Corrosion-Initiated Rotating Bending Fatigue Failure of a Fertilizer Conveyor Belt Head Roller
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CASE HISTORY—PEER-REVIEWED
Corrosion-Initiated Rotating Bending Fatigue Failure of a Fertilizer Conveyor Belt Head Roller P. Roffey
Submitted: 15 December 2014 / in revised form: 7 January 2015 ASM International 2015
Abstract This paper presents the experimental process and findings of a forensic investigation of a failed fertilizer plant conveyor belt head roller shaft. A 25-year service life was expected but failure occurred following only 10 years. A gearbox which provided direct power to the roller had been replaced only several weeks earlier. The primary aim for the plant owner was to determine if the gearbox replacement had caused failure of the shaft, or, if the two incidents were unrelated. If incorrect installment of the gearbox had caused failure, liability would fall on the gearbox suppliers in terms of mitigation. However, it was revealed that the shaft had failed due to low-stress rotating bending fatigue, over an extended period of time. A premeditated change of material selection at the manufacturing stage, substituting carbon steel for 304L stainless steel, resulted in reduced corrosion resistance. Fertilizer ingredients tend to pose limited problems in their dry form in terms of corrosion; in the presence of moisture, however, can change the ingredients into aggressive corrosion species. Thus, corrosion allowed multiple fatigue cracks initiated from corrosion pitting under rotating bending load. The cracks then eventually joined together to form a single fatigue crack which propagated through the cross section. Keywords Pitting corrosion Crevice corrosion Fatigue Welding Rotating bending
P. Roffey (&) Forensic Engineering & Materials, ESR Technology, Whittle House, 410 The Quadrant, Birchwood Park, Warrington, Cheshire WA3 6FW, UK e-mail: [email protected]
Introduction The following paper describes the failure investigation of a head roller shaft from a conveyor belt system used to transport fertilizer product from the drying process stage to the bagging area, see Fig. 1. The rollers rotate at very low revolutions per minute. Several grades of fertilizer were manufactured at the plant which typically contain varying amount of phosphates, potash sulfur, and ammonium nitrate. The increasing use of corrosion-resistant steels such as 304 or 316 grades for fertilizer equipment have seen a reduction in the extent of fertilizer plant corrosion [1]. The head roller shaft had been in service for approximately 10 years prior to failure although at least 25 years’ service was expected. A new gearbox had been installed to the head roller shaft several weeks prior to failure as part of a refurbishment program. The aim of the investigation was to determine if failure was caused due to incorrect installment of the gearbox, or if the failure was attributed to another cause. Construction The design drawing for the head roller specified that the shaft component should be forged 080A22 carbon steel (formerly EN43) with the end plate, drum wall, and boss constructed using grade 304L stainles
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