H21 Hot Work Tool Steel

H21 Hot Work Tool Steel is a low-carbon, tungsten hot work steel that exhibits excellent resistance to softening at elevated temperature. This grade is intended for elevated-temperature service in which the tool operates at a dull-red heat. For best results, H21 hot work steel should not be water cooled in service unless the tool design includes a continuous flow of internal water cooling. Spray cooling and thermal shock should be avoided. Tools should be thoroughly preheated before entering service.

APPLICATIONS: Recommended for difficult hot work tooling applications such as brass extrusion tooling, brass die casting dies, hot punches, and forging die inserts.

Composition

Relative Properties

Physical Properties

• Density: 0.296 lb/in³ (8193 kg/m³)
• Specific Gravity: 8.19
• Modulus of Elasticity: 30 x 10⁶ psi (207GPa)
• Machinability: 55-60% of a 1% carbon steel

Heat Treating Instructions

Hardening

Preheating: To minimize distortion in complex tools use a double preheat. Heat at a rate not exceeding 400°F per hour (222°C per hour) to 1150-1250°F (621-677°C), equalize, then raise to 1525°F (830°C) and equalize.

For normal tools, use only the second temperature as a single preheating treatment.

Austenitizing (High Heat): Heat rapidly from the preheat.
Furnace: 2125 - 2150°F (1164 - 1177°C)
Salt: 2100 - 2125°F (1149 - 1164°C)

Quenching: Air, pressurized gas, warm oil, or salt. For pressurized gas quenching, a minimum quench rate of approximately 50°F per minute (28°C per minute) to below 1000°F (538°C) is required to obtain the optimum properties in the steel.

For oil, quench until black, about 900°F (482°C), then cool in still air to 150-125°F (66-51°C).

For salt maintained at 1000-1100°F (538-593°C), equalize then cool in still air to 150-125°F (66-51°C).

Tempering: Temper immediately after quenching. The typical tempering range is 1000-1150°F (538-621°C). Hold at the tempering temperature for 1 hour per inch (25.4 mm) of thickness, but for 2 hours minimum, then air cool to ambient temperature. Double tempering is required. To maximize toughness and tool performance, a third temper is often used as a stress relief after all finish machining, grinding, and EDM work are completed on the tool.

Annealing

Annealing must be performed after hot working and before re-hardening.

Heat at a rate not exceeding 400°F per hour (222°C per hour) to 1600-1650°F (871-899°C), and hold at temperature for 1 hour per inch (25.4mm) of maximum thickness; 2 hours minimum. Then cool slowly with the furnace at a rate not exceeding 50°F per hour (28°C per hour) to 1000°F (538°C). Continue cooling to ambient temperature in the furnace or in air. The resultant hardness should be a maximum of 235 HBW.