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Correct option is (b) False

The best I can explain: Cold cracking is basically caused by residual STRESSES exceeding the limitation of the casting’s STRENGTH. It normally appears at the tensile sites of the castings, especially at the sites of stress concentration. Shot PEENING method, instead of avoiding this defect, it INCREASES the chances of cold cracking by inducing more stresses into the casting product.

Correct answer is (B) False

The explanation is: Residual stresses can be sufficient to cause any defects in the CASTING. It can split the metal part into two or more pieces abruptly without any APPLICATION of external load or force. Residual stresses can also RESULT in distortion of the casting component, which cannot regain its ORIGINAL shape easily.

The CORRECT choice is (b) False

The best I can explain: Residual stresses have the same role in a component’s strength as common MECHANICAL stresses. HOWEVER, the stresses due to external loads can be calculated with accuracy, but residual stresses are difficult to calculate. Therefore, it is very NECESSARY to have a reliable method able to measure them with minimum damage to the surface of the casting.

Right choice is (b) False

The BEST explanation: In natural aging treatment, generally the casting component is stored for several YEARS to make the residual stress released slowly. This method takes long time and it is only suitable for the production of small-batch and larger casting COMPONENTS. And heat treatment does not take MUCH of time depending on the type of heat treatment.

Correct option is (a) True

To explain: In the casting system, the RISERS must be placed at the heaviest sections to obtain better casting free from defects. And because the HEAVIER sections, themselves ACT as FEEDERS for the thin sections of the casting, and variations in the volume of riser should be done if the SHAPE and size of casting sections also vary.

Correct option is (a) True

To elaborate: By using RESIN bonded sand in the MAKING of mould, the deformability of the mould and core is always improved which helps in reducing the internal stresses in the casting produced. The stresses are also reduced by increasing the holding time of casting in the mould, so as to REDUCE the DIFFERENCE in temperature distribution.

Right CHOICE is (b) False

For explanation: Manufacturing PROCESS is not only the single consideration which can cause STRESS. Residual stresses may be present in any MECHANICAL component because of many causes. Residual stresses can be due to the technological process used to make the parts of casting, but manufacturing processes are the most common causes of residual stress.

Correct choice is (b) False

The best I can explain: The riser basically REPRESENTS waste metal that is to be separated from the casted part and re-melted to make other CASTINGS. It is always desirable to have a minimum volume of molten metal in the riser to make EFFICIENT production of castings. This increases the CASTING yield which SAVES metal and reduces the time of production too.

Correct OPTION is (a) True

To explain I would say: During solidification, prevention of sticking and tearing of the initial THIN shell of the casting is a major consideration. To MINIMIZE the shell sticking and tearing, friction between the casting surface and mould wall MUST be KEPT below a critical level which depends upon the shell strength of the casting.

The correct choice is (a) True

For explanation I WOULD say: Reducing the casting of residual stresses can improve the machinability of the casting and increases TOOL life also. On the other hand, the castings machined after reducing the residual stresses have good dimensional STABILITY, persistent precision and good reliability of the casting products.

The correct option is (a) True

Explanation: RESIDUAL stresses may play an important ROLE in explaining or preventing failure of a component at TIMES. For example, shot peening of component to induce surface COMPRESSIVE stresses, in which residual stresses prevent failure and also improves the fatigue life of the component. But the internal or residual stresses should be kept balanced in the CASTING component.

Right answer is (a) True

For explanation: Localized yielding of material can induce RESIDUAL stresses in the casting, because of a sharp notch or from VARIOUS surface TREATMENTS like shot peening or surface hardening. To relieve residual stresses from the casting. Usually, HEAT treatment of the casting is done which also enhances MECHANICAL properties in the casting product.

The correct option is (a) True

The best explanation: Residual stresses or locked-in stresses are defined as the stresses EXISTING WITHIN a body in the absence of external loading or thermal GRADIENTS. In other WORDS, residual stresses in a material or component are those stresses which exist in the body without the APPLICATION of any other external loads or forces.

The correct answer is (B) False

The explanation: The EFFECT of thermal contact resistance on the phase CHANGE process is very important to consider during solidification. Thermal contact resistance is VARIED by changing the heat transfer coefficient at the interface. Thermal contact resistance is always CONSTANT throughout the process and cannot be affected by impact or thermal parameters.

The correct choice is (d) Heat transfer

The explanation: The QUALITY of final casting MAINLY dependent on the rate of SOLIDIFICATION as rapid solidification MAKES fine grains structures with better MECHANICAL properties. The analysis of heat transfer during the casting and solidification of casting is mainly carried out to have better properties of casting material.

Right option is (a) True

Explanation: An alloy, UNLIKE a pure metal, does not have a sharply defined freezing temperature. DUE to this, the solidification of an alloy TAKES place over a RANGE of temperature. This makes the separate out of the solids parts at different temperatures which possess varying compositions in the MATERIAL.

Correct answer is (b) False

To EXPLAIN: During the solidification of CASTING in an insulating mould, the entire casting’s thermal resistance is offered by the mould MATERIAL. Hence, the ANALYSIS of freezing TIME is done by mainly considering the thermal resistance of the moulding material. This method usually takes more time for solidification as compared to the other methods.

Right choice is (b) False

To elaborate: In solidification, to find out the FREEZING time of COMPLEX contours of casting, only observations are made without any use of CALCULATIONS. This is mainly done to observe the basic features to know whether the analysis made underestimates or overestimates the actual freezing time of the casting material.

The correct choice is (b) False

The best EXPLANATION: Minimization of friction between the mould and CASTING surface and for the continuous release of the shell from the mould is achieved through the INTRODUCTION of mould oscillation AIDED by the lubrication. Refractory coating of the mould’s surface is BASICALLY done for the finishing aspect.

Right option is (c) SMALL CRYSTALS with random orientation

Explanation: When a liquid metal is poured into a mould, the mould face itself acts as the nucleus for CRYSTAL growth, and when the conductivity of the mould is high, that causes the formations of randomly oriented small crystals which grow NEAR the mould surface. Subsequently, a temperature GRADIENT results within the casting.

The correct answer is (b) False

For explanation I would say: The oscillation MARKS itself is a surface defect and its severity may prevent hot rolling. And increased thermal resistance at the bottom of mark reduces the LOCAL cooling rate which retards shell growth and coarsens the local MICROSTRUCTURE, this can disturb the WHOLE casting by inducing defects in the casting.

Right option is (a) True

The explanation: During SOLIDIFICATION of casting material, the interface temperature which generally prevails in the absence of CONTACT resistance has PHYSICAL MEANING when the contact resistance is in EFFECT. The interface temperature is independent to the magnitude of contact resistance and is dependent only on the thermal properties.

Right option is (b) False

Easy explanation: A gap of finite thickness separating the mould and casting interface always results in LOWERING of heat flux. It is the most important phenomena for controlling the SOLIDIFICATION of the casting in the metallic mould. The time of its FORMATION and DISTRIBUTION affects the TEMPERATURE distribution in the casting.

Right choice is (a) greater than 70%

Best explanation: The difficulty of feeding an alloy in a MOULD is expressed by a quantity which is commonly KNOWN as centre-LINE feeding resistance (CFR). It is basically defined as the ratio of a time INTERVAL between start and end of freezing at centre line to the TOTAL solidification time of casting. Normally, feeding is considered to be difficult if CFR is greater than 70%.

Right choice is (b) False

The explanation: The presence of solid crystals ahead of the solid-LIQUID interface does not make easy FEEDING, even this makes more DIFFICULT to FEED the liquid metal. This basically implies a greater risk of having voids within the casting which is normally referred to as centre-line SHRINKAGE defect.

Right option is (B) False

Easy explanation: Liquid metal always needs to be cooled below their freezing points before the solidification STARTS. This is because the energy is required to create surfaces for new CRYSTALS and the degree of super-cooling NECESSARY is also reduced by the presence of particles which serve as the initial NUCLEI for crystal growth.

The correct choice is (a) True

Easy EXPLANATION: The THERMAL gradient within the casting is the most important factor during the last stage of COOLING of casting. The minimum allowable thermal gradient generally depends on the shape and size of a cross-section of the casting. So to SET up the risers in the casting SYSTEM, area to volume (A/V) ratio of the casting is considered.

Right option is (b) False

For explanation: The CONSTANT air gap width at the INTERFACE cannot be assumed for SIMULATION. The magnitude of the air gap continuously increases with TIME. The time of start of the air gap at the corner of the casting is always less than that for the middle, and the air gap never starts simultaneously at all POINTS in the casting.

Correct answer is (b) False

For EXPLANATION I would say: When the air gap forms at the interface, the HEAT transfer across the interface drops rapidly. The MODE of heat transfer through the gap is considered to be due to both conduction and radiation, conduction is more DOMINANT mode at the lower temperatures while radiation depends upon the SURFACE temperatures and emissivities.

Right choice is (d) Convex

The explanation is: During the solidification of casting in an INSULATING MOULD, the divergent nature of heat flow is shown by the convex type of metal-mould interface and the convergent nature is shown by the concave type interface. And PLANE interface reveals the straight and parallel nature of heat flow. The nature of heat flow in all THREE INTERFACES is shown in below figure.

Correct ANSWER is (c) Q = h*∆T

Explanation: The transfer of heat across a gap (an interface between mould and casting) is given by q = h*∆T, where q is the heat FLUX, h is the heat transfer coefficient of the interface and ∆T is the TEMPERATURE difference between the source and the sink. Values of the heat transfer coefficient may vary from 10 to 102 for large air gap; 103 to 104 for normal castings.

The correct OPTION is (c) x = a/(y-b) + c

Easy EXPLANATION: The equation for a risering CURVE in the RISER system is given by: x = a/(y-b) + c, where a is the freezing constant for the molten metal, b is the contraction ratio from liquid phase to the solid phase and c is a constant depending on the DIFFERENT media around the riser and the casting.

Correct option is (b) False

For explanation: In CONTINUOUS CASTING technology, metallic mould is generally PREFERRED. The solidification is initiated in a water-cooled open-ended COPPER mould. And the casting which gets shaped in the mould includes a core of liquid metal which slowly solidifies as the strand moves through the caster during the solidification.

The correct choice is (a) True

Easiest explanation: In the SOLIDIFICATION process, extraction of heat can be achieved by applying a suitable cooling to the molten metal in ORDER to create a heat flux. The cooling rate can be deduced from a SIMPLE heat BALANCE only if the process is isothermal (low cooling rate) and the specific heats of the liquid and the solid are the same.

Correct OPTION is (b) Chaplets

Easy explanation: The SOLIDIFICATION time of the casting is generally dependent on the various parameters LIKE mould wall temperature, heat transfer coefficient at the metal and mould interface, mould wall THICKNESS and material of the mould. Chaplets are another part used for SUPPORTING the core in the mould.

Right option is (b) False

To explain I would say: DUE to predominant resistance in the mould of casting system, the heat transfer does not occur easily. However, it can be done forcefully, but in normal WAY the casting produced is of ductile NATURE as the heat is kept within the casting for a longer time. But, inaccurate way of solidification MIGHT cause defects in the casting.

Correct option is (B) False

To explain I would say: Thin wall of the MOULD and the water cooling makes the solidification rate FAST, which produces fine grain structure of the casting. The fine grain structure of casting is ALWAYS brittle and hard in nature because the process is somewhat similar to QUENCHING which also produces a casting of brittle nature.

Correct option is (b) False

The explanation is: After the STARTING of solidification of casting material, temperature gradient is set up and by progressing inwards, long COLUMNAR crystals are formed with their axes perpendicular to the mould face. This orientation of crystal growth is basically desirable from the POINT of strength of the CASTINGS.

Correct option is (b) CRYSTAL structure

To ELABORATE: A clear understanding of the mechanism of solidification and cooling of molten metal and alloys is very essential for the production of successful CASTINGS. During solidification, crystal structure of the casting material is mainly considered ALONG with its composition at different parts of the casting.

The correct choice is (a) True

Easy explanation: A proper CARE is must for the solidification of castings. Unless a proper care is TAKEN, defects like shrinkage cavity, hot tear, misrun and cold shuts are OCCURRED in the castings. To avoid defects, a proper technique should be implemented with clear understanding of material used for the formation of castings.

Correct choice is (a) True

Best explanation: The condition of no contact RESISTANCE exists only when the metal-mould contact is so close and tight that CAUSES the occurrence of perfect wetting that is the CASTING gets soldered to the moulding SURFACE. This causes the sticking problem and it becomes very DIFFICULT to extract casting from the mould cavity.

The correct option is (a) True

Best explanation: Metallic mould is made of metal capable of WITHSTAND high temperature and coated with refractory wash to REDUCE its chilling effect on the cast and also FACILITATES the removal of cast product from the mould cavity. It also helps in better surface FINISHING of the casting material.

Right CHOICE is (a) True

Explanation: The temperature variation with time of the CASTING is GENERALLY recorded by which cooling curves are obtained for the DETERMINATION of solidification time of the cast. This shows that the moulding thickness INCREASES the solidification time of the casting material during solidification.

Right option is (a) True

Best explanation: A dendritic crystal GROWTH STRUCTURE is produced when there is difference in the progress of crystallization that shows completion of crystallization at any POINT before with respect to another point. This is MAINLY caused by very small thermal gradient with a very high concentration difference. Dendritic structure FORMED is looks like as shown in below figure.

Correct answer is (B) False

To explain I would SAY: CONTINUOUS casting process nurtures the initial thin and fragile solid shells for the avoidance of breakout during the solidification of casting. And it also eliminates the SURFACE and internal defects from the cast product by having the proper solidification DYNAMICS in the casting process.