Shackle Material Selection Guide: How Steel Grade Affects WLL and Safety

As a professional rigging hardware manufacturer, we understand that different rigging products require different materials, and the choice of material directly affects load capacity, safety factor, and long-term performance.  We precisely match different steel grades to the performance requirements of different rigging products. Shackles require high tensile strength and yield strength, excellent toughness, the ability to withstand impact loads, stable performance after heat treatment, and compliance with industry standards (ASME, ASTM, US Federal Specifications). This article will analyze the selection of shackle materials from the perspectives of shackle load, core performance requirements, comparison of different steel grades, and the role of heat treatment in shackle performance.

Understanding Shackle Load Conditions

Before selecting shackle materials, it is essential to understand the load conditions the shackle will experience in actual applications.

Shackles typically undergo the following loads:

• Pure tensile load during straight lifting
• Dynamic loads caused by crane movement or load swinging
• Impact loads during sudden lifting or stopping
• Even if the rated load is for straight use, occasional lateral loads may occur

These load conditions require the shackle material to resist sudden failure while maintaining strength.

Core Performance Requirements for Lifting Shackles

From an engineering and manufacturing perspective, lifting shackles must meet the following performance standards:

• High Tensile Strength and Yield Strength

Tensile strength refers to the maximum load a component can withstand, and yield strength refers to the ability to resist permanent deformation.

The steel must be able to withstand the rated working load (WLL) and have a sufficient safety factor, typically: Proof load ≥ 2 × WLL, minimum breaking load ≥ 4–6 × WLL (depending on the standard)

• Excellent Toughness

Toughness refers to the ability to absorb impact and dynamic loads. Toughness is crucial to prevent brittle fracture under impact loads, low-temperature environments, and sudden overloads.

• Controlled Ductility

Ductility refers to the ability to resist cracking during forming or clamping. The shackle should deform before fracturing, providing a visible warning rather than catastrophic failure.

• Heat Treatment Stability

Consistency in performance across different batches of products depends on a predictable heat treatment response.

Characteristics and Comparison of Different Steels

Shackles are often made from C1045 carbon steel, Q235 carbon steel, and 40Cr alloy steel.The choice of each material is based on the functional requirements of specific rigging applications. We do not standardize on a single steel grade, but rather select materials based on actual usage conditions, not assumptions.

C1045 Carbon Steel

C1045 medium carbon steel is widely used in rigging hardware due to its reliable strength, good ductility, and excellent machinability. C1045 has stable forging and heat treatment properties, good resistance to deformation under rated load, and lower risk of brittle fracture compared to high-alloy steels. For many everyday lifting and rigging operations, C1045 offers the best balance between mechanical performance and production efficiency. C1045 material is widely used in US Type Bow Shackle G209, US Type Dee Shackle G210, US Type Bow Shackle G2130 body , and US Type Dee Shackle G2150 body.

Q235 Carbon Steel

Q235 is a low-carbon structural steel conforming to Chinese standards, with mechanical properties roughly equivalent to ASTM A36. With a relatively low carbon content, it has good weldability and formability, and moderate tensile strength. For shackles with non-rated or low loads, Q235 is a cost-effective choice. Q235 material is widely used in European large bow shackles, European large D shackles, JIS Type Dee shackles, and JIS type shackle without collar.

40Cr Alloy Steel

40Cr alloy steel (equivalent to AISI 5140) contains chromium, offering superior hardenability and fatigue performance, better performance under impact and cyclic loads, higher wear resistance in pin and thread areas, and more uniform strength after heat treatment. The pins of G209, G210, G2130, and G2150 are all made of 40Cr material.

Comparison of Different Steels

The table below shows how shackle material selection changes as Working Load Limits increase.

* Actual WLL depends on shackle size, design, and applicable standards.

The Role of Heat Treatment in Shackle Performance

Material selection alone does not guarantee performance. For shackles, quenching and tempering are crucial to achieving the following objectives. After heat treatment, the strength level, sufficient ductility, and more reliable impact resistance will be improved. The ultimate load will be six times the working load limit.

1. Improved Tensile Strength and Yield Strength

Heat treatment significantly increases tensile strength and yield strength, enabling shackles to withstand higher loads without permanent deformation.

For shackles with a rated load:

Higher yield strength prevents elongation deformation under rated load.

Higher tensile strength provides a greater margin for fracture load.

Without heat treatment, even high-quality steel may not meet the required Working Load Limit (WLL) standards.

2. Improved Toughness and Impact Resistance

Sudden load application, crane movement and load swinging, and accidental impact loads often take place in shackle operations. Heat-treated shackles have enhanced impact toughness, reducing the risk of brittle fracture under these conditions. This is especially important in construction, offshore operations, and heavy industrial lifting operations.

3. Controllable Ductility, Ensuring a Safer Failure Mode

Well-heat-treated shackles will undergo significant deformation before failure. It could provide warning signs such as bending or elongation, and reducing the risk of sudden catastrophic fracture. This characteristic is crucial for high-altitude lifting and personnel safety.

4. Higher and More Stable Working Load Limit (WLL)

Heat treatment ensures that the mechanical properties of the shackle are uniformly distributed throughout the shackle body and remain consistent between batches, even in larger or thicker sections. This consistency is critical for manufacturers producing high-capacity shackles, especially in heavy-duty applications such as those above 25 tons and between 85 and 150 tons.

5. Improved Fatigue Resistance

Many shackles are used repeatedly under cyclic loading. Heat treatment improves resistance to microcrack formation, fatigue life under repeated stress, and long-term dimensional stability, which directly extends service life and reduces replacement frequency.

6. Reliable Performance of Large and Heavy-Duty Shackles

Bigger size shackles are, the cross-sectional thickness and the stress distribution will be complex. Heat treatment ensures more durable strength and toughness throughout the cross-section, not only on the surface.

Conclusion

The choice of shackle material is influenced by the load conditions, safety requirements, and material properties. Sail Rigging foscus on the production of rigging products for 16 years, accumulating rich experience in forging production and processing. If you have any questions about shackle materials or would like to customize shackles made of other materials, please feel free to contact us.