This set of Powder Metallurgy Questions and Answers for Aptitude test focuses on “Tool Materials – Cemented Carbides and its Production – 2”.
1. ______ carbide alloys possess better resistance to diffusion wear under seizure conditions.
b) Ultra-fine grained
Explanation: Coarse-grained carbide alloys possess better resistance to diffusion wear under seizure conditions while fine-grained carbide alloys having the greater abrasion resistance and high hardness are suited for the resistance to the attrition type of wear in the region of partial contact and to shear stresses in the seizure area.
2. Which of the following is not a characteristic of vibratory mill used for milling of cemented carbide?
a) Shorter milling time (1-2 days)
b) Wear of balls and mills is reduced
c) Ratio of powders to balls in mills = 1:3.5 – 1:5.5
d) Ratio of powders to balls in mills = 1:1 – 1:3
Explanation: Vibratory mills are advantageous over rotary mills because of shorter milling time (1-2 days) in comparison to longer milling time (2-8 days) in rotary mills. Apart from this, wear of balls and mills and hence, the contamination of charge is reduced in the vibratory mill over the rotary mill. However, for rotary mill ratio of powders to balls in mills = 1:1 – 1:3 and for vibratory mill it is 1:3.5 – 1:5.5.
3. In which organic liquid is the lubricant, paraffin wax dissolved?
c) Light hydrocarbons
Explanation: The addition of lubricant, is done as a solution in an organic liquid such as paraffin wax dissolved in gasoline, camphor in ether or light hydrocarbons and glycol in alcohol and solutions are made in the mixers prior to compacting in order to facilitate pressing and to avoid the formation of defects and cracks in the compacts.
4. The classical method of production of cemented carbide tips is ________
a) Hot pressing
d) Liquid phase sintering
Explanation: The classical method of production of cemented carbide tips is double-sintering process, i.e., cold pressing to produce compacts in similar shapes such as round and rectangular blanks and pre-sintering to produce a material which is machined to final shape and sintered.
5. During _______, no liquid phase forms and ________ of only the binder metal occurs.
a) Pre-sintering, liquid phase sintering
b) Liquid phase sintering, pre-sintering
c) Pre-sintering, solid phase sintering
d) Solid phase sintering, pre-sintering
Explanation: During pre-sintering, no liquid phase forms and solid phase sintering of only the binder metal occurs i.e. little or no shrinkage take place. The pre-sintered compacts have a chalky structure and enough strength for further handling and to withstand machining or slicing operations to be carried out.
6. Milled powder mixtures are unsuitable for automatic pressing in comparison to agglomerated carbide-cobalt powder mixtures.
Explanation: Milled powder mixtures are unsuitable for automatic pressing due to their very poor flow properties, while the agglomerated carbide-cobalt mixture is required for compaction due to their high apparent density and good flow properties in automatic press. Hence, the powders containing about 2% lubricant are frequently agglomerated by compacting them (e.g. pressure= 3.5 tsi).
7. _________ is used to produce large carbide balls, bushes and hollow sections with more uniform density.
a) Isostatic pressing
b) Extrusion compaction
Explanation: Isostatic pressing is used to produce large carbide balls, bushes and hollow sections with more uniform density. The extrusion compaction of powders in the presence of binders and plasticizers is used for the production of fairly simple shapes such as cemented carbide rods, tubes or rectangular, circular or non-symmetrical cross section of greater length.
8. Which of the following is not a characteristic of hot-pressed material?
c) Wear- resistant
d) Zero residual porosity
Explanation: Hot pressed material is very hard, wear resistant and practically free from residual porosity. In this process, the powder mixture, without the presence of lubricant, is introduced into graphite dies and pressing and sintering operations are carried on simultaneously; the pressure varying from 0.5-2 tsi and the temperature from about 1300-1550°C with direct resistance or induction heating.
9. Which of the following is the correct sequence of grinding cemented carbides?
a) Rough grinding
b) Coarse grinding, polishing, and fine grinding
c) Rough grinding, and final grinding
d) Fine grinding
Explanation: All cemented carbides are ground in two steps. The rough grinding being the first step is usually performed with SiC grinding wheel. The final step is usually done with a metal bonded diamond wheel.
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