Hot-Wire TIG Welding Process for L415-N08825 Bimetallic Clad Pipe

Literature Overview

This study focuses on the development and optimization of a hot-wire TIG (HWTIG) welding process for fabricating L415 carbon steel pipe with N08825 (Inconel 625) nickel-based alloy cladding. N08825 is widely used in aggressive chemical environments such as sulfuric acid, phosphoric acid, and hot chloride solutions due to its exceptional corrosion resistance. The HWTIG process combines conventional TIG welding with a mechanically fed hot wire to increase deposition rate while maintaining the fine microstructure and low dilution characteristics of TIG welding. The study addresses the specific challenges of welding a nickel-based alloy overlay onto a low-alloy carbon steel substrate, including dilution control, residual stress management, and prevention of cracking in the weld and overlay layers.

Core Technical Findings

Process Parameter Optimization

The HWTIG process parameters were systematically optimized through orthogonal experimental design. The base TIG current was set at 80-120 A with an arc voltage of 12-16 V, while the hot wire feed rate ranged from 3-8 m/min with a wire diameter of 0.8-1.2 mm. The combination of base arc and hot wire produced a deposition rate of 0.8-1.5 kg/h, which is 2-3 times higher than conventional TIG welding. The dilution rate of the L415 base metal into the N08825 overlay was controlled at 8-15%, which is critical because excessive dilution can introduce carbon and sulfur that promote carbide precipitation and reduce corrosion resistance.

Microstructure and Properties

The overlay layer microstructure consists of columnar dendrites of the gamma phase with inter-dendritic M23C6 and Ni3B carbides near the fusion boundary. The hardness of the overlay layer ranged from 280-340 HV, while the base metal hardness was 180-220 HV. The hardness gradient at the overlay-base metal interface was steep, indicating minimal interdiffusion. Tensile testing of the overlay layer showed a yield strength of 550-620 MPa and elongation of 25-35%. The overlay layer exhibited excellent intergranular corrosion resistance per ASTM G48, with no measurable weight loss after 1000 hours in 66% boiling HNO3.

Cracking Prevention

The primary cracking concern in N08825 overlay is solidification cracking due to the high sulfur and phosphorus content in the weld metal. The study recommends strict control of sulfur and phosphorus in the filler wire to below 0.01% S and 0.02% P respectively. Preheating the base metal to 150-200°C and maintaining interpass temperature below 250°C were found to be effective in reducing residual stress and preventing hot cracking. Post-weld stress relief at 850-900°C for 1-2 hours further reduces residual stress without adversely affecting the overlay microstructure.

Engineering Practice Integration

For field application, the HWTIG process offers a practical solution for small-diameter and thin-walled piping where conventional cladding methods (such as ESW or strip cladding) are impractical. The process is particularly suitable for retrofitting existing carbon steel piping systems in chemical plants where N08825 corrosion resistance is required but full replacement is economically prohibitive. The deposition rate advantage of HWTIG over conventional TIG translates to significant labor cost savings, especially for multi-layer overlay applications requiring 3-5 passes.

Study Insights and Implications

The study demonstrates that HWTIG is a viable and efficient process for N08825 overlay on L415 pipe, offering a favorable balance between deposition rate, dilution control, and overlay quality. Engineers should note that the process requires careful monitoring of interpass temperature and wire feed stability to maintain consistent overlay quality. For critical applications in high-temperature sulfuric acid service, the study recommends verifying overlay thickness uniformity through UT mapping and conducting field corrosion coupon tests before commissioning. The HWTIG approach represents a significant advancement in field-applicable cladding technology for nickel-based alloy overlays on carbon steel substrates.