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 pulping this is accomplished by introducing energy into the wood chip or fiber bundles as the wood chips are passed between a set of counterrotating plates; these are up to 1.5 m in diameter and operate with a plate clearance on the order of 0.1 mm and plate peripheral velocity of 130 m s(-1). The working life of refiner plates is sufficient for the paper 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 refiner plates. It combined wear analysis of normally used plates obtained from the paper mills, of plates used in designed trials in paper mills, and evaluation of plate materials in a broad range of laboratory tests. The laboratory tests were selected and suitably modified to reproduce phenomena observed on worn plates. The laboratory tests included cavitation erosion, dry-sand-rubber-wheel abrasion, and particulate erosion with high pressure water or steam as carrier fluids. Plate materials 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 which 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 appearance as cavitation erosion were observed along plate bar edges near the periphery of the refiner plates where water is not thought to exist in a liquid form. Cavitation erosion tests in the laboratory duplicated the observed phenomena on the bar edges. Abrasion tests and erosion 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.