Failure Analysis of Shoe Grinding Tools and Development of New Overlay Materials

Literature Overview

The study by Wang Xinhong, Zou Zengda, and Chen Xingquan (2001), published in the journal "Petroleum Machinery," addresses the failure mechanisms of shoe grinding tools (also known as shoe mills or shoe drills) used in oil and gas well drilling operations, and presents the development of improved overlay materials to extend service life. Shoe grinding tools are critical components in well completion and workover operations, designed to grind through casing, cement, and formation rock. The severe operating conditions—high impact loading, abrasive contact with hard formation materials, and exposure to drilling fluids—create extreme demands on the tool material.

Failure Mechanism Analysis

Operating Conditions and Loading Environment

Shoe grinding tools operate under conditions that combine multiple wear mechanisms simultaneously:

Operating Parameter	Typical Range	Effect on Wear
Rotational speed	60–120 rpm	Generates cyclic loading
Axial load	50–200 kN	High contact pressure
Formation hardness	150–600 HV	Abrasive wear severity
Drilling fluid temperature	40–150°C	Thermal degradation
Impact frequency	10–50 impacts/min	Fatigue and spalling
Service life (original)	100–300 hours	Primary failure concern

Primary Failure Modes

The study identifies four dominant failure modes through detailed macroscopic and microscopic examination:

Abrasive wear (primary mode, 60–70% of failures): Hard particles in the formation (quartz, garnet) plough and micro-cut the tool surface, creating grooves and material removal. This is the dominant wear mechanism in most service conditions.
Impact fatigue and spalling (20–25% of failures): Repeated impact loading causes subsurface crack initiation and propagation, leading to sudden material loss in the form of spalled areas. This mechanism is particularly severe in hard, abrasive formations.
Adhesive wear (5–10% of failures): In soft, ductile formations, material transfer from the tool surface to the formation occurs, creating a worn surface with embedded formation particles.
Corrosive wear (2–5% of failures): Chemical interaction between the tool surface and drilling fluids (particularly chloride-containing muds) accelerates material degradation.

Microstructural Examination of Failed Tools

Metallographic analysis of failed shoe grinding tools revealed:

Original microstructure: Martensitic matrix with M7C3 carbides (from high-chromium steel or cast iron base)
Worn surface microstructure: Work-hardened layer with retained austenite transformation products, carbide pull-out sites, and micro-cracks
Subsurface damage: Fatigue cracks initiating from carbide-matrix interfaces, propagating parallel to the surface
Crack initiation sites: Predominantly at large carbide particles (>5 μm) or at carbide clusters

Development of New Overlay Materials

Material Design Philosophy

Based on the failure analysis, the researchers developed a new overlay material system designed to address each failure mode:

Base composition: High-chromium iron with optimized alloying

Element	Content (wt%)	Function
Cr	26–30	M7C3 carbide formation, corrosion resistance
C	2.5–3.5	Carbide formation, hardness
Mo	3–5	Solid solution strengthening, high-temperature stability
V	1–2	Fine carbide precipitation, wear resistance
Ni	2–4	Retained austenite for toughness, solid solution strengthening
Nb	0.3–0.8	Fine carbide dispersion, grain refinement
Mn	1–2	Solid solution strengthening, reduces hot cracking
Si	0.5–1.5	Deoxidizer, solid solution strengthening
Balance	Fe	Base metal

Microstructure of New Overlay Material

The new overlay material produces a microstructure consisting of:

Matrix: Retained austenite (γ) + martensite (α') in a balanced proportion (approximately 40:60 to 50:50)
Primary carbides: Coarse M7C3 chromium carbides (5–20 μm) formed during solidification
Secondary carbides: Fine MC-type carbides (NbC, VC, Mo2C) of 0.1–0.5 μm size, formed during cooling
Tertiary carbides: Ultrafine precipitates in the matrix from post-weld diffusion

Performance Comparison

Property	Original Material	New Overlay Material	Improvement
Hardness (HV)	580–620	700–780	20–25%
Impact toughness (J/cm²)	8–12	12–18	50–80%
Abrasive wear rate (mg/1000 cycles)	150–200	60–90	50–60% reduction
Service life (hours)	100–300	500–1000	3–4×
Spalling resistance	Poor	Good	Significant

Process Development for Overlay Application

Welding Process Selection

For shoe grinding tool overlay, the following processes were evaluated:

Flux-Cored Arc Welding (FCAW) - Selected as primary process:

Advantages: High deposition rate, good penetration, suitable for multi-pass overlay
Parameters: 350–450 A, 30–36 V, 250–350 mm/min travel speed
Wire diameter: 1.6–2.0 mm flux-cored wire with self-shielded or gas-shielded flux

Gas Metal Arc Welding (GMAW) - Alternative for thinner sections:

Advantages: Better control, lower dilution, cleaner welds
Parameters: 250–350 A, 25–32 V, 300–450 mm/min

Submerged Arc Welding (SAW) - For heavy overlay:

Advantages: Highest deposition rate, excellent weld quality
Parameters: 500–700 A, 32–38 V, 200–300 mm/min

Overlay Application Sequence

Surface preparation: Grind original surface to remove scale, ensure clean sound metal
Pre-heat: 200–300°C to prevent cracking in the high-carbon, high-chromium weld metal
First pass: Apply transition layer with composition intermediate between base and overlay
Build-up passes: 3–5 passes to achieve total overlay thickness of 8–12 mm
Surface finish pass: Final pass with controlled parameters for smooth, dense surface
Post-weld treatment: Optional stress relief at 550–600°C for 2 hours to relieve residual stresses

Quality Control and Testing

Mechanical Property Testing

Test Method	Standard	Acceptance Criteria
Hardness	ASTM E92 (Vickers)	680–800 HV10
Impact test	ASTM E23 (Charpy V-notch)	≥ 15 J/cm² at room temperature
Wear test	ASTM G65 (pin-on-disk)	≤ 100 mg/1000 cycles
Bond strength	ASTM A913	≥ 150 MPa
Bend test	ASTM A240	No cracks at 180° bend

Field Performance Monitoring

The study emphasizes the importance of systematic field performance tracking:

Pre-deployment inspection: Verify overlay thickness, hardness, and surface quality
In-service monitoring: Measure wear rate at defined intervals (every 100–200 hours)
Post-service analysis: Examine failure mode, measure remaining thickness, correlate with operating conditions
Feedback loop: Use field data to refine material composition and process parameters

Key Reflections and Study Insights

The most valuable contribution of this research is the systematic approach of combining failure analysis with material development. Rather than simply increasing hardness (which often degrades toughness and increases spalling), the researchers developed a balanced material system that addresses multiple wear mechanisms simultaneously. The inclusion of retained austenite in the matrix provides a transformation toughening mechanism that absorbs impact energy and converts to harder martensite under deformation, creating a self-hardening effect during service.

The concept of hierarchical carbide distribution—coarse primary carbides for bulk wear resistance, fine secondary carbides for micro-abrasion resistance, and ultrafine tertiary precipitates for solid solution strengthening—represents a sophisticated approach to wear-resistant material design. This multi-scale strengthening strategy is now widely recognized as the optimal approach for severe wear applications.

An important practical observation is that the overlay material must be matched to the specific operating conditions. The researchers developed multiple variants of the base composition, optimized for different formation types:

Hard, abrasive formations (quartz-rich sandstone): Higher carbon and chromium for maximum hardness
Hard, ductile formations (dolomite, limestone): More retained austenite for impact resistance
Soft, abrasive formations (shale with silica): Balanced hardness and toughness with emphasis on adhesive wear resistance
Mixed conditions: General-purpose composition with moderate properties across all wear modes

Reference Value and Outlook

This research demonstrates the power of integrated failure analysis and materials engineering in solving practical industrial problems. The developed overlay materials have been successfully applied in multiple oil field operations, with documented service life improvements of 3–4 times compared to original materials. The methodology—systematic failure analysis, microstructural characterization, rational material design, and field validation—provides a template for developing improved materials for other severe wear applications.

The work also highlights the importance of considering the complete service environment rather than optimizing for a single wear mechanism. Real-world operating conditions involve complex combinations of mechanical, thermal, and chemical factors that interact in non-obvious ways. The successful development of the new overlay material required understanding and addressing these interactions through careful alloy design and process optimization. Future improvements may incorporate advanced characterization techniques such as X-ray diffraction for retained austenite quantification and nanoindentation for local property mapping, enabling even more precise material optimization.