A
chip pretreatment process which comprises conveying the feed material through a
compression screw device having an
atmosphere of saturated steam at a pressure above about 5 psig, decompressing and discharging the compressed material from the screw device into a decompression region, feeding the decompressed material from the decompression region into a fiberizing device, such as a low intensity disc refiner, where at least about 30 percent of the
fiber bundles and fibers are axially separated, without substantial
fibrillation of the fibers. In a more specific form the invention is directed to a process for producing mechanical pulp, including the steps of fiberizing wood
chip feed material in a low intensity disc refiner until at least about 30 percent of the fibers are axially separated with less than about 5 percent
fibrillation, and subsequently refining the fiberized material in a
high intensity disc refiner until at least about 90 percent of the fibers are fibrillated. In another form the invention combines
chip fiberizing with chemical treatments, for improving the pulp property versus energy relationships.