MIG Vertical Down Cladding of Inconel 625 for Pressure Vessel Applications

Literature Overview

This research by Xu Bing from Datang Northeast Power Equipment Testing and Research Institute (2019) investigates the feasibility and optimization of MIG (MIG/MAG, gas metal arc welding) vertical down cladding of Inconel 625 alloy on pressure vessel components. The study addresses a significant industrial need: the ability to apply corrosion-resistant overlay layers to large-diameter pressure vessels in orientations that are traditionally considered unfavorable for arc welding. Vertical down welding is particularly relevant for large vertical vessels, heat exchanger shells, and reactor internals where access constraints and ergonomic considerations make conventional vertical up welding impractical.

Technical Background and Motivation

Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum superalloy containing approximately 62-70% Ni, 20-23% Cr, 8-10% Mo, and 3-4% Nb+Ta. It exhibits exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking in oxidizing and reducing environments, making it ideal for overlay applications in chemical processing, nuclear, and power generation industries. The alloy maintains good mechanical properties at temperatures up to 980°C and demonstrates outstanding weldability despite its high thermal expansion coefficient and low thermal conductivity.

The motivation for vertical down MIG cladding research stems from several practical considerations:

• Large vertical pressure vessels (diameter > 2 m) often require overlay on the interior surface, where vertical up welding is ergonomically challenging and requires extensive scaffolding.
• Vertical down welding allows the welder to work from above, providing better visibility and access to the workpiece.
• The process is faster than vertical up welding due to the gravitational assistance of the weld pool, potentially reducing labor costs by 20-30%.
• For multi-pass overlay builds, vertical down orientation facilitates easier wire feeding and torch manipulation.

Process Development and Parameter Optimization

Base Metal and Consumable Selection

The study investigates cladding on 16MnR (equivalent to SA-516 Gr.70) and Q345R carbon steel substrates, which are the most commonly used pressure vessel steels in China per GB/T 150 and NB/T 47002. The consumable selection follows a systematic approach:

The transition layer of ER309L is applied between the carbon steel substrate and Inconel 625 overlay to minimize dilution and prevent the formation of brittle intermetallic phases at the interface. The high chromium content (23-25%) and low carbon (<0.03%) of ER309L provide excellent crack resistance and ensure good metallurgical compatibility with both the ferritic substrate and the austenitic Inconel 625 overlay.

Welding Parameter Optimization

The study employed a systematic parameter optimization approach using orthogonal experimental design to identify the optimal combination of welding parameters for vertical down MIG cladding. The key parameters and their optimal ranges are summarized below:

Multi-Pass Build Strategy

For overlay thicknesses exceeding 3 mm, a multi-pass build strategy is essential. The study recommends the following sequence:

1. Pass 1 (Transition layer): ER309L, single stringer bead, 2-3 mm wide, 1.5-2 mm thick. This pass establishes metallurgical compatibility and provides a low-dilution interface.
2. Pass 2 (First overlay layer): ERNiCrMo-3, slightly wider bead (3-4 mm), with 50% overlap onto the transition layer. The dilution rate at this stage should be monitored and maintained below 30%.
3. Passes 3-5 (Build-up layers): ERNiCrMo-3, wider beads (5-8 mm) with 50-60% overlap. The bead width increases progressively to achieve a smooth, uniform surface profile.
4. Final pass (Surface finishing): ERNiCrMo-3, narrow bead to achieve a smooth, flat surface suitable for machining or direct service.

The total overlay thickness typically ranges from 3-5 mm for pressure vessel applications, depending on the expected service life and corrosion rate. For aggressive environments, thicknesses up to 8-10 mm may be specified.

Metallurgical Analysis

Microstructure Evolution

The study provides detailed metallographic analysis of the cladding interface and overlay microstructure. The key observations include:

Interface region: The transition from ER309L to ERNiCrMo-3 shows a gradual compositional gradient over approximately 100-200 μm. The ER309L layer exhibits a fine austenitic-ferritic structure (approximately 70% austenite, 30% ferrite), while the Inconel 625 overlay shows a fully austenitic matrix with delta ferrite (approximately 5-10%) and Nb-rich carbides.

Overlay microstructure: The Inconel 625 overlay exhibits a columnar grain structure in the first pass, transitioning to equiaxed grains in subsequent passes due to grain refinement from the previous pass. The grain size is typically 50-150 μm, which provides good resistance to cracking and adequate mechanical properties.

HAZ microstructure: The carbon steel HAZ shows a tempered martensite-ferrite structure after PWHT, with hardness values typically in the range of 200-250 HV. Without PWHT, the HAZ hardness can reach 350-400 HV, which is unacceptable for pressure vessel service.

Dilution Analysis

Dilution is a critical parameter that directly affects the corrosion resistance and mechanical properties of the overlay. The study measured dilution rates at different pass positions using OES analysis:

The data clearly demonstrates why a transition layer is essential: without ER309L, the first Inconel 625 pass would experience dilution rates of 50-60%, resulting in effective chromium and nickel contents well below the minimum required for corrosion resistance.

Defect Analysis and Quality Control

Common Defects in Vertical Down MIG Cladding

The study identifies several defect modes specific to vertical down welding orientation:

Undercut: Vertical down orientation promotes undercut at the trailing edge of the weld bead due to gravity pulling the molten pool downward. This is mitigated by using a slightly higher travel speed (250-300 mm/min) and maintaining a consistent torch angle.

Washout: Excessive current or voltage can cause the molten pool to flow beyond the intended bead width, resulting in washout. The study recommends limiting current to 220 A maximum for 1.2 mm wire and maintaining voltage below 26 V.

Cracking: Hot cracking in the Inconel 625 overlay is rare due to the alloy's excellent weldability, but cold cracking in the HAZ of carbon steel substrates can occur if preheat and interpass temperature controls are inadequate. The study reports zero instances of cracking when the specified preheat (100-150°C) and interpass temperature (≤200°C) were maintained.

Porosity: Porosity is primarily caused by inadequate shielding gas coverage. In vertical down orientation, the shielding gas must flow from above to below, which is less effective than in horizontal or flat positions. The study recommends using a minimum gas flow rate of 18 L/min and a tail cup to improve shielding coverage at the trailing edge.

Inspection and Acceptance Criteria

The quality control protocol for Inconel 625 cladding on pressure vessels follows a rigorous multi-level inspection approach:

Engineering Practice and Standards Compliance

Standards Framework

The vertical down MIG cladding process must comply with multiple standards simultaneously:

• NB/T 47014: Qualification of welding procedures for pressure vessels, requiring demonstration of weldability through coupon testing.
• GB/T 150: General rules for pressure vessel design and fabrication, specifying PWHT requirements and inspection protocols.
• ASME IX: Qualification of welding, brazing, and bonding procedures, providing the international framework for procedure qualification.
• ASME VIII Div.1: Unfired pressure vessels, specifying overlay requirements and thickness allowances.
• API 934: Standard for surface preparation and application of protective coatings, relevant for post-cladding surface finishing.

Procedure Qualification Requirements

Per NB/T 47014, the welding procedure qualification must include:

1. Coupon dimensions: Minimum 200 × 100 × 20 mm for multi-pass overlay qualification.
2. Test specimens: Tensile, hardness, and macrograph samples from the overlay and interface.
3. Acceptance criteria: No cracks, lack of fusion, or excessive porosity; hardness within specified limits; dilution rate documented.
4. Essential variables: Current, voltage, travel speed, wire diameter, shielding gas composition, preheat temperature, and interpass temperature.

The study demonstrates that the developed procedure meets all qualification requirements and produces overlay layers with excellent metallurgical quality and corrosion resistance.

Study Insights and Reflections

The research by Xu Bing makes a significant contribution to the practical application of Inconel 625 cladding in the power generation and chemical processing industries. Several key insights emerge from this study:

First, the successful implementation of vertical down MIG cladding challenges the traditional assumption that vertical up welding is the only viable orientation for overlay welding. The study demonstrates that with proper parameter optimization, vertical down welding can produce overlay layers of equivalent quality to vertical up welding, while offering significant advantages in terms of welder ergonomics, deposition rate, and access to large vertical surfaces.

Second, the systematic approach to dilution control through the use of a transition layer is a best practice that should be adopted universally for all Inconel 625 cladding applications. The data clearly shows that without a transition layer, the first overlay pass would have unacceptable dilution rates, compromising the corrosion resistance of the entire overlay build.

Third, the study highlights the importance of process parameter interaction. The optimal combination of current, voltage, and travel speed is not simply the sum of individual parameter optimizations but requires a holistic approach that considers the interaction between parameters and their combined effect on weld geometry, dilution, and metallurgical quality.

The practical implications of this research are substantial. For large vertical pressure vessels in power plants and chemical processing facilities, the ability to apply Inconel 625 overlay in vertical down orientation significantly reduces fabrication time, labor costs, and safety risks associated with working at height. The study provides a validated welding procedure that can be directly adopted for production use, subject to qualification testing per applicable standards.

The key takeaway from this literature is that vertical down MIG cladding of Inconel 625 is a technically feasible and economically advantageous approach for pressure vessel overlay applications. The critical success factors are proper consumable selection with a transition layer, careful parameter optimization to control dilution and bead geometry, and rigorous quality control through multi-level inspection. This approach should be considered as a standard option for large vertical vessel cladding projects where access and ergonomic constraints make conventional welding orientations impractical.