This set of Powder Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Tool Materials – Cemented Carbides and its Production – 1”
1. ______ enables us to shape a larger metallic object by removing a part or thin layer of the metal from its surface to the required shape.
a) Electrical contact materials
b) Bearing materials
c) Tool materials
d) Sintered Friction materials
View Answer
Explanation: Tool material is defined as the one which enables us to shape a larger metallic object by removing a part or thin layer of the metal from its surface to the required shape. Though the various tool materials differ in size and shape, all have a common feature of a wedge shape that is driven asymmetrically into the workpiece material.
2. Cemented carbides are commonly known as ‘Hard Metals’.
a) True
b) False
View Answer
Explanation: Cemented carbides are commonly known as ‘Hard Metals’, because of their high hardness, abrasion resistance, compressive strength and a considerable amount of toughness.
3. The modern production of hard metal is attributed to ________
a) Kieffer
b) Schroter
c) Schwarzkopf
d) Osram Studiegesellschaft
View Answer
Explanation: The modern production of hard metal is attributed to Schroter. The technique involves the blending of WC powder with up-to 20% by weight of a metal of the iron group, particularly Co as a binder metal, compaction of the powder mixture and sintering at a temperature close to the melting point of the binder metal.
4. Widia is a trade name of which cemented carbide?
a) WC-Co
b) WC
c) Co-Cr
d) Co-Ni-Cr
View Answer
Explanation: Cemented WC-Co materials with 4 – 13% cobalt was prepared in the same manner as the WC prepared by Schroter, by Fried Krupp in Essen were marketed under the trade name ‘Widia’.
5. The principle constituents of sintered hard metals are the ______ and _______
a) Metal, reinforcement
b) Metal, carbide powder
c) Non-metal, carbide powder
d) Non-metal, boride powder
View Answer
Explanation: The principle constituents of sintered hard metals or cemented carbides are the metal and carbide powders. The carbides are produced by either carburization of powdered metals with carbon, or by the reaction of metal oxide with carbon.
6. Recarburization is often necessary for carbide produced by the reaction of metal oxide with carbon.
a) True
b) False
View Answer
Explanation: In carbide powder production by direct reaction of metal oxide with carbon, accurate control of final carbon content is difficult due to a combination of reduction and carburization reactions in a single operation, thus often necessitating crushing and recarburization.
7. In hard metal industry, the carbide of _____ is produced by the direct metal-carbon reaction.
a) Tantalum
b) Titanium
c) Molybdenum
d) Niobium
View Answer
Explanation: In hard metal industry, the carbide of molybdenum and tungsten are produced by direct metal-carbon reaction while that of titanium, tantalum, and niobium are produced by metal oxide-carbon reaction since the former technique of production of these metal powders are comparatively expensive.
8. WC is produced by carburization of metallic W powders prepared by the reduction of _______
a) Scheelite
b) Wolframite
c) Tungstic acid
d) Limonite
View Answer
Explanation: WC is produced by carburization of metallic W powders prepared by the reduction of tungstic acid (H2WO4), tungsten trioxide (WO3), ammonium paratungstate. These compounds are produced from either the wolframite or scheelite ores.
9. Which of the following phases occur during sintering in WC powder due to carbon deficiency?
a) α, β, γ phase
b) θ, η, K – phase
c) δ, θ, η phase
d) β, γ, M – phase
View Answer
Explanation: Carbon deficiency in the tungsten carbide powder results in the formation of brittle carbide phases such as η-phase (Co3W3C), θ-phase (Co3W6C2) and K-phase (Co3W10C4) during sintering. If the material is under-carburized, i.e., the combined carbon content is below about 6.1%, carburization operation is repeated.
10. Why carbide balls are preferred over steel balls in wet milling?
a) Economy purpose
b) Higher iron pick-up
c) Faster production rate
d) Higher carbon pick-up
View Answer
Explanation: In wet milling, if steel balls (0.5 – 1.25 inch in diameter) are used, results in higher iron pick-up (>0.5%) in the charge. Hence, high wear resistant carbide balls (usually cylinders or rectangles with rounded ends) made by hot pressing are widely used.
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