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

Literature Overview

This research 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 Bureau, published in the Journal of Southwest Petroleum University (Natural Science Edition, 2007, Vol. 29, No. 6, pp. 145–148), addresses a critical wear protection challenge in drilling equipment. The shoe shoe (or shoe shoe) is a bottom-hole drilling tool component that directly contacts formation rock during drilling operations and is subject to severe impact wear. The study investigates how post-weld heat treatment—specifically quenching and cryogenic treatment—can enhance the impact wear resistance of tungsten carbide (WC) hardfacing layers on shoe shoes.

The work was supported by the Ministry of Education Key Laboratory project on Oil and Natural Gas Equipment (JZTZ-0501), reflecting the practical engineering significance of this research to the petroleum industry.

Core Technical Findings

Failure Modes of WC Hardfacing Layers

The authors identified three primary failure modes of conventional WC hardfacing layers on shoe shoes:

1. Fracture of WC particles: Brittle tungsten carbide particles fracture under high-impact loading, creating micro-cracks that propagate through the matrix
2. Abrasive wear: Progressive material removal from the hardfacing surface due to sliding contact with formation rock
3. Spalling/delamination: Bulk removal of hardfacing material due to fatigue cracking at the interface between the hard carbide phase and the binder phase

These failure modes are interrelated. Fracture of WC particles initiates micro-cracks, which under continued impact loading lead to spalling. Understanding this failure mechanism is essential for developing effective strengthening strategies.

Effect of WC-to-Binder Weight Ratio

The authors developed four proprietary welding electrodes with varying WC-to-binder (pipe skin) weight ratios. The experimental results demonstrate that this ratio directly governs the impact wear resistance and spalling tendency of the hardfacing layer.

The optimal WC-to-binder ratio represents a balance between hardness (provided by WC) and toughness (provided by the binder matrix). Too little WC results in insufficient hardness and rapid abrasive wear. Too much WC creates a brittle composite that is prone to spalling under impact. The optimal ratio achieves the best combination of hardness and toughness, with the WC particles being adequately supported by the binder phase.

Shadow Effect and Support Effect Mechanism

The authors propose a dual-mechanism model for understanding impact wear resistance:

• Shadow effect of hard phase: The WC particles act as a shield, protecting the underlying binder matrix from direct impact and abrasion. The shadow effect refers to the geometric shielding provided by the hard phase particles against the erosive medium.
• Support effect of binder phase: The binder matrix provides mechanical support to the WC particles, preventing their fracture and dislodgement. The binder phase absorbs impact energy through plastic deformation.

The interaction between these two effects is the fundamental mechanism for enhanced impact wear resistance. When the WC-to-binder ratio is optimal, the shadow effect is maximized (sufficient WC coverage) while the support effect is maintained (adequate binder volume).

Effect of Heat Treatment

The most significant finding of this study is the dramatic improvement in impact wear resistance achieved through post-weld heat treatment:

Microstructural Analysis

The microstructure of the hardfacing layer after cryogenic treatment consists of the following phases:

• η phase (η-Co3W, or in iron-based systems, η-Fe3W): A hard intermetallic compound formed during solidification
• M6C carbide (e.g., Fe3W3C): A stable carbide phase providing hardness
• M23C6 carbide: Chromium-rich carbide contributing to wear resistance
• M7C3 carbide: A common transition carbide providing moderate hardness

Importantly, the authors found that cryogenic treatment has minimal effect on the hard phase (WC particles and carbides). The improvement in impact wear resistance is attributed to changes in the binder phase composition, morphology, and distribution.

Heat Treatment Process Analysis

Quenching Treatment

Quenching of the WC hardfacing layer involves rapid cooling from an elevated temperature (typically above the austenitization temperature of the binder phase) to room temperature or below. The primary effects are:

1. Transformation of the binder phase from ferrite/austenite to martensite
2. Increased hardness of the binder phase
3. Improved mechanical support for WC particles
4. Potential residual stress development (which may be detrimental if excessive)

Cryogenic Treatment

Cryogenic treatment involves cooling the quenched component to liquid nitrogen temperature (−196°C) and holding for a specified duration. The effects include:

1. Transformation of retained austenite to martensite
2. Precipitation of fine carbides from supersaturated martensite
3. Refinement of the binder phase microstructure
4. Improved dimensional stability of the component

The cryogenic treatment is particularly effective for WC hardfacing layers because:

• The binder phase contains sufficient carbon and alloying elements to form additional fine carbides at low temperature
• The retained austenite in the as-quenched condition is transformed to hard martensite
• The fine carbides formed during cryogenic treatment provide additional hard points that enhance wear resistance
• The overall microstructure becomes more uniform and homogeneous

Engineering Practice Integration

For drilling equipment manufacturers and maintenance engineers, this study provides actionable guidance:

Electrode Selection Guidelines

• For high-impact, low-abrasion service: Use electrodes with moderate WC content and prioritize toughness
• For high-abrasion, low-impact service: Use electrodes with higher WC content and accept some brittleness
• For combined impact-abrasion service: Use the optimal ratio identified in the study and apply cryogenic treatment

Recommended Heat Treatment Cycle

1. Quenching: Heat to 850–950°C (depending on binder composition), hold 1–2 hours, quench in oil or air
2. Cryogenic treatment: Cool to −196°C, hold 2–4 hours, warm to room temperature
3. Tempering: Heat to 200–300°C for 1–2 hours to relieve residual stresses (if required)

Quality Control Considerations

Key Questions and Reflections

The study raises several important questions for further investigation. First, the long-term stability of the cryogenically treated hardfacing layer under cyclic thermal loading (as encountered in deep drilling with high bottom-hole temperatures) is not addressed. Second, the effect of cryogenic treatment on the residual stress state of the hardfacing layer and its implications for spalling resistance under fatigue loading requires further study. Third, the scalability of cryogenic treatment for large-diameter shoe shoes presents practical challenges in terms of cryogenic vessel size and treatment time.

From a standards perspective, cryogenic treatment of hardfacing layers is not yet widely codified in international standards. Engineers applying this technology should document the treatment process, verify the resulting microstructure and properties, and conduct wear testing under representative service conditions before full-scale deployment.

Study Insights and Implications

This study demonstrates that post-weld heat treatment, particularly cryogenic treatment, can dramatically improve the impact wear resistance of WC hardfacing layers without altering the hard phase microstructure. The 43% improvement in impact wear resistance through quenching plus cryogenic treatment is a significant engineering benefit that can translate to extended component life and reduced downtime. For drilling equipment engineers, the key recommendations are: (1) optimize the WC-to-binder ratio in the welding electrode for the specific service condition; (2) apply cryogenic treatment as a standard post-weld process for critical impact-wear applications; and (3) validate the treatment effectiveness through wear testing before production deployment. The shadow effect and support effect model provides a useful framework for understanding and optimizing hardfacing layer design for combined wear environments.