This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Nitriding, Cyaniding and Carbonitriding – 1”.
1. Nitriding involves the addition of ___________ for the hardening of surface.
Explanation: Nitriding is a diffusion process for surface hardening of steels. It involves the addition of nitrogen atoms to obtain the desired hardness of steel components.
2. Which of the following is required for the nitriding process?
Explanation: Nitriding differs from case-hardening as they contain nitrogen instead of carbon. Nitriding requires special steels called ‘Nitralloy’. This is because hardness depends on the formation of very hard compounds of nitrogen and metal such as aluminum, chromium, and vanadium present in the steel.
3. Why is nitriding not used for plain carbon steels?
a) Formation of oxides
b) Formation of cracks
c) Formation of iron nitrides
d) Formation of spots
Explanation: Nitriding is generally not preferred for plain carbon steels. This is due to the formation of iron nitrides formed to a considerable depth below the surface of the steel. This causes embrittlement of the material.
4. What is necessary to be done before the nitriding process?
a) Heat Treatment
b) Washing the material
c) Chamfering of sample
d) Shaping of material
Explanation: Before nitriding is carried out, it is required that the workpieces are heat treated. This is done in order to produce the required properties in the core. The parts are heat treated at just about 500oC for several hours.
5. How long should a steel component be heat treated before nitriding?
a) 5-20 hours
b) 40-100 hours
c) 100-200 hours
d) 300-500 hours
Explanation: Before nitriding is carried out, it is required that the workpieces are heat treated to produce the required properties in the core. The parts are heat treated at around 500oC for a period of 40 to 100 hours. This process takes place in a gas-tight chamber through which ammonia gas is allowed to circulate.
6. Why is atmospheric nitrogen inappropriate for use in the nitriding process?
a) Cannot be heated
b) Insufficient density
c) Inability to be absorbed
d) Expensive process
Explanation: Ordinary atmospheric nitrogen exists in the form of molecules (N2). As a result, these molecules cannot be absorbed by the steel, which is necessary for the nitriding process. Therefore, atmospheric nitrogen is not suitable for nitriding process.
7. Which of the following is not applicable for nitriding process?
a) Surface formed is extremely hard
b) Minimized cracking and distortion
d) High-temperature treatment
Explanation: Nitriding has an edge over case-hardening as the surface formed is extremely hard. The treatment is conducted at comparatively low temperatures due to which the cracking and distortion effects are minimized. Nitriding is generally an expensive process. However, it may be cheaper when a large number of components are to be treated.
8. Cyaniding is usually done for _________
a) Plain carbon steels
b) Cast irons
c) Stainless steels
d) Pig iron
Explanation: Cyaniding is a process of hardening the surface of steel components through the addition of nitrogen and carbon. It is done by immersing the workpiece is a cyanide bath. Usually, plain carbon steels and alloying steels containing 0.2% carbon are hardened using this process.
9. Cyaniding is carried out at a temperature of ___________
Explanation: Cyaniding is a process of hardening the surface of steel components through the addition of nitrogen and carbon. It is done by immersing the workpiece is a bath of molten sodium cyanide and sodium carbonate. This process is conducted at a temperature of 950oC. This causes the formation of hard iron nitrides which leads to surface hardening of the material.
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