Strengthening Technology for Cast Tungsten Carbide Iron-Based Composite Overlay on Shoe Shoes

Literature Overview

This research by Liu Chengjie, Qiu Yaling, Song Zhenhua, Zhuang Jia, and Liu Qingyou, published in the Journal of Southwest Petroleum University (2007, Vol. 29, No. 6, pp. 145-148), addresses the strengthening of cast tungsten carbide (WC) iron-based composite overlay layers on shoe shoes, a critical component in oil and gas drilling operations. The study was supported by the Ministry of Education Key Laboratory for Oil and Natural Gas Equipment (Project JZTZ-0501). The work directly tackles the failure modes of hard alloy overlay layers on shoe shoes, which include fracture, wear, and spalling under the severe impact loading conditions encountered during drilling operations.

Failure Analysis and Material Design

The study begins with a systematic failure analysis of existing WC hard alloy overlay layers on shoe shoes. Three primary failure modes were identified: (1) fracture of the hard phase under impact loading, (2) progressive abrasive wear of the overlay surface, and (3) spalling or delamination of the WC particles from the matrix. These failure modes indicate that the overlay layer design must balance hardness for wear resistance with toughness for impact resistance, a classic trade-off in tribological material selection.

Four proprietary welding electrodes were developed and tested, each with different WC-to-cladding mass ratios. The critical finding is that the WC-to-cladding weight ratio directly influences both the impact wear resistance and the tendency for WC particle spalling. This ratio represents a fundamental design parameter that must be optimized for the specific service conditions of the shoe shoe application.

WC Content Optimization and Performance

The concept of the "shadow effect" of hard phases and the "support effect" of the matrix and bonding phases on hard phases is introduced as the mechanism for improved impact wear resistance. When the WC-to-cladding ratio is appropriate, the hard WC particles are effectively supported by the ductile matrix, preventing particle fracture and spalling while maintaining the hardness needed for wear resistance.

Heat Treatment Strengthening

The study investigates two post-weld heat treatment approaches: quenching alone and quenching combined with cryogenic treatment. The results demonstrate significant improvements in impact wear resistance:

The microstructure of the overlay layer after cryogenic treatment remains η + M6C + M23C6 + M7C3, indicating that the cryogenic treatment does not significantly alter the hard phase composition. Instead, the improvement in impact wear resistance is attributed to changes in the bonding phase composition, morphology, and distribution. The cryogenic treatment promotes the transformation of retained austenite to martensite, increases the volume fraction of fine carbides, and modifies the microstructure of the matrix phase, collectively enhancing the overlay's ability to resist impact damage.

Microstructural Analysis of the Bonding Phase

The bonding phase in the WC-iron-based composite overlay is primarily a martensitic matrix with retained austenite. During cryogenic treatment at temperatures as low as -196°C, the following transformations occur:

1. Retained austenite undergoes martensitic transformation, releasing additional volume expansion that creates compressive residual stresses in the overlay.
2. Fine secondary carbides precipitate from the austenite, providing additional hardening.
3. The martensite lath structure refines, improving the toughness of the matrix.
4. The interface between WC particles and the matrix becomes more coherent due to the reduced thermal mismatch after transformation.

Process Parameters and Engineering Considerations

The welding process for depositing WC-iron-based composite overlays on shoe shoes typically employs submerged arc welding (SAW) or shielded metal arc welding (SMAW) with specialized flux-cored or solid electrodes containing WC particles. The welding parameters must be carefully controlled to minimize WC particle dissolution and agglomeration while ensuring adequate fusion with the base metal.

Typical process parameters for WC overlay welding include:

• Arc current: 300-450 A (SAW) or 150-250 A (SMAW)
• Arc voltage: 25-35 V
• Travel speed: 200-400 mm/min
• Preheat temperature: 200-350°C (depending on base material)
• Interpass temperature: 200-300°C
• Post-weld heat treatment: Quench in oil or air cool, followed by cryogenic treatment at -196°C for 4-8 hours

The interpass temperature control is critical to prevent excessive grain growth and to maintain the fine microstructure necessary for good impact wear resistance. The cryogenic treatment duration and temperature should be optimized for the specific overlay composition and thickness, with longer durations generally providing more complete austenite transformation.

Engineering Practice Integration

In drilling operations, shoe shoes are subjected to extreme impact loading from formation contact, high contact stresses from weight on bit, and abrasive wear from formation cuttings. The overlay layer must provide both wear resistance and impact toughness to survive these conditions. The cryogenic treatment approach offers a practical solution that does not require complex equipment beyond a liquid nitrogen or dry ice cooling system.

From a quality control perspective, the following inspection procedures should be implemented:

• Visual inspection for surface defects, porosity, and incomplete fusion
• Hardness testing (HV or HRC) at multiple locations across the overlay
• Microstructural examination of cross-sections to verify WC particle distribution
• Impact wear testing per standardized procedures to validate performance
• Spalling resistance testing to confirm adequate bonding strength

Key Questions and Reflections

The study raises important questions about the long-term stability of the cryogenic treatment benefits during actual service. While laboratory testing demonstrates significant improvements, the thermal cycling and mechanical loading experienced in the downhole environment may partially reverse the beneficial effects of cryogenic treatment. Additionally, the interaction between the overlay layer and the base shoe shoe material under sustained loading conditions warrants further investigation.

The development of proprietary welding electrodes represents a significant practical contribution, as the availability of appropriate consumables is often a limiting factor in implementing overlay technologies in field conditions. The optimization of WC-to-cladding ratio provides a clear design guideline that can be incorporated into electrode selection criteria for specific service applications.

Study Insights and Implications

This research demonstrates that cryogenic treatment is an effective and practical post-weld strengthening method for WC-iron-based composite overlay layers on shoe shoes. The 43.0% improvement in impact wear resistance achieved through quenching plus cryogenic treatment represents a substantial enhancement that can significantly extend the service life of shoe shoes in drilling operations. The mechanism-based understanding of how the bonding phase modifications contribute to improved performance provides a foundation for further optimization of the treatment process.

For engineering practice, the key implications are: (1) the WC-to-cladding ratio must be carefully optimized for the specific service conditions, (2) cryogenic treatment should be incorporated into the standard post-weld treatment procedure for WC overlay applications, and (3) the four proprietary electrode formulations provide practical options for different service severity levels. This work contributes to the ongoing development of surface engineering solutions for oil and gas drilling equipment, where the cost of component failure and downtime far exceeds the incremental cost of advanced overlay and heat treatment technologies.