. The thermal expansion of refractories bricks :- In general, the thermal expansion behaviours of unfired refractories are more complex than those of their fired counter-parts. During initial heating, drastic expansion or contractions can occur in an unfired material as a result of changes in the bonding structure, changes in mineralogy, and sintering effects. Fig shows thermal expansion of different refractories. Refractories heated below their firing temperature exhibit high thermal expansion rates with Magnesite and forsterite bricks having high rates fireclay, silicon carbide, zircon brick, and most insulating firebricks having low rates. Other refractories have intermediate values and uniform expansion over the working range temperature, except for silica bricks.

8. Constrained thermal expansion curve of the ZrO 2 -SiO 2  refractory upon cooling, calculated assuming the strain-free state to hold at the maximum temperature (1500 °C). The stress was applied just before the start of the phase transformation (see vertical arrow). Unconstrained thermal expansion curve of the ZrO2-SiO2 refractory. It is clearly observed that the applied stress directly affected the strain associated with the tetragonal to monoclinic transform . A tensile stress increased the strain associated to the T → M transformation, whereas a compressive one reduced it . Comparison of thermal expansion curve of the ZrO2-SiO2 refractory vs stress applied :-

9. Cold Modulus of Rupture:- Cold Modulus of Rupture: It measures of ability of the material to resist deformation under a bending load at ambient conditions or room temperature. Flexural strength is calculated by the formula: σ =3PL /(2bd 2 ) in 3-point test of rectangular specimen During thermal stress, normally combined with altered physical-chemical conditions because of infiltration, strain conditions occur in refractory brickwork which can lead to brick rupture or crack formation   In order to determine the magnitude of rupture stress, the resistance to deformation under bending stress (rupture strength) is measured. Determination of the modulus of deformation in the cold state is carried out, together with modulus of rupture, on a test bar resting on two bearing edges. In general, a high ductility is looked for in refractory bricks . i.e. a large deformation region without rupture, which means a high value of the ratio of modulus of rupture to modulus of deformation.

10. Refractoriness under load of refractory bricks :- Fig -Refractoriness under load curves of refractory bricks Examples of such an application are blast furnace stoves and carbon bake furnaces in which the refractories are heated relatively uniformly to high temperatures . In service, refractory materials are to support a load which, at its minimum, is equal to the lining above the reference point. The pressure which is exerted depends on the height of the lining and density of the material . Hence, for applications in which the entire lining component is at high temperatures, it is important to understand the load bearing capabilities of the selected materials.

11. It is the measurement of the refractoriness. Pyrometric cone equivalent (PCE) is the ability to withstand exposure to elevated temperature without undergoing appreciable deformation . Pyrometric cones are small triangular ceramic prisms which when set at a slight angle bend over in an arc so that the tip reaches the level of the base at a particular temperature if heated at a particular rate . The bending of the cones is caused by the formation of a viscous liquid within the cone body, so that the cone bends as a result of viscous flow. PCE is measured by making a cone of the refractory and firing it until it bends and comparing it with standard cone . PCE is useful for the quality control purpose to detect variations in batch chemistry changes or errors in the raw material formulation. F ig-PCE cones before and after firing Pyrometric cone equivalent (PCE) :-

12. Creep behaviour of refractories :- R efractories show creep behavior when exposed to high temperature. Majority of refractories show two characteristic stages of creep :- . Primary creep:- In the first stage the rate of subsidence declines gradually with time. Primary creep is normally short in duration. Secondary creep:- In the second stage, the rate of subsidence is constant. secondary creep normally provides a more meaningful comparison of refractories.  Creep curves of refractories bricks at 1500℃ Creep behaviours of refractory bricks cannot be predicted based only on chemistry. Important variables which affect creep behaviour are chemistry of the bonding phase and firing temperature. The result of the formation of low viscosity glassy phase is poor creep behaviour. larger aggregate and lower porosity giving better creep resistance

13. Mechanical stress in refractory brick:- Mechanical spalling of refractory brick is caused by the stresses resulting from impact or pressure. Shattering or spalling of brickwork can result from rapid drying of wet brickwork, inadequate provision for thermal expansion, pinching of the hot ends of brick, especially in the f urnace arches. Mechanical stress in refractory materials is the force or pressure applied to the material, frequently as a result of mechanical impact, thermal expansion, or spalling. External factors including vibrations, heat cycling, and mechanical loading impart mechanical stress to the refractory material. Bricks which are strongest and toughest at the operating temperatures have the highest resistance to mechanical spalling. Porosity and density Crushing Strength Abrasion Resistance Cold modulus of rupture Mechanical stress impacts on

14. Porosity:- Porosity is a measure of effective open pore space in the refractory and is expressed as the average percentage of open pore space in the overall refractory volume. Apparent Porosity and bulk density can be calculated by the formula: AP in % = (W-D)100 / (W-S) Where: D = weight of the dried sample, W = Soaked weight/ saturated weight, i.e. Weight of sample containing liquid in the surface pores but not in the free surfaces, S = Suspended weight of the sample when immersed in liquid The mechanical strength of a refractory material is largely determined by the true porosity which is consists of closed pores and open pores, the latter being either permeable or impermeable. By using water air displacement method, the open pores are classified either as permeable or impermeable pores. Low porosity of refractory bricks is preferred as it gives better mechanical strength . BD = DꝬ /(W-S)

15. Crushing strength and Abrasion resistance:- It is a measure of the mechanical strength of the refractory brick. In furnaces, cold crushing strength is of importance, because of bricks with high crushing strength is more resistant to impact from rods or during removal of slag than a brick with a low cold crushing strength. Higher the CCS of a material, greater is the resistance to abrasion, corrosion. CCS: P / A Where: P = Compressive load at which refractory disintegrates A = area on which load is applied Abrasion resistance  – The mechanical stress of refractory bricks is not caused by pressure alone, but also by the abrasive attack of the solid raw materials as they slowly pass over the brickwork and by the impingement of the fast moving gases with fine dust particles. Hence the cold crushing strength is not alone sufficient to characterize the wear of the refractories.. Crushing strength:-

16. Thermal conductivity of refractory bricks :- High  conductivity is desirable for refractories used in construction needing efficient transfer of heat through brickwork, as in retorts, muffles, by-product coke oven walls, and recuperators. However, in most types of furnaces or vessels, low thermal conductivity is desirable for heat conservation, but is normally less important than other properties of the refractories. In refractory structures built of insulating firebricks or very other porous refractories, the type of furnace atmosphere can have a very appreciable effect on heat loss through the refractory walls. Major factors which affects the thermal conductivity of a refractory material are mineral composition, the amount of amorphous material (glass or liquid) which it contains, its  porosity, and its temperature.  Within the temperature range seen in the majority of the applications, thermal  conductivity decreases with increasing porosity.

17. Thankyou