Which one of the following processes is used for hardening the surface of tool steel?

Explanation:

Carburizing:

• The most popular method of surface hardening is carburizing.
• Here, low-carbon steel's surface layers have up to a 10% carbon enrichment. 
• The carbon diffuses into the steel over time at a high temperature after entering the steel at the surface.
• At 920–950°C, carburizing is carried out.
• It is used for hardening low-carbon steel parts, high-alloy steel bearings,  tool steel, gears, shafts, and other components.
• In this process, the hardness and toughness of the low-carbon steel is improved to make high-carbon steel.

​Cyaniding:

• Cyaniding is done in a liquid bath of NaCN, with the concentration varying between 30 and 97%.
• The temperature used for cyaniding is lower than that for carburizing and is in the range of 800‐870^oC.
• The time of cyaniding is 1‐3 hr to produce a case depth of 0.25 mm or less.

Induction hardening:

• Induction hardening is accomplished by placing the part in a high-frequency alternating magnetic field.
• It differs from surface hardening in the way that the hardness of the surface is not due to the increase in carbon content but due to rapid heating followed by controlled quenching
• In this process, a high-frequency current is introduced in the metal surface and its temperature is raised up to a hardening range
• As this temperature is attained, the current supply is cut off, instantaneously water is sprayed on the surface
• It is used for hardening surfaces of crankshafts, camshafts, gear automobile components, etc.

Hardening:

• It is a heat treatment process in which steel is heated to the proper temperature based on its carbon content and maintained there for a long enough period of time to achieve uniform heating across the section.
• After that, a cooling liquid is used to quickly cool the steel.
• Depending on the steel's composition and the desired hardness, a cooling medium such as water, oil, molten salt, or air may be employed.

Thus, option (1) is the correct answer.