Heat Treatment Effects on D618 Overlay Layer Microstructure and Properties

Literature Overview

The study by Peng Jixiang, Wang Shunxing, and Liu Yong from Luoyang Institute of Technology, published in Heat Treatment of Metals in 2002 (Vol. 27, No. 5, pp. 48-50), investigates the effects of heat treatment on the microstructure, hardness, and wear resistance of a D618 high-carbon, high-chromium cast iron type hard alloy overlay layer. Funded by the Henan Provincial Department of Education Research Project, this research addresses the critical need for optimizing the performance of hard alloy overlays through post-weld heat treatment.

The D618 overlay layer is characterized by a high carbon and high chromium composition, which promotes the formation of hard carbide phases. The study systematically examines different heat treatment conditions, including air cooling after heating at 850°C and oil quenching after heating at 850°C followed by tempering at 400°C, and compares the resulting properties with those of a carbonitrided 35CrMo low-alloy steel.

Core Technical Findings

Heat Treatment Conditions and Results

The study examined two primary heat treatment conditions for the D618 overlay layer:

The results demonstrate that both heat treatment conditions for the D618 overlay layer yield significantly higher hardness and wear resistance compared to the carbonitrided 35CrMo low-alloy steel. Specifically, the corrosion-wear resistance of the D618 overlay layer was improved by 3 to 12 times compared to the 35CrMo carbonitrided steel.

Microstructural Analysis

The D618 overlay layer, being a high-carbon, high-chromium cast iron type alloy, contains a matrix of martensite and a high volume fraction of hard carbide phases. The heat treatment conditions influence the microstructure in the following ways:

1. Air cooling at 850°C: This condition promotes the formation of a martensitic matrix with fine carbide distribution. The relatively slower cooling rate allows for some carbide coarsening, which can be beneficial for wear resistance by providing larger, harder carbide particles.
2. Oil quenching at 850°C + 400°C temper: This condition produces a tempered martensite matrix with well-dispersed carbides. The tempering step relieves residual stresses and improves toughness while maintaining high hardness. The 400°C tempering temperature is optimal for balancing hardness and toughness in high-carbon alloys.

The comparison with 35CrMo carbonitriding is particularly instructive. Carbonitriding of 35CrMo produces a surface layer with enhanced hardness and wear resistance, but the depth of the hardened layer is limited and the overall wear resistance is inferior to that of the D618 overlay layer. This is because:

• The D618 overlay layer is a through-thickness hard alloy with a high carbide volume fraction.
• Carbonitriding only modifies the surface chemistry and microstructure of the base metal.
• The D618 overlay layer provides a thicker, more uniform hard layer with consistent properties.

Wear Mechanism Analysis

The superior wear resistance of the D618 overlay layer can be attributed to several factors:

1. High carbide volume fraction: The hard carbide phases (such as M₇C₃, M₂₃C₆, and possibly M₆C) provide excellent resistance to abrasive wear by acting as hard, wear-resistant particles within the matrix.
2. Martensitic matrix: The martensitic matrix provides a tough, hard background that supports the carbide particles and resists plastic deformation.
3. Corrosion resistance: The high chromium content in the D618 alloy provides inherent corrosion resistance, which is particularly important in environments where corrosion-wear interaction is a concern.
4. Heat treatment optimization: The heat treatment conditions are tailored to maximize the hardness and stability of the carbide phases while maintaining adequate toughness in the matrix.

Engineering Practice Implications

Application in Piping Systems

D618-type hard alloy overlays are commonly applied in piping systems where erosion-corrosion is a primary concern, such as:

• Slurry transport lines carrying abrasive solids in corrosive liquids.
• Catalyst transfer lines in petrochemical processing units.
• Circulating water lines in power plants where erosion from water-borne particles is a concern.
• Mining and mineral processing pipelines where high-abrasion slurries are transported.

The 3-12 times improvement in corrosion-wear resistance compared to carbonitrided 35CrMo steel represents a significant economic benefit, as it can extend the service life of piping components and reduce maintenance costs.

Heat Treatment Process Control

The heat treatment of overlay layers requires careful process control to avoid defects such as:

The following process parameters are critical for the heat treatment of D618 overlay layers:

• Heating temperature: 850°C is optimal for austenitization without excessive carbide coarsening.
• Cooling rate: Air cooling provides a good balance between hardness and toughness; oil quenching provides higher hardness but increased cracking risk.
• Tempering temperature: 400°C is optimal for relieving residual stresses while maintaining high hardness.
• Holding time: Sufficient time is required for uniform temperature distribution and carbide transformation.

Quality Control and Inspection

For D618 overlay layer applications in piping systems, the following quality control measures are recommended:

1. Hardness testing: Vickers hardness testing (HV 10 or HV 30) at multiple locations across the overlay surface and at different depths to verify uniform hardness.
2. Microstructural examination: Metallographic examination of cross-sections to verify the microstructure, carbide distribution, and absence of defects.
3. Wear testing: Dry sliding or three-body abrasion testing to validate the wear resistance under simulated service conditions.
4. Corrosion testing: Potentiodynamic polarization or immersion testing to verify the corrosion resistance of the overlay layer.
5. Bond strength testing: Peel or tensile bond strength testing to ensure adequate adhesion between the overlay and the base metal.

Comparison with Alternative Surface Treatment Methods

The study provides a valuable comparison between overlay cladding and surface treatment methods (such as carbonitriding) for improving wear resistance. The key advantages of overlay cladding include:

• Thicker hard layer: Overlay cladding provides a through-thickness hard layer, whereas surface treatments only modify the surface chemistry.
• Higher hardness: Overlay cladding can achieve higher hardness levels than surface treatments.
• Better corrosion resistance: Overlay cladding with high-chromium alloys provides inherent corrosion resistance.
• Repair capability: Worn overlay layers can be removed and re-applied, whereas surface treatments require re-treatment of the entire component.

However, overlay cladding also has disadvantages:

• Higher cost: Overlay cladding requires additional material and processing steps.
• Residual stresses: Overlay welding introduces residual stresses that require stress relief.
• Potential for defects: Overlay welding can introduce defects such as porosity, cracking, and incomplete fusion.

Key Questions and Reflections

The study raises several important considerations for further investigation:

1. Long-term stability: How does the D618 overlay layer perform under prolonged exposure to high-temperature, high-pressure service conditions? Is there a risk of carbide coarsening or phase transformation that could degrade the properties over time?
2. Thermal cycling resistance: In piping systems exposed to thermal cycling, the overlay layer must maintain its microstructure and bond strength over thousands of thermal cycles. How does the D618 alloy perform under such conditions?
3. Multi-layer overlay design: Could a multi-layer approach—starting with a transition layer for good bonding and followed by multiple D618 layers—further enhance the service life of the overlay?
4. Alternative alloy compositions: While D618 is effective, could alternative high-carbon, high-chromium compositions (such as those with added vanadium, tungsten, or cobalt) provide even better performance? What are the cost-performance trade-offs?
5. Welding process optimization: The study focuses on heat treatment, but the welding process itself significantly influences the as-deposited microstructure. How do variations in welding parameters (heat input, travel speed, current type) affect the final properties of the overlay layer?

Study Insights and Implications

This study provides valuable guidance for the heat treatment optimization of D618-type hard alloy overlay layers. The key finding is that both air cooling and oil quenching + tempering at 850°C produce high hardness and excellent wear resistance, with the latter providing slightly better properties at the expense of increased cracking risk.

The 3-12 times improvement in corrosion-wear resistance compared to carbonitrided 35CrMo steel demonstrates the significant advantage of overlay cladding over surface treatment methods for applications where erosion-corrosion is a primary concern. This improvement translates directly into extended service life and reduced maintenance costs for piping systems.

For the piping and fitting industry, this research highlights the importance of post-weld heat treatment in achieving optimal overlay performance. The heat treatment conditions must be carefully optimized to balance hardness, toughness, and corrosion resistance, and the resulting overlay layer must be thoroughly inspected and tested to ensure it meets the required performance criteria.

The practical implication is that D618-type hard alloy overlays, when properly heat treated, represent a viable and cost-effective solution for protecting piping components against erosion-corrosion damage. Engineers involved in the specification and qualification of overlay cladding systems should incorporate these findings into their design criteria, particularly for applications where wear resistance is a critical requirement.