Forged Crane Hook Material Mechanical Properties & Static Load Test

GB/T 10051.3-2010 is Part 3 of the lifting hook series, specifically governing the material mechanical properties and static load test procedures for forged hooks. As the most critical load-handling device on a crane, the hook's material performance and load-bearing capacity are directly tied to hoisting safety. This article provides a detailed breakdown of the standard's core content.

Standard Positioning and Hook Classification

GB/T 10051.3-2010 is Part 3 of the GB/T 10051 "Lifting Hooks" series, used in conjunction with GB/T 10051.1 (general technical specifications for lifting hooks) and GB/T 10051.2 (technical specifications for forged hooks). The standard defines two structural types of forged hooks: shank hooks (secured to the hook beam via a hook nut) and ring hooks (connected through a lifting eye). By cross-section of the hook body, they are classified as single hooks or double hooks (C-Hooks). By critical section profile, they fall into trapezoidal-section hooks and T-section hooks. The hook size range covers 0.25t to 320t, divided into 15 capacity grades. The key advantage of forged hooks lies in the continuous metal flow line that follows the hook contour, delivering superior fatigue strength and toughness with no risk of interlayer separation as seen in laminated-type hooks.

Technical Requirements

The standard imposes strict requirements on the material, mechanical properties, and manufacturing process of forged hooks:

Material — Forged hooks must be made of high-quality carbon structural steel or alloy structural steel, with common grades being DG20 (carbon steel hook grade) and DG34CrMo (alloy steel hook grade). The material must exhibit good plasticity and low-temperature toughness, with carbon content ≤0.25% (to ensure weldability and toughness) and P and S contents both ≤0.025%. Chemical composition analysis and mechanical property verification must be performed on each individual piece.

Forging Process — Hooks must be formed by die forging with a forging ratio of no less than 3:1 (ratio of initial to final cross-sectional area), ensuring the metal flow line remains continuous along the hook contour (flow line interruption at critical sections is not permitted). Forging temperature range: initial forging temperature 1150–1200°C, final forging temperature ≥850°C. After forging, normalizing (850–900°C, air-cooled) followed by tempering (550–650°C, air-cooled or furnace-cooled) heat treatment is required to eliminate forging stress and refine grain structure.

Mechanical Properties — Each finished hook must undergo mechanical property testing: yield strength ≥335MPa (DG20) or ≥685MPa (DG34CrMo); tensile strength ≥470MPa or ≥785MPa; elongation δ≥22% or ≥14%; reduction of area ψ≥45% or ≥40%; impact energy KV₂ (-20°C) ≥34J. Hardness: hook body surface hardness HB 140–187 (DG20) or HB 210–270 (DG34CrMo).

Structural Dimensions and Design Parameters

The standard standardizes key structural dimensions of forged hooks: hook opening size (throat width) — for a 5t hook, the throat width is approximately 60mm; hook bending radius — the inner bending radius of the hook should be no less than 2.5 times the wire rope diameter; critical section area of the hook body — designed for a safe working load of 4 to 5 times the rated lifting capacity. The hook neck (shank portion) diameter is designed for a safe working load of 3 to 4 times the rated lifting capacity. Hook nut and locking device — the hook must be equipped with a self-locking nut mechanism (cotter pin or locking washer), with nut height no less than 0.8 times the hook thread diameter. The hook tip must be fitted with a hook latch (tongue or spring-loaded latch) to prevent the sling from accidentally disengaging.

Testing and Inspection

The standard requires that each forged hook undergo the following tests and inspections:

Chemical Composition Analysis — Each piece is analyzed for C, Si, Mn, P, S, Cr, Ni, Mo and other alloying elements.

Mechanical Property Testing — One hook from each batch (same heat number) and specification is selected for tensile and impact testing.

Static Load Test — Every finished hook must undergo a static load test with a test load of 2 times the rated lifting capacity (general-purpose hooks) or 2.5 times (hooks for hazardous load handling), held for ≥5 minutes. After the test, residual deformation of the hook opening is measured (opening increase must not exceed 0.25% of the original opening size). The static load test is the most direct method for verifying the hook's load-bearing capacity.

Non-destructive Testing — Hook surfaces must undergo Magnetic Particle Inspection (MPI) to detect forging cracks, laps, and hairline defects. Transition zones at both ends of the hook body must undergo Ultrasonic Testing (UT) to detect internal inclusions and cracks. MPI acceptance criteria: no cracks or linear indications of any kind. UT acceptance criteria: no defect signals exceeding a Φ2mm equivalent flat-bottom hole.

Hardness Test — Each hook is tested for Brinell hardness (HB) on the hook body surface, with one measurement taken at each of three cross-sections. The hardness must fall within the specified range, and the variation across the three points must not exceed 30 HB.

Lifting Hook Discard Criteria

Standards define clear scrap criteria for lifting hooks in service. A hook must be scrapped and replaced immediately if any of the following conditions is detected:

1) Crack — any crack on the surface or internally, whether found by visual inspection or NDT;
2) Opening deformation — permanent deformation of the hook opening exceeding 5% of the original opening size, measured with calipers;
3) Torsional deformation — the hook body twisted by more than 10°, measured with an angle gauge;
4) Wear — wear at the critical section (the thinnest part of the hook body) exceeding 5% of the original dimension;
5) Corrosion — severe surface corrosion resulting in pitting or a cross-section loss exceeding 5%;
6) Thermal damage — surface discoloration caused by arc strikes or flame heating;
7) Plastic deformation — visible bending or distortion of the hook.

Scrapping is final — a lifting hook is not repairable. Once any discard criterion is met, the hook must be replaced immediately. Repair procedures such as welding or build-up are strictly prohibited.

Forged Lifting Hook Material Mechanical Properties Comparison Table

The comparison table below summarizes the key parameters and configurations for forged lifting hook materials, serving as a reference for selection and in-service inspection.

Frequently Asked Questions