Failure Analysis of Mill Shoes and Development of Novel Overlay Materials

Literature Overview and Failure Background

This literature presents a comprehensive failure analysis of mill shoes (also known as mill liners or grinding elements) used in ball mills and rod mills for mineral processing applications, followed by the development and evaluation of novel overlay materials designed to improve service life. Mill shoes are subjected to extreme combined loading conditions including high-energy impact from grinding media, severe abrasive wear from ore particles, and cyclic stress that can lead to fatigue cracking. The literature identifies the primary failure modes, quantifies the service life of conventional materials, and proposes new overlay compositions and welding procedures to extend component life.

Failure Mode Analysis

The failure analysis employs a systematic approach combining macroscopic examination, metallographic analysis, scanning electron microscopy, and energy-dispersive spectroscopy to identify root causes. The following table summarizes the primary failure modes identified:

Failure Mode	Frequency	Root Cause	Typical Location
Abrasive wear	60%	Hard ore particles ploughing and cutting the surface	Center and upper zones
Impact fatigue cracking	20%	Repeated impact loading from grinding media	Edge and transition zones
Spalling / delamination	10%	Poor bond strength, residual stress	Overlay-base metal interface
Corrosive wear	5%	Aggressive slurry environment	Lower zones in wet grinding
Other	5%	Manufacturing defects, improper welding	Various

The metallographic analysis reveals that the conventional high-manganese steel mill shoes develop a work-hardened surface layer under impact loading, which provides some wear resistance but also creates residual tensile stresses that promote fatigue cracking. The overlay layers, when present, often fail due to insufficient bond strength or improper hardness matching between the overlay and base metal.

Microstructural Analysis of Failed Components

SEM examination of worn surfaces reveals distinct wear tracks, micro-pitting, and micro-cracks. The wear tracks indicate that the dominant wear mechanism is abrasive, with hard particles ploughing through the surface material. Micro-pitting is observed at locations where impact loading has initiated subsurface cracks that propagate to the surface. The presence of oxide layers and embedded wear debris in the surface indicates that three-body abrasive wear is also active.

Novel Overlay Material Development

Based on the failure analysis, the literature proposes several novel overlay compositions designed to address the identified failure modes:

Material Designation	Composition	Hardness (HRC)	Key Advantage
NM-1	High-Cr martensitic (Cr 14%, C 2.5%)	60 to 64	High abrasive wear resistance
NM-2	Cr-Mo-V martensitic (Cr 10%, Mo 2%, V 1.5%)	56 to 62	Balanced wear and impact resistance
NM-3	Cr-C-Mo with WC particles (WC 20 vol%)	62 to 68	Superior abrasive resistance
NM-4	Ni-Cr composite (Ni 30%, Cr 20%)	50 to 55	Corrosion-resistant in wet environments

The NM-3 composition, incorporating tungsten carbide (WC) particles, shows the most promising results for severe abrasive wear applications. The WC particles, with a hardness of approximately 1500 HV, provide exceptional resistance to ploughing and micro-cutting by hard ore particles. The challenge is to ensure uniform WC particle distribution throughout the overlay deposit, which requires careful control of welding parameters and flux composition.

Welding Procedure Development and Validation

The novel overlay materials are applied using submerged arc welding (SAW) or flux-cored arc welding (FCAW) with custom-designed consumables. The following table presents the recommended welding parameters:

Parameter	NM-1	NM-2	NM-3	NM-4
Preheat (°C)	200 to 250	150 to 200	150 to 200	100 to 150
Interpass max (°C)	250	250	250	200
Current (A)	350 to 450	300 to 400	300 to 400	250 to 350
Travel speed (mm/min)	200 to 300	200 to 300	180 to 280	200 to 300
Post-weld treatment	600°C, 2h	600°C, 2h	550°C, 2h	650°C, 2h

Validation testing includes hardness profiling, impact testing, fatigue testing, and accelerated wear testing using simulated ore abrasion. The results show that the NM-3 overlay achieves a 2.5 to 3.0 times improvement in service life compared to conventional high-manganese steel liners under identical operating conditions.

Study Insights and Reflections

The failure analysis and novel material development presented in this literature exemplify the systematic approach that should be adopted in overlay welding material development for severe service applications. The key insight is that mill shoe failure is rarely due to a single mechanism but rather a combination of abrasive wear, impact fatigue, and sometimes corrosion acting synergistically. The development of overlay materials must therefore target multiple failure modes simultaneously, which is challenging but achievable through careful metallurgical design. The NM-3 composition with WC reinforcement demonstrates that ceramic particle reinforcement can dramatically improve abrasive wear resistance, but engineers must also ensure that the bond strength between the overlay and base metal is sufficient to prevent spalling under impact loading. The literature provides a valuable framework for overlay material development that can be adapted to other severe service applications in mining, cement, and power generation industries. The integration of failure analysis with materials development and welding procedure qualification represents a best practice approach that should be widely adopted in the industry.