Analysis of floating-head heat exchanger bolts failure

Abstract

As-received floating-head heat exchanger bolts were broken (BB) and deposite-coated. The aim was to estimatea cause of their failure. The new bolts of the same material were used as a reference material (reference bolt –RB). After visual and radiographic examination, their chemical composition, structure and room-temperaturemechanical properties were determined and compared. Comparison was made with the values set by standard,as well. Afterwards, fractography was performed on fractured surfaces of tensile specimens and originally(during exploitation) BBs to try to get an impression about bolts failure mechanism. Qualitative analysis ofdeposite was employed in order to confirm was there any possible influence of surroundings during their failurein terms of corrosion-assisted cracking. Chemical composition of RB and BB materials was analyzed by use ofspectrophotometry and structure properties with light optical microscope (LOM). Fractured surfaces of tensilespecimens and of BBs, as well as deposite chemistry, were analyzed by use of Scanning Electron Microscopywith Energy Dispersive System (SEM-EDS). BBs had an approximately three times higher sulphur content andlesser manganese content, lower ductility and higher strength values comparing to those of the RBs. Generally,fracture surfaces of both, RB and BB tensile specimens have a similar rosette-like macro-appearance. The onlydifference is that the radial marks in the case of the RBs are rougher. The surface has a more fibrous area andshear lip presence. Fracture mode can be characterized as dimple rupture and micromechanism as microvoidcoalescence. In the case of BB tensile specimens, the mixed presence of dimples and cleavage facets wasnoticed. The macrofractography of originally broken surfaces shows a rough and complex topography offracture surfaces indicating on a possibility that bolts failure has been a result of complex loading conditions.Presence of sulphur- and chlorine-containing particles on the fracture surfaces of BBs and in deposite reveals apossibility that failure was environmentally-assisted.