Hot Hardness of Cutting Tool Materials — Correct Increasing Order Select the correct order of increasing hot hardness (resistance to softening at elevated temperature) for common cutting tool materials.

Introduction / Context:
Hot hardness is critical in metal cutting because the tool tip experiences high temperatures. Materials that retain hardness at elevated temperatures enable higher cutting speeds and longer tool life. Typical tool materials include high speed steel (HSS), cemented carbides, and diamond (including CVD diamond and natural diamond for non-ferrous work).

Given Data / Assumptions:

• Comparative ranking at high cutting temperatures.
• Workpiece is generic; no chemical reactivity effects are considered.
• Diamond use is assumed on non-ferrous/non-steel materials to avoid graphitisation reactions at high temperature in steel cutting.

Concept / Approach:
HSS softens at lower temperatures than carbides; therefore, it permits lower cutting speeds. Cemented carbides (WC–Co and advanced grades) maintain hardness to higher temperatures, supporting faster machining. Diamond has the highest hot hardness among these, but is restricted by chemical reactivity with iron-based alloys at elevated temperatures. Ranking purely by hot hardness yields HSS < Carbide < Diamond.

Step-by-Step Solution:
Identify hot-hardness threshold for HSS (lowest of the three).Place cemented carbide next (higher hot hardness than HSS).Place diamond as the highest hot hardness material.Compile order from low to high → High speed steel, Carbide, Diamond.

Verification / Alternative check:
Tool material charts universally place diamond above carbides for hot hardness, with HSS below both.

Why Other Options Are Wrong:
Any order that puts HSS above carbides or carbides above diamond contradicts standard hot-hardness data.

Common Pitfalls:
Confusing hot hardness with fracture toughness; HSS may be tougher than carbide, but less hot-hard.

Final Answer:
High speed steel, Carbide, Diamond