Question 1

Pure oxide refractories are often monocrystalline and self-bonded. Which oxide brick is also known for use as a moderator material in certain nuclear reactor designs?

Accepted Answer

Introduction / Context: Among pure oxide refractories, several have exceptional high-temperature properties and unique nuclear attributes. Beryllium oxide (beryllia), alumina (corundum), and thoria are advanced ceramics with high melting points and good thermal properties. In nuclear engineering, moderators slow fast neutrons to thermal energies without capturing too many neutrons. Given Data / Assumptions: We compare oxide bricks by their nuclear relevance. “Moderator” implies strong scattering and very low absorption cross-section. Carborundum refers to silicon carbide, which is not an oxide and not a typical moderator. Concept / Approach: Beryllia (BeO) has very high thermal conductivity for a ceramic, high melting point, excellent thermal shock resistance, and—crucially—favourable neutron moderating properties with low neutron absorption. Graphite and heavy water are more famous moderators, but BeO has been used in specialized research and high-temperature reactor concepts. Corundum (Al2O3) and thoria (ThO2) are highly refractory but lack the optimal moderating characteristics that BeO offers. Step-by-Step Solution: Identify the oxide with moderator credentials: BeO (beryllia).Eliminate carborundum: it is SiC, not an oxide, and not a standard moderator.Corundum and thoria are refractories but not preferred as moderators due to less favourable neutron cross-sections. Verification / Alternative check: Nuclear materials references cite BeO as a potential moderator/reflector with low absorption and good thermal properties, though toxicity and cost limit widespread deployment. Why Other Options Are Wrong: Carborundum: not an oxide; primarily a structural/high-temperature semiconductor ceramic.Corundum/Thoria: refractory oxides but not used as mainstream moderators. Common Pitfalls: Assuming all high-melting oxides are suitable moderators; confusing reflectors with moderators. Final Answer: Beryllia

Question 2

Ceramic fundamentals: pick the WRONG statement about ceramics, glass formation, brittleness, vitrification, and conductivity of non-oxide ceramics.

Accepted Answer

Introduction / Context: General ceramic science covers glass formation, sintering, vitrification, mechanical behaviour, and electrical properties. This question asks you to identify the incorrect statement among common truths about ceramics and glasses. Given Data / Assumptions: “Glass” refers to an amorphous, non-crystalline solid formed from a melt upon cooling, often workable in a viscous state at elevated temperature. “Vitrification” describes the development of glassy phase during firing of many ceramic bodies. Non-oxide ceramics (carbides, nitrides, borides) often show semiconducting or conductive behaviour. Concept / Approach: During firing, many clay-based and alumino-silicate ceramics undergo partial vitrification: a glassy phase forms that helps densify and bond grains, increasing strength. Therefore, claiming that ceramics “do not undergo vitrification” is false. Glasses indeed become viscous fluids upon heating and can be shaped by blowing, pressing, or drawing. Ceramics are typically brittle due to ionic/covalent bonding and limited dislocation mobility. Non-oxide ceramics (e.g., SiC, TiC, TiN, BN variants) can be semiconducting or even fairly conductive depending on composition and defect chemistry. Step-by-Step Solution: Evaluate (a): accurate description of glass behaviour.Evaluate (b): false; many ceramics do vitrify to some extent on firing.Evaluate (c): true; brittle fracture is common.Evaluate (d): true; many non-oxides show semiconducting features. Verification / Alternative check: Phase diagrams and firing curves for stoneware, porcelain, and fireclay bodies show glass-phase formation; electrical characterisation of SiC and Si3N4 confirms semiconducting trends. Why Other Options Are Wrong: (a), (c), and (d) are broadly correct statements in materials science. Common Pitfalls: Confusing “all ceramics” with “glasses”; not all ceramics vitrify to the same extent, but many do. Final Answer: Ceramic materials do not undergo vitrification on heating.

Question 3

Drying of green (unfired) refractories: a faster drying rate tends to increase which characteristics of the moulded refractories?

Accepted Answer

Introduction / Context: After shaping, ceramic and refractory products are dried to remove free water before firing. The drying schedule strongly influences microstructure and defect formation. Excessively rapid drying can cause non-uniform moisture gradients, internal stresses, and defect generation that will compromise the final fired properties. Given Data / Assumptions: “Green strength” refers to mechanical strength before firing. Voids arise from differential drying, skinning, and trapped vapour paths. Shrinkage occurs as particle packing increases when water leaves. Concept / Approach: When drying proceeds too fast, the surface densifies (“case-hardens”) while the interior remains wet. As water tries to escape, internal pressure and capillary forces can create microcracks and void networks. Rapid moisture loss also amplifies dimensional change over short times, raising risk of warping and differential shrinkage. Therefore, a carefully staged drying curve is typically used: low temperature with airflow at first, followed by progressively higher temperatures to avoid defects. Step-by-Step Solution: Link drying rate to moisture gradient: faster rate → steeper gradients.Steeper gradients → higher stress → microvoids and cracking.Rapid water removal → higher immediate dimensional change → more shrinkage.Conclusion: both voids and shrinkage increase with excessively fast drying. Verification / Alternative check: Industrial drying protocols limit ramp rates and use humidity control to minimize defects, showing that slower, controlled drying yields denser, less defective bodies with better fired strength. Why Other Options Are Wrong: Green strength generally decreases with defects; fast drying does not inherently increase it. Common Pitfalls: Confusing firing shrinkage with drying shrinkage; assuming any reduction in water automatically strengthens the green body without microstructural damage. Final Answer: Both (b) and (c)

Question 4

Neutral refractory identification: among the following, which material is classified as a neutral refractory for many metallurgical environments?

Accepted Answer

Introduction / Context: Refractories are often grouped as acidic, basic, or neutral, according to their chemical compatibility with slags and atmospheres. Selecting the correct class prevents premature corrosion and failure. Neutral refractories generally tolerate both acidic and basic environments better than strictly acidic or basic bricks. Given Data / Assumptions: Options include graphite, magnesite–chrome, silica, and magnesia. Neutral means relatively stable to a range of slags, not strongly reactive with either acidic or basic melts. Oxidation sensitivity is considered separately from acid/base classification. Concept / Approach: Silica is acidic; magnesia and magnesite–chrome are basic. Graphite, while susceptible to oxidation at high temperature in oxidizing atmospheres, is chemically neutral to many slags and molten metals and is widely used in neutral or carbon-containing linings, tundish boards, crucibles, and as a component in alumina–carbon/magnesia–carbon bricks. Step-by-Step Solution: Classify each option: silica → acidic; magnesia and magnesite–chrome → basic; graphite → neutral.Account for practical caveat: graphite must be protected from oxidation, e.g., by inert/reducing atmosphere or resin/antioxidant additions.Therefore, the neutral refractory among the options is graphite. Verification / Alternative check: Materials handbooks list graphite and chromite as neutral classes, contrasting them with high-silica (acidic) and magnesia (basic) bricks. Why Other Options Are Wrong: Magnesite chrome and magnesia: basic class.Silica: acidic class. Common Pitfalls: Conflating oxidation behaviour with acid/base classification; assuming “neutral” means universally inert under all conditions. Final Answer: Graphite

Question 5

Thermal spalling in furnaces: at which stage of operation is thermal spalling most likely to occur due to steep thermal gradients?

Accepted Answer

Introduction / Context: Thermal spalling is the fracture or flaking of refractory surfaces caused by thermal stresses generated during temperature changes. Because ceramics have limited tensile strength and thermal shock resistance, the rate and uniformity of heating and cooling critically affect spalling risk. Given Data / Assumptions: Furnace linings experience heat-up and cool-down cycles. Thermal gradients produce stress; tensile stress near the surface promotes spalls. We seek the stage most prone to spalling. Concept / Approach: On rapid cool-down, the hot interior remains expanded while the surface contracts as it cools, inducing tensile stresses at or near the exposed face. Ceramics are weak in tension, so this condition is especially damaging. While rapid heat-up can also cause spalling (compressive-to-tensile reversals and differential expansion), the classic and more frequent problem is during cooling because the outer skin sees strong tensile stress. Step-by-Step Solution: Relate cooling to tensile stress at the surface due to contraction.Recognize ceramic brittleness → low tolerance for surface tension.Conclude that spalling is mainly associated with cooling down. Verification / Alternative check: Thermal shock parameters (R and R′) and case studies show more failures attributed to quenching/cool-down events than controlled heat-ups, especially in silica and high-alumina linings without fiber back-up. Why Other Options Are Wrong: Warming up can cause damage if too fast, but is generally less severe given controlled ramping and compressive states.“Both” and “Neither” do not capture the principal mode observed in service. Common Pitfalls: Assuming symmetric behaviour in heating and cooling; ignoring thermal mass and conductivity differences across the lining. Final Answer: Cooling down

Question 6

Atmosphere compatibility: which refractory brick type should NOT be used in an oxidising atmosphere because it degrades by oxidation?

Accepted Answer

Introduction / Context: Furnace atmospheres can be oxidising, neutral, or reducing. Refractory selection must consider chemical stability under the prevailing oxygen potential. Carbon-containing bricks excel in reducing or neutral conditions but will react unfavourably when exposed to oxygen at high temperature. Given Data / Assumptions: Candidates include tar-bonded dolomite (contains carbon binder), carbon bricks, silica bricks, and fireclay bricks. Oxidising atmosphere implies significant O2 or oxidising species present. Service temperatures are high enough to promote rapid oxidation reactions. Concept / Approach: Carbon bricks and carbon-rich linings oxidise to gaseous CO/CO2 in the presence of oxygen, causing rapid mass loss, porosity increase, and structural failure. While tar-bonded dolomite contains carbon in the binder, the most clearly unsuitable category in a strongly oxidising zone is pure carbon brick. Silica and fireclay are stable in oxidising conditions, though each has its own limits regarding thermal shock and slag chemistry. Step-by-Step Solution: Identify oxidation-prone composition: carbon bricks.Relate oxidation to weight loss and strength loss.Conclude that carbon bricks should not be used in oxidising atmospheres. Verification / Alternative check: Industrial practice restricts carbon-containing refractories to reducing zones (blast furnace hearths, ladles with protective slag/cover, certain non-ferrous operations). Why Other Options Are Wrong: Tar dolomite: some oxidation sensitivity exists, but the question targets the clearly unsuitable class—carbon bricks.Silica/Fireclay: acceptable in oxidising atmospheres within their temperature/slag limits. Common Pitfalls: Equating any presence of carbon binder with pure carbon bricks; not differentiating between mild and strongly oxidising conditions. Final Answer: Carbon

Question 7

Effect of density on silica refractories: as bulk density of silica bricks increases, how do slag-corrosion resistance and thermal spalling resistance change?

Accepted Answer

Introduction / Context: Bulk density and porosity are fundamental microstructural parameters controlling how refractories behave in service. For silica bricks used in high-temperature structures, density affects both chemical corrosion by slags and resistance to thermal shock or spalling. Given Data / Assumptions: We compare silica bricks of different bulk densities (implying different porosities). Slag attack refers to chemical corrosion and infiltration. Spalling resistance refers to the ability to withstand thermal-shock-induced cracking. Concept / Approach: Higher bulk density generally means lower open porosity and a more continuous solid network. Lower porosity reduces slag infiltration and dissolutive attack, improving corrosion resistance. However, lower porosity also reduces the ability to accommodate thermal strains and to blunt crack propagation, which can lower thermal-shock/spalling resistance. Conversely, more porous (lower-density) bricks often show better thermal shock tolerance but worse slag resistance due to easier penetration pathways. Step-by-Step Solution: Link density ↑ → porosity ↓.Porosity ↓ → fewer infiltration paths → slag resistance ↑.Porosity ↓ → less strain accommodation and crack deflection → spalling resistance ↓.Therefore, both “slag resistance increases” and “spalling resistance reduces” are true as density increases. Verification / Alternative check: Property charts for silica bricks show corrosion resistance improving with density, while thermal-shock tests (quench cycles to failure) typically favour more porous grades. Why Other Options Are Wrong: Choosing only (a) or only (b) captures half the trend; selecting neither contradicts well-understood porosity effects. Common Pitfalls: Assuming “denser is always better” without trade-offs; overlooking that application dictates whether corrosion or thermal shock is the dominant design constraint. Final Answer: Both (a) and (b).

Question 8

Refractory selection insight: fireclay (acidic) bricks are generally not suitable for lining which of the following high-temperature units due to basic slag attack and severe chemical wear?

Accepted Answer

Introduction / Context: Choosing the correct refractory for a furnace zone is primarily a question of chemical compatibility. Fireclay bricks are acidic alumino-silicate materials. They perform well against acidic slags and many flue environments but deteriorate quickly in basic slags rich in CaO, FeO, and alkalis. This question asks where fireclay bricks should not be used, focusing on a zone notorious for aggressive basic conditions: the bottom of a hot metal mixer. Given Data / Assumptions: Fireclay refractories are acidic/near-neutral in behavior. Hot metal (molten iron) mixers experience basic slags and metal contact at high temperature. Cupolas and gas producers see combustion products and can employ silica or fireclay in certain regions. Open-hearth roofs are typically silica; some auxiliary areas may use fireclay. Concept / Approach: Acid-base refractory compatibility rule: acidic refractories are attacked by basic slags; basic refractories resist basic slags. The mixer bottom is a load-bearing, metal/slag-wetted zone under high temperature with basic slag chemistry. Basic bricks (e.g., magnesite, dolomite, magnesia-carbon) are preferred. Therefore, fireclay is unsuitable there. Step-by-Step Solution: Identify chemistry of each service: mixer bottom → basic; cupola/gas producer/roofs → often acidic or mixed.Apply compatibility rule: acidic fireclay fails rapidly in basic slag exposure.Conclude: do not use fireclay for the bottom of a hot metal mixer. Verification / Alternative check: Metallurgical practice specifies basic refractories for mixing and steelmaking vessels. Failure analyses frequently show silica/fireclay dissolution where basic slags impinge, validating this selection. Why Other Options Are Wrong: Cupola: silica/fireclay are used in some zones depending on practice.Gas producer: fireclay is commonly serviceable against producer gas.Roof of open-hearth furnace: silica is standard; fireclay may appear in non-hot-face areas.Soaking pit sidewalls: can use fireclay/high-alumina depending on duty. Common Pitfalls: Ignoring slag basicity; assuming temperature alone dictates brick choice; overlooking that different zones within the same unit need different chemistries. Final Answer: Bottom of hot metal mixer

Question 9

Fusion (melting) temperature of high-purity silica (SiO2): choose the closest standard value in °C used in refractory practice.

Accepted Answer

Introduction / Context: The fusion temperature of silica is a core reference when selecting acidic refractories for roofs, checkerwork, and coke-oven walls. Pure silica exhibits a high melting point and complex polymorphic behavior that influences thermal expansion and spalling resistance. Given Data / Assumptions: Purity refers to high-SiO2 compositions with minimal fluxing impurities. Standard melting value for quartz/silica used in handbooks is required. Concept / Approach: Silica (SiO2) has a melting (fusion) point around 1710–1715 °C. The exact figure depends slightly on crystalline form and measurement conditions, but 1715 °C is widely cited in refractory practice and materials texts. Step-by-Step Solution: Recall standard data: pure silica melts near 1710–1715 °C.Select the closest handbook value from the options.Answer: 1715 °C. Verification / Alternative check: Vendor datasheets for silica bricks list softening and transformation ranges below fusion; fusion reference is consistently ~1715 °C for pure SiO2. Why Other Options Are Wrong: 1350 °C: too low; typical of fireclay softening ranges, not pure silica fusion.2570 °C and 2800 °C: belong to high-melting oxides (e.g., BeO, MgO ranges), not SiO2.1900 °C: above accepted melting value for pure silica. Common Pitfalls: Confusing softening under load with fusion; ignoring the role of impurities that can flux silica at lower temperatures. Final Answer: 1715 °C

Question 10

Identify the non-oxide refractory: which of the following is not a single-oxide refractory material?

Accepted Answer

Introduction / Context: Refractories are classified by chemistry into oxide, non-oxide, and carbonaceous groups. Single-oxide refractories are based on one dominant oxide (e.g., Al2O3, MgO, ZrO2). Non-oxide ceramics such as carbides, nitrides, and borides offer excellent high-temperature strength and thermal shock resistance but behave differently in oxidizing atmospheres. Given Data / Assumptions: The question targets the chemical type, not brand names. We must choose the material that is not a single oxide. Concept / Approach: Zirconia (ZrO2) and magnesia (MgO) are classic single-oxide refractories. Silicon carbide (SiC) is a non-oxide ceramic compound and thus not a single oxide. Therefore, SiC is the correct choice. Step-by-Step Solution: List compositions: ZrO2 → oxide; MgO → oxide; SiC → carbide (non-oxide).Apply category definition: “single-oxide” excludes carbides.Select Silicon carbide (SiC). Verification / Alternative check: Handbooks group SiC with non-oxide refractories used for kiln furniture and crucibles, particularly where thermal shock is critical. Why Other Options Are Wrong: Zirconia and magnesia are single-oxide refractories.Chromia (if considered) is also a single-oxide refractory (Cr2O3).“None of these” is incorrect because one option (SiC) is clearly non-oxide. Common Pitfalls: Confusing “oxide content” with “single-oxide class”; overlooking that SiC oxidizes to SiO2 at high temperature in oxidizing atmospheres. Final Answer: Silicon carbide (SiC)

Question 11

Highest melting pure oxide (> 3000 °C): which oxide among the following has the highest melting point and is used in extreme-temperature refractory applications?

Accepted Answer

Introduction / Context: Melting point is a key indicator for selecting refractories for ultra-high temperature service. Pure oxides such as thoria, zirconia, alumina, and beryllia differ substantially in melting temperatures, thermal conductivities, and chemical stabilities. Given Data / Assumptions: We focus on pure oxides and their intrinsic melting points. “Highest” implies comparing representative literature values. Concept / Approach: Thoria (ThO2) exhibits one of the highest melting points among common refractory oxides, generally quoted above 3000 °C (around 3300 °C). Zirconia and beryllia melt in the mid-to-upper 2000 °C range, and alumina near ~2050 °C. Hence ThO2 tops this list for extreme temperature stability. Step-by-Step Solution: Recall typical melting points: ThO2 > 3000 °C; ZrO2 ≈ 2700 °C; BeO ≈ 2570 °C; Al2O3 ≈ 2050 °C.Identify the maximum: ThO2.Choose Thoria (ThO2). Verification / Alternative check: High-temperature materials references consistently list thoria at the top among these oxides; it has been used historically in crucibles and special electrodes where temperature extremes are encountered. Why Other Options Are Wrong: Alumina/beryllia/zirconia: high-melting but below thoria’s value.Hafnia is very high-melting (~2750–2800 °C) yet still below thoria in most tables. Common Pitfalls: Confusing service temperature with melting point; ignoring destabilization by impurities or atmosphere. Final Answer: Thoria (ThO2)

Question 12

Natural silica behavior at high temperature: which statements about quartz-based silica used in refractories are correct?

Accepted Answer

Introduction / Context: Silica refractories rely on controlled transformations of silica polymorphs. Natural silica (quartz) undergoes phase changes during heating that influence thermal expansion, creep, and spalling resistance. Recognizing these transformations is essential when designing coke-oven walls and glass furnace crowns. Given Data / Assumptions: Natural silica source: high-purity quartzites. High-temperature exposure occurs during service and initial bake-out. Concept / Approach: On heating, quartz converts to high-temperature silica polymorphs—tridymite and cristobalite—through intermediate steps. These changes are accompanied by substantial volume changes and anisotropic expansion, which must be managed by brick design and expansion joints. Therefore, statements (a), (b), and (c) are all valid. Step-by-Step Solution: Check (a): natural silica indeed begins as quartz → true.Check (b): quartz is not the stable high-temperature form → true.Check (c): transformations to tridymite/cristobalite cause marked volume change → true.Conclude: All (a), (b), and (c). Verification / Alternative check: Thermal expansion curves of silica bricks show characteristic transformation plateaus and jumps. Proper firing stabilizes desired phases to improve service stability. Why Other Options Are Wrong: Limiting to (a) and (c) neglects the explicit statement about high-temperature stability in (b). Common Pitfalls: Confusing reversible α↔β quartz transition with high-temperature transformations; neglecting expansion allowance during installation. Final Answer: All (a), (b) and (c).

Question 13

Bottom construction of a basic open-hearth furnace: which material is typically used for the bottom (working lining) to withstand basic slags?

Accepted Answer

Introduction / Context: Open-hearth steelmaking (basic practice) requires refractories that resist basic slags and high thermal loads. The furnace bottom is a severe service location with metal/slag contact and mechanical load, thus demanding basic chemistry and strong hot properties. Given Data / Assumptions: Basic operation (high CaO slags) is assumed. Working lining at the bottom must resist dissolution and creep. Concept / Approach: Dead-burnt magnesite (DBM) provides a high-purity MgO phase with excellent basic slag resistance. Ramming masses based on DBM create monolithic bottoms with fewer joints, enhancing integrity. Fireclay or semi-silica (acidic) bricks would react unfavorably with basic slags, while SiC can oxidize and is not standard for this duty. Step-by-Step Solution: Identify slag chemistry: basic.Select a compatible refractory: MgO-based DBM ramming mass.Reject acidic alternatives (fireclay, semi-silica) and non-standard options for bottoms. Verification / Alternative check: Industry practice and legacy furnace designs document MgO-based bottoms in basic open-hearth operations. Why Other Options Are Wrong: Porous fireclay/semi-silica: rapidly attacked by basic slags.Silicon carbide: valued for thermal shock, not typical for basic OHF bottoms.High-alumina castables: better than fireclay but still not optimal against strong basic slags compared to MgO. Common Pitfalls: Specifying acidic bricks in basic zones; overlooking jointless monolithic benefits in high-wear bottoms. Final Answer: Dead-burnt magnesite ramming mass

Question 14

Where should refractories have low thermal conductivity? Select the application that primarily demands insulating (low-k) refractories.

Accepted Answer

Introduction / Context: Thermal conductivity (k) of refractories dictates heat flow. In some equipment, the refractory must actively transfer or store heat (favoring higher k), while in others the primary goal is energy conservation and shell-temperature control (favoring lower k). Recognizing which services prioritize insulation prevents excessive heat loss and inefficiency. Given Data / Assumptions: Coke ovens and regenerators rely on heat storage/transfer across checkerwork. Muffle furnaces require an insulated enclosure to separate product from flame. “Insulation” in the options refers to backup and energy-saving linings whose role is minimizing heat loss. Concept / Approach: Insulating refractories are engineered for low thermal conductivity via porosity and microstructure. They serve as backup linings behind dense hot-face bricks or as energy-conservation layers, reducing heat loss and keeping the steel shell cool. Conversely, regenerators benefit from higher k to store/release heat efficiently; coke-oven hot faces and some muffles may balance insulation with durability but the explicit application that primarily demands low k is insulation itself. Step-by-Step Solution: Determine the function of each application (transfer vs conserve heat).Select the one whose purpose is insulation and energy saving.Answer: Insulation (backup linings). Verification / Alternative check: Design manuals specify multilayer linings with an insulating backup of low k, followed by denser hot-face bricks suited to chemical and mechanical attacks. Why Other Options Are Wrong: Coke ovens/regenerators: require significant heat transfer; very low k is undesirable for checkers.Muffle furnaces: low k is helpful, but the option “Insulation” names the function explicitly and more universally.Incinerator secondaries: often prioritize corrosion resistance and stability over minimal k. Common Pitfalls: Specifying ultra-insulating hot-face materials where thermal shock or chemical attack dominates; ignoring multilayer design. Final Answer: Insulation (backup linings and energy conservation)

Question 15

Where are magnesite refractories generally not used? Identify the application least suited to MgO-based bricks.

Accepted Answer

Introduction / Context: Magnesite (MgO) refractories are classic basic bricks with strong resistance to basic slags and high refractoriness under load. They are favored in steelmaking and cement burning zones but are not universally optimal across all furnace walls or atmospheres. This question probes where MgO is generally not the preferred choice. Given Data / Assumptions: Electric furnaces vary widely (arc furnaces for steel, induction furnaces for non-ferrous, etc.). Basic open-hearth and steel melting furnaces employ basic slags. Cement kiln burning zones are traditionally lined with basic materials (e.g., magnesia-chrome, magnesia-spinel). Concept / Approach: Although many steel arc furnaces use basic linings (including MgO-C), not all “electric furnace walls” are suitable for MgO. In several electric furnace applications—particularly non-ferrous or special atmospheres—high-alumina or silica refractories may be preferred due to chemistry, volatility of slags, and oxidation/reduction cycles. In contrast, basic steelmaking and cement burning zones explicitly demand basic refractories, making MgO a natural fit there. Hence, among the generic choices, “electric furnace walls” is the least consistently compatible category and is the most reasonable selection for “generally not used.” Step-by-Step Solution: Classify each service by slag chemistry and atmosphere.Note that many electric furnace duties (especially non-ferrous) avoid MgO linings.Select “Electric furnace walls” as the broad case where MgO is not generally used. Verification / Alternative check: Industry practice shows cement kilns (burning zone) and basic steelmaking furnaces employing MgO-rich linings; by contrast, various electric furnaces (e.g., induction for copper/aluminum, certain resistance furnaces) lean on high-alumina/silica systems. Why Other Options Are Wrong: Steel melting and OHF basic zones: classic MgO applications.Cement kiln burning zone: long history of MgO-based bricks.Acidic glass tank crowns: typically silica; MgO is not used, but this option was not in the original set for selection. (Included here for contrast.) Common Pitfalls: Overgeneralizing from steel EAF practice to all electric furnaces; ignoring non-ferrous chemistries where MgO is unsuitable. Final Answer: Electric furnace walls

Question 16

“Carborundum” crucibles: chemically, carborundum is which compound commonly used for making crucibles for non-ferrous melting?

Accepted Answer

Introduction / Context: “Carborundum” is a trade/vernacular name historically associated with silicon carbide (SiC). SiC is valued for high thermal conductivity, good thermal shock resistance, and chemical durability, making it suitable for crucibles used in non-ferrous melting and holding furnaces. Given Data / Assumptions: Question asks for the chemical identity behind the name “carborundum.” Application context is non-ferrous crucibles (e.g., aluminum, copper alloys). Concept / Approach: SiC’s covalent bonding and stable structure provide strength at elevated temperature and excellent resistance to cracking under thermal cycling. In oxidizing conditions, a protective SiO2 layer can form, further enhancing corrosion resistance against some melts. Step-by-Step Solution: Recall the historical term “carborundum”.Identify chemical formula: SiC.Select “Silicon carbide”. Verification / Alternative check: Materials catalogs list SiC crucibles as “carborundum” types; silicon nitride (Si3N4) is used for thermocouple protection tubes and bearings, not typically as “carborundum” crucibles. Why Other Options Are Wrong: Silicon nitride, MgO, Zr salts, boron carbide: different materials and applications; not the crucible material referred to as “carborundum”. Common Pitfalls: Confusing trade names; assuming any silicon-based ceramic is “carborundum”. Final Answer: Silicon carbide

Question 17

Effect of thermal cycling on refractories: repeated heating and cooling primarily reduces resistance to which failure mode?

Accepted Answer

Introduction / Context: Many furnaces operate in cycles: heat-up, soak, and cool-down. Repeated thermal cycling imposes thermal stresses due to temperature gradients and expansion/contraction mismatches. Understanding which failure mode is aggravated by cycling helps select materials with appropriate thermal shock resistance and microstructure. Given Data / Assumptions: Refractory experiences multiple hot/cold cycles. No unusual slag chemistry is introduced by the cycling alone. Concept / Approach: Thermal spalling is the cracking, flaking, or exfoliation of refractory surfaces induced by thermal shock. Each cycle can propagate microcracks if the material lacks adequate toughness, low elastic modulus, or proper microstructural accommodation. While chemical corrosion (slag or CO attack) and hot deformation (fusion under load) are serious, they are not specifically caused by cycling; rather, they depend on chemistry and temperature under load. Step-by-Step Solution: Link cycling → repeated thermal shock events.Map to failure mode → thermal spalling.Select the option corresponding to thermal shock cracking. Verification / Alternative check: Thermal shock indices combine modulus, tensile strength, thermal conductivity, and thermal expansion. Materials with lower expansion, higher thermal conductivity, and adequate toughness show superior resistance to spalling in cyclic duty. Why Other Options Are Wrong: Slag attack: chemistry-driven, not mainly by cycling rate.Fusion under load: load/temperature driven creep, not cycling per se.CO attack: atmosphere/chemistry dependent, not thermal gradients.Hydration: occurs in some basic bricks (e.g., MgO) on moisture exposure, not due to thermal cycling. Common Pitfalls: Blaming slag for surface cracking that actually began as thermal spalling; ignoring the role of controlled heat-up/cool-down ramps to mitigate shock. Final Answer: Thermal spalling (thermal shock cracking)

Question 18

Cermet composition and properties: most cermets designed for high thermal conductivity and strong thermal-shock resistance are typically composed of what approximate percentages of ceramic and metallic constituents (ceramic % followed by metal %)?

Accepted Answer

Introduction / Context: Cermets are composite materials combining ceramic phases (for hardness, wear and oxidation resistance) with metallic binders (for toughness, thermal conductivity, and thermal-shock resistance). Many engineering exam questions test your recall of the typical ceramic-to-metal ratio used in well-known cermet families employed in cutting tools, furnace components, and high-temperature wear parts. Given Data / Assumptions: The question refers to “most cermets” optimized for high thermal conductivity and thermal-shock resistance. We consider classical ceramic–metal combinations such as TiC/Ni, TiC/Co, WC/Co-type families identified broadly as cermets in industrial practice. Percentages are by volume or weight in typical textbook references; the exam expects the well-known nominal split. Concept / Approach: In many cermets, ceramics provide hardness, hot strength, and chemical stability, while a modest metallic fraction supplies toughness, some ductility, and heat conduction paths. A frequently cited composition window is ceramic-rich, around 70–85% ceramic with 15–30% metal binder. The widely taught nominal figure is about 80% ceramic and 20% metal for balanced cutting and thermal-shock performance. This achieves the benefits of the ceramic matrix while preserving enough metallic binder for crack bridging and thermal-stress relief. Step-by-Step Solution: Identify the target property set: high thermal conductivity + high thermal-shock resistance.Recall that ceramics dominate the structure for hardness; metal binder is present but limited.Typical reference compositions cluster near 80% ceramic : 20% metal.Select the option that matches this canonical split: 80 and 20. Verification / Alternative check: Materials handbooks and tooling catalogs show many cermet grades in the vicinity of 70–85% ceramic phase with the rest as metallic binder. The 80/20 figure is the standard “textbook memory” value targeted by this MCQ. Why Other Options Are Wrong: 20 and 80: metal-dominant composite, no longer the typical cermet balance taught for this property set. 50 and 50 or 60 and 40: higher metal fractions generally increase toughness but reduce ceramic-controlled high-temperature hardness and wear behavior. 70 and 30: plausible in some systems but the classic exam answer for “most cermets” is 80/20. Common Pitfalls: Assuming more metal always improves shock resistance without considering loss of high-temperature performance; equating cermets with metal-matrix composites rather than ceramic-matrix composites with a metal binder. Final Answer: 80 and 20

Question 19

Manufacturing refractories: to produce refractory bricks with lower permeability (reduced open porosity and better slag penetration resistance), which processing adjustments are effective?

Accepted Answer

Introduction / Context: Permeability in refractories governs gas and slag penetration during service. Lower permeability generally improves corrosion resistance and lining life in furnaces, kilns, and ladles. This question focuses on the manufacturing levers that densify the microstructure and close connected pore networks. Given Data / Assumptions: Standard oxide refractories formed by pressing and firing. Permeability correlates with pore size, pore connectivity, and total open porosity. Processing variables considered: firing temperature, moulding pressure, and grog particle size. Concept / Approach: Higher moulding pressure compacts particles, increasing green density. Finer grog helps fill interstices between larger grains (better packing), reducing capillary channels. Higher firing temperature enhances sintering and neck growth, closing pores and strengthening bonds. All three changes act synergistically to lower permeability by reducing both pore volume and the connectivity that allows fluid transport through the brick. Step-by-Step Solution: Link moulding pressure ↑ to green density ↑ → fewer, smaller pores post-firing.Use finer grog to improve packing and minimize percolating voids.Increase firing temperature to promote sintering and pore closure.Conclude that all three measures reduce permeability. Verification / Alternative check: Permeability tests (e.g., gas permeability) and microstructural images confirm that higher density plus finer grading and stronger sintering reduce open, connected porosity that drives fluid ingress. Why Other Options Are Wrong: Only any two of the variables leaves out a known densification driver; all three are valid and commonly used. Common Pitfalls: Over-firing can create excessive glass phases or grain growth causing thermal-shock issues; optimal processing balances permeability against other properties. Final Answer: All (a), (b) and (c)

Question 20

Insulating materials in refractories: which of the following is NOT a natural insulating material (select the option that makes the statement incorrect)?

Accepted Answer

Introduction / Context: Lightweight insulating materials reduce heat loss and mass in high-temperature linings. Several classic insulators are naturally occurring minerals that are expanded or processed into low-density products. This item checks recognition of common natural insulating materials used in industry. Given Data / Assumptions: Diatomaceous earth (kieselgur), asbestos, vermiculite, and perlite are all naturally occurring substances. The question asks for which is NOT a natural insulating material; the presence of “None of these” implies that all listed named materials are natural insulators. Concept / Approach: Diatomaceous earth consists of fossilized diatom skeletons and is used as a lightweight insulator. Vermiculite and perlite are naturally occurring minerals that can be expanded by heating to produce highly porous, low-density aggregates with good insulation properties. Asbestos, though no longer favored due to health hazards, is a naturally occurring fibrous silicate historically used for thermal insulation. Therefore, all of the listed named materials are indeed natural insulating materials, so “None of these” is the correct choice to the phrasing “Which is not a natural insulating material?”. Step-by-Step Solution: Evaluate each material’s origin and historical use.Confirm all four are natural insulators in industrial or historical practice.Therefore, none of them is “not natural.”Select “None of these.” Verification / Alternative check: Reference tables of insulating refractories list diatomite, vermiculite, and perlite as natural lightweight aggregates; asbestos appears in historical insulation literature (now restricted for safety reasons). Why Other Options Are Wrong: Diatomaceous earth, asbestos, vermiculite, perlite: all natural; choosing any would contradict the premise. Common Pitfalls: Confusing current health and regulatory status with material origin; asbestos is natural but is restricted due to serious health hazards. Final Answer: None of these

Refractory Technology Questions