Effect of Heat Treatment on Microstructure and Wear Resistance of Hypereutectic Fe-Cr-C Overlay Alloy Containing Yttrium

Literature Overview

This study, published in the Journal of Heat Treatment of Materials in 2015 by researchers from XCMG Group, the State Key Laboratory of Metastable Materials Preparation Technology, and Yanshan University, addresses a critical challenge in hardfacing engineering: how post-weld heat treatment (PWHT) influences the microstructure and tribological performance of hypereutectic Fe-Cr-C alloys modified with rare earth elements, specifically yttrium. The work was supported by two National Natural Science Foundation grants (51271163 and 51471148), reflecting its significance in advancing hardfacing metallurgy.

Hypereutectic Fe-Cr-C alloys are widely employed in severe abrasion environments such as mining, cement grinding, and material handling. The addition of yttrium is motivated by its rare earth effects — grain refinement, oxygen scavenging, and modification of carbide morphology — which can significantly enhance wear resistance. However, the interaction between heat treatment parameters and these rare earth modifications remains poorly understood in the literature.

Core Technical Content

Microstructural Evolution Under Different Heat Treatments

The study examines how various heat treatment regimes affect the carbide distribution, matrix phase composition, and grain morphology in the overlay layer. Key findings include:

As-welded condition: The microstructure consists of primary M7C3 carbides (Cr7C3-type) dispersed in a martensitic matrix, with secondary carbides forming along grain boundaries during cooling. Yttrium enrichment at carbide-matrix interfaces is observed.
Solution treatment (1050–1100°C, 1–2h, water quench): Partial dissolution of primary carbides occurs, reducing their volume fraction from approximately 35–40% to 20–25%, while the matrix transforms toward retained austenite. Yttrium remains segregated at residual carbide boundaries.
Tempering (550–650°C, 2h, air cool): Secondary carbides precipitate within the matrix, and retained austenite partially transforms to tempered martensite. The carbide size distribution becomes more uniform.

Hardness and Wear Performance

Heat Treatment Condition	Surface Hardness (HV30)	Abrasion Volume Loss (mg, ASTM G65)	Wear Rate Index
As-welded	1450–1520	32–38	Baseline
Solution 1080°C/1.5h + QC	1320–1380	42–48	1.25–1.35
Solution + Temper 600°C/2h	1280–1350	28–34	0.88–0.95
Solution + Temper 650°C/2h	1180–1250	35–40	1.05–1.12

The optimal condition — solution treatment followed by tempering at 600°C — yields the best abrasion resistance despite lower hardness than the as-welded state. This demonstrates the well-known inverse relationship between hardness and toughness in hypereutectic hardfacing alloys, where excessive carbide network continuity leads to brittle fracture under impact-abrasive loading.

Role of Yttrium

Yttrium exhibits several metallurgical effects that persist through heat treatment:

Grain refinement in the dendritic structure (average grain size reduced by 15–20% compared to Y-free counterparts).
Inhibition of carbide coarsening during high-temperature solution treatment — carbide size remains below 2.5 μm even after 1080°C/1.5h.
Enhanced interfacial bonding between carbides and matrix, reducing debonding during wear.
Oxygen scavenging effect that reduces oxide inclusion content, improving ductility of the matrix phase.

Engineering Practice Implications

Process Selection Guidelines

For field applications where the overlay is applied to components experiencing combined impact and abrasion (e.g., crusher hammers, bucket teeth), the following practice is recommended:

Apply the hypereutectic Fe-Cr-C-Y overlay using submerged arc welding (SAW) with multi-pass technique to achieve a minimum overlay thickness of 8–12 mm.
Perform post-weld solution treatment at 1080°C for 1.5 hours followed by tempering at 600°C for 2 hours.
Verify hardness profile with a minimum of 1250 HV30 at 0.5 mm from the surface.
Conduct metallographic examination at 200× magnification to confirm carbide distribution uniformity and absence of carbide network connectivity.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking at overlay-base interface	Excessive cooling rate, high carbon content	Preheat to 200–250°C, use low-hydrogen consumables
Carbide network formation	Slow cooling, high Cr/C ratio	Increase cooling rate, control interpass temperature below 150°C
Yttrium oxidation loss	Open atmosphere welding	Use flux coverage or inert gas shielding
Poor bond strength after PWHT	Differential thermal expansion	Limit PWHT temperature to 1100°C maximum

Study Insights and Reflections

This research underscores a fundamental principle in hardfacing metallurgy: hardness is not the sole determinant of wear resistance, particularly in impact-abrasive environments. The optimal balance between carbide volume fraction, matrix ductility, and carbide morphology is achieved through controlled heat treatment rather than simply maximizing carbide content. The rare earth modification with yttrium provides an additional lever for microstructural control that is underutilized in industrial practice.

From a standards perspective, the findings align with requirements in NB/T 47014 and ASME IX regarding weld procedure qualification for overlay welding, though the specific heat treatment parameters must be established through supplementary testing beyond standard qualification requirements. Engineers involved in pressure vessel overlay repair should note that solution treatment above 1050°C may affect the base material properties, requiring careful evaluation of the heat-affected zone (HAZ) per GB/T 150 or ASME VIII Div.1 provisions.

The research also highlights the importance of understanding the Y-depletion effect near the overlay surface, where oxygen potential during welding can lead to yttrium oxide formation that reduces the effective rare earth concentration in the near-surface region. This has practical implications for surface preparation and post-weld machining operations.