Laboratory-based Failure Analysis of High Temperature Refractory Material
? Introduction
The refractory materials are inert inorganic solid materials which are stable at high temperature in contact with corrosive solid, liquid, and gas and can retain its physical shapes and structural strength at high temperature. These are mainly oxides, carbides, nitrides, and borides of aluminum, silicon, alkaline earth metals, and transition metals.
??Important Properties to be considered while performing RCA
? Physical Properties
?Mechanical Properties
? Thermal Properties
? Corrosion Properties
??Damage Mechanisms affecting High Temperature Refractory Material
??Thermal Reasons
??Overheating
Overheating is one of the important factors which cause severe damage to refractory lining. The overheating can be caused by thermal imbalance in the kiln or furnace or reactor when the input rate of feed and withdrawal rate of product get disturbed and the thermal input remains the same
?? Thermo-mechanical Load
The movements of refractory and steel shell are not really independent. Movement of one is restricted by the movement of other components. This fact is very important to develop the thermo-mechanical stress both during heat up and cool down and during the operation of the kiln or furnace or the reactor
?? Thermal Shock
Refractories in the furnaces or reactors experience temperature fluctuations in spite of its continuous type of operation. These variations may be due to fluctuations of feed to the furnace or reactor, due to variation in supply of primary or secondary air, variation in fuel input rate to burners.
??Creep
Creep in brickwork can be described as time and temperature-dependent deformation due to sustained load. A refractory's load-bearing strength or creep resistance is determined by the product's subsidence under a compressive load at elevated temperature. Creep resistance is the ability of a refractory to maintain dimensional stability under load at elevated temperatures, and can be measured following test procedures described in ASTM C832.
??Abrasion
There is a continuous countercurrent flow of gas and solid material in many of the furnaces and reactors. The flow of high-velocity dust-laden gases and the solid materials in the kiln system has a very abusive effect on the refractory life, because of the continuous abrasion.
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??Corrosion/Chemical attack
Refractory brick can be chemically attacked by the process environment in many ways. The design of the refractory lining; the mineralogical makeup of the refractory, the brick manufacturer pre-firing temperature; and the chemicals, compounds, or elements in the process environment and process temperature all impact the resistance of the refractory to corrosion and chemical attack. Corrosion or chemical attack can create sub-surface defects such as cracking, laminations and spalling. Impregnation of refractory may also affect the properties of the refractory, such as thermal conductivity, thermal shock resistance, crush strength, erosion resistance, refractoriness under load, creep resistance, and other critical properties.
??Failure Analysis Scheme for Refractory Material
?Visual Examination to characterize the damage
?Abrasion Resistance: To Evaluates the material's ability to resist surface wear.
?Air Permeability Test: To Determines the flow rate of gases through the material.
?Bulk Density: To Assesses the material's mass per unit volume.
?Apparent Porosity: To measures the volume of open pores.
?Cold Crushing Strength: To tests the material's load-bearing capacity in cold conditions.
?Pyrometric Cone Equivalent (PCE): Indicates the softening temperature range.
?Modulus of Elasticity, Rigidity, and Poisson's Ratio: Determines mechanical properties.
?Modulus of Rupture (MOR): Measures the flexural strength.
?Water Absorption: Evaluates the material's water uptake.
?Size & Dimensional Stability: Assesses the maintenance of shape under stress
?Petrographic Analysis by Optical Microscopy: Studies the internal structure.
?XRF (X-Ray Fluorescence): Analyses chemical composition.
?XRD (X-Ray Diffraction): Determines mineralogical composition.
?SEM/EDA/EDAX (Scanning Electron Microscopy/Energy Dispersive X-ray Analysis): Provides detailed microstructural information.
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