Discussion on Material Selection and Manufacturing Process of Hot Forging Die

Keywords: mold, material, heat treatment

I. Introduction

Molds play a crucial role in advanced manufacturing technologies that require minimal or no post-processing. They are widely utilized in modern industrial production. From practical usage, it is evident that the quality of a mold largely depends on its selection and heat treatment processes. Based on their operating conditions, molds can be classified into cold forming molds (including extrusion, cold drawing, and cold forging), warm forging molds, hot forging molds, plastic molding molds, casting molds, and others. 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: When the workpiece remains in contact with the mold at high temperatures for extended periods, the mold may soften and plastically deform. This typically results in collapse. Dies used for extrusion and high-load forging operations are particularly susceptible to this issue.

(2) Thermal Fatigue: Cracks often appear on the surface of molds subjected to repeated temperature changes. A large temperature difference between the workpiece and the die can lead to thermal fatigue cracks.

(3) Fracture: This occurs when the material cannot withstand the applied load, leading to cracking under unstable conditions. Types include brittle, ductile, fatigue, and corrosion fractures. Early failures in hot forging dies are often linked to excessive loads, improper handling, or stress concentration.

(4) Thermal Wear: This involves material loss due to relative motion between the mold's working surfaces and the processed material, including surface damage and dimensional changes. Factors like mold temperature, hardness, alloy composition, and lubrication influence wear. Areas with high movement or protrusions are more prone to wear.

Three Material Selection Principles and Heat Treatment Requirements

Based on the common failure modes 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 during heat treatment, and surface decarburization are also critical. Here, only the most important features are outlined.

1. Hot Hardness (Red Hardness): This refers to a mold’s ability to maintain its structure and performance at high temperatures, resisting softening. It depends on the material’s chemical composition and heat treatment process. Steels with high levels of V, W, Co, Nb, and Mo are commonly used.

2. Strength and Toughness: These depend on the mold’s load-bearing requirements, grain size, carbide distribution, and residual austenite content. The steel’s chemical composition, metallurgical quality, and heat treatment process significantly affect these properties.

3. Hardenability and Hardening Ability: Hardenability relates to the depth of hardness achievable after quenching, while hardening ability refers to the formation of martensite. Both depend on the steel’s composition. For example, punching dies prioritize surface hardness, while hot forging dies focus on uniform cross-sectional properties.

There are many factors affecting the life of hot forging dies. When choosing materials, it is essential to consider the specific working conditions. Below is a table summarizing the main materials used for two types of dies:

Mold Type

Recommended Materials

General Hardness Range

Hammer Forging Die

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

38–42 HRC

Hot Extrusion Die

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

44–55 HRC (48–52 HRC)

Specific applications, temperature ranges, and recommended hardness values can be found in the "Mechanical Engineering Handbook."

Fourth, Processing Technology and Its Impact on Die Life

The general 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 flow includes: blanking, forging + spheroidizing annealing – machining – vacuum quenching and tempering (to reduce distortion) – (cryogenic) – grinding and polishing – ion nitriding.

2.1 Blanking and Forging + Spheroidizing Annealing: Steel provided by manufacturers usually comes in forged billets or rods. Carbides tend to form along grain boundaries, which can lead to cracks during use if not properly processed. Through forging and spheroidizing annealing, carbides become more evenly distributed, improving microstructure and reducing the risk of early fracture.

2.2 Machining: It is best to perform machining before heat treatment unless the mold is extremely complex. This helps avoid surface tensile stresses that could reduce fatigue performance. Electric discharge machining can create a melted layer and heat-affected zone, lowering surface hardness and wear resistance. After heat treatment, it is better to minimize or avoid such processes.

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

(1) Use appropriate processes to reduce distortion, such as multi-stage heating. Avoid alloy element evaporation, and prefer vacuum or gas quenching when possible. For materials with poor hardenability, salt bath treatment may be preferred.

(2) Carburizing is recommended to prevent decarburization and improve surface wear resistance. This creates a compressive stress layer, enhancing fatigue resistance.

(3) Many mold steels contain elements like Cr, Mo, V, and W, which contribute to secondary hardening. Tempering at different temperatures can optimize performance. For hot forging dies, high-temperature tempering is essential to avoid performance degradation.

2.4 Shot Peening, Grinding, and Polishing: These treatments help form a compressive stress layer on the surface and eliminate defects, extending die life.

2.5 Ion Nitriding: This improves surface hardness and fatigue resistance. It is best to use N2 instead of NH3 to avoid hydrogen embrittlement. The nitriding temperature must be lower than the tempering temperature to prevent hardness reduction and distortion.

2.6 Cryogenic Treatment: This reduces residual austenite and increases hardness and wear resistance. However, safety precautions must be taken due to the risk of frostbite.

2.7 Preheating: Proper preheating (typically 200–250°C for at least 1 hour) is essential to extend die life, as insufficient preheating can drastically reduce service life.

Fifth, Surface Treatment Technologies

Common surface treatments for hot forging dies include coating (e.g., TiN, TiCN), plating (e.g., Cr, Ni-P), multi-component permeation (e.g., C, N, O), ion implantation, boronizing, PVD, CVD, and more. Among these, ion nitriding is most suitable. Each method has different performance characteristics, as shown in the following table.

Surface Treatment Method

Plating

NC Co-permeation

Ion Nitriding

Vacuum Nitriding

Sulphurizing

Boronizing

CVD

TiN

PVD

TiN

TD Method

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

Standard

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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Ion nitriding is a popular technique that enhances surface hardness and wear resistance through a vacuum glow discharge process. The theory behind it is still evolving, but it is known to produce a high-hardness alloy nitride layer. Ammonia decomposition gases are commonly used, consisting of about 25% N2 and 75% H2. While convenient, ammonia-based nitriding can result in brittle layers and may affect furnace temperature uniformity. It is best suited for less demanding applications.

The primary goal of ion nitriding is to strengthen the mold surface, increase hardness, and improve wear resistance. The resulting compressive stress can reach up to 600–800 MPa, significantly enhancing fatigue resistance and extending die life. The following table illustrates the application results of various mold materials after ion nitriding.

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

Seven Summary

Hot forging dies operate at higher temperatures and require hot die steel. Based on specific usage conditions and failure modes, rational selection is essential. To enhance die life, an optimized manufacturing process includes: blanking, forging + spheroidizing annealing – machining – vacuum quenching and tempering (to reduce distortion) – (cryogenic) – grinding and polishing – ion nitriding.

Ion nitriding improves die life by increasing surface hardness and creating compressive stress. Proper preheating (200–250°C for at least 1 hour) is also vital to prolong the lifespan of hot forging dies.