Development of Flexible Strip Overlay Welding Materials for Precision Internal Surface Applications

Literature Overview

This 2010 paper published in Welding by Song Dan, Li Deyuan, Liu Xiaoshu, and Peng Yang from Shenyang University of Technology and Shenyang Bolait Welding Materials Co., Ltd., funded by the Liaoning Provincial Department of Education Science and Technology Fund, reports on the development of a flexible strip-form hardfacing material designed for overlay welding on thin-walled pipe internals and other precision internal surfaces. This research addresses a specific practical challenge in the welding industry: the difficulty of applying hardfacing overlays to internal surfaces of small-diameter pipes and complex geometries where conventional electrode or wire feeding methods are impractical or impossible.

Core Technical Content

The Engineering Problem

Conventional hardfacing methods face significant limitations when applied to the internal surfaces of thin-walled pipes and small-diameter tubing. The challenges include:

Access limitations: Standard welding electrodes and filler wires cannot be easily positioned on internal surfaces of small-diameter pipes
Heat input control: Thin walls are susceptible to distortion and burn-through under conventional hardfacing heat inputs
Deposition quality: Achieving uniform, dense overlays on curved internal surfaces with standard methods is difficult
Multi-pass capability: Building up sufficient overlay thickness on confined internal surfaces requires materials that can be pre-positioned

These challenges are particularly acute in applications such as chemical processing piping, heat exchanger tubes, and hydraulic system components where internal surface protection is critical for performance and longevity.

Flexible Strip Material Development

The researchers developed a flexible strip-form hardfacing material by combining Ni60 alloy powder and WC (tungsten carbide) particles with various adhesive binders to form a pliable strip that can be pre-formed to fit the internal geometry of the pipe before welding. The flexibility of the strip allows it to conform to curved surfaces, and the pre-positioning capability eliminates the need for complex manipulator systems during welding.

Adhesive Binder Evaluation

A key aspect of the research was the systematic evaluation of different adhesive binder systems to optimize the strip's formability, handling characteristics, and welding performance. The researchers tested multiple adhesive systems including silicone-based binders and other polymer adhesives, evaluating their influence on:

Adhesive Property	Impact on Performance
Flexibility	Determines ability to conform to pipe internal surfaces
Thermal stability	Affects strip integrity during pre-heating and welding
Bonding strength	Ensures strip remains in position during welding
Residue formation	Influences weld quality and inclusion content
Curing characteristics	Affects production efficiency and shelf life

The silicone-based adhesive was identified as providing the optimal balance of flexibility, thermal stability, and weld quality, making it the preferred binder system for the final product.

Manufacturing Process

The flexible strip was manufactured through the following process:

Mixing of Ni60 alloy powder and WC particles with the selected adhesive binder
Forming into strip geometry using a calendering or casting process
Drying and curing to achieve the desired mechanical properties
Quality inspection for dimensional accuracy and material uniformity
Packaging and storage under controlled conditions

The manufacturing process is relatively straightforward and does not require specialized equipment beyond standard powder metallurgy and adhesive processing facilities, making it economically viable for commercial production.

Welding Application and Performance

The flexible strips were applied using TIG (GTAW) welding, which provides the low heat input necessary for thin-walled pipe applications. The welding process involves:

Pre-positioning the flexible strip on the internal surface of the pipe
Applying TIG welding to melt the strip material and fuse it to the substrate
Sequential application of multiple strips to build up the required overlay thickness
Inspection of the completed overlay for quality and thickness uniformity

The resulting overlay layers exhibited good metallurgical bonding to the substrate with acceptable hardness levels and microstructural characteristics consistent with Ni60-WC composite hardfacing deposits.

Microstructural and Mechanical Characterization

Overlay Microstructure

Metallographic examination of the overlay layers revealed a microstructure characteristic of Ni60-WC composite hardfacing, consisting of:

A nickel-based solid solution matrix (Ni, Cr, Fe, Si)
Hard WC and W2C carbide particles distributed throughout the matrix
Possible formation of Fe2W4C or other transition carbides depending on the welding thermal cycle
Good bonding at the overlay-substrate interface with no significant dilution

The microstructure quality was found to be dependent on the adhesive binder selection, with the silicone-based system producing the cleanest interfaces and fewest inclusions.

Hardness Distribution

The hardness of the overlay layers was measured using microhardness testing, with values expected to be in the range of HV 900-1100 for the Ni60-WC composite, significantly exceeding the substrate hardness. The hardness distribution across the overlay thickness was evaluated to ensure uniform performance throughout the deposit.

Engineering Practice Integration

Application Scenarios

The flexible strip overlay material is particularly suited for the following applications:

Chemical processing piping: Internal protection of small-diameter pipes carrying corrosive or erosive fluids
Heat exchanger tubes: Hardfacing of tube internals to resist erosion from flowing fluids
Hydraulic system components: Protection of cylinder bores and valve bodies from wear
Aerospace fuel system piping: Internal protection of thin-walled tubing in aggressive fuel environments
Medical device components: Surface modification of small-diameter tubing in implantable devices
Oil and gas industry: Protection of downhole tool internals and tubing connections

Comparison with Conventional Methods

Method	Applicable Diameter	Heat Input Control	Deposition Quality	Labor Intensity	Cost
Conventional electrode	>50 mm	Poor	Variable	High	Low
Cored wire	>25 mm	Moderate	Good	Moderate	Moderate
Submerged arc	>100 mm	Poor	Good	Low	Low
Flexible strip	>10 mm	Excellent	Good	Moderate	Moderate
Plasma spray	Any	Excellent	Variable	Low	High

The flexible strip method offers a unique combination of applicability to small diameters, excellent heat input control, and acceptable deposition quality that makes it competitive with more expensive spray methods for many applications.

Process Development Considerations

For successful engineering implementation, several process development activities are required:

Welding procedure qualification: Formal qualification of the TIG welding process with the flexible strip material for each specific application
Strip positioning fixtures: Development of simple fixtures to hold strips in position during welding, particularly for multi-pass builds
Inspection protocols: Establishment of NDT methods suitable for internal surface inspection (e.g., borescope examination, eddy current testing)
Training programs: Development of welder training materials specific to the flexible strip application technique
Quality control: Implementation of in-process and post-weld quality checks to ensure consistent overlay performance

FMEA for Flexible Strip Application

Potential Failure	Severity	Occurrence	Detection	RPN	Prevention Strategy
Strip detachment during welding	8	4	5	160	Optimized adhesive system, fixture support
Incomplete melting of strip material	7	6	4	168	Welding parameter optimization, pre-heat if needed
Adhesive residue inclusions	6	5	7	210	Binder selection, temperature control
Uneven overlay thickness	7	5	6	210	Fixture design, welder training
Cracking from thermal stress	8	3	6	144	Low heat input, interpass temperature control

Study Insights and Reflections

Innovation in Material Form Factor

The fundamental innovation of this research is the recognition that the form factor of the hardfacing material can be engineered to solve access and application challenges that cannot be addressed by composition optimization alone. By developing a flexible strip form, the researchers have created a material that can be pre-shaped to fit complex geometries, fundamentally changing the application paradigm for hardfacing in confined spaces. This approach is analogous to the evolution from stick electrodes to cored wires to flux-cored wires in welding technology, where each form factor innovation opened new application possibilities.

The Role of Adhesive Technology

The systematic evaluation of adhesive binders highlights an often-overlooked aspect of welding material development. The binder is not merely a processing aid but is a critical component that influences the final weld quality through its thermal decomposition behavior, residue formation, and interaction with the molten weld pool. The identification of silicone-based adhesives as the optimal system provides a practical solution, but further research into adhesive chemistry could yield even better performance through tailored thermal decomposition profiles that minimize residue formation.

Scalability and Commercial Viability

From a commercial perspective, the flexible strip approach offers several advantages:

Manufacturing simplicity: The production process uses standard powder metallurgy and adhesive processing equipment
Customization capability: Strips can be manufactured in various widths, thicknesses, and compositions to suit specific applications
Storage and handling: Flexible strips are easy to store, ship, and handle compared to rigid forms
Waste minimization: Strips can be cut to exact lengths, minimizing material waste
Skill requirement: The application technique requires standard TIG welding skills, with minimal additional training

Limitations and Development Opportunities

Several limitations of the current technology present opportunities for further development:

Production rate: The manual positioning and welding of individual strips limits throughput compared to automated methods
Thickness limitation: Building up thick overlays requires multiple strip applications, which increases labor time
Binder residue: Even with optimized binders, some residue may remain in the weld, potentially affecting performance in critical applications
Temperature sensitivity: The adhesive system has a limited temperature range for storage and handling
Cost premium: The additional processing steps increase material cost compared to conventional hardfacing electrodes

Future development could address these limitations through automated strip placement systems, improved binder chemistry with lower residue formation, and the development of thicker single-pass strips that reduce the number of applications required.

Conclusion

This research presents a practical and innovative solution to the challenge of applying hardfacing overlays to internal surfaces of thin-walled pipes and confined geometries. The development of a flexible Ni60-WC strip material with silicone-based adhesive binder, applied using TIG welding, provides a viable alternative to more complex and expensive methods such as plasma spraying or specialized equipment. The systematic evaluation of adhesive systems and the demonstration of acceptable overlay quality establish the technical feasibility of the approach. For engineers facing hardfacing challenges in small-diameter or internal surface applications, this research provides a practical technology option that balances performance, cost, and applicability. The key insight is that material form factor innovation can unlock application possibilities that composition optimization alone cannot achieve, and this principle has broad applicability to other welding material development challenges.