Order of increasing hot hardness of cutting tool material:

Hot hardness refers to the ability of a material to maintain its hardness at high temperatures. In the context of cutting tool materials, hot hardness is an important property as cutting tools are subjected to high temperatures during machining. The correct order of increasing hot hardness of cutting tool materials is:

HSS, Diamond, Carbide

1. High-Speed Steel (HSS):
High-Speed Steel is a type of tool steel that is commonly used for cutting tools. It is known for its relatively high hot hardness compared to other tool materials. HSS can maintain its hardness and cutting ability at elevated temperatures, making it suitable for high-speed machining operations. However, HSS has lower hot hardness compared to diamond and carbide.

2. Diamond:
Diamond is the hardest known material and has exceptional hot hardness. It retains its hardness and cutting ability even at extremely high temperatures. Diamond cutting tools are commonly used for machining non-ferrous metals and other abrasive materials. However, diamond tools are expensive and not suitable for all machining applications.

3. Carbide (Cemented Carbide):
Carbide, also known as cemented carbide or tungsten carbide, is a composite material made up of tungsten carbide particles bonded together with a metallic binder, usually cobalt. Carbide has excellent hot hardness and can maintain its cutting ability at high temperatures. It is widely used for cutting tools in various machining operations. Carbide tools are known for their high wear resistance and long tool life.

Conclusion:
The correct order of increasing hot hardness of cutting tool materials is HSS, Diamond, Carbide. High-Speed Steel has the lowest hot hardness among the three materials, while diamond has the highest hot hardness. Carbide falls in between and is widely used for cutting tools due to its excellent hot hardness and wear resistance.