Heat Treating Specifications for A2 Tool Steel

Annealing

• Annealing makes it easier to cut and shape metal. A2 tool steel is annealed by protecting the steel and heating it uniformly so that it reaches a temperature of 1550-1600 degrees Fahrenheit. The steel is protected either by keeping it in a container packed with neutral packing material or by using a controlled-temperature furnace. It then must be cooled in a cooling furnace by 20 degrees F per hour until it reaches 1200 degrees and the furnace turns black. It is then cooled in open air until it reaches room temperature. This process is performed on sheet steel before it is used to manufacture tools.

Stress Relieving

• Stress relieving is done after the steel is rough machined, or cut into the basic tool shape, and before it is hardened. It is meant to correct any stress which may have occurred during the machining process so that the tool hardens evenly and solidly. The specifications for stress relieving A2 tool steel call for heating the rough machined piece to 1200 degrees F for a minimum of an hour; the steel is then cooled slowly at least until it reaches 930 degrees F, after which it can be cooled more rapidly. At this point the tool can be machined more finely before hardening.

Hardening

• Hardening after stress relief should take place in two steps, the first being preheating to a temperature of 1200--1380 degrees F, and the second being austenitizing at 1720--60 F. Austenitizing causes the crystal structure of steel to change so that austenite, a carbon-based alloy that is the basis of stainless steel, forms. The austenitizing temperature is maintained for 20 minutes plus 5 minutes per inch of thickness before the steel tool form is air quenched or rapidly air cooled.

Tempering

• The correct temperature for tempering, which is the final hardening process that is performed after the tool is fully shaped, depends upon the desired level of hardness. The two most common temperatures used are 400 degrees F for hardness and strength (resistance to force) or 700 degrees F when strength is more important than hardness.