Die wear is affected by, but not limited to, steel strength and surface friction, contact pressure, sliding velocity, temperature, die surface treatment and lubrication. As the material strength and hardness is increased, an increase in die wear occurs which leads to quality issues in the stamping. In addition, frequent die wear requires more frequent die maintenance or replacement, affecting turnaround times, productivity and cost.
Actions can be taken to prevent excessive wear on die materials when forming Advanced High-Strength Steels. To prevent such wear, new die materials and better coatings have been developed to maintain their hardness without compromising the toughness of the material. Hard material coatings and nitriding have also been used to improve the tribological properties of die surfaces.
Tool and Die Materials
In general, the considerably higher strength levels associated with AHSS grades exerts proportionally increased forces on the die material. AHSS might reach strength levels four to five times higher than mild steels, approaching 2000 MPa. This is based on complex metallurgy that delivers high concentrations of martensite and very high work hardening rates.
The higher forces required to form AHSS require increased attention to tool and die material specifications. Key requirements that need to be specified are the stiffness and toughness of the die substrate, as well as the surface treatments and hardness of the die coating. Life and performance of a draw die is determined by the amount of wear/galling that accumulates during forming, which then defines specific die maintenance intervals. When selecting die materials, important considerations are:
- Sheet metal being processed, characterized by strength, thickness, and surface coating.
- Die construction, machinability, radii sharpness, surface finish, and die hardness, specifically on draw beads and radii.
- Cost per part.
Thickness reduction for weight saving is one primary reason for applications of AHSS. Unfortunately, the reduced thickness of the steel increases the tendency to wrinkle. Higher blankholder loads are required to suppress these wrinkles. Any formation of wrinkles will increase the local load and accelerate the wear effects. Figure 1 shows a draw die with severe die wear due to excessive wrinkling on a DP780 part. It is not uncommon to replace these high wear areas with a more durable tool steel insert to minimize this type of excessive wear condition.
Tool steel inserts for forming dies must be selected according to the severity of the forming. Surface coatings are recommended for DP 350/600 and higher grades. When coatings are used, it is important that the substrate has sufficient hardness/strength to avoid plastic deformation of the tool surface, even locally. Therefore, a separate surface hardening, such as nitriding, is recommended before the coating is applied. Prior to coating application, it is important to use the die in in pre-production to provide time to adjust forming conditions and establish die performance Surface roughness must be as low as possible before coating. Ra values below 0.2 µm are recommended. Steel inserts with a TiC/TiN coating are recommended for local high-pressure conditions that cause accelerated die wear and zinc flaking.
Table 1, provided by the Auto/Steel Partnership, describes recommended construction materials for stamping dies and components, based on significant research efforts. North American Automotive Metric Standards (NAAMS) are the product of a consortium between Ford, GM, FCA and various North America automotive steel producers.
Tool steels for cutting, trimming, and punching operations must be similarly selected. For these operations, tensile strength is more important than yield strength. Tool hardness between 58 and 62 HRC is recommended. Coatings may be used to reduce tool wear, but for the highest strength steels (above 1000 MPa tensile strength) coatings may fail due to local deformation of the die material substrate.
To prevent this, hardening of the substrate prior to coating is again strongly recommended. High performance tool steels, such as powder metallurgy (PM) grades, can be expensive but are justified because of their low wear rate and increased life. Ceramic tool inserts have extreme hardness for wear resistance, high heat resistance, and optimum tribological behavior, but have poor machinability and severe brittleness. High costs are offset by reduced maintenance and increased productivity. While not commonly used, the ceramic tool inserts offer a possible solution to high
interface loading and wear.