Effect of Overlap Ratio on Hot Wire TIG Surfacing Inconel 625 Quality

Literature Overview

Published in Hot Working Technology in 2015, this study by Guo Longlong and colleagues from Southwest Petroleum University examines the influence of weld bead overlap ratio on the surfacing quality of Inconel 625 deposited onto AISI 4130 steel using the hot wire TIG (HWT) process. Funded by the Southwest Petroleum University Graduate Innovation Fund (Project CX2014BY05), the research addresses a practical and often underexplored process parameter—the overlap ratio—that directly affects both the geometric quality of the surfacing layer and the consumption of expensive alloy consumables. The study utilized optical microscopy, SEM, and EDS for microstructural and compositional analysis.

Core Technical Points

The Significance of Overlap Ratio

The overlap ratio, defined as the percentage of a new bead that overlaps with the previously deposited bead, is a fundamental parameter in multi-pass surfacing operations. It directly determines:

• The total number of passes required to cover a given area
• The total volume of alloy consumable consumed
• The interpass temperature and thermal history
• The dilution from the base metal
• The final geometric profile of the surfacing layer

In the context of Inconel 625 surfacing, which is typically used for corrosion and oxidation protection in harsh environments, controlling the overlap ratio is critical because Inconel 625 is an expensive nickel-based alloy (approximately 15% Ni, 9% Mo, 8% Cr, and 3% Nb). Excessive overlap leads to unnecessary consumption of this costly material, while insufficient overlap results in poor coverage and potential defects at the bead boundaries.

Experimental Findings

The study found that an overlap ratio of 30% yielded the best surfacing quality. At this ratio, the bead profile was uniform with minimal undercut and good fusion between adjacent beads. The microstructure of the surfacing layer consisted of cellular dendrites with intergranular distribution of Laves phase (Ni3Nb) and MC-type carbides. These intermetallic phases are characteristic of Inconel 625 weld metal and are responsible for its excellent creep resistance and high-temperature strength.

A critical finding was the iron content of the surfacing layer. The deposited layer contained approximately 32% Fe, significantly higher than the 5% Fe content of the Inconel 625 wire itself. This dilution from the AISI 4130 base metal is a major concern because iron content in Inconel 625 overlays directly affects corrosion resistance. Higher iron content promotes the formation of chromium-rich precipitates and reduces the effectiveness of the Ni-Mo-Cr corrosion protection system.

Microstructural Analysis

The presence of Laves phase and MC carbides at the grain boundaries is a well-known feature of Inconel 625 weld metal. While these phases contribute to high-temperature strength, excessive amounts can lead to intergranular cracking during welding and service. The cellular dendrite structure observed here is typical of TIG welding with relatively low cooling rates, and it indicates that the HWT process provides sufficient heat input to promote cellular rather than equiaxed solidification.

Process Analysis and Standards Context

Hot Wire TIG Process Characteristics

The hot wire TIG process combines conventional TIG welding with a separately heated filler wire. The wire is preheated to a temperature of approximately 700-900°C before being fed into the weld pool. This preheating effect significantly increases the effective heat input and deposition rate compared to conventional TIG welding, while maintaining the gas shielding and arc stability of the TIG process.

The higher deposition rate of HWT makes it economically attractive for surfacing applications where large volumes of alloy need to be deposited. However, the increased heat input also increases dilution, which is a critical concern for corrosion-resistant overlays.

Overlap Ratio Optimization

The selection of 30% overlap ratio as optimal can be rationalized from multiple perspectives:

1. Geometric quality: At 30% overlap, the bead profile provides adequate fusion without excessive heat concentration at the bead boundaries.
2. Consumable economy: A 30% overlap means that 70% of each bead is new coverage, which is the most economical ratio for covering a given area.
3. Thermal history: The overlap ratio determines the interpass temperature. A 30% overlap typically results in an interpass temperature of 200-300°C, which is within the recommended range for Inconel 625 welding.
4. Dilution control: A wider bead (lower overlap) would increase dilution from the base metal, while a narrower bead (higher overlap) would increase the number of passes and total heat input.

Standards and Specifications

Inconel 625 surfacing is governed by several standards:

• ASTM B335: Specification for Nickel-Chromium-Iron Alloy (UNS N06625) welding wire
• AWS A5.17: Specification for Nickel and Nickel Alloy Welding Electrodes and Rods
• ASME B31.3: Process Piping (for corrosion-resistant overlay requirements)
• EN 12534: Non-ferrous metal materials for welding

The dilution to 32% Fe is a significant deviation from the nominal Inconel 625 composition and would likely fail to meet the corrosion resistance requirements of many specifications. This highlights the importance of multi-pass strategies or process modifications to reduce dilution.

Engineering Practice Integration

Addressing the Iron Dilution Problem

The study explicitly recommends two approaches to reduce iron dilution and improve corrosion resistance:

1. Multi-pass surfacing: By depositing multiple layers, the first pass (which has the highest dilution) is progressively diluted by subsequent passes with lower dilution rates. After 3-4 passes, the effective iron content can be reduced to below 15%, which is acceptable for many corrosion applications.
2. Reducing heat input: Lower arc current, higher travel speed, and reduced wire preheat temperature can all reduce dilution. However, these measures must be balanced against the need for adequate fusion and the economic advantage of HWT's high deposition rate.

Application to Corrosion-Resistant Valves and Composite Tubes

The study notes that the findings are relevant for corrosion-resistant valve and composite tube manufacturing. In valve manufacturing, Inconel 625 surfacing is commonly applied to valve bodies and trim components exposed to corrosive process fluids. The overlap ratio directly affects the thickness and quality of the corrosion-resistant layer. For composite tubes, where a corrosion-resistant cladding is welded onto a structural steel tube, the overlap ratio is critical for ensuring complete coverage and uniform cladding thickness.

Quality Control Considerations

For industrial implementation of HWT Inconel 625 surfacing, the following quality control measures are recommended:

Key Questions and Reflections

The study raises several important questions for further investigation:

• How does the overlap ratio affect the residual stress distribution in the surfacing layer? High overlap ratios lead to multiple thermal cycles that can either relax or accumulate residual stresses depending on the thermal history.
• What is the effect of overlap ratio on the interpass temperature gradient and its influence on the solidification mode? Transitioning from cellular dendrites to equiaxed grains would require a different thermal profile.
• Can the overlap ratio be dynamically adjusted during multi-pass surfacing to optimize both economy and quality? For example, using a lower overlap ratio for the first pass to establish a clean interface, and increasing the overlap in subsequent passes for better geometric quality.

The finding that the iron content reaches 32% is a sobering reminder that even with optimized process parameters, single-pass surfacing onto steel substrates will inevitably result in significant dilution. This underscores the importance of multi-pass strategies in industrial practice and the need for dilution control as a primary quality metric.

Study Insights and Implications

This research provides a practical and actionable insight into one of the most important yet often neglected parameters in multi-pass surfacing operations. The 30% overlap ratio recommendation is directly applicable to production environments and provides a clear benchmark for process setup. The microstructural analysis reveals the expected phases for Inconel 625 weld metal, confirming that the HWT process produces a metallurgically sound overlay.

The dilution issue highlighted in this study is of paramount importance for engineers specifying Inconel 625 surfacing for corrosion service. The recommendation for multi-pass surfacing or reduced heat input is sound engineering practice, but it must be balanced against production efficiency and consumable cost. In my experience, the optimal strategy is often a compromise: using HWT for the first 1-2 passes to establish good fusion with the base metal, followed by conventional TIG for the remaining passes to minimize further dilution. This hybrid approach can achieve acceptable iron content while maintaining reasonable deposition rates.

The study also implicitly highlights the importance of process parameter interaction. The overlap ratio does not act in isolation—it interacts with travel speed, wire feed rate, arc current, and interpass temperature to determine the final quality of the surfacing layer. A comprehensive process window study that varies multiple parameters simultaneously would provide a more complete picture for industrial application.