Shield Machine Tool Steel Brazing

Brazing of difficult materials and shield machine tool steel.
At present, the development of urban rail transit industry is in full swing, especially in recent years has maintained the momentum of rapid development. As a widely used machine in subway, railway, highway, municipal, hydropower and other tunnel projects, shield machine plays a decisive role. As an important part of the tunneling system, the cutter of shield machine plays a decisive role in the construction effect of shield machine. Today we’re going to learn about the cutting tools of shield machines.

1. Types and Cutting Principles of Common Cutting Tools on Shield Machines
For shield machine, the excavation system is the cutter head and its driving system. Tunneling system directly affects the construction effect of shield machine, and shield cutting tools have a great impact on the cost of shield machine, directly affecting more than 4% of the total cost of shield machine.
The cutter layout and shape of shield machine is an important part in the design of shield machine. Whether the cutter quality and the cutter shape are suitable for the geological conditions of the project directly affect the working effect of shield machine, the driving speed and the quality of the project. Therefore, the selection and quality of the cutter is an indispensable part of shield engineering.
The cutters of shield machine are classified according to their working mode, position and shape. The common classification methods are working principle, mainly cutting cutter and hob. Cutting cutter is divided into gear cutter, scraper, advance cutter and so on. According to different shield design and soil conditions, customers need to choose different cutters for investment. For example, when geology is hard rock, hobs are usually used in conjunction with soft rock. When geology is soft rock, gear cutters are generally used. Cutters can be selected to work between the two.
Principles of Hobs and Cutters
The cutting principle of hob is mainly that the cutter relies on extrusion to break rock, which is generally used in the excavation of rock tunnel. Hob cutters can also be used when there are large size gravels (diameter greater than 400 mm) passing through loose strata and the content reaches a certain proportion. In addition, the hob cutter may be used in the complex shield machine when the geological conditions of the tunnel are complex and changeable, and the rock (strength is not too high) interlaces with the general soil (or clay or sand) frequently.
The cutting principle of cutting cutter is that while the shield machine advances forward, the cutter rotates with the cutter disc to produce axial (along the tunnel direction) shear force and radial (cutter disc rotation tangent direction) cutting force on the excavation surface soil, which continuously cuts down the soil in front of the excavation. Cutting cutters are generally suitable for loose strata such as sand, pebble, sand and clay with particle size less than 400 mm.

2. Selection of cutter head materials
Shield cutting tools generally consist of two or three kinds of materials, namely, matrix materials, cemented carbide materials, tool edge surfacing materials and so on. The following is a case study of Herrick’s cutter. The cutter body of its peripheral cutter is made of steel St52-3 made in Germany. Its performance is equivalent to that of domestic low carbon steel Q235. The material has good toughness but low hardness. The edge of the peripheral cutter mainly bears abrasive wear with high stress, and its face-to-face brazing has spherical teeth. The spherical teeth are made of cemented carbide, which are mainly used to bear abrasive wear and impact of gravel and pebble. The knife is made of cemented carbide steel and high wear-resistant alloy.

3. Tool Failure Form
The main failure modes of cutting tools are as follows:
Tool wear: 80% of tool replacement.
Ring eccentric grinding: Hobs do not rotate in the process of excavation, and hobs in the central area are prone to this phenomenon.
Ring breakage: Excessive thrust or rolling of the ring on hard rock may occur, which often occurs in edge hobs and front hobs.
Surface defect: metal or other hard material damages the local surface of the tool ring.
Ring retainer wear or fall off: Once the ring retainer falls off or the weld is worn seriously, the tool should be replaced.
Hob Oil Leakage: Sealing oil may leak due to damage of seals, resulting in damage to oil seats and hub.
Hub wear: Cutter disc and dregs are continuously friction, which makes the hob hub wear.
Oil Seal Damage: Oil leakage.

4. Cause analysis of tool wear
Tool wear can be divided into the following categories:
Adhesive wear: Its appearance is pitted; the main reason is the interaction of materials. The process of adhesive wear is actually the formation and separation of atomic bonds between two friction pairs.
Abrasive wear: abrasion, grooves, streaks, etc. The main reason is that hard particles or hard protrusions cause friction surface damage.
Fatigue wear: cracks and pitting; material surface is subjected to cyclic contact stress, resulting in repeated deformation, leading to crack and particle separation
Tribochemical wear: reaction products, membranes, particles, etc. appear on the surface; chemical or electrochemical reactions occur under the action of friction
Fretting wear: abrasion of debris; superimposed by the above four mechanisms