CLADDING TECHNOLOGY SHANXI CO., LTDGTAW Overlay of Inconel 625 on ASTM A4130 Steel Substrate
Literature Overview
This 2014 publication by Guo Biyong from Jiangsu Hongda Fu Petroleum Equipment Co., Ltd., published in the journal "Petrochemical Equipment," presents a detailed welding process development study for the overlay of Inconel 625 (UNS N06625) onto ASTM A4130 carbon steel using automatic gas tungsten arc welding (GTAW). The application context is pressure vessel and heat exchanger fabrication for petrochemical service, where the base material provides structural strength while the overlay layer offers resistance to high-temperature oxidation and sulfur-containing corrosive environments.
Process Development and Parameter Optimization
The automatic GTAW process was selected for its ability to produce consistent, repeatable weld beads with controlled heat input and minimal dilution. The process parameters were optimized through a systematic experimental approach, varying wire feed speed, travel speed, shielding gas flow rate, and interpass temperature.
| Process Parameter | Optimized Value |
|---|---|
| Wire diameter | 1.6 mm |
| Wire feed speed | 2.5–3.5 m/min |
| Travel speed | 150–200 mm/min |
| Shielding gas | 99.99% Ar (15–20 L/min) |
| Backing gas | 99.99% Ar (5–8 L/min) |
| Current (DCEN) | 140–180 A |
| Arc voltage | 18–22 V |
| Preheat | 150–200 °C |
| Interpass temperature | ≤ 200 °C |
| Overlay thickness | 2.5–4.0 mm (2 passes) |
The dilution rate was controlled at below 20% through the use of a pure Inconel 625 first pass followed by a slightly lower-current second pass. Metallographic examination revealed a columnar-to-equiaxed transition in the overlay microstructure, with the columnar zone near the interface and equiaxed grains toward the surface.
Microstructure and Mechanical Properties
The overlay microstructure consists primarily of austenite (γ) with a minor amount of delta ferrite (δ), consistent with the Schaeffler diagram prediction for Inconel 625 weld metal. The hardness of the overlay layer ranges from 220 to 260 HV, while the base material hardness is approximately 180 HV. The interface bond strength exceeds 300 MPa in peel test specimens, well above the acceptance threshold specified in ASTM A263. The dilution zone shows a gradient of nickel and chromium enrichment extending approximately 0.5 mm into the base material, creating a beneficial diffusion barrier against carbon migration and carburization.
Quality Assurance and Inspection Protocol
The quality control plan follows NB/T 47014 for welding procedure qualification and NB/T 47002 for material requirements. Non-destructive examination includes dye penetrant testing (PT) for surface defects, ultrasonic testing (UT) for interface bonding quality, and radiographic testing (RT) for volumetric discontinuities. The weld procedure qualification includes tensile testing of the overlay layer, bend testing, and macrograph examination with acid etching to reveal the full weld cross-section. Hydrogen-induced cracking susceptibility is evaluated through a delayed cracking test at ambient temperature for 24 hours.
Engineering Practice Implications
The automatic GTAW process offers significant productivity advantages over manual welding for large-area overlay applications on pressure vessel shells and heads. The key engineering consideration is the thermal management during multi-pass overlay, particularly for thick-walled components where the cumulative heat input can cause temper softening in the base material heat-affected zone. Engineers should implement a thermocouple monitoring system to maintain the interpass temperature within the specified window. The Inconel 625 overlay is particularly suitable for hydrocracker reactor internals, hydrotreater heat exchangers, and sulfur recovery unit components where temperatures exceed 600 °C and the environment contains hydrogen and hydrogen sulfide.