Tempering Steel

• Unlike the annealing, hardening, and normalizing processes, all tempering occurs below the low-critical point of steel. Tempering lasts at least 1 hour. For each inch added to the thickness of the steel, the tempering process lasts an additional hour. Operators determine what temperature to use for the steel, based on the desired end product. The higher the temperature of tempering, the lower the hardness of the material will be. Tempering causes a thin oxide layer to develop on the steel's surface. This oxide layer changes color based on the temperature, indicating different molecular changes in the material. By examining this color, the operator can determine when tempering is finished.
  
  

Tempering Glass

• Tempered glass is up to five times stronger than pre-tempered glass. The tempering process involves heating annealed glass to the point where the glass begins to slump under its own weight -- called the softening point -- and then quickly cooling it. The result is a safety glass; when it breaks, it shatters into many small blunt shards, rather than sharp pieces. Fully tempered glass is used in windowpanes and windshields. Other benefits to tempering glass include a higher edge strength and resistance to sunlight breakage, in which the uneven distribution of heat from the sun may cause untempered glass to distort.
  
  

Tempering Precipitation-Hardened Alloys

• Precipitation-hardened alloys are previously malleable materials, such as alloys of magnesium, copper or nickel, that have undergone a tempering process to strengthen them. The process disassociates certain particles in the alloy and hardens them, preventing the movement of other molecules in the crystalline structure of the metal. Tempering controls the mechanical properties of the alloy for specific industrial uses. Because the tempering of precipitation-hardened alloys is done at lower temperatures and for a longer period of time, it is sometimes called "aging."
  
  

Blacksmith Tempering

• Blacksmiths use the tempering process to harden steel. Tempering in a forge is considered an art for two reasons: the temperature of the fire may not be accurately gauged, and it may not be easy for the blacksmith to see the steel color at certain temperatures. The color is the usual indicator that the steel should be taken out of the flame and quenched. Heating the steel too quickly or too slowly can cause structural defects that are not noticeable at first.