Current Research Status of Iron-Based Wear-Resistant Overlay Alloys

Literature Overview

This review article, published in 2012 in the journal "Electric Welding Machine," was authored by researchers from the School of Materials Science and Engineering at Henan University of Science and Technology and the Henan Provincial Engineering Technology Research Center for Wear-Resistant Materials. Supported by the Henan Provincial Science and Technology Program, the paper provides a comprehensive overview of the development and current state of iron-based wear-resistant overlay alloys, which are extensively used in mining, agriculture, construction, and power generation industries.

Classification and Composition of Iron-Based Overlay Alloys

Iron-based wear-resistant overlay alloys are broadly classified according to their microstructure and hardening mechanism. The primary categories include martensitic, martensitic-carbide, austenitic, and austenitic-carbide alloys. Each category offers distinct combinations of hardness, toughness, and wear resistance suited to different service conditions.

The dilution rate in iron-based overlay welding is typically lower than in nickel-based systems, with values ranging from 5% to 25% depending on the welding process and substrate material. The lower dilution is advantageous because it allows the overlay composition to more closely match the intended alloy design, resulting in more predictable mechanical properties.

Welding Processes for Iron-Based Overlay Deposition

Multiple welding processes are employed for depositing iron-based wear-resistant overlays, each with distinct advantages and limitations. Submerged arc welding (SAW) is widely used for thick overlay deposits on large components due to its high deposition rate and low spatter. Flux-cored arc welding (FCAW) offers good deposition rates with wire and flux combinations that can be tailored to specific alloy compositions. Gas metal arc welding (GMAW) provides good control over dilution and is suitable for thin overlay layers on smaller components. Gas tungsten arc welding (GTAW/TIG) is reserved for repair and low-dilution applications where precise heat input control is required.

Process Comparison for Iron-Based Overlay Welding

Microstructure and Wear Mechanisms

The wear resistance of iron-based overlay alloys is primarily governed by the hardness, toughness, and morphology of carbide phases. In martensitic-carbide alloys, the carbides are typically M₇C₃, M₂C, or MC type, depending on the carbon and alloying element content. The size, distribution, and volume fraction of these carbides directly influence the wear resistance. Fine, uniformly distributed carbides provide superior wear resistance, while coarse, segregated carbides can lead to premature wear through microcracking and spalling.

The matrix microstructure also plays a critical role. Martensitic matrices offer high hardness but limited toughness, making them susceptible to cracking under impact loading. Austenitic matrices, while softer, provide excellent impact resistance through work hardening, making them suitable for applications involving abrasive impact. The optimal alloy design often involves a composite structure where hard carbides are dispersed in a tough matrix, combining the benefits of both microstructural features.

Engineering Applications and Quality Control

Iron-based overlay alloys find extensive application in components subject to severe wear, including excavator buckets, crusher hammers, mill liners, conveyor rollers, and pump impellers. In pressure vessel fabrication, overlay welding is used to protect heat exchanger tubes, reactor internals, and piping systems from erosion-corrosion in slurry service.

Quality control for iron-based overlay welds involves several critical steps. First, the substrate surface must be prepared by machining or grinding to remove scale, rust, and contaminants. The overlay weld itself must be inspected for porosity, lack of fusion, and cracking using MT, PT, or UT methods. The overlay thickness and dilution rate must be verified through macrographical examination, with the bond line typically required to be less than 1 mm per applicable standards such as NB/T 47002 or ASME VIII Div.1.

Key Questions and Reflections

A persistent challenge in iron-based overlay welding is the formation of hot cracks in the overlay weld metal, particularly in high-carbon martensitic-carbide alloys. The susceptibility to cracking is influenced by the sulfur and phosphorus content of the consumables, the cooling rate, and the restraint imposed by the substrate. Engineers must select low-sulfur, low-phosphorus consumables and employ appropriate preheating and post-weld cooling strategies to minimize cracking risk.

Another important consideration is the effect of welding sequence on the residual stress distribution and distortion of the overlaid component. For large structural components such as mill liners or conveyor rollers, the welding sequence must be carefully planned to minimize distortion while ensuring uniform overlay thickness. The use of back-step welding, stitch welding, or symmetric welding sequences are practical strategies for controlling distortion.

Study Insights and Implications

This review provides a valuable synthesis of the knowledge base surrounding iron-based wear-resistant overlay alloys. The findings emphasize that alloy selection must be driven by a clear understanding of the service conditions, including the type of wear (abrasive, adhesive, impact, or erosion-corrosion), the operating temperature, and the environmental factors. A martensitic-carbide alloy may be optimal for dry abrasive wear, while an austenitic-carbide alloy may be preferred for wet or corrosive environments.

For engineers involved in pressure vessel and equipment fabrication, the practical implication is that overlay welding specifications must be rigorously developed and qualified. The welding procedure specification (WPS) must define all critical variables, including consumable selection, preheat temperature, interpass temperature, heat input, and post-weld treatment. Performance qualification testing must demonstrate that the overlay meets the required hardness, wear resistance, and bond strength criteria. The review also underscores the importance of ongoing research into novel alloy compositions and welding processes that can further enhance the performance of iron-based overlay systems.