Heat Treatment – Change the Properties of Metal through Heat Treatment

Key Terminology for Carbon Steel:

  • Annealing: Heating metal and cooling Slowly to remove internal stresses and Increase its Toughness (ability to deform without fracturing/ability to absorb energy).
  • Normalizing: An annealing process to give the material a uniform fine-grained structure and make it less brittle. It involves heating the steel to 20-50°C above its upper critical point, soaking it for a short period at that temperature and then allowing it to cool.
  • Hardening: This process increases the metals Hardness(wear resistance) and Strength but this comes at the cost of increasing its brittleness(breaks when subjected to small amounts of stress - e.g. glass), and reducing its toughness and plasticity. Carbon steel is heated to its upper critical point and soaked for a short period of time, then Quenched/cooled rapidly (Usually placed in water/oil).
  • Tempering: This process removes much of the Brittleness from hardened steel whilst allowing it to retain an amount of its Strength/Wear resistance. This process can be adjusted to balance the wear resistance with brittleness. To temper carbon steel, the metal (within 1 hour of hardening) is reheated to between 300 to 600f (depending on the type of steel used) for a period of time, usually no less than 2 hours but can require even longer.

 

Example Heat Treatment Process for O1 Tool Steel

Preheating: Heat at a rate not exceeding 400F per hour (222centigrade) to 1200-1300F (649-704 centigrade) and equalize.

Austentizing (High Heat): Heat slowly from the preheat to 1475-1500F (802-816 centigrade). Soak for 30 minutes for the 1st Inch of thickness, plus 15 minutes for each additional inch.

Quenching: Oil Quench (Canola oil/Quenching oil) to a temperature no lower than 150-125F (66-51 centigrade). O1 susceptible to quench cracking, especially in changes between section thickness and sharp internal corners. Recommended for oil should be hot at 300-400F (149-204 centigrade).

Tempering: Temper immediately after quenching and do not allow part to cool below 125F (51 Centigrade). Typical tempering range is 350-400F (177-204 centigrade). Hold at temp for 1 hour per inch/minimum 2 hours.Then air cool to ambient temp