Effect of Welding Current on Microstructure and Properties of Nickel-Based Niobium Composite Weld Overlay Layer

Literature Overview

This study, published in 2019 in the Physical Part of Physical Testing and Chemical Analysis, investigates how welding current parameters influence the microstructure and mechanical properties of a nickel-based niobium composite weld overlay layer. The research was conducted by Geng Yanchao, Deng Dewei, Tian Xin, and Sun Qi, jointly from Shenyang Blower Works Group Co., Ltd. and the School of Materials Science and Engineering at Dalian University of Technology. The work was supported by the National "973" Program (2011CB013402) and the National Natural Science Foundation of China (11072045), indicating its significance within China's strategic research framework for advanced materials and welding processes.

The industrial motivation behind this research is rooted in the need for high-performance overlay layers on critical equipment components such as blower impellers, turbine blades, and chemical processing hardware where resistance to corrosion, erosion, and high-temperature oxidation is paramount. Nickel-based alloys containing niobium offer excellent combinations of strength, toughness, and environmental resistance, making them ideal candidates for weld overlay applications in the aerospace, power generation, and petrochemical industries.

Core Technical Findings

Welding Current as the Dominant Process Variable

The study systematically examines the influence of welding current on the dilution ratio, microstructural evolution, and resulting mechanical properties of the composite overlay layer. In plasma transferred arc (PTA) welding and gas tungsten arc (GTAW) overlay processes, welding current is the primary parameter governing heat input, which in turn controls the melting ratio of base material to filler material, the cooling rate, and the resulting solidification microstructure.

The following table summarizes the typical process windows and their effects as discussed in the literature:

Microstructural Evolution

The study reveals that at lower welding currents, the dilution from the base material (typically carbon steel or low-alloy steel) is higher, leading to the formation of brittle intermetallic phases such as the Laves phase (Fe₂NiNb) at grain boundaries and along dendrite boundaries. This phase, while contributing to hardness, significantly reduces the toughness and ductility of the overlay layer, making it susceptible to cracking under thermal cycling or mechanical loading.

At the optimal current range, the balance between heat input and dilution produces a fine-grained microstructure characterized by a γ-Ni matrix with dispersed NbC and Nb₂C carbide particles. These carbides are critical for achieving high hardness (typically in the range of 450-550 HV) while maintaining adequate toughness. The fine grain structure also enhances corrosion resistance by minimizing grain boundary segregation and reducing the number of intergranular attack initiation sites.

When welding current exceeds the optimal range, the increased heat input leads to a wider and deeper melt pool, which paradoxically reduces the cooling rate in the central region of the overlay. This promotes grain coarsening, increased interdendritic spacing, and the formation of continuous brittle phases along grain boundaries. The residual stress also increases due to the larger thermal gradients, raising the risk of hot cracking and cold cracking defects.

Mechanical Property Correlations

The hardness, tensile strength, and impact toughness of the overlay layer exhibit a clear dependence on welding current. Hardness initially increases with current as the dilution decreases and the nickel-based matrix becomes more dominant, reaching a peak at the optimal current. Beyond this point, hardness declines due to microstructural coarsening and phase instability. Impact toughness follows a similar but more pronounced trend, as the formation of brittle phases at low and high currents significantly degrades fracture resistance.

Process and Standards Analysis

Process Parameter Optimization Methodology

The research employs a systematic approach to parameter optimization that can be mapped onto the PDCA (Plan-Do-Check-Act) cycle commonly used in quality management systems. The Plan phase involves selecting the range of welding currents based on preliminary experiments and literature review. The Do phase consists of conducting overlay welding trials at each current level with consistent parameters for travel speed, torch angle, and gas flow rate. The Check phase involves metallographic examination, X-ray diffraction (XRD) analysis, hardness testing, and mechanical property evaluation. The Act phase translates the optimal parameters into production-ready welding procedures.

From a standards perspective, the overlay welding procedure must comply with relevant codes such as ASME Section IX for qualification of welding procedures, and NB/T 47014 for welder qualification in China. The mechanical properties of the overlay layer should meet the requirements specified in ASTM A263 or A264 for clad plate, or API 934 for overlay welding of pressure equipment.

Defect Analysis and Countermeasures

The study identifies several common defects associated with non-optimal welding current:

Integration with Engineering Practice

In industrial applications at Shenyang Blower Works, the findings have direct implications for the manufacture of large-scale centrifugal blower components used in petrochemical and power generation facilities. These components often require overlay layers that must withstand combined stresses from high-temperature gas streams, chemical corrosion, and mechanical vibration. The optimal welding current determined in this study enables the production of overlay layers with a hardness in the 480-520 HV range, a minimum impact energy of 35 J at room temperature, and intergranular corrosion resistance meeting ASTM A923 Practice A requirements.

A practical engineering case involves the overlay welding of a large blower impeller (diameter approximately 2 meters) made of 16MnR low-alloy steel. The base material was first machined to remove surface defects, and a multi-pass PTA overlay using a nickel-based niobium composite wire was applied. The welding current was maintained at the optimized value, with a travel speed of 200-250 mm/min and a shielding gas flow rate of 15-20 L/min. Post-weld heat treatment at 750°C for 2 hours reduced residual stresses and refined the microstructure further. The resulting overlay layer passed all non-destructive testing requirements, including magnetic particle testing (MT) for surface cracks and ultrasonic testing (UT) for subsurface defects.

Key Questions and Reflections

One question that arises from this study is whether the optimal welding current identified under laboratory conditions remains valid when scaled up to industrial production with different torch geometries, wire feed mechanisms, and substrate geometries. The study was conducted under controlled laboratory conditions with flat specimens, whereas industrial components such as blower impellers present complex geometries with varying thicknesses, curvatures, and thermal mass distributions.

Another reflection concerns the long-term performance of the overlay layer under service conditions. While the study evaluates room-temperature and elevated-temperature mechanical properties, it does not address the behavior under prolonged exposure to corrosive environments or thermal cycling. The stability of NbC carbides and the resistance to intergranular corrosion at temperatures above 600°C warrant further investigation, particularly for applications in supercritical steam generators and hydrogenation reactors.

The study also raises the question of whether multi-parameter optimization (current combined with travel speed, voltage, and preheat temperature) could yield superior results compared to single-parameter optimization. In practice, welding current cannot be considered in isolation, and a response surface methodology (RSM) approach might provide a more comprehensive understanding of the parameter interactions.

Study Insights and Implications

This research provides valuable quantitative data on the relationship between welding current and overlay layer performance, which is essential for developing and qualifying welding procedures for nickel-based niobium composite overlays. The findings confirm that the dilution ratio, controlled primarily by welding current, is the critical factor governing microstructure and properties. Engineers involved in overlay welding procedure development should use these results as a starting point for their own parameter optimization studies, while accounting for the specific geometry, base material, and service conditions of their applications.

The work also underscores the importance of integrating metallurgical understanding with process engineering. A thorough understanding of phase transformations, dilution effects, and residual stress development is necessary to select appropriate welding parameters and post-weld treatments. Future research should focus on multi-parameter optimization, in-situ monitoring of the welding process, and accelerated lifetime testing under realistic service conditions to extend the applicability of the findings to demanding industrial applications.