Brittle fracture of a crane hook

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Abstract

The objective of this work is to identify the causes that led to a failure of the crane hook in service. The study of the accident includes: (1) a summary and analysis of the peculiarities inherent to the standards that determine the manufacture and use of this type of device, (2) metallographic, chemical and fractographic analyses, (3) assessment of the steel mechanical behaviour in terms of Vickers hardness profile, its tensile strength and fracture energy, and (4) simulation of the thermal history of the hook. The visual and microstructural inspections reveal some evidences that a weld bed was deposited on the hook surface. Several cracks grew from that area into the material. Fracture surface shows features typical of brittle failures (transgranular cleavage fracture). The unalloyed, low-carbon steel contains a relatively low aluminium (<0.025%) and high non-combined nitrogen (>0.0075%) content. All the gathered evidences are in agreement with a strain-aging process triggering the embrittlement of the material, with the fracture starting from a crack generated at the heat affected zone of an uncontrolled welding of the hook.

Introduction

In the frame of the social policy, Article 118 of the EEC treaty established that member states shall endeavour to promote the improvement of working environment to protect the safety and health of workers.

The laws, standards and techniques designed to prevent occupational risks, ensuring safe and healthy working conditions vary [1], [2]. Particularly, the standards UNE 58-509-79 [3], UNE-EN 1677-1 [4] and UNE-EN 1677-5 [5] summarize and describe the terms and conditions that are to be considered in the design, manufacture, use, and control of lifting hooks. Their observance will prevent the fall of the load during service and minimize the risk to which people are exposed in the danger zone. Within this framework, the objective of this work is to identify the causes that originated the fracture of a lifting hook in service which caused the death of a worker.

Section snippets

Details of the standards governing the manufacturing and use of lifting hooks

The steel must be manufactured using electrical melting furnaces and oxygen converters. The austenitic grain size should remain finer than 5 (according to UNE-EN-ISO 643:2004 standard [6]). Al content should be higher than 0.025%, but N cannot exceed 0.0075%.1

Visual and macroscopic observation, evaluation of dimensions

Initial examination of the hook (Fig. 1, Fig. 2) reveals the presence of general surface damage: burrs, beatings and remnants of rust and a welding band on one of the fracture surface edges.

The fracture origin (bright zone in Fig. 2b) is located on the original surface of the hook, adjacent to the welding band, as can be traced from propagation lines emanating from that point. Also, splashing of molten material is visible (Fig. 2c).

The received part (weight and size) was compared to the

Discussion and conclusions

The main results of the tests and studies carried out can be summarized as follows:

  • The lifting hook did not meet the requirements of the UNE-EN standards governing the use of such devices, including:

    • insufficient content of aluminium (less than 0.025%) and, at the same time, a high proportion of nitrogen (greater than 0.0075%), a situation that marks this steel as a candidate for weakening by strain-aging,

    • extensive general surface damage (burrs, beatings, remnants of rust and the presence of a

Acknowledgements

The authors wish to thank to the laboratory technicians Jesús Pinto (Y. Torres and J.M. Gallardo) and José Guareño (J. Domínguez) for carrying out the microstructure characterization and mechanical testing.

References (16)

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