Hot-work tool steels must have the ability to resist deteriorating core-hardness values when operating under high-temperature, hot-forming operating conditions such as steel and aluminum forging, die-casting dies and aluminum extrusion dies. The hot-work die steels can be categorized into three specific groups as follows:

  • Chromium hot-work die steels = AISI H10 to H19
  • Tungsten hot-work die steels = AISI H21 to H26
  • Molybdenum hot-work die steels = AISI H42

We will focus on the chromium hot-working tool steels (AISI H10 to H19). This group of steels contains the following nominal general analysis of the following alloying elements (there are variations on the individual chromium hot-work steels):

  • Carbon = 0.40% nominal
  • Chromium = 5% (nominal, except for H10 and H19)
  • Molybdenum = 1.5% (nominal, except for H10 and H19)
  • Vanadium = 0.80% (nominal, except for H10 and H19)
  • Tungsten = (used only in H12, H14, H19)
  • Cobalt = 4.25% (nominal, only used in H19)

The nominal analysis of the most popular chromium hot-work steels (H13) is as follows (nominal values):

  • Carbon =0.40%
  • Manganese = 0.35%
  • Chromium = 5%
  • Molybdenum = 1.5% 
  • Nickel = 0.30% max
  • Vanadium = 1.00% 

This grade of hot-work chromium steel demonstrates good shock resistance and (as previously stated) good resistance to losing hardness at elevated operating temperatures. Care should be exercised to protect the surface of the steel during its heat-treatment cycle, particularly during the austenitizing procedure. This is necessary to protect the surface against surface oxidation and decarburization.

It is almost mandatory and strongly recommended that one of the following methods of heating is conducted to prevent a surface attack: atmosphere (with a balanced atmosphere carbon potential in equilibrium), salt bath and vacuum.