Technically, cold working is the forming of metal at room temperature, with the maximum temperature not exceeding 200°C (390°F). Tool and die steel used for such processing must have extremely high hardness and wear resistance, making it suitable for stamping, cold drawing, embossing, shearing, and cold forging.
Critical Properties Governing Tool Steel Selection
1. Hardness and Wear Resistance
In cold forming applications, tools and dies must withstand not only immense mechanical pressure but also severe friction. Therefore, materials must possess high hardness and high wear resistance. Cold-forming dies, especially punches, endure significant stress during continuous production.
According to data, cold work dies sometimes endure pressures as high as 3000 MPa. If the steel lacks sufficient strength, the die will undergo plastic yielding, leading to dimensional deviations in the product. Hardness directly determines resistance to deformation. Typically, cold work dies require a hardness of approximately 60 HRC. High hardness generally translates to high wear resistance.
2. Toughness and Shock Resistance
In cold-working applications, the relentless pursuit of high hardness often comes with significant risks. Toughness is the steel’s ability to resist chipping and cracking.
However, as hardness increases, materials typically become more brittle. At high hardness levels, insufficient toughness can lead to microchipping at mold edges. These minor defects can rapidly propagate, ultimately leading to complete mold failure. In thick-plate machining or heavy-duty applications—such as chisels, thick-plate shear blades, or cold-heading dies—toughness considerations may outweigh hardness requirements.
3. Dimensional Stability and Hardenability
In cold work die manufacturing, dimensional stability refers to the ability of tool and die materials to maintain their original dimensions and shape after heat treatment. If materials are improperly selected, the die may warp, expand, or shrink, leading to costly subsequent grinding corrections or even outright scrapping.
Hardenability is a critical indicator. Hardenability does not refer to the maximum hardness achievable, but rather the depth to which hardness can penetrate into the core of the steel. Tool and die steels with poor hardenability may develop a hard surface after heat treatment, yet retain a soft core. This can lead to mold collapse during use.
Selection of Cold-Work Tool Steel Grades (AISI Classification)
The AISI system classifies cold-work tool steels into three main groups—Air-hardening (A), High-Carbon, High-Chromium (D), and Oil-hardening (O)—along with specialized Shock-Resisting (S) and Water-Hardening (W) categories.
D-Series: The Wear Resistance Champions
The D series features high carbon and high chromium content. Chromium combines with carbon to form numerous hard carbide particles. These particles resist severe wear during stamping and shearing processes, making them ideal for long-run blanking dies, forming rolls, and cold extrusion dies.
Among all cold work tool steels, D2 (1.2379) is the most widely used and the highest-volume grade shipped from our factory. It serves as the benchmark for cold work tool steels. If you’re unsure which grade to choose, selecting D2 is usually a safe bet. It is highly suitable for long-run blanking dies, forming rolls, and cold extrusion dies.
If you require higher wear resistance than D2 and do not experience severe impact or vibration, D3 (1.2080) offers a more cost-effective solution. We typically maintain a stock of this grade of steel, particularly in round bar form.
O-Series: Oil-Hardening Grades for Medium Runs
O stands for Oil-Hardening. This type of steel must be quenched in oil to achieve full hardness. The most well-known representative is O1 (1.2510). Although O1 is less hard than D2, it offers superior toughness. Due to its hardness, O1 also has better machinability than D2, making it easier to turn, mill, and drill—saving you significant machining labor costs.
As a low-alloy tool steel, O1 offers lower costs compared to high-alloy grades like D2 or D3. For medium-run production molds, O1 offers better value for money and is suitable for bending, forming, blanking, and drawing dies.
A-Series: Air-Hardening Toughness
If D2 lacks sufficient toughness and is difficult to machine, while O1’s wear resistance falls short, then AISI A2 (DIN 1.2363) is a superior choice. The “A” in the A series (Air-Hardening) denotes air-hardening. A2 possesses exceptional hardenability, allowing it to be hardened by air cooling. Unlike O1, which requires oil quenching, A2 only needs mild air quenching, resulting in minimal movement or distortion. For molds with complex geometries or large dimensions, using A2 maximizes the avoidance of post-heat-treatment cracking or dimensional deviations.
S-Series: Maximizing Shock Resistance
The S series features lower carbon content, such as S1, S5, and S7. While reduced carbon content lowers hardness and wear resistance, it enhances shock resistance. It can absorb significant impact energy, such as that of spring steel, without fracturing. S7 is currently the most widely used and versatile grade on the market. It is suitable for Cold Chisels & Pneumatic Tools; Large Shear Blades; and Cold Forging Dies.
W-Series: Economical and Shallow Hardening
The W series is essentially high-carbon tool steel, such as W1 and W2. These grades contain minimal alloying elements, relying almost entirely on carbon content to provide hardness. Since they lack expensive molybdenum (Mo), vanadium (V), or significant chromium (Cr), the W series offers the lowest raw material cost among all tool steels. If cost is a primary concern and the mold structure is simple, the W series represents the most economical choice. Despite its affordability, the W series achieves a surface hardness of 50-64 HRC through water quenching, which is sufficient for handling routine wear and tear.
After heat treatment, W-series steel exhibits extremely low hardenability, allowing its surface to cool rapidly and harden while the core remains relatively soft. This hard-outside-soft-inside structure is ideally suited for cold-heading dies. During screw and nut manufacturing, dies endure immense impact forces; the soft core of W-series steel effectively cushions these impacts, preventing die fracture.
When water-quenched, this type of steel undergoes a highly violent process that causes significant material changes. It is unsuitable for precision stamping dies with complex geometries, sharp corners, or thin walls, as these conditions would likely result in cracking or scrapping.
High-Speed Steels and Hot-Work Steels in Cold-Work Applications
When the wear resistance of D2 or D3 still fails to meet requirements, high-speed steels such as M2 (1.3343) and M4 are viable alternatives. Originally designed for cutting tools, M2 is used in cold-work dies for manufacturing extrusion punches. With proper heat treatment, it achieves a remarkable hardness of 60-66 HRC—the highest hardness and wear resistance among all cold work options. However, it also exhibits the lowest toughness and highest brittleness.
In certain cold working processes, such as deep coining and heavy forming, dies endure severe impact forces. Using high-hardness tool steel may result in instantaneous fracture. In such cases, we employ H13 (1.2344) hot-work tool steel with a hardness of 45-52 HRC. While H13 is less hard than other cold-work tool steels, its superior toughness allows it to withstand substantial deformation energy without fracturing.
Material Selection for Specific Die and Punch Applications
The choice between grades often depends on the specific loading and geometry of the tool component:
| Application | Tool Component | Preferred Steel Grade | Required Properties |
| Cold Extrusion | Punches/Die Inserts | M2, D2, A2, M4 | High wear resistance (D2 for long runs), balanced with toughness (S7) |
| Shrink Rings | H11 (high toughness) | High toughness to prestress the die inserts | |
| Cold Heading | Dies/Punches | W1, W2, L6, M4, D2 | High hardness (≈62 HRC), resistance to wear and galling. Shallow hardening preferred for W1/W2 dies |
| Blanking/Punching | Dies/Punches | D2, A2, O1, S7 | High wear resistance (D2 for long runs), balance with toughness (S7) |
| Coining | Dies | W1, O1, A2, D2, H11/H13 | Resistance to wear, sinking, and cracking. D2/A2 offer great compression resistance; H-series offer extreme toughness for deep designs. |
| P/M Compaction | Punches/Core Rods | A2, D2, S7, H13 | High wear resistance and toughness. |