Notes on Japanese Sword Craftsmanship and Metal Strength

Introduction

• Discussion on legendary Japanese swords with extraordinary powers (e.g., Ishikiri Maru)
• Scientific exploration of sword strength based on craftsmanship
• Focus on the effects of grain size on steel

Overview of Metal Strength

• Metals with identical appearances can have different properties
• Example: 99.9% copper bar
  • Difficult to bend at room temperature
  • Becomes soft and easy to bend after heat treatment

Importance of Internal Structure in Steel

• Steel's structure consists of crystal grains of varying orientations
• Smaller grain sizes lead to higher strength

Crystal Grain Formation

• Upon heating and cooling, small crystals form and grow
• Larger crystals can lead to defects and easier deformation
• Smaller crystals act as barriers against deformation

Grain Boundaries

• Larger grain boundary area allows for better impact absorption and dispersion
• Impact energy is absorbed and spread out by smaller grains

Relationship Between Grain Size and Strength

• Hall-Petch Equation: Predicts material strength based on grain size
  • Smaller grains = higher tensile strength
  • Example: Steel with 25 micrometers grain size vs. several micrometers grain size (strength can double)

Techniques for Achieving Smaller Crystal Grains

• Comparative Example: Crushing ice while preventing melting
• Maintaining lower temperatures is crucial to avoid larger grain formations
• Heating above certain temperatures can cause larger crystals to form
• Best to work below 700°C to prevent grain growth
• Effective striking technique (rounded hammer) can help in deformation

Historical Context of Japanese Swords

• Distinction between swords made before and after the Edo period (Koto vs. Shinto)
• Claims of Koto swords having superior qualities (sharpness, color, feel)
• Study by Masahiro Kitada on 30 katanas from Kamakura to Edo period
  • Found that Edo period swords have larger crystal grain sizes
• Kamakura swords: grain size 1-5 micrometers; Edo swords: 20-30 micrometers
  • Smaller grains in Kamakura swords indicate higher hardness

Implications of Findings

• Changes in heating techniques and processing potentially affect grain size
• May contribute to rumors about the superiority of swords from certain periods
• Note: Findings are based on a limited sample size (30 swords)

Conclusion

• Discussed only one method of strengthening steel (grain size)
• Other strengthening principles exist (e.g., crystal structure, solid solution strengthening)
• Future videos may cover additional strengthening methods.