News

The manufacturing process of ultra-heavy crane hooks (such as those used in 10,000-ton shipbuilding gantry cranes, nuclear power dome hoisting, deep-sea platform installation, etc.) faces extremely high technical challenges and needs to break through multiple bottlenecks such as materials, design, heat treatment and testing. The following are the core difficulties and solutions:

1. Material Challenges

Pain Points

• The contradiction between ultra-high strength and toughness :
  Traditional alloy steels (such as 34CrNiMo6) have difficulty in achieving both strength (≥800MPa) and low-temperature impact toughness (-40℃ KV≥60J) in ultra-large cross-sections (diameter>500mm).

• Segregation and inclusion defects :
  Hundred-ton steel ingots are prone to central segregation during solidification, resulting in sulfide/oxide inclusions inside the hook, which become the source of cracks.

Solution

• Special metallurgical process :

  • Use electroslag remelting (ESR) or vacuum arc remelting (VAR) to reduce impurities (S, P content ≤ 0.005%).

  • Use high purity steel (such as American ASTM A788 Grade F) and add Ni, Cr and Mo to improve hardenability.

• Composite material design : Metal ceramic layers (such as WC-Co)
  are embedded in stress concentration areas (such as hook neck) to enhance wear resistance.

2. Forging process challenges

Pain Points

• The capacity limit of a 10,000-ton press :
  Ultra-large hooks (single weight > 50 tons) require a hydraulic press of more than 50,000 tons to achieve full-section forging. Only a few companies in the world have this capability.

• Fiber streamline control :
  Insufficient forging ratio will cause metal streamline breakage and reduce fatigue life.

Solution

• Multi-directional segmented forging :
  Using three-dimensional die forging (such as the German Siempelkamp process) to optimize metal flow through multiple deformations.

• Isothermal forging :
  Slow forming (rate <1mm/s) at the material recrystallization temperature to avoid surface cracking.

3. Heat treatment problems

Pain Points

• The temperature difference in the cross section causes quenching cracks :
  When quenching ultra-thick sections (such as more than 300mm), the core cooling rate is insufficient, and the surface and core structures are very different (surface martensite vs. core bainite).

• Temper brittleness risk :
  When tempered at 350-550℃, impurity elements such as P and Sn will segregate at the grain boundaries, causing brittleness.

Solution

• Differential temperature heat treatment technology : spray + immersion composite quenching
  is adopted , and the cooling rate of each part is controlled by computer simulation.

• Cryogenic treatment :
  Cryogenic treatment at -196℃ liquid nitrogen is performed immediately after quenching to transform the retained austenite and improve dimensional stability.

4. Difficulties in machining

Pain Points

• Hook body curved surface processing accuracy :
  The contour of asymmetric curved surfaces (such as S-shaped hook bodies) is required to be ≤0.1mm, which is difficult to achieve with traditional turning.

• Thread processing vibration :
  Extra large threads above M300 are prone to vibration during cutting, resulting in excessive pitch error.

Solution

• Five-axis CNC machining :
  Using the combination of slow-wire cutting and contour milling , the surface accuracy can reach IT6 level.

• Vibration suppression tool :
  adopts hydraulic damping tool bar and ultrasonic assisted cutting to reduce the vibration mark of thread processing.

5. Testing and verification

Pain Points

• Internal defect detection blind area :
  The ultra-thick section causes severe attenuation of the ultrasonic testing (UT) sound beam, and the detection rate of small defects (<2mm) is low.

• Risks of full-scale load testing :
  When a 10,000-ton hook is subjected to a 1.25-fold static load test, the test bench itself may fail.

Solution

• Multimodal nondestructive testing :
  TOFD (Time-of-Flight Diffraction) + Phased Array Ultrasonic (PAUT) combined testing, defect resolution increased to φ1mm.

• Digital twin verification :
  Finite element analysis (FEA) + actual strain gauge testing can replace some physical load tests.

Typical Cases

• China Second Heavy Industries : Manufactured 12,000-ton hooks for the "Blue Whale" crane ship, using ESR steel ingots + three-dimensional forging + differential temperature quenching technology, with a fatigue life of over 100,000 times.

• RUD of Germany : Developed a titanium alloy composite hook (Ti-6Al-4V + steel matrix), which reduces its own weight by 40% and is used in space launch sites.

Future Trends

• Additive manufacturing : Selective laser melting (SLM) forms a gradient material hook with customizable local properties.

• Smart hook : Embedded fiber optic sensors monitor stress/cracks in real time and predict remaining life.

The manufacture of super-heavy hooks is the intersection of metallurgy, machinery and materials science, and its technological breakthrough directly represents the cutting-edge level of the country's heavy equipment.