When many people hear the term 'oil immersed sleepers', they may imagine the image of wood soaking in an oil tank. Indeed, over a hundred years ago, when Europeans first attempted to treat sleepers with coal tar, they used the method of soaking them in soil outdoors. But the wood with oil stains on its surface often doesn't last for a few years on damp railway tracks before starting to rot. It was not until engineers discovered the mystery of high temperature and pressure that the lifespan of sleepers was truly extended to over thirty years.
Why pine wood has become the best partner for railway tracks
In the vast forest sea of Northeast China, the straight and upright bodies of red pine and larch are particularly prominent. These coniferous tree species with a growth cycle of about 20 years have a price per cubic meter that is nearly one-third cheaper than hardwood, but they have a unique structure. The early part of pine wood is as soft as a sponge, while the late part is densely covered with tough resin channels. This wavy fiber arrangement not only buffers the impact of trains, but also leaves natural channels for the penetration of preservatives. More importantly, the resin inherent in pine wood can react chemically with anti-corrosion oil to form a natural waterproof barrier.
The fatal flaw of traditional soaking method
At the beginning of the 20th century, railway workers discovered that simply soaked sleepers were like sugar coated apples - shiny on the surface but still fragile inside. Anti corrosion oil cannot penetrate the surface layer. After rainwater seeps into the wood through the cracks of the ballast, fungi will multiply wildly in the humid environment under the oil film. Even worse, thermal expansion and contraction can cause the surface oil film to crack, forming new channels for water seepage. This approach of treating the symptoms rather than the root cause led to the large-scale replacement of railway sleepers every few years at that time.
Chemical reaction between steam and pressure
In modern sleeper processing plants, cylindrical pressure tanks are undergoing miraculous changes. After the dried pine wood is sent into the pressure tank, the operator will first evacuate the air in the autoclave and then inject high-temperature anti-corrosion oil into the tank. The pressure is gradually increasing. During the six hour high-temperature and high-pressure operation, the anti-corrosion oil not only penetrates deeper into the sleepers, but also undergoes esterification reaction with the resin, forming a dense anti-corrosion layer inside the wood.
The guardian who penetrates through the annual rings
The pine wood that undergoes this process will undergo a complete transformation. The penetration depth of anti-corrosion oil can reach over 13 millimeters, which cannot be compared to ordinary soaking methods. Even in rainy areas in the south, these sleepers can successfully resist termite invasion, extending the replacement cycle from a few years to several decades. The slight creaking sound of the sleepers as the train passes is a testament to the perfect integration of industrial civilization and natural materials.
Walking beside today's railway tracks, those oil immersed sleepers with a black luster are silently telling the wisdom of industrial civilization. The pine wood harvested from the Northeast Forest Farm undergoes a "transformation" through high-pressure tanks and ultimately transforms into the foundation that carries the steel dragon. This seemingly simple processing technique embodies the profound understanding of materials science by three generations of engineers.
