Q1: Case hardening is not done by

ANS:D - both b & c

Case hardening is not done by both b & c, which are hot dipping and electroplating. Explanation:

1. Nitriding: Nitriding is a case hardening process that involves diffusing nitrogen into the surface of a metal to create a hardened outer layer. This process is typically done by exposing the metal to ammonia gas or a nitrogen-rich atmosphere at elevated temperatures. Nitriding is effective for increasing the surface hardness, wear resistance, and fatigue strength of ferrous metals such as steel.
2. Hot dipping: Hot dipping, also known as hot dip galvanizing, is a process used to apply a protective coating of zinc to the surface of metal components. While hot dipping provides corrosion protection to the metal, it does not significantly increase the surface hardness or wear resistance. Therefore, it is not considered a case hardening process.
3. Electroplating: Electroplating is a process in which a metal coating is deposited onto a conductive surface using an electric current. While electroplating can provide decorative finishes or corrosion protection to metal surfaces, it does not involve altering the metallurgical properties of the base metal to increase hardness or wear resistance. Therefore, it is not a case hardening process either.

Both hot dipping and electroplating primarily provide surface treatments for corrosion protection or decorative purposes and do not involve the diffusion of hardening elements into the metal substrate. In contrast, nitriding specifically aims to increase surface hardness through the diffusion of nitrogen into the metal substrate, making it a true case hardening process.