Relationship Between Cladding Layer Interface Characteristics and Crack Formation

Literature Overview

This 2003 study by Zhou Xiying, Li Peiyao, Tong Jianhua, Li Manping (Shanghai University of Engineering Science, School of Materials and Engineering) and Liu Handing (Yichun First Machinery Factory) investigates the relationship between the microstructural characteristics of the cladding layer interface and the initiation and propagation of cracks in hardfaced coal mining machinery components. The research bridges fundamental metallurgical understanding with practical failure analysis in the demanding environment of underground mining equipment.

Core Technical Points

Interface Microstructure in Coal Mining Applications

Coal mining machinery components — including bucket teeth, scraper chains, haulage tracks, and conveyor rollers — are subject to severe abrasive and impact loading in the presence of moisture and coal dust. Hardfacing overlays applied to these components typically use high-chromium iron (26Cr, 32Cr) or high-speed steel-based alloys deposited via submerged arc welding (SAW), gas metal arc welding (GMAW), or oxy-fuel processes.

The interface between the cladding layer and the low-carbon or low-alloy steel substrate is the critical region where cracks most frequently initiate. The study identifies several interface features that promote or inhibit crack formation:

Dilution zone width: Higher dilution (>25%) introduces more ferrite into the cladding layer, reducing hardness and potentially creating a soft zone that acts as a crack path.
Carbide network continuity: Continuous M7C3 carbide networks along the interface create preferential crack paths due to the inherent brittleness of these phases.
Microsegregation patterns: Compositional segregation at the interface creates local variations in thermal expansion and hardness, generating stress concentrations.
Residual stress distribution: Tensile residual stresses at the interface, caused by differential cooling rates between overlay and substrate, promote crack initiation.

Crack Formation Mechanisms

Crack Type	Location	Driving Force	Interface Feature Involved
Solidification cracking	Near interface, first pass	Low ductility of solidification zone	High Cr/C content; rapid cooling
Hot cracking	Interface boundary	Thermal stress + embrittling phases	Continuous carbide network
Cold cracking	Interface HAZ	Hydrogen diffusion + high hardness	High carbon equivalent; martensitic structure
Thermal fatigue cracking	Interface	Cyclic thermal stress	High residual stress; brittle microstructure
Stress corrosion cracking	Interface	Moisture + tensile stress	Cracking-sensitive microstructure

Interface Characterization Methods

The study employs multiple characterization techniques to correlate interface features with crack susceptibility:

Technique	Information Obtained	Application
Optical metallography	Grain structure; carbide morphology; dilution width	Primary screening
SEM-EDS	Compositional mapping; interdiffusion zone	Interface chemistry
XRD	Phase identification; residual stress	Phase analysis
Hardness profiling (HV)	Hardness gradient across interface	Mechanical property mapping
Micro-fracture analysis	Crack path; initiation site	Failure analysis

Engineering Practice and FMEA Integration

For coal mining equipment manufacturers, understanding the interface-crack relationship enables systematic failure prevention through FMEA (Failure Mode and Effects Analysis):

Failure Mode	Effect	Severity	Cause	Occurrence	Detection	RPN	Mitigation
Interface crack	Component failure; production stoppage	10	Excessive dilution + high residual stress	6	4	240	Control first-pass heat input; post-weld stress relief
Spalling	Loss of overlay; accelerated wear	8	Poor bonding; thermal fatigue	5	3	120	Improve surface prep; reduce interpass temp
Cracking at weld toe	Progressive crack growth	7	Stress concentration; brittle microstructure	5	4	140	Use multi-pass technique; temper after welding

Key Process Recommendations

Based on the interface-crack relationship established in the study, the following process guidelines are recommended for hardfacing coal mining components:

Substrate preparation: Grind to bare metal with 60–80 grit; remove all scale, rust, and previous coatings. Surface roughness should be Ra 6.3–12.5 μm for optimal mechanical interlock.
First-pass control: Use the lowest practical heat input to minimize dilution. Consider using a pre-alloyed surfacing layer (e.g., 13% Cr martensitic steel) to reduce dilution in subsequent passes.
Interpass temperature: Maintain at 150–250 °C to balance crack prevention (requires higher temp) against microstructure refinement (requires lower temp).
Post-weld treatment: Stress-relief annealing at 550–650 °C for 2 hours per 25 mm thickness, followed by controlled air cooling. This reduces residual stresses by 40–60% without significantly reducing hardness.
NDT protocol: Visual inspection (VT) followed by magnetic particle testing (MT) or dye penetrant testing (PT) on all overlay surfaces. For critical components, phased array ultrasonic testing (PAUT) of the interface is recommended.

Study Insights

This research provides a fundamental metallurgical framework for understanding why hardfaced components fail in service. The key insight is that crack formation is not a random event but is systematically governed by interface microstructure, which in turn is controlled by process parameters. For engineering teams responsible for welding procedure development and qualification, this means that interface characterization should be a standard part of procedure verification — not merely mechanical testing of bulk properties. The study also underscores the importance of multi-scale analysis: from macro-scale residual stress measurements to micro-scale carbide morphology observations, each scale reveals different aspects of the crack susceptibility picture. For coal mining equipment engineers, the practical implication is clear: investing in proper interface control during hardfacing operations yields significantly longer service life and fewer unplanned maintenance events.