Zhuojin Pipe Fitting Co., LtdFailure Analysis of Milling Shoes and Development of New Hardfacing Materials
Literature Overview
The paper by Wang Xinhong, Zou Zengda, and Chen Xingquan (2001), published in Petroleum Machinery, presents a comprehensive failure analysis of milling shoes used in oil well operations and the subsequent development of new hardfacing materials. The study examines eight failed milling shoes using multiple analytical techniques including optical microscopy, scanning electron microscopy, electron probe microanalysis, microhardness measurement, and X-ray diffraction. The research is particularly relevant to engineers working in the oil and gas industry, where milling shoes are critical tools for removing fallen objects from wellbores.
Failure Analysis Results
The failure analysis revealed a complex set of failure modes, reflecting the severe service conditions experienced by milling shoes during operation:
| Failure Mode | Description | Root Cause |
|---|---|---|
| Hard alloy fracture and detachment | Tungsten carbide particles breaking away from matrix | Brittle fracture under impact loading |
| Overall hardfacing layer detachment | Complete separation of weld overlay from base | Poor interface bonding, residual stresses |
| Excessive wear of hard alloy phase | Progressive material removal from carbide particles | Thermal fatigue, impact fatigue, high-temperature oxidation |
Detailed Failure Mechanism Analysis
The scanning electron microscopy examination of the fracture surfaces revealed distinct features for each failure mode. The hard alloy fracture surfaces showed typical brittle fracture characteristics with flat, featureless regions. The interface between the tungsten carbide particles and the copper-nickel-zinc matrix showed poor bonding in several cases, indicating either inadequate welding procedure or excessive residual stress.
The electron probe microanalysis revealed significant compositional changes at the interface between the hardfacing layer and the base steel. Carbon diffusion from the base into the hardfacing layer created a decarburized zone in the steel and a carburized zone in the hardfacing, both of which weakened the interface.
Operating Conditions and Their Effects
Milling shoes operate under extremely harsh conditions:
- High impact loading from contact with wellbore obstacles
- Thermal cycling from frictional heating during milling
- High-temperature oxidation from frictional heat reaching several hundred degrees Celsius
- Abrasive wear from contact with rock and metal debris
- Corrosive environment from drilling fluids and formation fluids
These combined factors create a synergistic degradation mechanism that accelerates wear far beyond what any single factor would cause independently.
New Hardfacing Material Development
Based on the failure analysis findings, the authors developed a new hardfacing electrode using YT769 and YT5-35 tungsten carbide as the hard phase and a Cu-Zn-Ni alloy as the matrix. The key design principles were:
Material Selection Rationale
| Component | Selection | Rationale |
|---|---|---|
| Hard phase | YT769 (high Co content) and YT5-35 (high WC content) | Combined toughness and hardness |
| Matrix | Cu-Zn-Ni alloy | Good wetting, moderate hardness, ductile |
| Electrode type | Flame-hardfacing electrode | Compatible with field welding conditions |
The YT769 tungsten carbide grade provides higher toughness due to its higher cobalt content, while YT5-35 provides higher hardness due to its higher tungsten carbide content. The combination of these two grades creates a graded hardness structure that resists both impact fracture and abrasive wear.
Performance Results
The new hardfacing electrode produced a weld overlay with the following characteristics:
| Property | Value |
|---|---|
| Hard alloy phase macrohardness | 91-92.5 HRA |
| Matrix hardness | 40-50 HRC |
| Bond strength | Good (no detachment observed) |
| Wear resistance | Significantly improved over previous design |
| Impact resistance | Improved over single-grade tungsten carbide design |
Engineering Practice Integration
The findings from this study have direct applications in my practice with oil and gas equipment. Milling shoes are used extensively in well intervention operations, and their failure can result in significant production losses. The new hardfacing material developed in this study represents a practical solution to a well-recognized industry problem.
A key practical consideration is the flame-hardfacing process used for application. Flame hardfacing is preferred for field applications because it requires minimal equipment and can be performed in remote locations. However, flame hardfacing produces higher heat input than arc welding, which can affect the microstructure. The authors' selection of materials that are tolerant of flame hardfacing conditions is an important practical consideration.
Study Insights and Implications
This paper demonstrates the value of systematic failure analysis in guiding material development. The approach of analyzing multiple failed components using multiple analytical techniques provides a comprehensive understanding of the failure mechanisms, which is essential for developing effective solutions.
The material development approach taken by the authors—using a combination of hard phases with different properties to create a synergistic effect—is a strategy that can be applied to other hardfacing applications. The concept of graded hardness structures, where the hard phase properties vary through the thickness of the deposit, is particularly promising for applications involving combined loading conditions.