Failure Analysis of a Fractured Hook-Shaped Steel Rod from a Lifting Weight Bridge Beam
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CASE HISTORY—PEER-REVIEWED
Failure Analysis of a Fractured Hook-Shaped Steel Rod from a Lifting Weight Bridge Beam George A. Pantazopoulos
Submitted: 30 August 2009 / in revised form: 6 November 2009 / Published online: 2 December 2009 ASM International 2009
Abstract A broken hook-shaped steel rod from a weightlifting bridge beam was received from a plant for failure analysis. Visual, stereo, and light optical microscopy as well as hardness testing, used for fractographic, microstructural, and strength evaluation, were used as the principal analytical techniques in the investigation. Macrofractographic investigation suggests strongly that failure initiated at the weld area and was followed by torsional overload fracture of the hook body. The fracture probably originated from abnormal operation of the crane lift. Keywords Steel hook-shaped rod Torsional overload Macrofractography Weld defects Ferrite-pearlite steel
Introduction and Background Information A hook-shaped steel rod working in a lifting crane that had been welded on a steel beam was delivered by an industrial customer for failure investigation. The load being carried at the time of fracture was a bundle of metallic products hung from belts. This load was out of balance and as a consequence of fracture dropped down without causing any human injuries. The incident was characterized as a ‘‘near-miss’’ accident, and, therefore, plant health and safety strategy activated an incident rootcause investigation. The material failure analysis was considered as a vital part of the entire investigation. The steel construction material was specified by the
G. A. Pantazopoulos (&) ELKEME Hellenic Research Centre for Metals S.A., 252 Pireaus Str., 17778 Athens, Greece e-mail: [email protected]
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manufacturer as St-37 (low-carbon steel), and the maximum load-carrying capacity was 1.0 tn. A picture showing the operating hook-shaped rods welded on steel beam is shown in Fig. 1(a), while the broken sample under investigation is shown in Fig. 1(b) and 2. Stereo and optical microscopy and hardness testing were used as the principal analytical techniques.
Experimental Procedure Optical examination of rod fracture topography was performed using a stereomicroscope. Microstructural evaluation was conducted on mounted cross sections, after suitable grinding up to No. 1200 grit SiC abrasive papers, followed by fine polishing using diamond and silica suspensions, respectively. To reveal the microstructure, immersion etching was performed in a 2% nital solution followed by alcohol cleaning and hot air-stream drying. Metallographic observations were performed using an inverted light optical microscope. Hardness testing on various polished sections was performed using a Vickers indentation device under 1 kg applied force.
Investigation Optical Microscopy and Hardness Testing Metallographic sections were prepared in locations close to and remote from the fracture area for comparison purposes. The microstructure consists mainly of ferrite and lamellar pearlite c
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