Lifting Hook Mechanical Properties & Material Grades

GB/T 10051 is the series standard for crane lifting hooks, with Part 1 released and implemented in 2010. The standard specifies two mechanical property classes for hooks: Class M (standard) and Class P (premium), each with distinct material and heat treatment requirements. As the most critical load-handling device on a crane, the hook's safety and reliability directly affect the safety of personnel and suspended loads. When selecting a hook, a sufficient safety margin must be provided above the rated lifting capacity.

Hook Mechanical Property Classes Explained

The standard classifies hooks into Class M (standard) and Class P (premium) based on mechanical properties. Class M hooks are manufactured from DG20 or DG20Mn steel, with a minimum yield strength of 225 MPa, minimum tensile strength of 400 MPa, and minimum elongation after fracture of 22%. These hooks are suitable for conventional operating environments and general-purpose cranes. Class P hooks use DG20Mn or DG34CrMo steel, offering a minimum yield strength of 315 MPa, minimum tensile strength of 510 MPa, and minimum elongation after fracture of 20%. They are designed for cranes operating under high-intensity, high-frequency duty cycles.

The superior performance of Class P hooks comes from their alloy composition and heat treatment process. DG34CrMo contains higher levels of alloying elements, and after quenching and tempering, it achieves significantly better comprehensive mechanical properties — delivering approximately 25% higher load-bearing capacity than Class M at the same hook dimensions. The standard specifies that Class P hooks must have a grain size of no less than Grade 5, non-metallic inclusion content not exceeding Grade 2, and Ultrasonic Testing (UT) must show no cracks or white spots. The class designation is permanently marked on the hook during the forging or heat treatment process, and user units should verify the class marking when replacing hooks.

The lifting capacity of a hook is strictly tied to its shank diameter. The standard's appendix provides Load Capacity Charts for both Class M and Class P hooks across all dimension series, and design personnel should select the appropriate hook diameter based on actual operating conditions. For example, a Class M hook with a 40 mm shank diameter has a rated lifting capacity of approximately 5 t, while a Class P hook of the same diameter can handle 6.3 t. A Class M hook with a 63 mm shank diameter is rated at approximately 12.5 t, while Class P reaches 16 t. Choosing a Class P hook increases load-bearing capacity without increasing hook size — an advantage in applications where space is limited but higher capacity is required.

Hook Materials and Manufacturing Requirements

The standard imposes strict requirements on hook materials. Hooks must be forged from killed steel melted in an open-hearth, electric, or oxygen converter furnace, and the steel must be accompanied by a Factory Certificate and certificate of quality. Hooks formed by the forging process must undergo normalizing or quenching and tempering after forging. Normalizing is performed at 850–900°C, while quenching and tempering consists of quenching followed by high-temperature tempering at no less than 550°C. After heat treatment, hook hardness must be controlled within HB140–225 for Class M and HB180–260 for Class P, with a maximum hardness deviation of HB30 across different sections of the same hook.

Regarding the manufacturing process, hooks must be produced by integral forging — welding to assemble or build up hooks is strictly prohibited. The forging ratio must be no less than 3:1 (the ratio of billet cross-section to forged cross-section) to ensure continuous, uninterrupted metal flow lines. Hook surfaces must be free from cracks, folds, and overheating defects. Local defects not exceeding 5% of the shank diameter may be removed by grinding, provided the ground area transitions smoothly into the surrounding surface. The hook body must undergo Magnetic Particle Inspection (MPI) or penetrant testing in accordance with JB/T 6062. The acceptance criteria require zero crack-type defects, and any single non-crack defect must not exceed 2 mm in its largest dimension.

The standard also sets out marking requirements. Each hook must bear permanent markings in a non-stressed area: the manufacturer's trademark, hook class (M or P), rated lifting capacity in tonnes, material grade, and batch number. Markings must be clearly legible with a minimum depth of 0.3 mm. Each hook shipped from the factory must be accompanied by a Certificate of Conformity stating the manufacturer's name, standard number, hook class, rated lifting capacity range, material grade, heat treatment condition, inspection results, and date of manufacture. The certificate must be delivered to the user unit together with the hook and serves as the basis for Installation Acceptance and future replacements.

Discard Criteria for Lifting Hooks

The standard provides explicit discard criteria for lifting hooks, which form the core basis for Daily Inspection and safety management. A hook must be scrapped if any of the following conditions is present: ① cracks on the hook surface — cracks in any direction or of any length are unacceptable, and a hook is condemned immediately upon detection by visual inspection or MPI; ② the throat opening has increased by more than 15% over its original dimension, indicating plastic deformation; ③ the hook body has twisted more than 10°; ④ wear on the critical cross-section (the smallest shank diameter) exceeds 5% of the original dimension; ⑤ permanent deformation has occurred in the hook neck (at the thread relief groove) or the hook body.

Daily Inspection of hooks must be performed by the operator at the start of each shift, with visual inspection focusing on cracks in the hook body, permanent deformation of the throat opening, and the condition of the Hook Latch. Periodic Inspection must be carried out quarterly, using MPI or penetrant testing on the hook body and the thread relief groove area. Annual inspections must measure the throat opening and shank diameter and compare them against the original recorded data. All inspection results must be documented in the equipment file. Hooks found to be approaching the discard limits should have their inspection interval shortened to once per month. The standard further emphasizes that hooks meeting the scrapping criteria must never be repaired by welding and returned to service — they must be replaced entirely.

Hook Grades and Scrapping Criteria Comparison

The comparison table below summarizes the key performance differences and scrapping criteria for Grade M and Grade P hooks, providing a practical reference for user units during selection, procurement, and daily inspection.

Hook Safety Latch and Safe Use

Standards require that all hooks be equipped with a hook safety latch (spring-loaded or locking plate type) to prevent slings from accidentally disengaging during hoisting and travel operations. The latch must have sufficient elasticity and strength to spring back and lock automatically when compressed by the sling. User units must not remove the hook safety latch. The spring mechanism of the latch should be inspected monthly, and replaced if it shows signs of weakened tension or sticking. In applications where slings are changed frequently, the latch wears faster and the inspection interval should be shortened accordingly.

For hooks used in low-temperature environments (below -20°C), a Grade P hook material with certified low-temperature impact toughness is required; DG34CrMo meets the requirements for service down to -40°C. In high-temperature environments such as metallurgical foundries, the hook surface temperature must not exceed 200°C. Hooks must not be overloaded, and angled or oblique lifting is strictly prohibited—such practices subject the hook to horizontal force components, placing additional bending stress on the hook mouth and accelerating deformation and fatigue. Kelude Heavy Industry equips all cranes at the factory with Grade P forged hooks and hook safety latches matched to the rated lifting capacity, ensuring the safety and reliability of the load-handling device.

FAQ: Hook Safety, Inspection & Maintenance