Release time：2020-01-16 18:44 Browse：
Moderate Hardenability:
Avoid materials with excessively high hardenability (e.g., steels rich in alloying elements like Ni, Mo, Cr). High hardenability materials readily form hard and brittle martensite throughout the cross-section during quenching, generating enormous transformational stresses that cause severe bending or even cracking in ultra-long screws.
Materials with low to medium hardenability should be selected. This allows the screw's core to transform into tougher non-martensitic structures (like troostite or bainite) during quenching, while the surface becomes hard martensite. This "tough core, hard shell" structure meets surface wear resistance requirements while the core absorbs stress, significantly reducing deformation.
Good Processability:
Good Carburizing Response: Efficiently absorbs carbon atoms and forms a uniform hardened case.
Low Quench Distortion Sensitivity: The material itself is less sensitive to thermal expansion/contraction and phase transformation during heat treatment.
Good Machinability: Facilitates machining (e.g., thread rolling) before carburizing.
Based on the above principles, here are recommended materials suitable for carburizing heat treatment of long screws, listed in order of commonality and applicability:
Characteristics: These steels have very low alloy content (mainly Mn), low hardenability, and are low cost.
Applications: Suitable for long screws under not extremely high loads and with general requirements for core strength. For example, long shafts in conveying machinery, ultra-long tie rods in non-critical areas, etc.
Note: Due to their lower core strength, they are not suitable for critical components承受高冲击、高剪切力 (withstanding high impact, high shear forces).
This is the mainstream and optimal choice for carburizing ultra-long screws, offering the best balance between strength, toughness, and controllable hardenability.
20CrMnTi (GB/T 3077)
This is the most classic and commonly used carburizing steel, almost born for such applications.
Composition Advantages: Cr and Mn elements improve hardenability and strength. The Ti element refines grain size, significantly improving material toughness and inhibiting overheating tendency, thereby greatly enhancing heat treatment deformation stability.
Properties: High surface hardness and wear resistance after carburizing, high strength and good toughness in the core. Its medium hardenability is very suitable for controlling deformation in ultra-long workpieces.
Applications: Widely used in long shafts, gears, screws in heavy machinery, automobiles, mining equipment, etc.
20CrMo (GB/T 3077)
Composition Advantages: Cr improves hardenability and strength. Mo improves hardenability while significantly reducing temper brittleness and enhancing high-temperature strength.
Properties: Similar performance to 20CrMnTi, with good strength and toughness. The addition of Mo makes its properties more balanced.
Applications: An excellent alternative to 20CrMnTi, also very suitable for long screws.
20MnV, 20MnTiB
These are economical alternatives to 20CrMnTi, using elements like Mn, V, B to replace some Cr, Mo, achieving decent performance at lower cost, but requiring slightly higher heat treatment process control.
If the screw needs to withstand huge impact, fatigue loads, or operate in harsh environments, consider the following materials, though the difficulty of controlling heat treatment deformation increases accordingly.
20CrNiMo (GB/T 3077)
Composition Advantages: The addition of Ni greatly improves core toughness and strength. Mo improves hardenability and eliminates temper brittleness.
Characteristics: Excellent comprehensive mechanical properties, very high core strength and toughness.
Trade-off: Higher content of Ni, Cr, Mo means higher hardenability than 20CrMnTi, making heat treatment deformation control more difficult and requiring more sophisticated processes (e.g., milder quench media).
Applications: Used for critical long screws in high-end heavy machinery, ships, aerospace, etc., where performance requirements are extremely high.
17Cr2Ni2Mo
This is a higher-grade carburizing gear steel with excellent performance, but also places very high demands on heat treatment and is expensive.
| Material Grade (Chinese Std) | Main Characteristics | Application Scenario | Deformation Control Difficulty | Cost |
|---|---|---|---|---|
| 20#, 20Mn | Low hardenability, low core strength | Low load, non-critical long screws | Easy | Low |
| 20CrMnTi | Good comprehensive properties, excellent strength & toughness | Preferred choice for most heavy-duty applications | Medium | Medium |
| 20CrMo | Balanced properties, good resistance to temper brittleness | Excellent alternative to 20CrMnTi | Medium | Medium |
| 20CrNiMo | Extremely high core toughness, optimal overall properties | Extreme heavy load, high impact conditions | Difficult | High |
Top Recommendation: For the vast majority of industrial applications for "long screws," 20CrMnTi is the most balanced and reliable choice. It offers the best balance between performance, processability, and cost.
Consult Suppliers: Before finalizing the material,务必咨询您的热处理服务供应商 (must consult your heat treatment service provider). They can recommend the most suitable material based on their equipment capabilities (like agitation strength of the quench tank, furnace uniformity) and experience, as the final material choice needs to match the heat treatment process.
Process and Material are Both Important: Remember, for ultra-long screws, even the best material requires special heat treatment processes (like vertical hanging quenching) to leverage its advantages and control deformation. The material is the foundation, but the process is the key.
