Powder Metallurgy Multiple Choice Questions on “Tool Materials – Properties and Testing of Cemented Carbides ”.
1. ______ of cemented carbide is the measure of the completeness of sintering operation.
a) Density
b) Volume
c) Strength
d) Ductility
Answer: a
Clarification: Density of cemented carbide is the measure of the completeness of sintering operation owing to the occurrence of contraction or shrinkage on final sintering, depends on the composition. Increasing Co content and TiC addition reduce the density of cemented carbide.
2. Cemented carbide grades can be classified into _____ groups, according to their density by using Hg as a buoyancy medium.
a) 2
b) 3
c) 4
d) 5
Answer: a
Clarification: Cemented carbide grades can be classified into three groups, according to their density by using Hg as a buoyancy medium; WC-Co grades will sink in Hg while WC-TiC-Co and WC-TiC-Ta (Nb)C-Co grades will float on the surface.
3. _________ of cemented carbide tools is used as a measure of wear performance.
a) Hot hardness
b) Transverse rupture strength
c) Compressive strength
d) Hardness
Answer: d
Clarification: Hardness of cemented carbide tools is used as a measure of wear performance. It is customary to adopt Rockwell as a speed control method. Increasing Co content decreases while increasing TiC addition increases the hardness of cemented carbide.
4. Transverse-Rupture strength of cemented carbide is a measure of _________
a) Hardness
b) Wear resistance
c) Toughness
d) Compressive strength
Answer: c
Clarification: Transverse-Rupture strength of cemented carbide is a measure of toughness. Moreover, lower sintering temperature and over sintering lead to a pronounced decrease in transverse-rupture strength.
5. The oxidation resistance of WC-Co composition is higher than that of the WC-TiC-Co composition.
a) True
b) False
Answer: b
Clarification: The oxidation resistance of WC-Co composition is considerably lower than that of the WC-TiC-Co compositions because of the formation of non-dense and non-adherent oxide layer at elevated temperatures.
6. Lower thermal conductivity of cemented carbides leads to formation of _________
a) Brazing cracks
b) Comb-type cracks
c) Grinding cracks
d) Crumbling
Answer: c
Clarification: Higher thermal conductivity of cutting tools is necessary in order to dissipate larger amount of heat, generated from the cutting action, by conduction through the tool, otherwise overheating of the cutting edges will result in the formation of grinding cracks thereby rapid tool failure.
7. Which of the following property of cemented carbide increases on increase in Co content?
a) Coefficient of thermal expansion
b) Hardness
c) Density
d) Thermal conductivity
Answer: a
Clarification: The addition of Co slightly increases the coefficient of thermal expansion, whereas the other properties like hardness, density, and thermal conductivity of cemented carbide decrease with increase in Co content.
8. The cutting performance of cemented carbide can be plotted as _________
a) T-V curve
b) C-P curve
c) I-V curve
d) M-V curve
Answer: a
Clarification: T-V curves (T=service or tool life in minutes and V=cutting speed in meters per minute) are plotted which are used for comparing the cutting efficiency of cemented carbides of different grade under varying cutting conditions.
9. The microstructure of WC-Co alloy contains ____ phases.
a) 1
b) 2
c) 3
d) 4
Answer: b
Clarification: The microstructure of WC-Co alloy contains only two phases- α phase (WC), and γ phase (Co with WC in solid solution). The α phase (WC grains) appear as regular shape such as triangles or trapeziums.
10. The microstructure of WC-TiC-Co alloy contains ____ phases.
a) 1
b) 2
c) 3
d) 4
Answer: c
Clarification: In WC-TiC-Co composition, apart from α and γ phase, a second carbide phase, referred to as β phase (TiC-WC solid solution), usually more rounded than α, forms which is distinguished from α by electrolyte etching in a mixture of hydrofluoric and nitric acids or by heat tinting technique which causes the darkening of β phase.