Question 1

In the blast furnace hearth, a solidified 'skull' does not adhere to the carbon (graphite) blocks upon cooling primarily because of which material property?

Accepted Answer

Introduction / Context: Hearth linings in blast furnaces commonly use carbon blocks for their chemical compatibility and thermal properties. After shutdowns or localized cooling, metal/slag may freeze and form a “skull.” Whether this adheres to the lining depends on interfacial wetting and chemistry as well as temperature gradients. Given Data / Assumptions: Lining material: carbon/graphite blocks. Phases contacting the lining: molten iron and slag. Cooling causes temporary solidification (skull formation). Concept / Approach: Wetting describes how a liquid spreads on a solid surface. Poor wetting reduces adhesion of the solidified layer. Carbon surfaces are not readily wetted by molten iron and many slags, particularly in the presence of reducing conditions. Thus, even when a skull forms, it tends to separate rather than bond strongly to the carbon lining. Step-by-Step Solution: Identify interface: molten iron/slag vs carbon block.Recall interfacial property: non-wetting → low adhesion upon solidification.Conclude that non-wetting explains the lack of skull bonding. Verification / Alternative check: Operational experience and literature on hearth wear note the benefit of carbon’s non-wetting behavior in limiting skull adherence and chemical attack. Why Other Options Are Wrong: High thermal conductivity: Influences heat flow but not direct adhesion. Crushing strength: A mechanical bulk property unrelated to interfacial bonding. None of these: Incorrect because non-wetting is the key factor. Common Pitfalls: Confusing bulk mechanical properties with surface/interfacial phenomena that govern adhesion and skull formation. Final Answer: Non-wetting characteristic of carbon by iron/slag

Question 2

For annealing furnaces (walls, roofs, and combustion chambers), which refractory brick class is commonly specified to handle the moderate service temperatures and cycling?

Accepted Answer

Introduction / Context: Annealing furnaces typically operate in moderate temperature ranges (often below the extreme temperatures of smelting or steelmaking). Refractory selection balances cost, thermal shock, and chemical compatibility. This question focuses on the standard lining choice for walls, roofs, and combustion chambers in annealing service. Given Data / Assumptions: Service: annealing (moderate temperature, controlled atmosphere). Need: adequate refractoriness plus reasonable cost and thermal shock tolerance. No highly corrosive slags or extreme flame impingement conditions assumed. Concept / Approach: High-duty fireclay bricks (alumino-silicate) provide sufficient refractoriness and mechanical strength for many annealing applications and are cost-effective. Silica bricks are optimized for very high, steady temperatures (domes/crowns), and mullite or SiC bricks are used when higher temperature strength, abrasion, or special atmospheres demand them—often at higher cost. Step-by-Step Solution: Match annealing temperature range with material capability.Select economical, reliable option → high-duty fireclay.Exclude specialty bricks used for harsher, higher-temperature zones. Verification / Alternative check: Industry practice and specifications commonly list fireclay bricks for general-purpose furnace linings in annealing service. Why Other Options Are Wrong: Silica: Better suited to very high-temperature crowns with low thermal shock; over-specified here. Mullite/SiC: Higher performance and cost; chosen when specific stresses require them. Common Pitfalls: Defaulting to the most refractory brick, ignoring cost-performance optimization for moderate service. Final Answer: High-duty fireclay bricks

Question 3

When ground refractory material is shaped into bricks by hand moulding, the approximate water content required in the mix is closest to which value?

Accepted Answer

Introduction / Context: Forming refractory bricks can be done by hand moulding, soft-mud, stiff-mud extrusion, or pressing. The moisture content of the mix directly affects plasticity, workability, drying time, and the risk of cracking. This question tests typical moisture ranges for hand moulded bricks. Given Data / Assumptions: Method: hand moulding (soft, highly workable mass). Objective: sufficient plasticity to fill the mould and release cleanly. Drying and firing will follow to remove moisture and sinter the body. Concept / Approach: Hand moulding generally requires a higher moisture content than stiff-mud extrusion or dry pressing. Typical values for hand moulding are on the order of about 20% (sometimes a bit more depending on clay/plasticity). Values like 5% are far too low to provide plasticity; extremely high values such as 40–55% would cause severe drying shrinkage and cracking. Step-by-Step Solution: Identify forming method → hand moulding (soft).Recall typical moisture band → around 20% for workable plasticity.Choose 20% as the realistic, widely cited value. Verification / Alternative check: Manufacturing references list soft-mud/hand-moulding moisture contents near 20–30%, with 20% as a typical target for many bodies. Why Other Options Are Wrong: 5%: Too dry for hand moulding. 40% or 55%: Excess water leads to high drying shrinkage, cracking, and long drying cycles. Common Pitfalls: Confusing dry-pressing low-moisture mixes with hand moulding; underestimating shrinkage risks at very high water contents. Final Answer: 20%

Question 4

In furnace construction, the steel framing member used to resist and hold the thrust of the refractory brickwork is called a:

Accepted Answer

Introduction / Context: Large refractory structures such as furnaces require external steelwork to counteract the lateral thrusts caused by thermal expansion and arch action of brick linings. Knowing the correct terminology for these structural members is essential for design communication and maintenance. Given Data / Assumptions: Refractory-lined furnace with significant internal pressure and thermal expansion. Need to prevent outward bulging and maintain geometry. Standard industrial vocabulary is used. Concept / Approach: A buckstay is a heavy steel member (often an I-beam) attached to the exterior of furnace walls to resist the thrust of the brickwork and maintain alignment. It works with tie rods, anchors, and buckstays at intervals to keep walls straight and prevent structural failure as the lining expands and contracts with temperature cycles. Step-by-Step Solution: Identify the function: resist brickwork thrust.Match to the term: buckstay is the dedicated member for this role.Eliminate terms that indicate other features (breast wall, baffle, etc.). Verification / Alternative check: Furnace engineering texts and drawings label external vertical stiffeners as buckstays, often tied with horizontal members. Why Other Options Are Wrong: Breast wall: A wall portion near the furnace front. Armouring: Protective cladding, not the thrust-resisting frame. Baffle: Flow-directing internal plate, not a structural restraint. Common Pitfalls: Confusing internal flow-control components with external structural members. Final Answer: Buckstay

Question 5

In refractory technology, “grog” refers to which of the following material descriptions?

Accepted Answer

Introduction / Context: Grog is a widely used term in ceramics and refractories. It affects drying, firing shrinkage, and thermal shock behavior of clay bodies and refractories. Understanding what grog is and why it is added helps explain body formulation and performance under heat. Given Data / Assumptions: Grog derives from previously fired (calcined) material. Intended to reduce plasticity and shrinkage, improve thermal shock resistance. Typically alumino-silicate composition from crushed firebrick or calcined clays. Concept / Approach: Grog is non-plastic, pre-fired aggregate (often crushed firebrick), usually containing alumina and silica. Adding grog to a plastic clay body reduces drying/firing shrinkage, controls cracking, and can improve permeability and thermal shock performance by interrupting continuous clay matrix shrinkage paths. Step-by-Step Solution: Define grog: crushed, calcined refractory aggregate.Note composition: commonly alumino-silicate from firebrick.State function: non-plastic temper to tune body properties. Verification / Alternative check: Ceramics processing references consistently describe grog as non-plastic, calcined aggregate used to temper bodies and reduce shrinkage. Why Other Options Are Wrong: Each individual statement is true; hence the inclusive option (all of the above) is correct. Common Pitfalls: Assuming grog is raw clay or a chemical additive; overlooking its pre-fired, non-plastic nature. Final Answer: All of the above

Question 6

For high-alumina refractories, how does refractoriness (softening temperature) change as the Al2O3 content is increased, assuming other factors remain comparable?

Accepted Answer

Introduction / Context: Refractoriness is the ability of a refractory to withstand high temperatures without softening or deforming. In alumino-silicate systems, changing Al2O3 content has a pronounced effect on softening temperatures and slag resistance. This is a fundamental selection rule in refractory engineering. Given Data / Assumptions: Comparing high-alumina refractories of similar microstructural quality. Focus on Al2O3 content variation, not porosity or impurity effects. Service involves neutral to mildly basic conditions. Concept / Approach: Generally, increasing alumina content raises the softening temperature and improves refractoriness under load. High-alumina phases (e.g., mullite, corundum) possess higher melting/softening points than silica-rich phases. Therefore, as Al2O3 increases (and free silica decreases), refractoriness increases, assuming proper bonding and low impurity levels. Step-by-Step Solution: Consider phase equilibria: higher Al2O3 → more mullite/corundum → higher softening point.Hold other variables constant to isolate composition effect.Conclude refractoriness increases with Al2O3 content. Verification / Alternative check: Standards and datasheets for fireclay vs high-alumina bricks show progressive increases in refractoriness under load and softening points as Al2O3 rises from ~30% toward 90%+. Why Other Options Are Wrong: Decreases/Remains same: Contradict composition–property trends. Unpredictable: While microstructure matters, the dominant trend with higher alumina is increased refractoriness. Common Pitfalls: Ignoring impurity and porosity effects; although they modulate values, the core trend with Al2O3 is upward refractoriness. Final Answer: Increases

Question 7

In refractory materials engineering, the thermal diffusivity (α) of a brick is defined as α = k / (ρ * cp). For a given refractory brick, thermal diffusivity will be high when which property is high?

Accepted Answer

Introduction / Context: Thermal diffusivity tells us how quickly a temperature disturbance travels through a solid. In refractory selection for furnaces, kilns, and ladles, knowing which material property most increases thermal diffusivity helps engineers balance heat storage against heat flow. The classic definition used in heat transfer is α = k / (ρ * cp). Given Data / Assumptions: Thermal diffusivity α relates to thermal conductivity k, density ρ, and specific heat cp. We compare the effect of increasing k, ρ, and cp on α, assuming the other two remain constant. Brick behavior is considered under steady, single-phase, homogeneous conditions relevant to refractories. Concept / Approach: From α = k / (ρ * cp), thermal diffusivity rises with larger k and falls with larger ρ or cp. Physically, k measures how effectively heat is conducted, while ρ * cp measures the material’s thermal inertia (how much heat it stores per unit volume per degree). Step-by-Step Solution: Start from the definition: α = k / (ρ * cp).Hold ρ and cp constant and increase k: α increases because the numerator increases.Hold k constant and increase ρ: α decreases because the denominator increases.Hold k constant and increase cp: α decreases for the same reason.Therefore, a high thermal conductivity makes α high. Verification / Alternative check: Dimensional analysis confirms α has units of m^2/s. Increasing k (W/m·K) while keeping volumetric heat capacity (ρ * cp) constant raises α, meaning faster heat penetration—consistent with observed transient heating of high-k refractory inserts. Why Other Options Are Wrong: Density: Increasing ρ raises heat storage per volume, reducing α. Specific heat: Increasing cp also increases thermal inertia, lowering α. None of these: Incorrect because k clearly increases α. Common Pitfalls: Confusing thermal conductivity (steady-state property) with thermal diffusivity (transient response). A material can have high k yet lower α if its ρ * cp is also very high. Final Answer: thermal conductivity

Question 8

Silica (SiO2) refractories: Which property most directly indicates the degree of firing/vitrification achieved in a silica brick during manufacture?

Accepted Answer

Introduction / Context: Quality control of silica bricks relies on markers that reflect the extent of firing, phase transformation, and vitrification. While several properties are measured, not all directly reveal the degree of firing attained during manufacture. Given Data / Assumptions: Silica brick develops different SiO2 polymorphs (quartz, tridymite, cristobalite) as firing progresses. Manufacturing aims to achieve controlled conversion and vitrification for dimensional stability and performance. We compare specific gravity, fusion point, and RUL as indicators. Concept / Approach: Specific gravity (true density) of silica brick correlates with mineralogical conversion and degree of vitrification. Fusion point is an inherent high-temperature property that does not vary sensitively with small changes in firing extent. RUL measures performance under load at high temperature but is influenced by porosity, microstructure, and impurities, not solely the degree of firing. Step-by-Step Solution: Identify target: a parameter that tracks how far firing/vitrification has progressed.Specific gravity changes with phase development and glassy phase content, reflecting firing severity.Fusion point remains essentially characteristic of silica chemistry, offering limited resolution for firing level.RUL is a functional test, not a direct firing gauge.Therefore, specific gravity best indicates degree of firing. Verification / Alternative check: Materials practice correlates true specific gravity with phase analysis (e.g., XRD) to audit firing consistency in silica refractories. Why Other Options Are Wrong: Fusion point: Too insensitive to incremental firing differences. RUL: Performance metric under load; not a direct indicator of firing extent. None of these: Incorrect because specific gravity is a valid indicator. Common Pitfalls: Equating high RUL with high firing—RUL can improve via composition or grain size control without strictly reflecting firing degree. Final Answer: specific gravity

Question 9

Polymorphic transformations of silica: Identify the correct sequence of stable forms of SiO2 as temperature increases.

Accepted Answer

Introduction / Context: Silica (SiO2) has several polymorphs—quartz, tridymite, and cristobalite—each stable over different temperature ranges. These phase changes are crucial in refractory processing, as they influence thermal expansion, microcracking, and dimensional stability of silica bricks and castables. Given Data / Assumptions: Temperature ranges are approximate because kinetics and impurities can shift transformation temperatures. We are discussing equilibrium stability fields under typical ceramic firing conditions. Statements concern stability ranges rather than instantaneous kinetics. Concept / Approach: As temperature rises, silica shifts from quartz to tridymite to cristobalite. Standard ceramic references place the quartz–tridymite change near 870°C and the tridymite–cristobalite transition around 1470°C. Correct identification of these phases helps explain thermal shock behavior and firing schedules for silica refractories. Step-by-Step Solution: Check statement (a): Quartz stable to ~870°C — correct.Check statement (b): Tridymite stable roughly 870–1470°C — correct.Check statement (c): Above ~1470°C cristobalite is stable — correct.Therefore all three statements are true → choose 'All of the statements (a), (b) and (c) are correct.' Verification / Alternative check: Phase diagrams and high-temperature XRD studies corroborate the sequence and ranges used in refractory texts and standards. Why Other Options Are Wrong: Any single statement alone is incomplete; the correct answer acknowledges the full sequence. Common Pitfalls: Confusing cristobalite formation temperature or assuming irreversible conversion; on cooling, metastable retention can occur, impacting thermal expansion cycles. Final Answer: All of the statements (a), (b) and (c) are correct.

Question 10

Thoria (ThO2) as a refractory: Which combined statements about its properties and uses are correct?

Accepted Answer

Introduction / Context: Thoria (ThO2) is a premium refractory oxide used when extreme refractoriness and chemical stability are required. Understanding its strengths and limitations guides selection for crucibles, high-temperature windows, and specialized furnace components. Given Data / Assumptions: Thoria’s melting/fusion temperature exceeds 3000°C. We are comparing thermal shock resistance, slag resistance, and application use cases. Cost and radioactivity constraints influence practical adoption. Concept / Approach: Thoria has an exceptionally high fusion point and excellent chemical stability, especially toward basic slags, but like many high-modulus ceramics, it is susceptible to thermal shock due to relatively low fracture toughness and significant thermal expansion mismatch in rapid temperature cycling. Its expense and radioactivity limit usage to niche, high-purity metal processing where contamination must be minimized. Step-by-Step Solution: Confirm (a): Very high fusion temperature with poor thermal shock resistance — consistent with oxide ceramics behavior.Confirm (b): Thoria resists basic slags due to chemical stability of ThO2 in alkaline environments.Confirm (c): Used in crucibles for specialty melting despite cost/radioactivity, because it resists contamination and withstands very high temperatures.Therefore, all three statements are correct. Verification / Alternative check: Reference data for oxide refractories list ThO2 among the highest melting oxides with strong slag resistance but caution on thermal shock performance. Why Other Options Are Wrong: Choosing any single statement ignores that all are true for thoria. Common Pitfalls: Assuming that very high melting point guarantees thermal shock resistance; thermal shock depends on thermal expansion, modulus, and toughness, not only on melting point. Final Answer: All of the statements (a), (b) and (c) are correct.

Question 11

Pyrometric Cone Equivalent (PCE): For a ‘‘superduty’’ refractory (Seger cone number > 33), the corresponding temperature is closest to which value (°C)?

Accepted Answer

Introduction / Context: Pyrometric Cone Equivalent (PCE), often expressed via Seger cone numbers, provides a comparative measure of refractoriness under a standardized heating schedule. Classification into duty classes (e.g., superduty) guides selection for severe high-temperature service such as coke ovens, glass tanks, and steel furnaces. Given Data / Assumptions: ‘‘Superduty’’ is defined as PCE greater than 33. We use standard cone–temperature correlations for oxidizing conditions. We select the temperature that best represents cone > 33. Concept / Approach: Seger cone numbers map to characteristic softening temperatures. For superduty refractories (cone > 33), the corresponding temperature is above about 1730°C. This class exhibits maximal refractoriness among common silica/aluminosilicate compositions used industrially. Step-by-Step Solution: Identify threshold: superduty → PCE > 33.Match cone > 33 to approximate temperature → ≳ 1730°C.Select the closest listed temperature → 1730°C. Verification / Alternative check: Manufacturer datasheets and standards cite PCE grades with approximate equivalent temperatures; superduty aligns with ≳ 1730°C, validating the selection. Why Other Options Are Wrong: 1520, 1630, 1670°C: These correspond to lower cone numbers and do not meet the superduty threshold. Common Pitfalls: Confusing PCE with simple fusion point; PCE reflects a softening behavior under load and a heating schedule, not just a single-point melting temperature. Final Answer: 1730

Question 12

Blast furnace troughs and runners: Hot metal runners are commonly lined with which class of refractory bricks for serviceability and economy?

Accepted Answer

Introduction / Context: In blast furnaces, hot metal (liquid iron) and slag are guided through troughs and runners. The lining selection must balance thermal shock resistance, corrosion/erosion resistance, cost, and ease of maintenance. Different materials are favored for hot metal versus slag handling. Given Data / Assumptions: Question targets the typical, widely used lining class for hot metal runners. Service involves high temperature, molten iron contact, and intermittent thermal cycling. Economy and availability also affect choice. Concept / Approach: Fireclay and high-alumina refractories have long been used in hot metal runners owing to acceptable resistance to iron erosion and thermal cycling at reasonable cost. Silicon carbide is used in some severe-wear zones, but broadly and economically, fireclay-based shapes and ramming mixes have been prevalent, especially historically and in general practice. Silica refractories are more suited to coke ovens and hot blast stoves; magnesite is a basic refractory better matched to basic slags in furnaces, not the standard choice for hot metal runners. Step-by-Step Solution: Identify common industry practice: fireclay linings for hot metal runners.Evaluate alternatives: SiC is specialized and costlier; silica and magnesite do not match typical runner requirements as broadly as fireclay.Choose fireclay as the most representative answer. Verification / Alternative check: Operations manuals and refractory vendor literature list fireclay/high-alumina castables and bricks as the standard baseline for runners, with SiC used selectively. Why Other Options Are Wrong: Silica: Excellent at high temperatures in stoves but not the go-to for molten iron runners. Carborundum (SiC): Used in high-wear areas; not the most common general lining historically. Magnesite: Basic slag service; not standard for hot metal runners. Common Pitfalls: Assuming the most wear-resistant material (SiC) is always chosen; cost and repairability matter in large-scale ironmaking logistics. Final Answer: fireclay

Question 13

Refractoriness/fusion point classification: ‘‘Superduty’’ refractories fall in which fusion temperature range (°C)?

Accepted Answer

Introduction / Context: Duty classifications (medium, high, superduty) help users quickly match refractories to service temperatures. ‘‘Superduty’’ indicates the highest class among common siliceous/aluminosilicate refractories, associated with very high softening/fusion behavior under standard tests such as PCE/Seger cones. Given Data / Assumptions: PCE cone > 33 aligns with ‘‘superduty’’ class. Equivalent temperatures are approximate but standardized for comparison. We pick the range that correctly represents superduty behavior. Concept / Approach: Superduty refractories exhibit fusion points exceeding about 1730°C, allowing duty in severe, continuous high-temperature operations. Options with lower ranges correspond to lower duty classes and do not meet superduty thresholds, while a blanket ‘‘> 2000°C’’ is too restrictive for the class definition. Step-by-Step Solution: Map class → temperature: superduty ≳ 1730°C.Evaluate choices: only ‘‘> 1730’’ matches standard classification.Select ‘‘> 1730’’ as correct. Verification / Alternative check: Correlation tables used in refractory datasheets confirm that superduty aligns with temperatures above 1730°C. Why Other Options Are Wrong: 1520–1630 and 1630–1670°C: Too low for superduty. > 2000°C: Exceeds typical superduty threshold; applies only to elite oxide refractories. Common Pitfalls: Equating ‘‘superduty’’ with absolute highest melting oxides; the term is specific to established PCE-based classes. Final Answer: > 1730

Question 14

RUL (Refractoriness Under Load) testing: In which heating environment is the standard test piece normally heated for RUL determination?

Accepted Answer

Introduction / Context: Refractoriness Under Load (RUL) measures how a refractory deforms under a specified mechanical load as temperature rises. Accurate temperature control and uniform heating are essential for comparable results across laboratories and batches. Given Data / Assumptions: Standard RUL tests use controlled heating rates and constant loads on cylindrical test pieces. The apparatus must maintain precise temperature profiles. Atmosphere may vary for special tests, but a controlled lab method is implied. Concept / Approach: RUL rigs are typically electrically heated furnaces (vertical dilatometers or load frames) because electric furnaces provide accurate, programmable temperature ramps and stable measurement conditions. While oxidizing or reducing atmospheres can be introduced for special studies, the canonical answer describing the heating environment/equipment is the electric furnace used to conduct the test. Step-by-Step Solution: Identify the core requirement: precise, uniform, ramped heating under load.Match requirement to equipment: electric furnaces meet precision needs for RUL.Choose ‘‘electric furnace’’ as the standard heating environment/equipment. Verification / Alternative check: Standards and vendor manuals show RUL frames integrated with electric furnaces, supporting consistent test conditions and temperature control. Why Other Options Are Wrong: Oxidising/reducing/neutral atmosphere: These describe gas chemistry, not the primary heating medium/equipment. The question targets the heating setup, which is an electric furnace. Common Pitfalls: Confusing furnace atmosphere with furnace type; RUL focuses on load-bearing deformation during controlled electric heating. Final Answer: electric furnace

Question 15

Chromite refractories: Select the correct combined statements about bonding, thermal fatigue resistance, and slag resistance.

Accepted Answer

Introduction / Context: Chromite refractories (based on FeO·Cr2O3 spinel compositions) are widely used in metallurgical furnaces where a neutral-to-basic refractory is advantageous. Their behavior against thermal cycling and basic slags influences lining life in steel and nonferrous operations. Given Data / Assumptions: Bonding systems for chromite shapes include lime and clay additions. Thermal fatigue (spalling) resistance depends on thermal expansion, thermal conductivity, and microstructural stability. Basic slag resistance is a key selection criterion for certain vessels. Concept / Approach: Chromite (especially low-silica grades) exhibits good resistance to basic slags and thermal shock/fatigue relative to some silica and magnesite products in specific operating windows. Traditional manufacturing used lime/clay bonding; modern products may use synthetic spinel or resin bonds, but the statement remains broadly correct in a classical context. Step-by-Step Solution: Validate (a): Lime/clay bonding for chromite bricks is historically established.Validate (b): Silica-free chromite tends to show favorable thermal fatigue resistance versus pure silica and some magnesite grades.Validate (c): Chromite’s chemistry gives good basic slag resistance.Hence, all three statements are correct. Verification / Alternative check: Handbooks on refractories list chromite as neutral-basic refractory with the cited properties and bonding systems, supporting the combined statement. Why Other Options Are Wrong: Choosing any single option omits the full, correct property set; the combined choice reflects established practice. Common Pitfalls: Assuming all magnesite bricks outperform chromite in every respect; resistance comparisons depend on grade, impurities, and operating environment. Final Answer: All of the statements (a), (b) and (c) are correct.

Question 16

Thermal shock behavior of refractories: The ability of a refractory brick to withstand sudden temperature changes without cracking is known as what?

Accepted Answer

Introduction / Context: Refractory linings experience rapid heating and cooling during start-ups, shutdowns, and process fluctuations. A key property determining service life under such conditions is the material’s resistance to thermal shock, often discussed in terms of resistance to spalling (fragmentation or flaking due to thermal stresses). Given Data / Assumptions: Thermal shock is induced by steep temperature gradients and resulting thermal stresses. Terminology distinguishes between high-temperature strength (refractoriness, RUL) and thermal shock performance (spalling resistance). We seek the correct property name for sudden temperature change endurance. Concept / Approach: Spalling resistance is the recognized term describing a refractory’s ability to resist crack initiation and propagation under rapid temperature cycling. It depends on thermal expansion, thermal conductivity, elastic modulus, and microstructural features such as porosity and crack-bridging phases. It is distinct from refractoriness (ability to withstand high temperature without softening) and RUL (deformation under load at temperature). Step-by-Step Solution: Define the phenomenon: sudden temperature changes → thermal shock → risk of spalling.Match to property: resistance to spalling = spalling resistance.Differentiate from refractoriness and RUL.Select 'spalling resistance' as the correct term. Verification / Alternative check: Thermal shock testing methods (e.g., water quench, flame impingement cycling) are used to quantify spalling resistance, confirming the property’s distinct identity. Why Other Options Are Wrong: Refractoriness: Measures softening/melting behavior, not thermal shock endurance. RUL: Measures creep/deformation under load, not crack resistance during quench. None of these: Incorrect because spalling resistance is the standard term. Common Pitfalls: Assuming high refractoriness implies good thermal shock resistance; many ultra-refractory oxides can be brittle and spall readily under rapid cycling. Final Answer: spalling resistance

Question 17

In refractory engineering, highly porous bricks exhibit which combined behaviors in service? Select the most complete statement.

Accepted Answer

Introduction: Refractory bricks with high porosity are widely used in furnaces, kilns, and reactors for insulation and lining. Understanding how porosity affects penetration by slags and gases, thermal conductivity, strength, and thermal-shock behavior is essential for selecting the right brick for each zone of a high-temperature unit. Given Data / Assumptions: “High porosity” means a larger fraction of interconnected pores within the refractory body. Service involves contact with slags/metals/flue gases and cyclic heating. Choice requires comparing multiple properties simultaneously. Concept / Approach: Porosity influences transport, strength, and thermal response. More pores generally reduce thermal conductivity (good for insulation) but diminish bulk strength and allow easier penetration if pores are open and connected. However, pores also blunt thermal stresses, improving thermal-shock (spalling) resistance because the material accommodates expansion gradients. Step-by-Step Solution: Relate porosity to permeability: higher open porosity increases pathways for slag/metal/gas ingress → poorer penetration resistance.Relate porosity to thermal transport: more voids mean lower solid-to-solid contacts → lower heat conductivity.Relate porosity to strength: reduced load-bearing cross-section → lower cold crushing strength and modulus.Relate porosity to thermal shock: pores distribute and arrest thermal stress → better spalling resistance.Hence all listed effects occur together. Verification / Alternative check: Materials handbooks consistently show inverse relationships between porosity and both strength and thermal conductivity, alongside improved resistance to thermal spalling for lightweight or more porous refractories. Why Other Options Are Wrong: Only any one benefit (like improved spalling) ignores the trade-offs in strength and permeability. Claiming insulation improves “with no other trade-offs” is incorrect; strength and penetration resistance decline. Common Pitfalls: Confusing closed porosity (beneficial for insulation) with open porosity (raises permeability). Also, assuming higher porosity always better—zone duty determines the acceptable porosity level. Final Answer: All of the above

Question 18

Silica (SiO2) polymorphs vs. temperature: which crystalline form of silica is thermodynamically stable between 1470°C and its melting point near 1713°C?

Accepted Answer

Introduction: Silica exhibits temperature-dependent polymorphism. Correctly identifying the stable phase at high temperatures is important for choosing silica bricks and predicting dimensional changes and thermal-shock behavior in furnaces. Given Data / Assumptions: Temperature range: 1470°C up to the melting point (~1713°C). Common polymorphs: quartz (low/high), tridymite, cristobalite. We are concerned with equilibrium stability domains, not metastable persistence. Concept / Approach: Equilibrium phase relationships for SiO2 indicate that quartz is low-temperature, tridymite is intermediate (~870–1470°C), and cristobalite is the high-temperature phase stable from ~1470°C to melting. Bricks and linings must account for the volume changes associated with these transformations. Step-by-Step Solution: Identify the upper-temperature stable polymorph: cristobalite.Confirm boundaries: tridymite stabilizes below ~1470°C; quartz at still lower temperatures.Therefore, for 1470–1713°C, cristobalite is the correct phase. Verification / Alternative check: Phase diagrams and refractory textbooks list cristobalite as the stable phase approaching silica’s melting point. Why Other Options Are Wrong: Tridymite: stable at intermediate temperatures below ~1470°C. Quartz: low-temperature region; transforms before reaching 1470°C. “None of these” and glassy silica: do not represent the equilibrium high-T crystalline silica. Common Pitfalls: Confusing metastable persistence (e.g., retained tridymite) with true equilibrium stability, and overlooking large volume changes during phase transitions. Final Answer: Cristobalite

Question 19

Industrial applications of fireclay bricks: identify where fireclay bricks are generally not used due to service temperature and chemical demands.

Accepted Answer

Introduction: Fireclay refractories (alumino-silicate) are widely used below ultra-high temperatures, whereas silica refractories dominate extremely hot zones like coke oven crowns and regenerators. Choosing the wrong brick can cause rapid creep, deformation, or chemical attack. Given Data / Assumptions: Fireclay bricks have moderate refractoriness and good thermal-shock resistance. Coke oven regenerators experience very high steady temperatures and thermal cycling. Other listed units may run at temperatures compatible with fireclay in selected zones. Concept / Approach: Silica refractories excel at very high temperatures and resist creep, making them standard for coke oven chambers and regenerators. Fireclay is common in lower-temperature zones, backup linings, or where thermal shock is frequent but peak temperatures are lower. Step-by-Step Solution: List typical service temperatures: coke oven regenerators approach high steady temperatures favoring silica bricks.Assess each candidate unit’s lining practice: the regenerator’s hot-end design relies on silica’s stability.Conclude that fireclay is generally not used in the coke oven regenerators. Verification / Alternative check: Standard refractory selection guides recommend silica for coke oven walls and regenerators because of dimensional stability at high temperature and resistance to creep. Why Other Options Are Wrong: Beehive coke oven and some stove chambers can employ fireclay in certain zones at lower peak temperatures. The combustion chamber of BF stoves may incorporate fireclay in selected sections depending on duty. General furnace crowns vary; many hot crowns use silica but not exclusively; question targets the unit most clearly ill-suited to fireclay: the coke oven regenerator. Common Pitfalls: Assuming one refractory type fits all zones; actual practice uses graded linings with silica in the hottest areas and fireclay below. Final Answer: Coke oven regenerators

Question 20

Among common refractory types, which brick typically has the lowest firing temperature window (about 1250–1400°C) during manufacture?

Accepted Answer

Introduction: Refractory manufacturing requires firing the green body to develop desired phases and strength. Different chemistries need different peak firing temperatures. Knowing these ranges helps in cost estimation and quality control. Given Data / Assumptions: Fireclay: alumino-silicate composition. Silica, magnesite, and direct-bonded basic bricks typically demand higher firing temperatures to achieve strong bonding. Target window named: 1250–1400°C. Concept / Approach: Fireclay bodies sinter effectively at lower temperatures than pure silica or basic (MgO-rich) systems. Direct-bonded basic bricks especially require higher temperatures to develop direct periclase or spinel bonding. Step-by-Step Solution: Compare typical firing windows: fireclay ~1250–1400°C.Silica bricks often >1400–1500°C to stabilize cristobalite/tridymite bonding.Magnesite/basic bricks require still higher temperatures.Hence the minimum window given corresponds to fireclay. Verification / Alternative check: Manufacturers’ datasheets and handbooks align with these firing ranges for standard grades. Why Other Options Are Wrong: Direct-bonded basic/magnesite/silica: need higher peak temperatures to densify properly. Chromite often fired higher than fireclay for adequate bonding. Common Pitfalls: Confusing service temperature capability with firing temperature; basic bricks may work at higher service temperatures but also require hotter firing during manufacture. Final Answer: Fireclay brick

Refractory Technology Questions