Discussion on Material Selection and Manufacturing Process of Hot Forging Die

Keywords: mold, material, heat treatment

I. Introduction

Molds are essential technological equipment in advanced manufacturing processes that require minimal or no post-processing. They are widely utilized across modern industrial applications. From practical usage, it is evident that the quality of a mold largely depends on its selection and heat treatment process. Molds can be classified based on their operating conditions, such as cold forming molds (including extrusion, cold drawing, and cold forging), warm forging molds, hot forging molds, plastic molding molds, casting molds, and more. This article focuses on the selection and manufacturing processes of hot forging molds, especially the heat treatment procedures.

Failure Modes

The main failure modes of hot forging dies include deformation, thermal fatigue, thermal wear, and fracture.

(1) Deformation occurs when the mold softens and plastically deforms due to prolonged exposure to high temperatures during operation. This typically results in collapse, particularly in extrusion and forging dies subjected to high loads and elevated temperatures.

(2) Thermal fatigue manifests as crack formation on the mold surface due to repeated temperature fluctuations. Hot forging dies with large temperature differences between work and cooling cycles are especially prone to this type of failure.

(3) Fracture happens when the material cannot withstand the applied load, leading to instability and cracking. This includes brittle, ductile, fatigue, and corrosion fractures. Early fractures in hot forging dies are often linked to excessive loading, improper material handling, and stress concentration points.

(4) Thermal wear refers to surface degradation caused by relative motion between the mold and the workpiece. Factors like mold temperature, hardness, alloy composition, and lubrication influence this wear. Areas with significant movement or protrusions are more susceptible to wear-related failures.

III. General Material Selection Rules and Heat Treatment Requirements

Based on the common failure mechanisms of hot forging dies, key considerations in material selection include thermal properties, toughness, hardenability, decarburization sensitivity, and thermal fatigue resistance. From a heat treatment perspective, factors such as wear resistance, hardness, distortion control, and surface decarburization are also critical. Only a few of the most important aspects are discussed here.

1. Hot hardness, also known as red hardness, refers to a mold’s ability to maintain its structure and performance under high-temperature conditions. It is mainly influenced by the material’s chemical composition and heat treatment system. Steels with high levels of vanadium, tungsten, cobalt, niobium, and molybdenum are commonly used for this purpose.

2. Strength and toughness depend on the mold’s load-bearing requirements. Factors like grain size, carbide distribution, and residual austenite content significantly affect these properties. Proper steel composition, metallurgical quality, and heat treatment processes are crucial for achieving optimal strength and toughness.

3. Hardenability refers to the range of hardness achievable after quenching, while hardenability relates to the ability to form martensite. Both are influenced by the steel’s chemical composition. For example, punching dies benefit more from high surface hardness, whereas hot forging dies require uniform performance throughout the cross-section.

Many factors impact the lifespan of hot forging dies. When choosing materials, it is essential to consider the specific working conditions of the mold. Below is a table listing the main materials used for two types of hot forging dies:

Mold Type

Recommended Materials

General Hardness Range

Hammer Forging Die

5CrMnMo, 5CrNiMo, 5SiMnMo, 4SiMnMo, 3Cr2W8V (SKD5), 4Cr5MoSiV (H11, SKD6), 4Cr5MoSiV1 (H13, SKD61), 4CrMnSiMoV

38~42HRC

Hot Extrusion Die

3Cr2W8V (SKD5), 4Cr5MoSiV (H11, SKD6), 4Cr5MoSiV1 (H13, SKD61), 4CrMnSiMoV

44~55HRC (48~52HRC)

Specific material use, temperature ranges, and recommended hardness values can be referenced in the "Mechanical Engineering Handbook."

IV. Manufacturing Process and Its Impact on Die Life

The standard mold manufacturing process includes: blanking, forging + spheroidizing annealing - machining - quenching and tempering - (cryogenic treatment) - finishing (such as electrical discharge machining) - grinding and polishing - ion nitriding.

An optimized process would be: blanking, forging + spheroidizing annealing - machining - vacuum quenching and tempering (to reduce heat distortion) - (cryogenic) - grinding and polishing - ion nitriding.

2.1 Blanking and Forging + Spheroidizing Annealing: Steel provided by mills is usually forged billets or rods. The carbides within the steel are distributed along the grain boundaries. Without further forging, cracks may develop and propagate along these boundaries, reducing the mold's load capacity and potentially causing early failure.

Through forging and subsequent spheroidizing annealing, the carbides become uniformly fine and dispersed, improving the internal microstructure and preparing the mold for final heat treatment. This helps avoid stress concentration-induced cracking and enhances mold life. The following figure illustrates the rapid spheroidizing annealing process for several mold materials. The temperature range can be found in the "Heat Treatment Manual" or "Mechanical Engineering Manual."

T1: 3Cr2W8V, 1050°C; 3Cr3Mo3VNb, 1030°C; 5Cr4W5Mo2V, 1100°C

T2: 3Cr2W8V, 850~870°C; 3Cr3Mo3VNb, 850~870°C; 5Cr4W5Mo2V, 850~870°C

2.2 Machining: Unless the mold is extremely complex, it is best to machine before heat treatment. This prevents tensile stresses on the surface, which could reduce the mold’s fatigue performance. Electric discharge machining (EDM) involves melting the material, creating a melted layer and heat-affected zone that reduces surface hardness and wear resistance. It is advisable to avoid EDM after heat treatment or minimize machining allowances. Instead, grinding and polishing are preferred to reduce surface damage and extend die life.

2.3 Heat Treatment: The temperature and time for heat treatment should follow guidelines in the "Heat Treatment Manual" or "Manual Engineering Manual." Key considerations include:

(1) Use a reasonable process to minimize distortion, such as multi-stage heating to prevent cracking. Avoid evaporation of alloying elements, and prefer vacuum heat treatment or gas quenching if possible. For materials with poor hardenability, salt bath treatment is preferable.

(2) Supersaturated carburizing is recommended to prevent surface decarburization and enhance wear resistance. This creates a compressive stress layer that improves fatigue resistance.

(3) Many mold steels contain elements like Cr, Mo, V, W, and Nb, which contribute to secondary hardening during tempering. Therefore, the tempering temperature should be selected based on the mold’s operating temperature. However, for hot forging dies, high-temperature tempering is necessary to avoid secondary hardening effects that could reduce performance. Multiple temperings are typically required to ensure stability.

T1: 550~560°C; T2: 820~830°C; T3: 1070~1090°C;

T4:560~580°C; T5:220~260°C; T6:220°C;

P1: Quenching (oil cooled or air cooled), rest: air cooled

T1: 120; t2: 60; t3: 10; t4: 15; t5: 30; t6: 120~180; t7: 120.

2.4 Shot Peening, Grinding, and Polishing: After quenching and tempering, shot peening can create a compressive stress layer on the surface, improving the stress state of the hardened surface. Polishing removes surface defects and increases mold life, typically done manually.

2.5 Ion Nitriding: To enhance fatigue resistance and wear resistance, it is better to use N₂ instead of NH₃ to avoid hydrogen embrittlement. The ion nitriding temperature must be below the tempering temperature after quenching to prevent hardness reduction and deformation.

2.6 Cryogenic Treatment (Liquid Nitrogen): This process reduces residual austenite, forms a surface compressive stress layer, and improves hardness and wear resistance. Safety precautions are essential, as improper use can cause burns. The general procedure is: mold at room temperature → liquid nitrogen (-196°C) for 2 hours → return to room temperature → 160–170°C for 4 hours → air cooling.

Preheating is critical for hot forging dies, as they undergo repeated thermal cycles. Insufficient preheating can severely shorten the mold’s life. A typical preheating temperature is 200–250°C, with an insulation time of at least one hour before forging begins.

V. Application of Surface Treatment Technology

Common surface treatments for hot forging dies include coating (e.g., TiN or TiCN via vacuum coating), plating (e.g., Cr or Ni-P), multi-component permeation (e.g., C, N, O or C, N, O, S), ion implantation, boronizing, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Among these, ion nitriding is most suitable. The application scope of different surface strengthening methods is shown in the table below.

Surface Treatment Method

Plating

NC Co-permeation

Ion Nitriding

Vacuum Nitriding

Sulphurizing

Boronizing

CVD

TiN

PVD

TiN

TD Method (Borax Salt Bath V, Nb, Ti, Cr, etc.)

Superhard Alloy

Tool Steel

Performance

Cr

Ni-P

VC

NbC

Cr7C3

Hardness

Good

Good

Good

Good

Good

General

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Standard

Wear Resistance

Good

Good

Good

Good

Good

Good

Good

Excellent

Excellent

Excellent

Excellent

Good

Excellent

Standard

Hot Tack Resistance

Good

Good

Good

Good

Good

Good

Good

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Standard

Anti-Occlusion

Good

Good

Good

Good

Good

Good

Good

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

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Impact Resistance

General

General

General

General

General

Standard

General

Standard

Standard

Standard

Standard

Standard

General

Standard

Anti-Stripping

General

General

Good

Good

Good

Excellent

General

Good

Good

Good

Good

Good

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Anti-Deformation Cracking

General

General

Good

Good

Good

Excellent

Good

Good

Good

Good

Good

Good

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This article mainly introduces the ion nitriding process and its application. Ion nitriding uses a vacuum glow discharge process to create a high-wear-resistant, high-hardness alloy nitride layer on the part’s surface. While the theory is still debated, the sputtering and deposition theory was among the earliest proposed. Today, media such as N₂+H₂, ammonia, and its decomposition gases are used for ion nitriding. Ammonia decomposition gas is considered a mixture of 25% N₂ and 75% H₂.

Directly using ammonia gas for ion nitriding is convenient but can result in a brittle nitriding layer. The decomposition rate of ammonia gas is affected by the furnace’s air intake, temperature, and starting area, which can impact temperature uniformity. This method is suitable only for less demanding parts.

The purpose of ion nitriding for molds is to form an alloy nitride layer on the surface, enhancing surface hardness and wear resistance. After quenching and tempering, the high hardness of the surface nitride layer combined with the substrate’s hardness difference creates a compressive stress layer of up to 600–800 MPa, thereby improving fatigue performance and extending the mold’s life. The following table shows the ion nitriding process and application results for several mold materials.

Mold Name

Mold Material

Technology

Effect

Shower

W18Cr4V

500~520°C × 6h

Increase 2~4 times

Aluminum Die Casting

3Cr2W8V

500~520°C × 6h

Increase 1~3 times

Hot Forging Die

5CrMnMo

480~500°C × 6h

Increase 2~3 times

Cold Extrusion Die

W6Mo5Cr4V2

500~550°C × 2h

Increase 1~2 times

Rolling Die

C12MoV

500~520°C × 6h

Increase 5~6 times

VII. Conclusion

Due to the high operating temperatures, hot forging dies require the use of hot die steel, and rational selection must be made according to specific usage conditions and failure modes. To improve the service life of the die, the optimal manufacturing process is: blanking, forging + spheroidizing annealing - machining - vacuum quenching and tempering (to reduce heat distortion) - (cryogenic) - grinding and polishing - ion nitriding.

Ion nitriding treatment enhances mold life by increasing surface hardness and forming a compressive stress layer.

The life of the hot forging die is also influenced by proper preheating at the beginning and during use. The preheating temperature is generally 200–250°C.