Emergency welding repair of the hottest shield cut

Emergency welding repair of shield cutterhead

welding operation on the excavation surface in the earth bunker is limited by many factors such as operation space and ventilation conditions. The operation is difficult, and there are also high requirements for the operation level of welders. Because the stratum condition of the excavation surface is relatively hidden during this operation, and the measures are effective, the welding operation is carried out smoothly

1. Steel selection: cutter head repair work, time is tight, and the task is heavy. However, we have made serious investigation, analysis and preparation. First, adopt st steel grade according to the cutter head body. Equivalent to domestic low carbon steel. The repair width of the cutter head body and the cutter installation base is 350mm, the thickness is 100mm, and the height is 150mm. Considering the mechanical properties, weldability and other factors of the material, and after studying with the welding experts of relevant units, it is finally decided to use domestic 20# steel to repair the cutter head and installation base

2. Optimization of cutter head repair scheme: how to ensure that the performance of the repaired cutter head body is not lower than the level of the original design and manufacture, we have conducted in-depth analysis and research, and processed sample blocks for simulated assembly analysis on site

3. Considering the welding process, the scheme of left and right blocks plus support blocks is adopted: in the original design repair scheme, two 150mm thick steel blocks are used to weld and repair the left and right sides of the cutter head boom. Through the analysis and the simulation installation of the sample block, we found that in the soil bin, due to the limitation of the operation space, the welding operation can only be carried out from two sides, and the cutter head cannot be welded from the front. On the width of 345mm, weld two 150mm steel blocks. The first one can be welded, while the weld between the part adjacent to the second one and the first one and the cutter head body cannot be welded. Therefore, the two-piece 3 balancing machine (including on-site balancing instrument) is used instead; 100mm thick steel block plus intermediate support plate; Weld the left and right pieces first, and then weld the intermediate connecting plate. This not only ensures the accessibility and convenience of welding operation, but also ensures a high connection strength between the restoration and the cutter head body

4. Determination of the number of connecting bolts of the peripheral knife: the middle bolt connecting hole on the knife mounting base around the knife head is ground off by 1/2, and half of the bolt hole cannot be welded and repaired. We initially decided to use only the connecting holes at both ends for bolt connection and fixation, and the middle one is not fixed. After our analysis and research, we believe that: if the middle bolt hole is not connected, first, the surrounding cutter alliance was jointly established by China synthetic resin supply and Marketing Association and China Automobile Industry Association, and the connection strength with the cutter head is reduced by 1/3; Second, if the bolt connection hole is not blocked, sediment will pass through the hole during the rotation of the cutter head, and because this part is located at the edge of the cutter head, its linear speed is large, which is easy to cause the wear of the cutter head and the tool; Therefore, it is decided to connect and block the bolt hole. The specific method is: process a smooth sleeve with internal thread hole, weld it with the cutter head residue at the position of the original sleeve, and connect and fix the bolt with the internal thread of the sleeve. This not only blocks the holes on the cutter head, but also improves the mechanical connection strength between the surrounding cutter and the cutter head body

5. Increase the weld height to improve the connection strength between the restoration and the cutterhead matrix: the equipment manufacturer confirmed that when the cutterhead is made, the weld height between the peripheral knife mounting base and the cutterhead body is 3 layers of 5mm. During the repair, we specified the weld height of 5 layers of 8mm. It ensures that the repair quality is not lower than the welding in the original manufacturing process, and better understands the failure mechanism and strength of the material under low-speed impact

6. Take a number of measures to ensure welding quality:

(1) select welders with rich experience and high welding technology for continuous operation

(2) choose DC welding machine to ensure the stable combustion of welding arc, so as to improve the quality of weld formation

(3) select j506 welding rod, dry the welding rod before welding, and use an incubator to keep the welding rod warm during welding operation. So as to reduce the possibility of pores in the weld

(4) polish the welding surface with sandpaper and grinder, and wipe the welding surface with acetone to ensure that the welding surface is clean

so as to ensure the weld quality

(5) strictly take waterproof measures to ensure the quality of welds

in order to shorten the downtime of the shield, we have adopted the method of 24-hour continuous operation. The operation is divided into two shifts, each of which is 12 hours, and the two welders rotate within 12 hours. In this way, the repair of the cutter head was successfully completed within a week, and the worn tools were replaced

7. Technological measures to improve the repair accuracy: because the peripheral cutter is fixed to the cutter head with three connecting bolts. Therefore, how to ensure the positioning accuracy of the bolt hole on the restoration in the welding operation has become the key to judge the quality of this repair. The technicians thought carefully and studied carefully, formulated the detailed construction process, and compiled the construction process flow chart, as shown in Figure 4; The welder is required to weld symmetrically and cross during the welding operation to ensure that the weldment is heated evenly and avoid displacement of the workpiece due to uneven heating. Due to the specific and reasonable formulation of various measures and the active cooperation of operators, the positioning accuracy after welding is ensured, and there is no rework. While ensuring the quality, it also ensures the operation progress

8. Some suggestions and thoughts: from this welding repair work, we found and thought of some problems

1) several times of warehousing inspection should be strengthened: welding operations should be carried out on the excavation surface, which has high requirements for the stability of the formation. The excavation surface of our shutdown stratum is very stable, and we can dig out the working space forward. If the self stability of the soil on the excavation surface is poor, we need to strengthen the soil first, which will take a long time. This requires us to enter the warehouse in time in the future to check the wear of tools and cutterhead, and make inspection records

2) the cutter head overhaul process specification should be formulated as soon as possible: from the situation of this emergency repair, the cutter head is seriously worn. When completing the TBM hole out of the next section, the cutterhead must be overhauled. How to repair and how to ensure the quality? We have no idea about these problems. Therefore, we must invest a certain amount of human and financial resources to carry out this work as soon as possible, carry out the necessary scheme research and test, and formulate a reasonable cutter head repair construction process and surface strengthening treatment scheme

3) brief analysis of the causes of sharp tool wear: from the analysis of the construction process, the sharp wear of tools and cutterhead occurred after the 500th ring. Before the 500th ring, each shift can normally complete the tunneling of the 4th ring in 8h, while after the 500th ring, the advancing speed is significantly slowed down. The obvious increase of thrust and torque results in the aggravation of tool wear; And the tool wear intensifies, which leads to the increase of the torque of the cutter head with the simple and fast upgrading of the software using the world's popular high-level language. The two are cause and effect, which makes the situation worse and worse. This is only the author's preliminary analysis and judgment. The real reason still needs to be analyzed and studied by experts in depth based on the factors of construction records, construction parameters and so on, so as to give a correct conclusion. I won't go into details here

4) selection of TBM localized cutter head materials: this repair work also provides a valuable reference for our TBM localization in material selection. And because the carbon content of 35 # steel is higher, the hardness is higher and the wear resistance is better; In addition, the weldability of 35 # steel is acceptable, and the Cutterhead of domestic shield machine can choose 35 # steel as the parent material