CO2 Arc Welding of Iron-Based Overlay Alloys Microstructure and Performance

Literature Overview

The paper published in 2011 in the journal "Ordnance Materials and Science and Engineering" by Shi Haifang, Zhang Bo, Hu Shijie, and Jiang Xintong from Liaoning Technical University and Fuxin Higher Vocational College investigates the microstructure evolution and mechanical properties of iron-based alloy overlay deposits produced using CO2 gas metal arc welding (GMAW-C). This work is particularly relevant for engineers dealing with heavy-duty wear-resistant cladding applications in mining, construction, and power generation equipment. The authors systematically examined how the CO2 shielding gas environment influences the dilution rate, phase composition, hardness distribution, and wear resistance of the overlay layer deposited on carbon steel substrates.

Core Technical Points

The study focuses on several critical aspects of CO2 arc welding overlay technology. The CO2 shielding atmosphere creates a highly oxidizing environment that promotes the formation of hard carbide phases, particularly Fe3C and Cr7C3, which are essential for achieving high hardness in the overlay deposit. The iron-based alloy consumables used typically contain chromium in the range of 12 to 30 percent by mass, with additional alloying elements such as molybdenum, vanadium, and tungsten to enhance carbide formation and improve thermal stability.

The microstructure of the CO2 arc welded overlay is characterized by a dendritic cellular grain structure with carbide particles precipitating along the interdendritic regions. The authors observed that the primary phase consists of martensite or bainite depending on the cooling rate and alloy composition, while the secondary hard phase comprises a mixture of chromium carbides and iron carbides. The hardness of the overlay layer typically ranges from 50 to 65 HRC, which is significantly higher than the base metal hardness of 20 to 25 HRC.

Key Process Parameters

Microstructure Analysis and Performance Evaluation

The microstructural examination reveals that the CO2 atmosphere leads to a distinct composition gradient from the fusion line to the surface of the overlay. Near the fusion line, the dilution rate is highest, resulting in a lower carbon and chromium content and consequently reduced hardness. Moving away from the fusion line toward the free surface, the alloying element concentration approaches that of the consumable wire, and the hardness increases correspondingly. The authors reported that the transition zone between the base metal and the overlay typically exhibits a narrow diffusion layer of 0.1 to 0.3 mm, which is critical for bond strength evaluation.

The wear resistance testing, conducted using the pin-on-disk method against alumina counterfaces, demonstrated that the CO2 arc welded overlay provides a 3 to 5 times improvement in wear life compared to the uncladded base metal. The wear mechanism is predominantly abrasive, with the hard carbide phases acting as the primary wear-resisting components. However, the study also identified a potential concern: the high dilution rate associated with CO2 welding can lead to localized soft spots if the process parameters are not carefully controlled, particularly at the beginning and end of each weld pass.

Common Defects and Countermeasures

Engineering Practice Implications

From an engineering practice standpoint, this research highlights the importance of dilution rate control in CO2 arc welding overlay applications. In actual production environments, such as the cladding of excavator buckets, crane hooks, and crusher hammers, the dilution rate can vary significantly due to operator technique, joint preparation, and environmental conditions. The study provides a valuable reference for establishing process windows that ensure consistent hardness and wear resistance across the entire cladded surface.

A practical consideration that emerges from this work is the trade-off between deposition efficiency and overlay quality. CO2 arc welding offers excellent productivity compared to more controlled processes like plasma transferred arc (PTA) or laser cladding, but achieving uniform microstructure and properties requires rigorous process monitoring. Engineers should consider implementing multi-pass strategies with intermediate grinding to reduce the effective dilution rate and improve the homogeneity of the overlay layer.

Key Reflections and Study Insights

The most significant insight from this paper is the recognition that CO2 arc welding, despite its simplicity and cost-effectiveness, can produce overlay deposits with competitive performance when properly executed. The study reinforces the principle that the final properties of a weld overlay are not solely determined by the consumable composition but are equally dependent on the process parameters and the resulting dilution behavior. For engineers working in the field of bimetal product manufacturing, this means that process qualification testing in accordance with NB/T 47014 or ASME IX should be conducted with particular attention to the dilution rate measurement and hardness profiling across the overlay cross-section.

Furthermore, the research underscores the need for proper heat input management. Excessive heat input leads to grain coarsening and carbide dissolution, which degrades both hardness and wear resistance. In practice, this translates to the need for careful selection of wire diameter, current, voltage, and travel speed combinations, as well as the potential use of multi-pass techniques with controlled interpass temperatures. The study serves as a reminder that even conventional welding processes can be optimized to deliver high-performance overlay solutions when the metallurgical fundamentals are well understood and applied systematically.