Tool materials — red hardness of HSS High-speed steel (HSS) cutting tools are known to retain useful hardness up to approximately what temperature under cutting conditions?

Introduction / Context:
Red hardness (hot hardness) indicates a tool material’s ability to retain hardness, and therefore cutting ability, at elevated temperatures generated during machining. HSS became a breakthrough because it maintains useful hardness significantly above that of plain carbon tool steels.

Given Data / Assumptions:

• Conventional HSS grades (e.g., M2, T1) under typical cutting conditions.
• No advanced coatings or high-pressure coolant assumed.
• Comparison with much higher hot hardness of carbides and ceramics.

Concept / Approach:
Carbon tool steels rapidly lose hardness above roughly 200–250°C. HSS remains serviceable to temperatures around the 500–600°C range, enabling higher cutting speeds than carbon steel tools. Carbides and ceramics extend this much further.

Step-by-Step Solution:

Verification / Alternative check:
Textbook data cites HSS operational hot hardness range near 550°C; 500°C is a standard rounded figure for exam contexts.

Why Other Options Are Wrong:
250°C / 350°C: more indicative of carbon tool steels; underestimate HSS performance.900°C: closer to carbide/ceramic realms; unrealistic for HSS.

Common Pitfalls:
Assuming HSS equals carbide performance; in reality carbides operate far hotter with superior wear resistance, enabling much higher speeds.

Final Answer:

500°C