Controllable Hardfacing Repair of Shield Tunneling Machine Drive Shell Using Flux-Cored Wire

Literature Overview

This technical case study, published in China Surface Engineering (Vol. 23, No. 3, 2010, p. F0003), presents an application of surface engineering technology to the repair of a shield tunneling machine (TBM) drive shell. The drive shell is a critical structural component of the TBM, fabricated from cast steel, and is subject to severe wear during tunneling operations. The paper highlights that TBMs are high-value assets — priced between 30 to 80 million RMB — and that failure of the drive shell due to wear or other damage can result in complete equipment scrapping. The repair approach employs flux-cored wire arc welding (FCAW) with a controllable hardfacing process to restore the worn areas of the drive shell to serviceable condition.

Technical Context and Challenge Analysis

Shield tunneling machines are used for underground tunnel construction in urban transit, water supply, and transportation infrastructure projects. The drive shell (also referred to as the cutterhead support ring or front shield) is the component that supports the cutting tools and transmits the thrust force from the hydraulic jacks to the tunnel face. During operation, the drive shell is exposed to:

Abrasive wear from soil particles, rock fragments, and gravel in the tunnel face.
Impact loading from rocks and hard inclusions in the soil profile.
Corrosive attack from groundwater containing dissolved salts, sulfates, and other aggressive species.
Thermal cycling from the heat generated by cutting operations and the ambient temperature variations in the tunnel environment.

The drive shell is typically fabricated from a high-strength cast steel, such as ZG270-500 or equivalent, which provides adequate strength and castability but limited wear resistance. When the drive shell develops significant wear — particularly in areas where cutting tools are mounted or where the shell contacts the tunnel lining — the remaining material thickness may fall below the minimum acceptable limit, compromising structural integrity and safety.

The Controllable Hardfacing Approach

The term "controllable" in the context of this repair application is significant. It implies that the hardfacing process is designed to achieve specific targets for:

Overlay thickness: The hardfacing layer must be thick enough to restore the wear allowance but not so thick as to interfere with the geometry of the drive shell or the mounting of cutting tools.
Overlay composition: The hardfacing alloy must provide adequate wear resistance while maintaining compatibility with the cast steel base metal.
Overlay hardness profile: The hardness should be sufficiently high to resist wear but not so high as to cause excessive brittleness or cracking during subsequent machining or service.
Stress state: The hardfacing process must be designed to minimize residual stresses and distortion, preserving the geometric accuracy of the drive shell.

The flux-cored wire approach offers several advantages for this application:

High deposition rate: FCAW with flux-cored wire can deposit material at rates of 5–15 kg/h, significantly faster than manual arc welding methods. This is important for large-area repairs where productivity is a key consideration.
Good process flexibility: FCAW can be performed in various positions, including overhead and vertical, which is essential for repairing a large, complex geometry like a TBM drive shell.
Alloy control: The flux core can be designed to incorporate specific alloying elements to tailor the hardfacing composition for the required wear resistance and toughness balance.
Reduced spatter: Compared to GMAW with solid wire, FCAW produces less spatter, which is advantageous for maintaining the cleanliness of the surrounding components.

Repair Process Design

The repair process for the TBM drive shell typically involves the following steps:

Inspection and assessment: The worn areas are inspected using ultrasonic thickness measurement (UT) to determine the remaining material thickness. The extent of wear, the location of critical areas, and the required repair dimensions are documented.
Surface preparation: The worn areas are ground or machined to remove damaged material, oxide scale, and any contaminants. A proper weld groove is prepared to ensure adequate fusion between the hardfacing overlay and the base metal.
Preheating: The cast steel drive shell is preheated to reduce the risk of hydrogen-induced cracking. The preheat temperature is typically 150–250 °C, depending on the carbon equivalent of the base metal and the thickness of the component.
Transition layer deposition: A transition layer with a composition compatible with both the base metal and the hardfacing alloy is deposited. This layer reduces the dilution mismatch and minimizes the risk of cracking at the interface.
Hardfacing overlay deposition: The hardfacing alloy is deposited in multiple passes to achieve the required overlay thickness. The welding parameters are controlled to maintain a consistent heat input and dilution rate.
Post-weld stress relief: The repaired area is stress-relieved by post-weld heat treatment (PWHT) to reduce residual stresses and minimize the risk of delayed cracking.
Machining and finishing: The overlay surface is machined to the required geometry and dimensional accuracy. The final surface finish is inspected for any defects.
Quality verification: The repaired area is inspected using non-destructive testing methods (MT, PT, UT) and mechanical property testing (hardness, impact) to confirm that the repair meets the acceptance criteria.

Key Process Parameters

Parameter	Typical Value	Purpose
Preheat temperature	150–250 °C	Reduce hydrogen-induced cracking risk
Interpass temperature	≤ 250 °C	Control grain growth and residual stress
Heat input	15–30 kJ/mm	Balance dilution and microstructure
Wire diameter	1.2–1.6 mm	Control deposition rate and penetration
Travel speed	300–600 mm/min	Control heat input and bead profile
Shielding gas (if used)	CO₂ or Ar/CO₂ mixture	Stabilize arc and reduce oxidation
Number of hardfacing passes	2–4	Achieve required overlay thickness
Overlay thickness	5–15 mm	Restore wear allowance

Engineering Practice and Reuse Economy

The repair of TBM drive shells is not merely a technical exercise — it is an economic necessity. Given the high cost of TBM equipment, scrapping a drive shell due to wear would represent a significant financial loss. The surface engineering repair approach extends the service life of the component, reduces the need for new material, and supports the broader goals of energy conservation and emission reduction in the transportation infrastructure sector. This aligns with the principles of remanufacturing and circular economy, which are increasingly important in the Chinese heavy equipment industry.

The controllable hardfacing approach also addresses a practical challenge: the geometry of the drive shell is complex, with curved surfaces, mounting bosses for cutting tools, and bolted connections to the main TBM body. The hardfacing repair must restore the original geometry without introducing distortion that would affect the alignment of cutting tools or the sealing interface with the tunnel lining. This requires careful planning of the welding sequence, with symmetric deposition patterns and controlled heat input to minimize distortion.

Quality Assurance and Acceptance Criteria

The quality of the hardfacing repair is verified through a combination of non-destructive and destructive testing methods:

Visual inspection (VT): The overlay surface is inspected for porosity, undercut, cracks, and other surface defects.
Magnetic particle inspection (MT) or liquid penetrant inspection (PT): Used to detect surface and near-surface cracks, particularly at the weld toe and along the overlay boundaries.
Ultrasonic testing (UT): Used to detect internal defects such as lack of fusion, slag inclusion, and internal porosity.
Hardness testing: The hardness of the overlay is measured at multiple locations to ensure uniformity and to verify that the hardness meets the specification for the hardfacing alloy.
Metallographic examination: Cross-sections of the overlay are examined to verify the microstructure, dilution profile, and absence of internal defects.
Impact testing: If required by the specification, Charpy V-notch impact testing may be performed on coupon specimens welded under the same conditions as the repair.

Summary

This case study illustrates the practical application of controllable flux-cored wire hardfacing to the repair of a high-value TBM drive shell, demonstrating that surface engineering technology can extend the service life of critical equipment components and reduce the economic and environmental impact of equipment replacement. The key technical elements — preheat control, transition layer design, multi-pass overlay deposition, and post-weld stress relief — are well-established practices in surface engineering, but their application to a complex, large-scale component like a TBM drive shell requires careful process planning and rigorous quality control. For engineers involved in heavy equipment repair and remanufacturing, this case study provides a clear example of how surface engineering can be integrated into maintenance strategies to achieve significant cost savings and operational reliability improvements. The principles of controllable hardfacing — precise control of overlay thickness, composition, and stress state — are applicable to a wide range of repair scenarios in the mining, construction, and transportation sectors.