A Detailed Explanation of the Chain Forging Process for Hand Chain Hoists

A Detailed Explanation of the Chain Forging Process for Hand Chain Hoists

The lifting capacity and safety performance of a hand chain hoist hinge crucially on the quality of its forging. As a critical component that directly bears the load, it must possess a breaking stress exceeding 800N/mm² (GB/T 30026-2021), an impact toughness of ≥30J at -20°C, and fatigue resistance under long-term, repeated stress. Each link of the chain undergoes 12 core processes, from raw material screening to finished product inspection, each step imbued with an unwavering commitment to safety.

First, Raw Material Pretreatment: Triple Screening Lays the Foundation for Safety

Chain quality begins with steel selection; substandard raw materials can directly lead to lifting accidents. Hand chain hoist chains (TH/VH grade) must undergo a three-step process: material screening, composition testing, and dimensional pre-processing.

1. Rigorous Selection of High-Quality Steel

According to the GB/T 30026-2021 standard, high-quality carbon structural steel or alloy structural steel must be used, meeting three stringent requirements:

Metallurgy Standard: Smelted in an electric furnace or oxygen-blown converter, ensuring high steel purity

Composition Control: Total sulfur and phosphorus content ≤ 0.045% (to prevent hot and cold brittleness), aluminum content ≥ 0.025% (to avoid strain aging)

Grain Requirements: Austenite grain size ≥ Grade 5 (grain refinement improves overall performance)

Commonly used materials, such as 40Mn2 and 35MnB alloy structural steels, offer a balance of tensile strength and toughness ideal for lifting applications.

2. Comprehensive Raw Material Testing

Each batch of steel must be accompanied by a manufacturer's warranty and undergo dual verification in the company's laboratory:

Physical and Chemical Testing: Spectrometer analyzers measure chemical composition to ensure compliance with standard ranges

Appearance Screening: No surface cracks, interlayers, deep scratches, or burrs on either end

Steel from different heats and specifications must be stacked separately and labeled to prevent the risk of material mixing.

3. Precision Cutting of Bar Stock

Cut to length using a CNC saw according to chain specifications (nominal sizes of 3mm-13mm):

Dimensional Accuracy: Length tolerance is controlled within ±0.5mm to ensure consistent link spacing in subsequent chains

Cutting: End burrs are removed immediately after sawing to avoid stress concentration during heating

After cutting, the dimensions of each bar must be checked and recorded. Bars from a single heat must be produced continuously to ensure consistent performance.

II. Hot Forging: The Four Core Steps in Chain Link Creation

Hot forging is the key step in giving the chain its basic form. It requires a continuous process of "heating - bending - welding - deburring" under precise temperature control. Any deviation in this step will result in a scrapped chain link.

1. Temperature Control for Medium-Frequency Heating

The bar stock is heated in a medium-frequency induction furnace, with a temperature profile set according to the unit specifications:

Temperature Range: Small Units (5#/6#): 730°C-880°C (dark cherry red); Large Units (11#): 850°C-1000°C (bright cherry red)

Temperature Control Key Points: The temperature difference between the core and the surface of the end face should be ≤50°C to avoid uneven heating and cracking during forming.

After heating, the bar stock must be immediately transferred to the bending process. Waiting for the bar stock to cool down at the furnace mouth is strictly prohibited.

2. Geometric Forming of Hot Bending Rings

On a CNC chain-making machine, the heated bar material is bent along the core into rings.

Temperature Window: The bending temperature must be no less than 700°C (dark cherry red) to ensure material plasticity.

Dimension Control: The inner width and pitch of the chain links are precisely positioned using a die, with a gap clearance of ≤3mm (≤50mm).

Symmetry Assurance: A "Break-Symmetry Measurement Auxiliary Template" provides real-time monitoring to ensure zero deviation between the break center and the theoretical center. This tool is four times more efficient than a vernier caliper and offers high-temperature self-test capabilities.

3. Flash Butt Welding for Strong Fusion

Welding is the lifeblood of chain link forming. A fully automated flash butt welding process is used:

Welding Principle: Electric current generates resistance heat, melting the joint. Forging pressure displaces impurities, causing the metal to recrystallize.

Key Parameters: Ring back temperature 600°C-700°C (maroon), forging pressure tolerance ±1.0 MPa, cooling time ≥ process specifications.

Quality Control: Observe continuous flash welding during welding. Ensure uniform weld fillet around the weld after forging. Use a welding recorder to monitor the weld curve for each link, ensuring traceability.

4. High-Temperature Deburring and Edge Finishing

After welding, chain links are deburred at temperatures above 550°C (dark maroon).

Process Requirements: Use a dedicated fixture to remove burrs from the weld joint to avoid deformation caused by cold deburring.

Quality Standards: After deburring, the weld joint should be smooth, free of obvious knife marks or pinch marks, and free of raised burrs on the inside that could affect chain engagement.

III. Finishing and Strengthening: A Key Process for Improving Chain Performance

After forming, chain links undergo a process called "shaping, heat treatment, and surface treatment" to meet the performance requirements of lifting operations. Heat treatment is crucial for improving strength and toughness.

1. Dimensional Calibration for Pressing and Shaping

Precision shaping of chain links using a hydraulic press:
Calibration focuses on correcting welding distortion and ensuring chain link flatness ≤ 0.5mm/m.

Stop Treatment: Chains above Grade 3 require welded stoppers to enhance lateral bending resistance.

After shaping, chain links must pass a go/no-go gauge inspection. Any links that do not meet the required inner width dimensions will be immediately rejected.

2. Performance Enhancement Through Quenching and Tempering Heat Treatment
The "quenching + high-temperature tempering" quenching and tempering process is key to achieving the required chain performance:
Quenching: Heating to a temperature above the Ac3 point (approximately 860°C) followed by rapid cooling in oil increases the hardness to ≥360 HV10.
Tempering: Low-temperature tempering at 200°C-300°C reduces brittleness and achieves the required impact toughness.
Process Results: After heat treatment, the chain's breaking stress increases from the original material's 500N/mm² to over 800N/mm², a 60% increase.

3. Surface Treatment for Corrosion and Wear Resistance
Select the treatment method based on the operating environment:
Conventional Treatment: Tumble blasting to remove scale, followed by varnishing, with a film thickness of ≥60μm.
Hardening Treatment: Plastic spraying or galvanizing, rust-free after a 48-hour salt spray test, suitable for humid and corrosive environments.
Before surface treatment, thoroughly clean any oil and scale to ensure coating adhesion.

IV. Factory Quality Inspection: Six Lines of Defense for Safe Delivery

As a safety component, hand hoist chains undergo rigorous testing far exceeding that of ordinary industrial products. Unqualified products are strictly prohibited from leaving the factory.

1. Comprehensive Dimensional Accuracy Inspection

Batch-by-Batch Inspection Using Vernier Calipers and Image Measuring Instruments:

Critical Dimensions: Link Diameter, Pitch, and Inner Width: Tolerance ≤ ±0.1mm

Weld Inspection: The maximum weld diameter must not exceed the standard limit, and there must be no dents or pores.

2. Internal Inspection Using Nondestructive Testing

Magnetic Particle Inspection for Welded Joints:

Inspection Range: Welds and Heat-Affected Zones, with a sensitivity of detecting cracks larger than 0.1mm

Defective Handling: Any cracks detected are immediately marked and removed, and links from the same batch can be traced. 3. Destructive Testing of Mechanical Properties

Three specimens are sampled from each heat for testing:

Tensile Test: Breaking stress is measured on a universal testing machine; the breaking stress must be ≥800N/mm².

Low-Temperature Impact: Impact energy absorption must be ≥30J at -20°C to prevent brittle fracture in cold regions.

Fatigue Test: No breakage after 1 million load cycles, simulating actual use.

4. Actual Verification of Load Testing

Finished chains must undergo rated load testing:

Loading Method: Apply 1.25 times the rated load for 10 minutes.

Acceptance Criteria: No permanent deformation, chain elongation ≤0.2%.

5. Complete Labeling and Traceability Information

Each chain must be labeled with the following:

Basic Information: Specification, Grade (TH/VH), Breaking Load

Traceability Information: Heat Number, Production Date, Inspector Code

The labeling is laser-etched to ensure it remains in place for extended use.

6. Factory Packaging Protection Measures

Use moisture-proof cartons and pallets for packaging:

Internal Isolation: Soft paper is used to separate every 10 links to prevent impact and wear during transportation.

Moisture-proof Treatment: Desiccant is placed inside the box, and the outer packaging is labeled "Handle with Care" and "Moisture-Proof."

Conclusion: The Safety Commitment Behind Forging

A qualified chain hoist chain undergoes over 20 tests and 12 core processes. The pass rate from steel to finished product is only approximately 85%—beneath this high threshold lies a deep respect for lifting safety. When you use a chain hoist to lift heavy objects, the chain in your hands is not just a metal component, but a rigorously tested safety barrier.

When selecting a chain, be sure to look for products that comply with the GB/T 30026-2021 standard and check for a complete material report and factory inspection certificate. After all, lifting operations are crucial, and chain quality is paramount.