This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Carburizing”.
1. The treatment of steel to get a stronger casing while maintaining a soft core is called _________
a) Surface hardening
d) Surface lining
Explanation: In many applications, it is required for the surface to be harder while the core of the material remains soft. The treatment of steels to achieve this is known as surface heat treatment or surface hardening.
2. Which of the following is not a diffusion method of surface heat treatment?
c) Induction hardening
Explanation: The treatment of steels to achieve a hard casing and softer core is known as surface heat treatment or surface hardening. The diffusion methods of surface heat treatment are classified as carburizing, cyaniding, nitriding, and carbonitriding. Thermal methods include flame hardening and induction hardening.
3. Steels containing ________ carbon are used for carburizing.
Explanation: Carburizing the process by which carbon atoms are introduced onto the surface of low-carbon steels to produce a hard casing while the core remains soft. Steels used in carburizing contain carbon content of 0.10-0.20%.
4. Which of the following is not a method of carburizing?
a) Pack carburizing
b) Gas carburizing
Explanation: Carburizing the process by which carbon atoms are introduced onto the surface of steels to produce a hard surface while the interior remains soft. When a piece of low-carbon steel is placed in a carbon saturated temperature, the carbon diffuses into the steel and causes carburizing. Pack carburizing, gas carburizing, and liquid carburizing (cyaniding) are three methods of carburizing.
5. The energizer in pack carburizing makes up ________ of the total composition.
Explanation: The carburizing mixture contains a carbon-rich material along with an energizer making up 40% of the total composition. The energizer is composed of soda ash and barium carbonate. It is used to accelerate the carburizing process.
6. What kind of carbon-rich material is used in the carburizing mixture?
a) Charred leather
b) Sodium carbonate
c) Barium carbonate
d) Carbon black
Explanation: The carburizing mixture contains a carbon-rich material like charcoal or charred leather, along with an energizer. The energizer is composed of sodium carbonate (soda ash) and barium carbonate.
7. What is the carburizing temperature?
Explanation: The components to be heat treated and the carburizing mixture are packed in steel boxes and their lids are fixed on the boxes. This is then heat treated at 900-950oC and maintained at this temperature for up to six hours. After carburizing, the components are quenched slowly in the box.
8. Which of the following holds true for pack carburizing?
a) Efficient heating
b) Uniform temperature
c) Easy to handle
d) Not readily adoptable
Explanation: Pack carburizing is a process of carburizing which has several drawbacks. It has inefficient heating, the temperature is not uniform, and is difficult to handle. Furthermore, pack carburizing is not readily available to continuous operation.
9. Prevention of carburizing of components can be done by ___________
a) Adding filler
c) Cold working
d) Adding water
Explanation: Sometimes it is desired that a few parts of the surface of the component are prevented from carburizing. In such cases, electroplating the area with copper achieves the desired result. The thickness of electroplating with copper is usually between 0.07 mm to 0.1 mm.
10. Gas carburizing differs from pack carburizing due to the use of ________
a) Silicon-providing gas
b) Mercury-providing gas
c) Carbon-providing gas
Explanation: Gas carburizing overcomes the drawbacks of pack carburizing by replacing the solid carburizing mixture with a carbon-providing gas. In general, natural gas, propane, or generated gas atmospheres are used in these cases.
11. The molten baths used in liquid carburizing are composed of _________
a) 20-50% sodium cyanide, 40% sodium carbonate
b) 40% sodium cyanide, 20-50% sodium carbonate
c) 10% sodium cyanide, 20% sodium carbonate
d) 20% sodium cyanide, 10% sodium carbonate
Explanation: Liquid carburizing is carried out in molten salt baths. These baths contain 20-50% sodium cyanide, 40% sodium carbonate, along with varying quantities of sodium or barium chloride. It is also known as salt carburizing.
12. Liquid carburizing temperature range lies between ________
Explanation: Liquid carburizing is carried out in baths of molten salt containing 20-50% sodium cyanide, 40% sodium carbonate, and with varying quantities of sodium or barium chloride. The cyanide-rich mixture is heated in iron pots to a temperature of 870-950oC. The workpiece is immersed for a little longer than 5-minute periods according to the depth of the case required.
13. Liquid carburizing can be used for producing ________ cases.
a) 0.05-0.1 mm
b) 0.1-0.25 mm
c) 0.3-0.6 mm
d) 0.7-1.0 mm
Explanation: Liquid carburizing is carried out in baths of molten salt baths containing 20-50% sodium cyanide, 40% sodium carbonate, and with varying quantities of sodium or barium chloride. The cyanide-rich mixture is heated in iron pots to a temperature of 870-950oC. This process is suitable for producing shallow cases of 0.1-0.25 mm.
14. Which of the following is not applicable for liquid carburizing?
a) Uniform temperature
b) Dirty work surface
c) Temperature controlled by pyrometers
d) High cost of carburizing
Explanation: Liquid carburizing takes place due to the decomposition of sodium cyanide at the surface of the steel. It has major advantages that the temperature of the salt bath is uniform, the surface of work remains cleans, and that the temperature can be accurately controlled by pyrometers. It also has the disadvantage that the salt pots require batch processing and the cost of carburizing salt is high.
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