How to make metal stronger by heat treating, alloying and strain hardening

Introduction

• Dr. Billy Wu discusses how processing can enhance the strength of materials.

• Previous videos covered equilibrium phase diagrams and steels.

Importance of Material Processing

• Mechanical properties of materials depend on composition and microstructure.

• Many manufacturing processes occur under non-equilibrium conditions.

• Cooling rates significantly impact mechanical properties.

• Examples:

  • Sword making (quenching to increase strength).

  • Gears needing hard outer surfaces with ductile cores (e.g., induction heating).

Approaches for Strengthening Materials

• Three main strengthening methods:

  1. Solid Solution Hardening:

     • Alloying with impurities increases tensile and yield strength.

     • Impurities impede dislocation movement in crystalline materials.

  2. Strain Hardening (Cold Working):

     • Involves plastic deformation to increase strength.

     • Accompanied by a loss of ductility.

  3. Precipitation Hardening:

     • Formation of a second, dispersed phase enhances strength.

     • Specific heat treatment processes are utilized.

Solid Solution Hardening

• High purity metals are typically softer than alloys.

• Increased impurities lead to stronger materials by impairing dislocation movement.

• Impurity size matters:

  • Smaller impurities cause tensile lattice strain.

  • Larger impurities introduce compressive lattice strain.

Grain Size Influence on Mechanical Properties

• Grain size affects mechanical properties due to dislocation movement across boundaries.

• Fine-grained materials are harder and stronger.

• Processing conditions greatly influence grain size:

  • Annealing at 550 °C produces smaller grains than at 650 °C.

• Hall-Petch Equation relates yield strength to grain size:

  • Yield strength = sigma i + k / √(average grain size).

Strain Hardening (Cold Working)

• Strength increases through plastic deformation characterized by the amount of cold working.

• Increased dislocation density makes further deformation harder.

• Smaller grains due to strain hardening provide additional barriers against dislocation movement.

Precipitation Hardening Process

• Two stage process to strengthen materials:

  1. Solution Heat Treatment:

     • Supersaturated single phase created by rapid cooling.

  2. Precipitation Heat Treatment:

     • Material reheated to form small dispersions of a second phase.

• Example: Silver-copper alloy phase diagram illustrates solid solubility and supersaturation.

• Rapid cooling leads to insufficient atom diffusion, creating a stable structure at high temperature.

Effects of Heat Treatment on Steel

• Types of alloys in low alloy steels: Iron and carbon.

• Eutectoid composition at room temperature consists of pearlite (ferrite and cementite).

• Cooling rates determine final structures:

  • Slow cooling results in coarse pearlite.

  • Faster cooling yields finer pearlite or bainite.

  • Quenching forms martensite, a very hard but brittle phase.

• Tempering helps restore ductility in brittle martensite.

Time-Temperature-Transformation (TTT) Diagram

• TTT diagrams are essential for understanding non-equilibrium structures formed at various cooling rates.

• Eutectoid temperature separates stable and unstable phases.

• Features:

  • Various regions show formation points for structures like pearlite and martensite.

• Shallow gradient lines on the plot indicate slower cooling leading to pearlite formation.

Summary of Material Strengthening

• Properties significantly vary with composition and processing methods.

• Solid solution hardening enhances strength via impurities.

• Controlling grain size through thermal and plastic processes impacts mechanical strength.

• Precipitation hardening through heat treatment creates dispersed phases that scatter dislocation movement.

• Steel’s strength is sensitive to heat treatments, revealing diverse microstructures based on cooling rates.

Conclusion

• Dr. Wu concludes with an invitation to review earlier material for deeper understanding.