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⋅Δ𝑇
Where:
α is the coefficient of thermal expansion.
ΔT 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 ZrO2-
SiO2 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 /(2bd2) 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.
Fig-PCE cones before and after firing
Pyrometric cone equivalent (PCE) :-
12. Creep behaviour of refractories :-
• Refractories 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 furnace 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.
