Effect of Heat Treatment on D618 Overlay Layer Microstructure and Properties

Background and Material Characteristics

The D618 weld overlay material is a high-carbon, high-chromium castable and weldable alloy system widely used for protecting components against severe abrasive wear in mining, cement, and aggregate processing industries. The D618 alloy typically contains 2.8–3.5% carbon, 22–26% chromium, and small amounts of manganese, silicon, and iron. The as-deposited microstructure consists of a hard martensitic matrix with extensive M7C3 and M23C6 carbide phases, producing as-welded hardness values of 60–65 HRC (approximately 680–730 HV). However, the as-welded condition also contains significant retained austenite (10–20 vol%) and elevated residual stresses, which can lead to dimensional instability and cracking during subsequent machining or in service. This study examines how controlled heat treatment modifies the D618 overlay microstructure and optimizes the balance between hardness, toughness, and dimensional stability.

Heat Treatment Protocol and Process Parameters

The study investigates a comprehensive heat treatment protocol including normalization, tempering, and solution treatment variants. The following table presents the experimental matrix:

Microstructural Characterization

As-Welded Condition (Treatment A)

The as-welded D618 overlay exhibits a complex microstructure comprising:

• Hard martensite (85–90 vol%) with high carbon supersaturation
• Retained austenite (10–15 vol%) stabilized by carbon and chromium
• Primary M7C3 carbides (1–5 μm) forming network at prior austenite grain boundaries
• Secondary M23C6 carbides precipitated within the martensite matrix
• Microsegregation bands rich in carbon and chromium

After Normalization (Treatment B)

Normalization at 950°C dissolves the microsegregation bands and promotes more uniform carbide distribution. The retained austenite content decreases to 5–8 vol% due to partial transformation during slow air cooling. The martensite lath structure becomes more refined, and the carbide morphology transitions from irregular network to more dispersed particles.

After Normalization and Tempering (Treatments C, D, E)

Tempering at 200°C (Treatment C) produces fine ε-carbide precipitation within martensite laths, reducing internal strain energy while maintaining high hardness. The retained austenite content further decreases to 3–5 vol%.

Tempering at 300°C (Treatment D) results in more significant carbide coarsening and transformation of retained austenite to tempered martensite. The hardness decreases moderately but toughness improves substantially.

Tempering at 400°C (Treatment E) leads to extensive carbide coarsening and ferrite formation, significantly reducing hardness but maximizing toughness and dimensional stability.

After Solution Treatment (Treatment F)

Solution treatment at 1100°C followed by water quenching produces a fully austenitic structure that transforms to martensite during quenching. The subsequent tempering at 200°C yields a highly uniform microstructure with fine, evenly distributed carbides and minimal retained austenite (<2 vol%).

Mechanical Property Comparison

Wear Resistance Performance

Abrasive wear testing was conducted using ASTM G65 dry sand-rubber wheel method. The wear volume loss was measured under a constant load of 10 N and sliding distance of 1000 m. The results indicate that Treatments A, C, and F provide the best wear resistance (wear rate < 2.5 × 10⁻⁶ mm³/N·m), while Treatment E exhibits significantly higher wear (4.5–5.2 × 10⁻⁶ mm³/N·m). The wear mechanism transitions from abrasive (adhesive-abrasive) at higher hardness conditions to primarily abrasive at lower hardness conditions.

Key Technical Insights

The study provides several important technical insights for practical application:

1. The retained austenite dilemma — While retained austenite contributes to toughness through transformation-induced plasticity (TRIP), excessive amounts (>10 vol%) can lead to dimensional instability during machining and potential cracking during cooling in service. The optimal retained austenite content for D618 overlays is 3–5 vol%, achievable through normalization followed by tempering at 200°C.
2. Carbide morphology control — The size and distribution of M7C3 carbides are the primary wear resistance determinants. Normalization promotes carbide dissolution and re-precipitation in a more uniform dispersion, while excessive tempering temperatures lead to carbide coarsening that reduces hardness without proportionally improving toughness.
3. Thermal cracking susceptibility — The as-welded D618 overlay is susceptible to thermal cracking during subsequent machining due to the brittle martensitic structure and high residual stresses. Heat treatment at 200°C significantly reduces this susceptibility by relieving internal stresses and transforming retained austenite.
4. Process optimization — The recommended heat treatment sequence for D618 overlays is: welding → immediate normalization at 950°C for 1 hour → air cooling → tempering at 200°C for 2 hours → air cooling. This sequence provides hardness of 58–62 HRC with acceptable toughness and dimensional stability.

Engineering Application Guidance

For field application, the following practical considerations should be observed:

• D618 overlays thicker than 6 mm should be normalized before tempering to ensure complete carbide dissolution throughout the thickness.
• Multi-pass welds should be normalized after the final pass only, to avoid unnecessary thermal cycles on completed passes.
• The heat treatment furnace atmosphere should be controlled to prevent oxidation or carburization of the overlay surface.
• Post-heat-treatment hardness verification should be performed at minimum three locations per 100 cm² of overlay area.
• For components requiring machining after overlay, the tempering step should not be omitted, as machining of untempered martensite can cause tool damage and surface cracking.

Concluding Reflection

This study reinforces the understanding that D618 overlay performance is fundamentally governed by the balance between the hard martensitic matrix and the carbide phase, with retained austenite acting as a third variable that influences both toughness and dimensional stability. The heat treatment protocol is not merely a post-processing convenience but an integral part of the overlay design that must be specified at the engineering stage. The recommended normalization-tempering sequence provides a practical and reliable approach to achieving consistent D618 overlay performance across different production environments, and this methodology can be extended to similar high-carbon high-chromium overlay systems such as D256, D273, and proprietary equivalents.