Tungsten Carbide Welding to Enhance the Shield Cutters

At present, China's major cities are booming in the construction of rail transit projects. Due to the great advantages, the shield tunneling method has been widely used in the construction of rail transit tunnels.

Rail transit lines generally pass through urban core areas such as commercial centers, dense residential areas, and large public places. These places are generally located with towering buildings or large bridges, and their pile foundations are usually deep and thick. For the large-diameter pile foundation, the biggest difficulty lies in how to cut the coarse steel bar, which is almost impossible with the conventional material shield cutters. The other difficulty is the cutting of mass concrete containing coarse aggregate. This poses a huge challenge to the wear resistance and impact resistance of the cutter.

Since the existing shield cutter does not consider the cutting of the reinforced concrete pile foundation, and basically does not have the ability to directly cut the pile. So the traditional method is done by manual operation to remove the pile foundation. Although these traditional methods are simple and safe, they have drawback of high cost, huge environmental impact and long construction period. Thus, if the cutter can be improved, the shield can directly cut the pile foundation. It will save construction investment and reduce the environmental impacts.

Targeting the poor wear resistance and impact resistance of shield cutters, some enterprises adopt a welding method to enhance the wear resistance of shield cutters. It is welded with a layer of tungsten carbide surfacing layer by plasma arc surfacing on both sides of the outer edge of the cutters. The production process of tungsten carbide powder is:

Step 1: Mix 55% to 60% by weight of ammonium metatungstate aqueous solution and nano carbon black powder to form a fluid, and adding into a mold to form a precursor;

Step 2: Move the precursor into the freezing room for instant freezing, with a temperature of -150 ~ -160 °C for 200 ~ 240mins;

Step 3: Move the frozen precursor into a vacuum freeze-drying chamber;

Step 4: Turn up the vacuum freeze-drying chamber. And the carbonization reduction reaction is performed at a temperature of 900 to 1200 ° C to obtain a tungsten carbide block;

Step 5: The tungsten carbide block is pulverized into a tungsten carbide powder by a pulverizer.

The freeze-dried precursor block is only a microporous honeycomb block formed by sublimation of water molecules. The position of the tungsten source and carbon source with small and uniform volume change does not change, and the fixed carbon source and its surrounding location are also relatively fixed. The tungsten source reacts and is positionally homogenized to finally form uniform tungsten carbide, and is also a nano-sized tungsten carbide block.

Due to the uniformity of the microporous pores of the block after lyophilization, it is relatively easy to achieve uniform particle size. The microporous nano-scale tungsten carbide has a relatively uniform heat absorption in the plasma arc surfacing process, and can be rapidly melted and combined with the outer edge of the cutters, and the strength of the combination is improved. Correspondingly, the dilution ratio of the weld overlay is further reduced, and the dilution rate should be controlled below 5%.

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