Failure Analysis of Mill Shoes and Development of New Overlay Welding Materials

Literature Overview

This comprehensive failure analysis and materials development paper by Wang Xinhong, Zou Zengda, and Chen Xingquan (2001), published in Petroleum Machinery (Vol. 29, No. 9, pp. 41-43), investigates the failure mechanisms of mill shoes (reamer shoes) used in oil well downhole operations and develops new overlay welding materials to address the identified failure modes. The study combines detailed failure analysis with materials engineering to develop improved solutions.

Service Environment and Failure Modes

Operating Conditions

Mill shoes are downhole milling tools used to remove well debris, stuck pipe, and formation obstructions. They operate under extremely harsh conditions:

Failure Mode Classification

The analysis of eight failed mill shoes revealed four primary failure modes:

Detailed Failure Analysis Results

The analytical techniques employed included:

• Metallographic microscopy: Revealed microstructural degradation in the overlay layer
• Scanning electron microscopy (SEM): Identified fracture surfaces and wear mechanisms
• Electron probe microanalysis (EPMA): Mapped elemental distribution at failure sites
• Microhardness measurement: Quantified hardness profile through the overlay layer
• X-ray diffraction (XRD): Identified phase transformations and oxidation products

The SEM fractography showed that hard alloy fracture was predominantly transgranular with some intergranular features, indicating that both impact loading and microstructural weakness contributed to the failure. The overall layer spalling was associated with interface debonding caused by thermal cycling stresses between the hard alloy particles and the matrix.

New Overlay Welding Material Development

Design Philosophy

The new overlay welding material was designed based on the following principles:

1. Enhanced bonding strength: Improve the metallurgical bond between hard alloy particles and the matrix
2. Improved thermal fatigue resistance: Reduce the thermal expansion mismatch between phases
3. Maintained hardness: Preserve the high hardness required for milling operations
4. Oxidation resistance: Add alloying elements to improve high-temperature stability

Material Composition

The selection of YT769 and YT5 35 cermet (cemented carbide) as the hard phase provides a combination of hardness and toughness that is superior to conventional carbides in impact loading conditions. The Cu-Zn-Ni matrix was chosen because its thermal expansion coefficient more closely matches that of the steel substrate, reducing thermal cycling stresses at the interface.

Performance Results

The new overlay welding material demonstrated the following performance characteristics:

Engineering Practice and Implementation

Flame Heating Overlay Process

The flame heating overlay method was selected over conventional arc welding because:

• Lower peak temperature reduces thermal stress in the substrate
• More uniform heating reduces thermal gradients
• Better control of the thermal cycle reduces residual stress
• Lower dilution maintains the designed composition of the overlay layer

The process involves:

1. Preheating the substrate to 400-500°C
2. Applying hard alloy particles to the prepared surface
3. Heating with oxy-fuel flame to melt the matrix material
4. Controlling the cooling rate to promote good bonding
5. Post-weld stress relief treatment

Quality Control Considerations

For mill shoe overlay welding, the following quality control measures are essential:

• Visual inspection for surface defects and porosity
• Magnetic particle testing for interface cracks
• Hardness testing across the overlay layer
• Bond strength testing (peel or shear test)
• Wear testing under simulated service conditions

Key Reflections

This paper exemplifies the integrated approach of failure analysis driving materials development. Rather than simply identifying the failure modes, the authors developed new materials specifically designed to address each failure mechanism. This is the ideal approach in engineering practice: understanding the failure, then designing solutions that address the root causes.

The selection of cermet (YT769/YT5 35) over conventional cemented carbide is particularly insightful. Cermet combines the hardness of carbide with the toughness of a metallic binder, making it more resistant to impact fracture. This is directly relevant to the service conditions of mill shoes, which experience repeated impact loading during milling operations.

The choice of Cu-Zn-Ni matrix is also noteworthy. While iron-based matrices are conventional, the thermal expansion mismatch between iron and the steel substrate contributes to thermal fatigue cracking. The copper-based matrix, with its thermal expansion coefficient closer to that of steel, reduces this driving force for cracking.

For engineers in the oil and gas industry working with downhole tools, this research provides a clear methodology for improving tool life through materials development informed by failure analysis. The approach can be extended to other downhole tools that experience similar harsh operating conditions.