Study on Microstructure and Mechanical Properties of Multi-Pass Submerged Arc Cladding

Literature Overview

This research article, published in 2015 in the Journal of Shanghai University of Engineering Science and funded by the National Natural Science Foundation of China (Grant No. 51305253), was authored by He Xiaofeng, Lu Qinghua, Peng Birong, Chen Xuan, and Zhang Jing from the School of Materials Engineering at Shanghai University of Engineering Science. The study investigates the microstructural evolution and mechanical properties of multi-pass submerged arc welding (SAW) cladding deposits, providing fundamental understanding of how welding parameters and pass sequence influence the final cladding quality. This work bridges the gap between fundamental materials science and practical cladding engineering, offering insights that are directly applicable to the design and optimization of multi-pass cladding procedures for pressure vessels and other critical equipment.

Core Technical Analysis

Multi-pass SAW cladding is the predominant method for achieving thick corrosion-resistant overlays on carbon steel or low-alloy steel substrates, particularly in applications requiring overlay thicknesses exceeding 5 mm where single-pass or strip electrode methods are insufficient. The fundamental challenge in multi-pass cladding lies in the dilution control and microstructural uniformity across passes. Each subsequent pass is deposited on a previously solidified layer that has already undergone a thermal cycle, creating a complex thermal history that influences the final microstructure and properties.

The study examines how the welding current, voltage, travel speed, interpass temperature, and the number of passes affect the dilution ratio, grain structure, phase composition, and mechanical properties of the cladding deposit. Dilution is defined as the percentage of base metal alloying elements present in the final cladding composition, and it is critical because excessive dilution can compromise the corrosion resistance of the cladding layer by reducing the chromium and molybdenum content below the required threshold for the intended service environment.

Microstructural Evolution

The microstructure of multi-pass SAW cladding typically consists of a columnar grain structure in the early passes near the base metal interface, transitioning to a more equiaxed structure in the later passes near the surface. This columnar-to-equiaxed transition is governed by the thermal gradient and growth rate, which are influenced by the interpass temperature and welding parameters. The columnar grains in the first pass grow perpendicular to the fusion line, and if the interpass temperature is too high, these grains can grow through subsequent passes, creating a continuous columnar structure that may be susceptible to transverse cracking.

The study highlights that the heat input per pass, calculated as Q = (I × V) / v where I is current, V is voltage, and v is travel speed, is the primary parameter controlling the grain structure. Higher heat input promotes coarser grains and greater dilution, while lower heat input results in finer grains but may lead to incomplete fusion. The optimal heat input window for multi-pass cladding typically falls between 15 and 30 kJ/mm, depending on the specific alloy system and required properties.

Mechanical Properties

The mechanical properties of multi-pass cladding deposits, including hardness, tensile strength, and impact toughness, vary significantly across the cladding thickness due to the differing thermal histories of each pass. The first pass, deposited directly on the base metal, experiences the highest thermal gradient and typically exhibits the highest hardness due to the formation of hard intermetallic phases at the dilution boundary. Subsequent passes, deposited on cooler, previously solidified material, generally exhibit lower hardness and better toughness.

The study presents data showing that the hardness profile across a multi-pass cladding deposit can vary by 50-100 HV from the base metal interface to the free surface. This hardness gradient is a direct consequence of the varying dilution and microstructure at different depths. For engineering applications, the hardness of the cladding layer should be maintained below 350 HV to avoid brittle fracture, and the hardness difference between the cladding and the base metal should be limited to prevent stress concentration at the interface.

The following table summarizes typical mechanical property requirements for multi-pass SAW cladding deposits:

Process Optimization Insights

The research demonstrates that controlling the interpass temperature is as critical as controlling the welding parameters themselves. An interpass temperature that is too low can lead to incomplete fusion between passes, while an interpass temperature that is too high promotes excessive grain growth and dilution. The recommended interpass temperature range for austenitic stainless steel cladding is typically 150 to 250 degrees Celsius, with monitoring performed using infrared thermometers or thermocouples.

The study also addresses the effect of welding sequence on the final properties. A zigzag or weave pattern within each pass, combined with a proper pass sequence that avoids overlapping high-heat-input zones, can improve the uniformity of the cladding deposit. The recommended pass sequence for multi-pass cladding is to start from the center and work outward, or to use a backstep sequence that limits the maximum interpass temperature.

Study Insights and Engineering Implications

This research provides a rigorous scientific foundation for the optimization of multi-pass SAW cladding procedures. The key insight is that microstructure and properties in multi-pass cladding are not simply the sum of individual pass contributions but are the result of a complex interaction between the thermal history of each pass and the existing microstructure of the previously deposited material. Engineers must therefore approach multi-pass cladding as a systems engineering problem, where each pass is designed not only for its own quality but also for its contribution to the final property profile of the entire cladding deposit. The emphasis on dilution control, interpass temperature management, and pass sequence optimization represents a mature understanding of cladding metallurgy that should inform all procedure qualification and production welding activities.