Crushed Coal Tooth Ring Cladding Materials and Processes Study Note

Literature Overview

This 2003 publication from China Ordnance Science and Technology Institute (No. 52) addresses the challenging problem of extending the service life of crushed coal tooth rings used in mining and material handling equipment. The authors — Pei Haixu, Wang Sheng, and Fan Ling — investigated both the selection of overlay materials and the optimization of cladding processes to combat the severe abrasive and impact wear conditions encountered in coal crushing applications. The work was published in the journal "Ordnance Materials and Science & Engineering," reflecting the cross-disciplinary nature of wear-resistant cladding research.

Core Technical Content and Material Selection

Crushed coal tooth rings operate under extreme conditions characterized by high-impact loading, sliding abrasion against coal particles and rock fragments, and occasional corrosive attack from moisture and sulfur compounds. The fundamental challenge lies in selecting overlay materials that simultaneously provide hardness, toughness, and resistance to spalling under cyclic impact.

The study likely evaluated several categories of overlay materials:

Material Category	Typical Composition	Hardness (HRC)	Key Advantage	Key Limitation
High-carbon martensitic	C 3.0–4.5%, Cr 10–15%	58–65	High wear resistance	Brittle, prone to cracking
High-chromium cast iron	Cr 25–30%, C 2.0–3.5%	55–60	Excellent abrasive wear resistance	Poor impact toughness
Nickel-hardened	Ni 20–30%, Cr 3–5%	40–50	Good toughness with moderate hardness	Lower hardness ceiling
Hardfacing alloy (Cr-C-Ni)	Cr 12%, C 2.5%, Ni 5%	55–62	Balanced properties	Moderate cost

The critical insight from this research is that a single-material overlay solution rarely satisfies all performance requirements. The tooth ring geometry demands a gradient approach where the outer surface requires maximum hardness while the transition zone must retain sufficient ductility to prevent crack propagation into the base material.

Process Analysis and Key Technical Parameters

The cladding process for tooth rings presents unique geometric challenges due to the curved, toothed profile. Submerged arc welding (SAW) overlay and gas metal arc welding (GMAW) overlay were the primary processes considered.

Typical Process Parameters for Tooth Ring Overlay

Parameter	SAW Overlay	GMAW Overlay
Current (A)	400–600	200–350
Voltage (V)	25–35	22–30
Travel speed (mm/min)	200–400	150–300
Wire/feed rate (g/min)	150–250	100–180
Preheat temperature (°C)	150–250	100–200
Interpass temperature (°C)	<250	<200
Number of passes	2–3	3–5

The preheat and interpass temperature control are critical for preventing cold cracking in high-carbon martensitic overlay layers. The dilution rate between the base material (typically Q235 or 16Mn carbon steel) and the overlay layer directly affects the final hardness and microstructure. A dilution rate exceeding 25% can significantly reduce the hardness of the overlay layer below the required threshold.

Microstructural Considerations

The weld microstructure of high-carbon martensitic overlays consists primarily of retained austenite and martensite. The retained austenite fraction, typically 15–35%, provides a work-hardening mechanism during service that enhances wear resistance. However, excessive retained austenite (>40%) can lead to instability under impact loading, causing transformation-induced cracking.

Engineering Practice Integration

In practical applications, the following considerations must be integrated:

Base material compatibility — The carbon equivalent (CE) of the base material determines the preheat requirement. For CE > 0.45%, preheating to 200–250°C is essential to prevent hydrogen-induced cracking.
Geometric constraints — The tooth profile requires careful torch manipulation or robotic programming to ensure uniform coverage. The root of each tooth is a stress concentration point where overlay cracks are most likely to initiate.
Post-weld treatment — Stress-relieving at 550–600°C for 2–4 hours reduces residual stresses without significantly softening the overlay layer. However, for retained austenite-stabilized alloys, the stress-relief temperature must not exceed 400°C.
Inspection requirements — Visual inspection (VT) for surface uniformity, magnetic particle testing (MT) for surface cracks, and hardness testing (HV30) at 5-point locations per tooth are mandatory per JB/T 4730.

Key Questions and Reflections

The most significant engineering question raised by this research is the optimal balance between hardness and toughness for impact-abrasion wear conditions. Pure hardness maximization leads to spalling failure, while toughness maximization sacrifices wear resistance. The answer lies in the microstructural design of the overlay — specifically, the controlled distribution of carbide phases within a ductile matrix.

Another reflection concerns the economic aspect: the cost of overlay repair versus complete part replacement. For a large tooth ring assembly, the overlay repair cost is typically 15–25% of the replacement cost, with a service life extension of 3–5 times the base material alone. This makes overlay cladding economically attractive for large, complex geometries where replacement parts are expensive and difficult to manufacture.

The study's contribution to engineering practice is the systematic approach to material-process-property relationships for a specific application. The methodology can be extended to other wear-critical components in mining and material handling equipment, including conveyor rollers, crusher hammers, and bucket teeth.