Notes on Japanese Sword Strength and Steel Grain Structure

Introduction

• Discussion of legendary Japanese swords (e.g., Ishikiri Maru, Ishidoro) and their exaggerated strengths.
• Examination of swordsmith craftsmanship and how it affects sword performance.
• Overview of the scientific aspects of creating strong swords, focusing on grain size in steel.

Importance of Metal Processing

• Slight changes in metal processing can significantly alter strength.
• Example: Copper bar (99.9% pure) behaves differently before and after heat treatment.
• Heat treatment can make metals softer or harder.

Internal Structure of Steel

• Steel consists of crystal grains, which can vary in size.
• Orientation of crystal grains affects strength and performance.
• Smaller grain size leads to higher strength due to two main reasons:
  1. Deformation Resistance:
     • Defects within larger crystals cause easier deformation.
     • Smaller grains act as barriers, preventing deformation.
  2. Impact Absorption:
     • More grain boundaries can absorb and disperse impact effectively.

Grain Size and Strength Relationship

• Hall-Petch Equation:
  • Strength increases as grain size decreases.
  • Example: Steel with 25 micrometers grain size vs. several micrometers grain size—strength can double.
• Research ongoing for industrial applications regarding grain size effects.

Creating Smaller Crystal Grains

• Avoid heating metals to high temperatures to prevent grain growth.
• Comparison with crushing ice: need to apply large energy while maintaining low temperatures.
• Optimal temperature for steel processing is below 700 degrees Celsius.
• Use rounded hammers for greater deformation.

Historical Insights on Grain Size

• Distinction between two periods of sword making:
  • Koto: Pre-Edo period.
  • Shinto: Post-Edo period.
• Koto swords are believed to have superior qualities.
• Study by Masahiro Kitada on 30 katanas showed:
  • Edo period swords have larger crystal grains (20-30 micrometers) compared to Kamakura swords (1-5 micrometers).
  • Smaller grains in Kamakura swords lead to higher hardness and better performance.

Conclusion

• Changes in sword-making techniques likely affected grain size over time.
• These changes could explain the myths surrounding the superiority of swords from certain periods.
• Acknowledgement that the study had limitations (only 30 swords analyzed).
• The discussion on grain size is one aspect of steel strengthening; other factors also contribute.

Future Topics

• Potential exploration of other principles for strengthening steel, including crystal structure and solid solution strengthening.