Selection of Tool Steel for Hot Forging Dies
Hot forging forms metal through repeated impact or continuous pressure, exposing dies to high contact stress, rapid thermal cycling, and severe surface wear. Contact pressures commonly exceed 1000 N/mm² (145 ksi), which places the die surface at constant risk of plastic deformation.
The die surface can reach 650–700°C during operation, while rapid heating and cooling generate strong thermal gradients. These cyclic temperature changes induce thermal stress, which drives crack initiation and growth.
At the same time, high-speed material flow, up to 50 m/s, produces continuous abrasive and adhesive wear on the die surface.
Failure occurs mainly through three mechanisms: abrasive wear, plastic deformation, and thermal fatigue cracking. Tool steel selection must therefore be based on which failure mode dominates under actual service conditions.
Key Selection Factors
Wear resistance and toughness define the primary trade-off. Higher hardness and carbide content improve resistance to abrasive wear, but reduce impact resistance. In forging dies, insufficient toughness leads to edge cracking or fracture under repeated loading. Practical hardness must therefore be set near the upper limit that still maintains sufficient toughness.
Hot hardness determines resistance to plastic deformation at elevated temperature. As temperature increases, yield strength drops rapidly. If the hot strength is insufficient, the die surface deforms, leading to dimensional loss and early failure.
Resistance to thermal fatigue controls how quickly heat checking develops. Repeated heating and cooling cycles generate surface cracks. Steels with higher thermal conductivity, combined with adequate high-temperature strength and ductility, slow the initiation and propagation of cracks.
Selection should follow the dominant service condition: impact-dominated applications require higher toughness, while long contact time and higher temperatures require greater hot strength and wear resistance.
Recommended Tool Steels
AISI H11 Tool Steel | 1.2343 | SKD6
H11 is used where impact loading is the primary failure driver. Its high toughness and resistance to thermal shock make it suitable for hammer forging, where contact time is short but impact is severe.
Working hardness is typically 38–54 HRC. The material tolerates limited water cooling without immediate cracking, which helps control die temperature in high-cycle operations.
H13 Tool Steel Supplier | 1.2344 | SKD61
H13 is used where both wear resistance and toughness are required. Its vanadium content improves resistance to abrasive wear while maintaining sufficient toughness for most forging conditions.
Working hardness is typically adjusted based on service conditions. Lower ranges of 40–44 HRC are used for higher impact resistance, while 44–50 HRC is used when wear resistance becomes more critical.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is selected for high-temperature applications where resistance to softening is the primary requirement. It retains hardness up to approximately 620°C, making it suitable for sustained high-heat conditions.
Working hardness is typically 43–52 HRC. However, its lower toughness makes it sensitive to thermal shock. Water cooling must be avoided, as rapid temperature changes can cause immediate cracking.
AISI H12 (Chromium-Tungsten Base)
H12 provides higher hot strength than H11 due to the addition of tungsten. It is better suited for press forging, where dies operate at higher temperatures and longer contact times.
Working hardness is typically 38–55 HRC. Compared with H11, it offers improved resistance to softening and wear, but with reduced toughness.
Low-Alloy Proprietary Steels (e.g., 6F2 / 6F3)
Ni-Cr-Mo steels are used for large die blocks and holders where structural strength is required rather than surface wear resistance.
With hardness levels of 341–375 HB, these steels provide high resistance to bulk deformation and impact. They are suitable for applications with moderate surface temperatures and large tooling sizes where cost and structural reliability are critical.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage | Cooling Method |
| AISI H11 | 38–54 HRC | High toughness for impact-dominated forging | Water cooling acceptable |
| AISI H12 | 38–55 HRC | Higher hot strength for press forging | Air or limited water |
| AISI H13 | 40–50 HRC | Balanced wear resistance and toughness | Air or controlled water |
| AISI H21 | 43–52 HRC | Maximum hot hardness at high temperature | No water cooling |
| 6F2 / 6F3 | 341–375 HB | High structural strength for large dies | Air or oil |