Selection of Tool Steel for Hydroforming Dies
Hydroforming is a high-pressure forming process in which fluid—typically an oil-water emulsion—forces sheet or tubular blanks into a die cavity. Internal pressures commonly exceed 200 MPa (30 ksi), thereby generating bursting forces and die-clamping loads. At the same time, sliding contact under load produces frictional shear at the die surface.
In warm hydroforming, die temperatures can reach up to 400 °C (750 °F), especially when forming aluminum or magnesium alloys. Under these combined conditions, die failure is typically driven by plastic deformation, galling, surface wear, or, at elevated temperature, thermal fatigue.
Selection Factors
Tool steel selection for hydroforming dies is governed by three main risks: deformation under pressure, surface damage from friction, and structural failure under combined loading.
Compressive Strength
Hydroforming applies sustained hydrostatic pressure and axial clamping force. The die material must resist plastic deformation to maintain dimensional accuracy. Once yielding occurs, die geometry is permanently altered, leading to part deviation and early tool rejection.
Wear Resistance vs. Toughness
Friction between the workpiece and die surface leads to adhesive wear (galling) and abrasion. Increasing hardness improves wear resistance but reduces toughness. Under high internal pressure, localized tensile stresses can develop, making brittle steels prone to cracking.
Selection must therefore follow the dominant failure mode. In wear-driven conditions, higher hardness and carbide content improve die life. Where cracking risk is present, toughness becomes the limiting factor.
Polishability
Surface finish directly affects friction and material flow. Poor surface quality increases galling and accelerates surface damage. Tool steels with a uniform microstructure allow stable polishing and better surface retention during production.
Thermal Stability
In warm hydroforming, the steel must retain hardness and resist thermal fatigue under repeated heating and cooling. Softening accelerates wear, while insufficient thermal fatigue resistance leads to surface cracking. This becomes critical once the die temperature rises beyond typical cold-forming conditions.
Recommended Tool Steels
AISI D2 Tool Steel |1.2379 | SKD11
AISI D2 is a high-carbon (about 1.5%) and high-chromium (about 12%) cold-work tool steel suited for cold hydroforming where friction and wear dominate. At 60–62 HRC, it provides high abrasion resistance and compressive strength, helping maintain die geometry over long production runs.
Its limitation is low toughness due to high carbide content. Under high internal pressure or insufficient structural support, cracking can occur. D2 is best suited for rigid die designs, stable loading conditions, and high-volume production, where galling is the primary failure mode.
AISI A2 Tool Steel | 1.2363 | SKD12
AISI A2 is selected when the risk of cracking cannot be ignored. Operating at 57–60 HRC, it offers lower wear resistance than D2 but significantly higher toughness. This allows it to withstand bursting forces and stress concentration without premature fracture.
Its air-hardening characteristic reduces distortion during heat treatment, which is beneficial for complex die geometries requiring dimensional stability. A2 is a practical choice when both wear and fracture risks are present, especially in less predictable loading conditions.
H13 Tool Steel Supplier | 1.2344 | SKD61
For warm hydroforming up to 400 °C, AISI H13 is the standard solution. As a chromium hot-work tool steel, it maintains hardness at elevated temperatures and resists thermal fatigue.
At 46–54 HRC, H13 provides lower wear resistance than cold-work steels but much higher toughness and resistance to heat checking. It is suitable for applications where thermal cycling and pressure loading occur simultaneously, and where cold-work steels would soften or crack.
AISI O1 Tool Steel | 1.2510 | SKS3
AISI O1 is used mainly for prototyping or short production runs. At 58–60 HRC, it offers moderate wear resistance and good machinability, enabling faster die manufacturing and lower tooling costs.
However, its lower hardenability and limited resistance to wear and pressure make it unsuitable for long runs or severe loading conditions. It is best used where tooling cost and lead time are more important than service life.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage | Best Application |
| AISI D2 | 60–62 HRC | Maximum wear and galling resistance | High-volume cold hydroforming |
| AISI A2 | 57–60 HRC | Higher toughness, reduced cracking risk | Complex dies, high pressure |
| AISI H13 | 46–54 HRC | Thermal fatigue resistance | Warm hydroforming (≤400 °C) |
| AISI O1 | 58–60 HRC | Machinability and low cost | Prototyping and short runs |