In the field of metal forming, coining places extremely high demands on aço para ferramentas performance. Unlike conventional bending or blanking, the coining process relies on immense pressure to flow the metal, achieving exceptional dimensional accuracy and flawless surface detail.
The core principle of embossing lies in full-depth yielding. To imprint intricate patterns onto workpieces, dies must force the workpiece metal to yield across its entire thickness, subjecting the dies themselves to immense compressive forces. According to industry data, the compressive stresses endured by embossing dies typically exceed 325 ksi (approximately 2240 MPa). Under such extreme conditions, if the compressive strength of the tool steel is insufficient, the die will fracture instantly or undergo plastic deformation.
Many tool and die manufacturers focus solely on the material being processed—whether it’s aluminum, copper, or stainless steel—when selecting mold steel. However, in stamping processes, the critical factors determining tool and die steel longevity are often not the processed material itself, but rather the manufacturing method of the tooling and the type of stamping equipment.
Critical Performance Factors for Coining Die Steels
In the coining process, dies endure immense pressure and often face the risk of failure within extremely short timeframes. For die engineers, the most vexing issues are threefold: wear, sinking, and cracking. Five Causes of Embossing Die Failure:
1. High compressive strength and hardness
During embossing, dies endure extremely high cyclic pressures. If the steel lacks sufficient hardness, the die surface will develop indentations or even undergo plastic deformation. The surface hardness of cold embossing dies typically requires reaching approximately 62 HRC. Adequate surface hardness not only resists wear but is also the most critical factor in preventing die deformation under high pressure.
2. High Toughness
The harder the tool steel, the lower its toughness. However, in the embossing process, molds must withstand significant mechanical loads and fatigue. Therefore, material selection must balance high hardness requirements with sufficient toughness. Particularly for molds with deep cavities or complex designs, insufficient toughness can easily lead to cracking or splitting.
3. Dimensional Stability During Heat Treatment
Impression parts typically demand extremely high dimensional accuracy. Therefore, tool and die steels must exhibit excellent dimensional stability during heat treatment. For large or complex-shaped precision molds, steels that deform little during hardening are preferred.
4. Wear Resistance and Anti-Sticking Properties
Intense metal flow across mold surfaces causes severe abrasive wear. Therefore, selecting tool and mold materials with excellent wear resistance effectively counters metal erosion from workpieces and prevents galling or metal pickup.
5. Wear Resistance and Anti-Sticking Properties
Intense metal flow across mold surfaces causes severe abrasive wear. Therefore, selecting tool and mold materials with excellent wear resistance effectively counters metal erosion and prevents galling or metal pickup.
Recommended Steels for Coining Die Applications
There is no single best material for embossing dies; only the most suitable one. Based on production volume, die design complexity, and failure modes, we categorize commonly used steels into the following four tiers.
1. Water-Hardening Tool Steels (W Series)
For embossing processes involving materials such as silver, copper alloys, or stainless steel, W Series tool steels are the preferred choice, typically heat-treated to a hardness of 59-61 HRC.
A key characteristic of W-series steels is cold hubbability. Compared to air-hardening steels like A2, W1 steel exhibits exceptional cold-hardenability in the annealed condition. Under equivalent pressure, W1 steel achieves twice the indentation depth of A2 steel.
Molds made from W1 steel may fail after repeated use due to surface wear or the development of shallow cracks. Rather than being scrapped immediately, they can be revitalized through recozimento. The old cavities are machined away, followed by cold extrusion molding (hubbing). Finally, heat treatment is applied, restoring the mold to like-new condition. This recyclability significantly reduces production costs.
The W series tool steel exhibits shallow hardenability. After heat treatment, a high-hardness wear-resistant layer forms on the surface while the core retains good toughness. While this may sound like a drawback, this hard-outside-tough-inside structure effectively absorbs impact forces, preventing the entire mold from fracturing. It is highly suitable for die stamping and drop-forging applications.
W series tool steel is typically only suitable for smaller molds. If your mold features deep cavities, W series tool steel carries a risk of deep cracking. In such cases, you must decisively abandon the W series and instead select steel grades with deeper hardenability and superior toughness, such as S1 or L6.
2. Oil- and Air-Hardening Cold-Work Steels (O, A, and D Series)
For medium- to long-run production and when greater dimensional stability is required, alloyed cold-work steels are preferred.
| Grau | Characteristics & Applications | Dureza de Trabalho (HRC) | |
| Oil-Hardening | O1 | Good hardness and wear resistance; generally used for machined or hubbed dies for production quantities up to 100,000 pieces. Good combination of properties for coining dies. | 58-60 |
| Air-Hardening | A2 | Provides very high wear resistance and deep hardening. Used for long-run circular and flat dies. Commonly used as a die insert (prestressed by shrink rings) in progressive dies for quantities over 10,000 pieces due to the high required pressures | 56–58 |
| High-Carbon, High-Chromium | D2 | Provides very high wear resistance and deep hardening. Used for long-run circular and flat dies. Commonly used as a die insert (prestressed by shrink rings) in progressive dies for quantities over 10,000 pieces due to high required pressures | 56–58; up to 62 HRC for small aluminum parts |
Air-hardening grades like A6 are often used for large dies (over 4 inches in diameter) that must be hardened to 59-61 HRC due to their low distortion during heat treatment. These air-hardening steels offer better wear resistance than water- or oil-hardening types, making them excellent choices for long runs.
3. Shock-Resisting and Aços para trabalho a quente (S and H Series)
In deep designs or when dies frequently chip or fracture during operation, material selection must prioritize high toughness. Low-alloy steels from the S series (e.g., S1, S5) are the preferred solution. These steels withstand immense impact forces while maintaining excellent toughness at hardness levels of 57-59 HRC. Aço para ferramentas S1 is explicitly designated by industry standards as the dedicated material for manufacturing embossing dies and coin production tools. L6, as a low-alloy tool steel, also demonstrates outstanding toughness.
Many mistakenly believe that H13 (1.2344) is only suitable for hot forging or die casting. However, in cold-stamping applications, H13 is also the material of choice for addressing deep-cavity cracking. Compared to cold work tool steels, H13 exhibits superior resistance to extreme splitting stresses. In such applications, its hardness is typically controlled between 45-52 HRC. While this approach sacrifices some surface wear resistance, it ensures the die’s overall structural integrity, preventing failure from cracking.
Tratamento térmico
In the manufacturing of embossing dies, selecting high-quality steel is merely the first step. The ultimate performance of the die—whether it proves durable or fractures prematurely—largely depends on the quality of the heat treatment process. Below are three key heat treatment guidelines distilled from industry experience.
1. The Balance Between Hardness and Toughness
Heat treatment isn’t solely about making steel harder. Its core purpose is to transform the steel’s microstructure into martensita through quenching, followed by adjustment via têmpera. Many customers obsess over high hardness while neglecting toughness. Adequate heat treatment must strike a perfect balance between hardness and toughness. For embossing dies, high hardness without toughness results in brittleness, making the die prone to sudden fracture during use.
2. Multiple Tempering
After quenching and cooling, steel often retains unstable “retained austenita” internally. During subsequent use of the mold, residual austenite undergoes stress-induced transformation, causing mold volume expansion and leading to dimensional instability or even stress cracking. For high-load impression molds, double tempering is standard practice, and triple tempering is often employed. This ensures complete transformation of residual austenite into stable tempered martensite, significantly enhancing the mold’s strength and toughness.
3. Stress-relief annealing
Residual stresses from machining are the hidden culprits behind mold cracking. If left unchecked, complex mold geometries can amplify the destructive effects of these stresses. Stress-relief annealing must be performed to release internal stresses generated during cutting, ensuring dimensional stability during final heat treatment.
Aprimoramento de superfície
In practical applications of embossing dies, the surface requires extremely high hardness for wear resistance; however, the core must maintain sufficient toughness to prevent overall die fracture. Surface treatment can achieve both hardness and toughness.
Below are the three most mainstream process solutions.
1. Hard Chromium Plating
This method forms a protective layer through electrochemical deposition, particularly suitable for extending the service life of coining punches. When lower-hardness tool materials are selected, the mold surface is prone to scoring or scratches. Hard chromium plating effectively prevents such surface damage. However, we recommend that the base steel hardness be at least 50 HRC to provide a stable substrate for the plating layer.
2. Nitriding and Soft Nitriding
If your mold is made of H13 (1.2344) steel, nitriding is the optimal choice. Nitriding forms an extremely hard compound layer on the mold surface, significantly enhancing wear resistance while preventing material adhesion. Nitrided H13 parts also exhibit greatly improved fatigue resistance, making them highly suitable for high-frequency impression molds.
3. PVD Coatings (e.g., TiN)
Applying physical vapor deposition (PVD) coatings, particularly golden-yellow titanium nitride (TiN), to embossing punches can extend mold service life by 3 to 4 times. PVD coatings are extremely thin. If the substrate steel is too soft, the coating will fracture under heavy pressure due to substrate deformation. Therefore, when using PVD coatings, high-hardness substrate materials must be selected to prevent coating spalling.