This set of Engineering Materials and Metallurgy Problems focuses on “Metal Spraying and Hard Facing, Age Hardening”.
1. Usage of ___________ for metallizing helps spray the metal powder in the correct place.
a) Air, oxygen, and combustible gas
b) Oxyacetylene powder gun
c) Silicon dioxide
d) Hydrogen peroxide
Explanation: One method of metalizing employs the usage of air, oxygen, and combustible gas. This method forms loose and friable oxides. Another method employs have of oxyacetylene gunpowder, which can spray the metal powder in place or over almost any contoured surface.
2. Plasma is metal spraying can obtain temperatures up to ________
a) 10,000 F
b) 20,000 F
c) 30,000 F
d) 60,000 F
Explanation: Plasma is a luminous stream of ionized gas produced by passing a gas through an electric arc. Temperatures up to 30,000 F are economically obtained. Thus, the use of plasma permits deposition of the highest melting metals.
3. Which of the following can be used for hard coating applications?
a) High-chromium stainless steel
b) Heat-treated stainless steel
c) Hardened brass
Explanation: High-chromium (13.5%) stainless steel can usually be used in applications where a hard coating is required, even where corrosion resistance is not necessary. The material has high strength and elongation, low shrinkage, and little tendency to crack on shafts. Typical applications include armature shafts, cylinder liners, hydraulic rams etc.
4. What is the hardness of molybdenum powder?
a) 168-183 Vickers
b) 246-263 Vickers
c) 321-368 Vickers
d) 330-390 Vickers
Explanation: Molybdenum wire and powder, and tungsten wire and powder are a few plasma sprayed coatings of metal. Molybdenum powder has a hardness of 321-368 Vickers, whereas tungsten powder is of 330-390 Vickers.
5. How does the amount of cobalt affect the wear of alloys?
a) High cobalt high wear
b) Low cobalt high wear
c) No cobalt high wear
d) Cobalt does not affect the wear
Explanation: Higher cobalt concentration of about 13-17% results in 30-40% faster wear or the alloys. However, it provides better resistance to mechanical shock. The basic tungsten-carbide composition containing 6-8% cobalt is recommended for general wear applications.
6. Carbon steels containing ________ carbon are easier to hard-face.
a) < 0.35%
d) > 6.77%
Explanation: Hard facings can be applied to most ferrous metals, but with a few exceptions. Carbon steels containing less than 0.35% carbon are relatively easy to hard-face. Welding becomes more difficult with increasing carbon content and hence must be preheated and postheated for hard-facing.
7. Increase of hardness of an alloy with an increase in time is known as ________
a) Age hardening
b) Solution hardening
c) Dispersion hardening
d) Phase transformation hardening
Explanation: The strengthening of an alloy resulting from the precipitation of a finely dispersed second phase from a supersaturated solid solution is called age hardening. The finely dispersed second phase precipitates with age. This type of hardening is known as age hardening.
8. What is the relation of solubility and temperature for an age-hardening alloy?
a) Increases with decrease in temperature
b) Decreases with decrease in temperature
c) Increases irrespective of temperature
d) No change due to temperature
Explanation: The most important requirement of an age-hardening alloy is that its solubility must decrease as the temperature decreases. This gives a supersaturated solid solution necessary for age-hardening of alloys.
9. Which of the following is not a stage of the age-hardening process?
Explanation: For age-hardening of alloys, it must be first solutionized by heating into a single phase region. Then the alloy must be rapidly quenched into the two-phase region. Lastly, fine-scale transition structure as small as 100 angstrom is formed due to aging above room temperature.
10. What is the ductility of age-hardened Al-4.5%Cu alloy?
Explanation: Al-4.5%Cu is an example of an age-hardened alloy. At the age hardened stage, it achieves as ductility of 20% with a tensile strength of 4225 kg/cm2. As it overages, the ductility drops to 20%.
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