DETERIORATION BEHAVIOR OF THERMOMECHANICAL REFINER PLATES

To make paper from wood raw materials, wood chips must be processed to turn them into individual fibers or fiber bundles. In mechanical pulp ing this is accomplished by introducing energy into the wood chip or f iber bundles as the wood chips are passed between a set of counterrota ting plates; these are up to 1.5 m in diameter and operate with a plat e clearance on the order of 0.1 mm and plate peripheral velocity of 13 0 m s(-1). The working life of refiner plates is sufficient for the pa per industry; however, the deterioration of the fine surface detail of the plates reduces refining efficiency. This research was focused on the behavior of the plate materials in the refining environment, with the emphasis on understanding the deterioration behavior of the refine r plates. It combined wear analysis of normally used plates obtained f rom the paper mills, of plates used in designed trials in paper mills, and evaluation of plate materials in a broad range of laboratory test s. The laboratory tests were selected and suitably modified to reprodu ce phenomena observed on worn plates. The laboratory tests included ca vitation erosion, dry-sand-rubber-wheel abrasion, and particulate eros ion with high pressure water or steam as carrier fluids. Plate materia ls tested included 25Cr and 20Cr2Mo1Cu (both wt.%) high chromium white cast irons, Ni-hard white cast irons, and 440C and 17-4 PH stainless steels. Plate failure analysis revealed a variety of wear processes wh ich varied with plate location and plate running time in the refiner. Cavitation erosion was clearly identified in the center of the plates where water was definitely present. Damage patterns with the same appe arance as cavitation erosion were observed along plate bar edges near the periphery of the refiner plates where water is not thought to exis t in a liquid form. Cavitation erosion tests in the laboratory duplica ted the observed phenomena on the bar edges. Abrasion tests and erosio n tests lacked such a close correlation to the observed phenomena. For all observed wear phenomena the role of the microstructure appears to be more important than the bulk hardness and for pitting-type damage such as that produced by cavitation erosion, the microstructure has a major influence on the wear process. (C) 1997 Elsevier Science S.A.