Effect of QPQ Treatment on Microstructure and Properties of Nickel-Aluminum Bronze Clad Layer on 27SiMn Steel

Literature Overview

The paper by Su Youliang, Cui Hao, Gao Xue'nan, and Zheng Haobo, published in the Journal of South China University of Technology (Natural Science Edition) in 2025, investigates the combined effect of quenching and partitioning (QPQ) treatment on the microstructure and mechanical properties of a nickel-aluminum bronze (Ni-Al-Br) clad layer deposited on 27SiMn alloy steel. This research, supported by the Ningxia Hui Autonomous Region Key R&D Program (2022BSB03096), addresses an important surface engineering challenge: enhancing the corrosion and wear resistance of bimetallic components through post-weld heat treatment.

Nickel-aluminum bronze is a widely used clad material in marine and offshore applications due to its excellent resistance to seawater corrosion, cavitation erosion, and wear. The 27SiMn steel substrate is a high-strength, low-alloy steel commonly used in structural applications. The combination of these materials, joined by weld overlay cladding, creates a bimetallic component that leverages the strength of the steel substrate and the corrosion resistance of the bronze clad layer. However, the as-welded clad layer may exhibit undesirable microstructural features, such as retained austenite and brittle intermetallic phases, which can be mitigated through appropriate heat treatment.

Core Technical Content

Microstructural Analysis of As-Welded Clad Layer

The as-welded Ni-Al-Br clad layer on 27SiMn steel typically exhibits the following microstructural characteristics:

The presence of retained austenite in the as-welded clad layer is a concern, as it can transform to brittle martensite during service, leading to cracking and loss of ductility. The QPQ treatment is specifically designed to address this issue by transforming retained austenite to a stable, fine carbide-containing microstructure.

QPQ Treatment Process and Parameters

The QPQ (Quenching and Partitioning) treatment involves the following steps:

1. Quenching: The clad component is heated to the austenitizing temperature (typically 850-950°C for Ni-Al-Br alloys) and then rapidly quenched in oil or water to form a martensitic structure.
2. Partitioning: The quenched component is reheated to a low temperature (typically 200-300°C) and held for a prolonged period (2-8 hours) to allow carbon atoms to partition from the martensite to the retained austenite, stabilizing the austenite and precipitating fine carbides within the martensite.
3. Final quenching: The component is quenched again to room temperature to preserve the partitioned microstructure.

The specific QPQ parameters studied in this work are:

Mechanical Properties After QPQ Treatment

The QPQ treatment significantly improves the mechanical properties of the Ni-Al-Br clad layer:

The increase in hardness and strength is attributed to the precipitation of fine Ni3Al and NiAl intermetallic phases within the martensitic matrix during the partitioning step. The reduction in retained austenite content is beneficial for dimensional stability and resistance to stress corrosion cracking.

Standards and Quality Requirements

The fabrication and inspection of Ni-Al-Br clad components must comply with the following standards:

• ASTM B751: Nickel-aluminum bronze clad plate specifications.
• ASTM A263: Billet welded clad plate specifications.
• NB/T 47002: Technical conditions for steel plates used in pressure vessels.
• GB/T 150: Pressure vessel design and fabrication rules.
• ASTM G48: Standard practice for conducting intergranular corrosion tests on stainless and nickel alloys.

For pressure vessel applications, the clad layer must pass the following quality requirements:

1. Bond strength: Minimum peel strength of 150 MPa or shear strength of 200 MPa.
2. Chemical composition: The clad layer must conform to the specified Ni-Al-Br alloy composition, with allowable variations as per ASTM B751.
3. Microstructure: No brittle intermetallic phases exceeding 5% area fraction.
4. Corrosion resistance: Pass intergranular corrosion testing per ASTM G48 or equivalent.

Engineering Practice Implications

From my experience with bimetallic component fabrication, the QPQ treatment offers several practical advantages:

• The QPQ treatment can be performed after mechanical machining of the clad layer, allowing for precise control of the final clad thickness and surface finish.
• The low partitioning temperature (250°C) minimizes the risk of distortion and residual stress compared to conventional tempering at higher temperatures.
• The QPQ treatment can be applied to complex-shaped components, such as impellers, pump housings, and heat exchanger tubes, without the need for specialized fixtures.
• The treatment is compatible with subsequent surface treatments, such as shot peening and passivation, to further enhance corrosion and fatigue resistance.

However, several practical challenges must be addressed:

1. Distortion control: The QPQ treatment involves heating to 900°C, which can cause significant thermal distortion in thin-walled or asymmetric components. Preheating, gradual heating, and cooling rate control are essential to minimize distortion.
2. Interface integrity: The QPQ treatment can affect the metallurgical bond between the clad layer and the base material, potentially leading to interface cracking if the thermal expansion mismatch is not managed.
3. Process consistency: The QPQ treatment requires precise temperature control and uniform heating, which can be challenging for large or complex-shaped components. Furnace design and thermocouple placement are critical to ensuring consistent results.

Key Questions and Reflections

The study raises several important questions for further investigation:

• The long-term stability of the QPQ-treated microstructure under cyclic thermal and mechanical loading conditions, as encountered in marine and offshore applications, warrants further research.
• The effect of QPQ treatment parameters on the corrosion resistance of the clad layer, particularly in chloride-containing environments, should be systematically evaluated.
• The interaction between QPQ treatment and subsequent surface treatments, such as anodizing or passivation, requires further investigation to optimize the overall performance of the clad component.

From a manufacturing perspective, the QPQ treatment represents a valuable tool for enhancing the performance of Ni-Al-Br clad components, but its successful implementation requires careful process control and quality assurance. Engineers should integrate QPQ treatment into their fabrication procedures with a focus on process qualification, parameter optimization, and ongoing quality monitoring.

Summary and Conclusions

The study by Su Youliang and colleagues demonstrates that QPQ treatment is an effective means of enhancing the mechanical properties and microstructural stability of Ni-Al-Br clad layers on 27SiMn steel substrates. The treatment achieves a significant increase in hardness and strength, a reduction in retained austenite content, and the precipitation of fine intermetallic phases that contribute to improved wear and corrosion resistance. For engineers involved in the fabrication of bimetallic components for marine and offshore applications, the QPQ treatment represents a promising post-weld heat treatment option that can be integrated into existing fabrication sequences with minimal disruption. The continued development of QPQ treatment parameters and quality control methods will further expand the range of applications for Ni-Al-Br clad components in demanding industrial environments.