Cold extrusion is a near-net-shape forming process, in which metal blanks undergo plastic deformation inside the die cavity under room temperature. It is widely applied in mass production of fasteners, motor rotors, automotive connectors, battery casings and other components. Under working conditions, the dies bear ultra-high contact compressive stress up to thousands of megapascals, severe friction from flowing metal, and fatigue loads caused by repeated impacts. Premature chipping or wear will directly affect the dimensional accuracy of finished parts and production delivery schedule. Therefore, selecting the right mold steel together with proper heat treatment are two core factors determining the service life of cold extrusion dies.

1. Core Performance Requirements for Cold Extrusion Die Materials

Under cold extrusion working conditions, mold steel must satisfy four key properties simultaneously:

• High hardness and compressive strength to resist cavity collapse and minor plastic deformation;
• Excellent wear resistance to maintain dimensional accuracy after long-term production;
• Sufficient toughness: hard surface with tough core to prevent chipping and spalling;
• High multi-impact fatigue resistance to adapt to cyclic loads of dozens of strokes per minute.

Material selection is essentially balancing hardness and toughness. Excessively high hardness tends to cause die chipping, while overemphasizing toughness will lead to rapid wear.

2. Common Cold Extrusion Mold Steels and Application Scenarios

3. Material Selection Guide: Match Grades by Production Batch and Load

• Small batch, trial production, aluminum / mild steel soft materials:Conventional cold work steel such as Cr12MoV and 9SiCr are sufficient with minimum die manufacturing cost.
• Medium batch, carbon steel or low-alloy structural steel：Prioritize high-toughness grades including 6W6, LD and 65Nb. Slightly higher material cost brings multiple times longer service life for optimal overall economy.
• Mass production, high-strength steel / stainless steel extrusion：Adopt high-speed steel (M2) or powder metallurgy HSS, paired with cemented carbide die inserts to avoid frequent die repair and production downtime.
• Complex cavity, uneven wall thickness and high cracking risk：Avoid high-carbon high-chromium steel. Choose air-quenched low-distortion grades like 7CrSiMnMoV and A2 to solve cracking issues first before optimizing wear resistance.

4. Premium Steel Requires Proper Heat Treatment & Surface Treatment

For the same mold steel, different heat treatment processes can result in several-fold differences in die service life:

• Quenching and tempering:Cold extrusion dies are normally hardened to HRC60–64. For punches under heavy impact, hardness shall be reduced appropriately to improve toughness.
• Surface strengthening:TD treatment, PVD coating (TiN/TiAlN), gas soft nitriding or hard chrome plating can further enhance surface wear resistance, delivering prominent benefits for stainless steel and superalloy extrusion.
• Toughening treatment:Low-temperature quenching and multiple tempering for M2 and 6W6 high-speed steel can greatly boost toughness with almost no hardness loss.

Conclusion

There is no universal best mold steel for cold extrusion dies. The optimal choice depends on production volume, workpiece material hardness and die structure. This is the mature selection principle widely verified in the industry.