Is SCM435 suitable for producing high-strength bolts?

SCM435 is an alloy steel grade from the Japanese Industrial Standards (JIS), classified as a Chromium-Molybdenum Steel. It is one of the core materials for high-strength fasteners widely used in China, Japan, South Korea, and Southeast Asia. It is equivalent to the Chinese grade 35CrMo and the American grade ASTM A29/ A29M-04 4137.

The "SCM" prefix denotes its chemical composition:

• S: Steel

• C: Chromium

• M: Molybdenum

The "435" indicates a carbon content of approximately 0.35%.

Due to the presence of alloying elements like chromium and molybdenum, SCM435 achieves excellent strength, good toughness, and superior fatigue resistance after quenching and tempering heat treatment. This makes it particularly suitable for manufacturing high-strength fasteners such as bolts, screws, studs, and nuts of Property Class 8.8 to 12.9.

1. Chemical Composition
The chemical composition of SCM435 is the foundation of its properties. Its typical composition (according to JIS G4105) is as follows:

• Carbon (C): Provides the fundamental strength and hardness of the steel.

• Chromium (Cr): Increases hardenability, strength, and wear resistance, and enhances some corrosion resistance.

• Molybdenum (Mo): Significantly improves hardenability, reduces temper brittleness, and helps the steel maintain strength at elevated temperatures.

2. Mechanical Properties (After Heat Treatment)
The superior properties of SCM435 are achieved through quenching and tempering heat treatment. The typical mechanical properties after heat treatment are:

• Tensile Strength (σb): ≥ 930 MPa (Can meet the requirements for Property Class 10.9 and 12.9 fasteners)

• Yield Strength (σs): ≥ 785 MPa (For Property Class 12.9 fasteners)

• Elongation (δ): ≥ 12%

• Reduction of Area (ψ): ≥ 50%

• Impact Energy (Akv): ≥ 40 J (Indicating good toughness)

3. Key Characteristics

• Excellent Combination of High Strength and High Toughness: This is the core advantage of SCM435. It can be used to manufacture bolts that withstand high loads without being brittle and fracturing under impact.

• Good Hardenability: The chromium and molybdenum allow for a uniform microstructure and mechanical properties throughout the cross-section, even in larger-diameter fasteners.

• Good Fatigue Resistance: Fasteners made from SCM435 have a long service life under cyclic loading conditions.

• Good Elevated Temperature Strength: The addition of molybdenum reduces the loss of strength at high temperatures, making it suitable for applications up to 400-450°C.

• Resistance to Temper Embrittlement: Molybdenum effectively suppresses the embrittlement that can occur in some steels during the tempering process.

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4. Primary Applications
SCM435 is primarily used to manufacture high-strength fasteners that require high reliability and safety. Common applications include:

• Automotive Industry: Engine connecting rod bolts, cylinder head bolts, wheel hub bolts, and critical chassis components.

• Construction Machinery: High-strength bolts for excavators, cranes, road rollers, and other heavy equipment.

• Wind Power Industry: Wind turbine tower flange bolts and blade bolts.

• Steel Structure Buildings: High-strength bolts for friction-grip connections in steel structures.

• Railway and Bridge Construction: High-strength connections for critical sections.

SCM435, as a base material, is referenced and specified in multiple standard systems.

1. Material Standards

• JIS G4105: Chromium Molybdenum Steel - This is the fundamental source standard for SCM435.

• GB/T 3077: Alloy Structure Steels - The equivalent Chinese grade is 35CrMo.

• ASTM A29/A29M - The equivalent American grade is 4137 or G41370.

• DIN EN 10083-3 - A similar material in the European standard is 34CrMo4.

2. Fastener Product Standards
Fasteners manufactured from SCM435 must conform to relevant product standards, which specify mechanical property classes, dimensions, and tolerances.

• JIS B1186: Hexagon Head Bolts - Property Class 10.9 and 12.9 bolts often use SCM435.

• GB/T 3098.1: Mechanical Properties of Fasteners - Bolts, Screws and Studs - Corresponding to Property Class 10.9 and 12.9.

• ISO 898-1: Mechanical properties of fasteners made of carbon steel and alloy steel - The international standard, also corresponding to Property Class 10.9 and 12.9.

• GB/T 3098.2: Mechanical Properties of Fasteners - Nuts - Corresponding to Property Class 10 and 12 nuts.

3. Heat Treatment Standards
To ensure the performance of SCM435 fasteners, the heat treatment process must follow relevant standards.

• GB/T 3098.1 Appendix A specifies strict requirements for the heat treatment (quenching and tempering) of Property Class 12.9 bolts, including temperature and hardness.

• Automotive manufacturers and large equipment builders often have their own internal corporate standards with more detailed requirements for process control, microstructure, and decarburization depth.

Summary:

SCM435 is a high-performance chromium-molybdenum alloy steel. After quenching and tempering, it achieves an excellent balance of high strength and toughness, making it an ideal material for manufacturing Property Class 8.8 to 12.9 high-strength, high-reliability fasteners. It holds an irreplaceable position in critical industrial sectors such as automotive, machinery, and wind power. When selecting it, it is essential to confirm not only the material grade but also that the final product complies with the relevant fastener property class standard (e.g., 10.9 or 12.9) to ensure safety and reliability in application.