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

Literature Overview

This paper by Liu Chengjie, Qiu Yaling, Song Zhenhua, Zhuang Jia, and Liu Qingyou from Southwest Petroleum University and Chengdu General Machinery Factory of Sichuan Petroleum Administration addresses a critical tribological challenge in oilfield equipment: the failure of tungsten carbide hardfacing layers on shoe shoes (shoe shoes, or shoe shoes used in well completion operations). Published in the Journal of Southwest Petroleum University (Volume 29, Issue 6, 2007, pages 145-148), the study investigates the strengthening of cast WC/iron-based composite hardfacing layers through heat treatment. The work was supported by the Ministry of Education Key Laboratory for Oil and Natural Gas Equipment (Project JZTZ-0501) and directly addresses field failure modes including fracture, wear, and spalling of hardfacing layers.

Failure Analysis and Material Design

The authors begin with a systematic analysis of failure modes observed in service. The three primary failure mechanisms identified are: fracture of the hardfacing layer, abrasive wear, and spalling (delamination) of the tungsten carbide particles from the matrix. This failure taxonomy is consistent with the FMEA (Failure Mode and Effects Analysis) approach and provides a clear framework for understanding the design requirements for improved hardfacing materials.

Electrode Development

Four types of hardfacing electrodes were self-developed for this study. The critical design parameter identified is the weight ratio of tungsten carbide (WC) to the electrode sheath (iron-based binder alloy). This ratio directly influences both the impact wear resistance and the tendency for WC particle spalling. An optimal balance must be achieved: too low a WC content results in insufficient hardness and wear resistance, while too high a WC content leads to poor bonding between the carbide particles and the matrix, promoting spalling under impact loading.

Synergistic Mechanism of Wear Resistance

The study identifies a dual-mechanism model for impact wear resistance improvement:

1. Shadow effect of hard phases: The hard WC particles shield the softer matrix from direct contact with the counterface, reducing matrix wear.
2. Support effect of binder phase: The ductile iron-based binder phase supports and retains the hard WC particles, preventing their dislodgement under impact loading.

The interaction between these two mechanisms is the key to achieving high impact wear resistance. This insight is particularly valuable for engineering design, as it suggests that optimizing the WC/binder ratio is more effective than simply maximizing the WC content.

Heat Treatment Strengthening Results

The study demonstrates significant performance improvements through post-weld heat treatment of the cast tungsten carbide hardfacing layers.

The cryogenic treatment (typically involving cooling to -78°C to -196°C) provides a substantially greater improvement than conventional quenching alone. This is a significant finding with direct practical implications for production processes.

Microstructural Analysis

The microstructure of the hardfacing layer consists of the eta phase (η, a Co-W or Fe-W based carbide), M6C, M23C6, and M7C3 carbide phases. Importantly, the cryogenic treatment does not significantly alter the hard phase morphology or distribution. The improvement in wear resistance is attributed to changes in the binder phase composition, morphology, and distribution. This is a critical distinction: the strengthening mechanism operates through modification of the ductile matrix rather than through changes in the hard carbide particles.

The binder phase modification likely involves:

• Transformation of retained austenite to martensite during cryogenic cooling, increasing matrix hardness
• Refinement of the binder phase grain structure
• Improved bonding between the binder phase and WC particles due to differential thermal contraction during cryogenic treatment

Engineering Practice Applications

Process Optimization

The findings from this study suggest a clear process improvement pathway for shoe shoe hardfacing:

1. Select electrodes with an optimized WC/sheet weight ratio to balance hardness and bonding strength
2. Apply quenching followed by cryogenic treatment to the completed hardfacing layer
3. Monitor the microstructure evolution through metallographic examination to verify the binder phase transformation

The 43% improvement in impact wear resistance achieved through cryogenic treatment represents a substantial extension of component service life, potentially reducing the frequency of shoe shoe replacement and associated downtime costs.

Quality Control Considerations

For production implementation, the following quality control measures are recommended:

• Verify the WC/binder ratio in the electrode composition through chemical analysis
• Document the heat treatment cycle (quenching medium, cryogenic temperature, holding time, tempering temperature)
• Measure hardness profiles across the hardfacing layer thickness to confirm uniform hardening
• Perform impact wear testing on coupon samples to validate performance improvement

The cryogenic treatment adds a step to the production process, but the 43% improvement in wear resistance likely justifies the additional cost and processing time for critical applications. The process should be validated through a pilot production run before full-scale implementation.

Study Insights and Reflections

This study exemplifies the value of combining failure analysis, material design, and post-processing optimization to solve practical engineering problems. The identification of the dual shadow/support mechanism provides a rational basis for electrode design, while the cryogenic treatment results demonstrate that post-weld heat treatment can significantly enhance the performance of cast composite hardfacing layers. The finding that cryogenic treatment improves wear resistance through binder phase modification rather than hard phase alteration is a nuanced insight that deepens our understanding of the tribological behavior of composite hardfacing materials. For oilfield equipment manufacturers, this research provides a clear pathway to improving the reliability and service life of shoe shoes, with the cryogenic treatment offering a relatively simple and cost-effective solution to a persistent field problem.