Characteristics of 4Cr2NiMoV Hot Work Mold Steel
I. Chemical Composition and Alloy Design
4Cr2NiMoV is a nickel (Ni)-containing hot work mold steel with a carefully optimized chemical composition to achieve a balance between high-temperature performance and toughness:
Carbon (C): 0.35%-0.45%, providing foundational strength and hardness while avoiding brittleness caused by excessive carbon.
Chromium (Cr): 1.80%-2.20%, forming carbides (Cr₇C₃) to enhance wear resistance and improve corrosion resistance.
Nickel (Ni): 1.10%-1.50%, significantly improving the steel's toughness and resistance to thermal fatigue while reducing crack susceptibility at high temperatures.
Molybdenum (Mo): 0.45%-0.60%, enhancing tempering stability and suppressing high-temperature softening.
Vanadium (V): 0.10%-0.30%, refining grain size and strengthening thermal stability.
Silicon (Si) ≤ 0.40%, Manganese (Mn) ≤ 0.40%: Controlling impurity content to optimize machinability.
II. Mechanical Properties and Thermal Stability
High-Temperature Strength and Hardness:
Operates at temperatures exceeding 600°C, far surpassing conventional hot work mold steels (e.g., the 500°C limit of 5CrMnMo), making it suitable for high-temperature applications such as die casting and hot extrusion.
Annealed hardness ≤ 220HBW; post-quenching hardness can reach 50-55HRC (adjustable based on process parameters). For example, die-casting molds use 48-52HRC to balance wear resistance and toughness.
Tempering Resistance:
Hardness decreases slowly during tempering, with a secondary hardening peak at 550°C. Even after tempering at 650°C, it retains relatively high hardness (~40HRC), ensuring reliable performance under high-temperature service conditions.
Thermal Fatigue Resistance:
The addition of nickel significantly enhances thermal fatigue resistance. After quenching at 910°C and tempering at 610°C, its thermal fatigue crack initiation life (TFCN) outperforms traditional steel grades, making it ideal for applications involving frequent temperature fluctuations (e.g., die-casting cycles).
Toughness:
The synergistic effect of nickel and molybdenum maintains excellent toughness at high temperatures, reducing the risk of brittle fracture. This makes it particularly suitable for hot forging dies subjected to impact loads.
III. Process Performance and Applicability
Heat Treatment Process:
Quenching: Oil quenching at 910-960°C to prevent surface decarburization (0.5-2.0mm thickness) and ensure uniform hardness.
Tempering: Double tempering at 590-610°C for 2 hours each to relieve quenching stresses and stabilize the microstructure.
Austempering: Further enhances toughness for molds with extreme crack resistance requirements (e.g., connecting rod hot forging dies).
Machinability:
Forging temperature range: 1130-1160°C (start), 850-900°C (finish) to avoid cracking.
Excellent cutting performance due to low hardness in the annealed state, facilitating easy machining.
Surface Quality and Lifespan Optimization:
Nitriding treatment (e.g., gas nitriding at 520°C) forms a 0.1-0.2mm hardened layer with a surface hardness of 1000-1200HV, significantly improving wear resistance.
