Failure Analysis of Mill Shaft Roll
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CASE HISTORY—PEER-REVIEWED
Failure Analysis of Mill Shaft Roll A. Babakr Æ R. Bradley Æ A. Al-Ahmari
Submitted: 11 March 2008 / in revised form: 16 January 2009 / Published online: 13 February 2009 ASM International 2009
Abstract A strip mill roll shaft failure was investigated. The investigation revealed that the fatigue strength of the shaft had been compromised by a combination of several factors that included: inadequate fillet radius size, the use of dissimilar filler materials to rebuild the shaft surface, the presence of slag inclusions at the weld/alloy interface, and machining defects on the shaft surface. Such failures have occurred repeatedly, and this paper presents a case study investigating the cause of the problem and discusses possible remedial measures to prevent similar occurrences. Keywords Cyclic loads Filler Fracture Inclusions Reclamation Rotational bending fatigue Shaft Slag Stress risers Weld Weld porosity
Introduction Welding and reclaiming equipment by using weld overlays to build up thickness are frequently conducted according to internally developed standards and welding procedures. Such reclamation processes are common and repeatedly used in the petrochemical, chemical, petroleum, gas, energy, and utility industries. Unfortunately, these processes are also behind many failures in the reclaimed equipment [1, 2]. Reclamations are mainly driven by financial demands and economic gains. As newer welding processes evolve, many welding practices become simpler, but the pitfalls in using weld overlay practices do not disappear. The weld itself may lead to unwelcome additions, A. Babakr (&) R. Bradley A. Al-Ahmari SABIC Technology Center, P.O. Box 11669, Jubail Industrial City 31961, Saudi Arabia e-mail: [email protected]
such as porosity and inclusions [1, 3] to the reclaimed part. These additions are part of the weld material [4] and may become stress risers, which are located in regions of high residual stress [2, 5] and are associated with regions of the component that had previously experienced deterioration. Thus, weld defects frequently become sites leading to fatigue failures [4, 6]. There is generally no way for the welder to forecast failures, but the success of the repair is generally dependent on the welder’s understanding of the need for high-quality welds. Bad welds are often instigators of failure in reclaimed rotating equipment and blamed many times for failures in stationary equipment such as water wall boiler tubes [7]. A hurdle that is always faced during a failure investigation, especially for reclaimed components, is that the history of equipment is not properly recorded and cannot be retrieved. Such information related to failures, especially when is the failures are repetitive, can only be acquired with methodical investigation and frequent discussions with personnel. Often such discussions provide insight into the failure process and into techniques for failure avoidance. In this case, the filler material used to make the fillet welds on the
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