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Advantages of Vacuum Heat Treatment for Tool Steels (part 2)

July 12, 2012
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The heat treatment of tool steels (and any other steel heat-treatment procedure) relies on a given set of metallurgical principles.

Vacuum heat treatment has another advantage, particularly for hot-work tools such as forging dies, shot sleeves, aluminum die-casting dies, magnesium die-casting dies and other hot-work applications.

It was mentioned in part 1 that there will be no intergranular surface oxidation, and this was a distinct advantage of the vacuum heat-treatment method. If there is any intergranular surface oxidation, it has initiated a small surface (micro)crack, which is migrating into the surface of the steel. That small crack is the initiation point of heat-checking.

Further, the surface microcracks will strongly influence the die life and its performance. Microcracks can occur in service as a result of prior microstructure (e.g., occurring as a result of the die heat treatment) when heat treated in a furnace that contains an atmosphere (not vacuum or low pressure).

The conditions that will affect the cause of surface microcracks during, for example, hot-forging operations and die-casting operations are:
  • The accomplished microstructure from the heat-treatment procedure
  • Grain size accomplished from the heat-treatment procedure
  • Hardness uniformity throughout the maximum cross section
Steel analysis is another condition that will affect the cause of surface microcracks. Some of the ways include tool steels that contain:
  • Tungsten tto increase the toughness/resistance to mechanical damage as well as resistance to softening by the billet temperature
  • Molybdenum to improve wear resistance/abrasion that occurs particularly on forging dies due to scale build up on the die from the heated billet and sliding wear as a result of the deformation of the billet being deformed during the forge operation. It will also reduce the resistance to die softening as a result of the forged billed contact with the die surface.
  • Vanadium will also assist in the resistance of wear resistance/abrasion as well as improve the resistance to softening of the die surface due to the forged billet surface contact with the die.
Vacuum heat treatment at non-radiant temperatures is not an economical method of heating because of the lack of conduction that occurs due to the absence of a conduction gas (such as air at normal atmospheric temperature and pressure). Therefore, the practice of heat-up in vacuum could be effectively conducted in a uniform and efficient manner simply by:
  • Evacuating
  • Backfilling with either nitrogen or argon gas (depending on the application of the workpiece) to a pressure just below atmospheric pressure
  • Transmitting thermal energy heat from the heat source to the workpiece by gas molecular energy interaction
Once at a radiant temperature of, say, approximately 1250°F, the conductance gas can be evacuated and heating by radiation will occur.

Care will need to be taken in selecting the temperature ramp rate up to the final austenitizing temperature in relation to the steel analysis, the maximum material (steel) cross section and the part geometry.

The benefits of vacuum heat treatment will most certainly reduce (not eliminate) the risk of die-surface heat checking.        
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