Tube-Sheet Nickel Alloy 625 Cladding Welding Process Design and Quality Control

Literature Overview and Application Context

This study addresses the welding process design and quality control for Inconel 625 cladding on pressure vessel tube sheets, a critical component in heat exchangers, reactors, and distillation columns used in petrochemical, hydrogen, and chemical processing industries. Tube sheets are subjected to combined mechanical and corrosion loading, and the cladding layer must provide reliable protection against the process medium while maintaining structural integrity under cyclic thermal and pressure loading.

The Inconel 625 alloy is selected for tube sheet cladding due to its exceptional resistance to oxidation, sulfidation, and a wide range of corrosive media, including hydrofluoric acid, hydrochloric acid, and high-temperature hydrogen environments. The cladding process must produce a uniform, defect-free overlay with controlled dilution, adequate bond strength, and acceptable mechanical properties throughout the cladding thickness.

Process Design and Parameter Selection

The cladding process for tube sheets typically involves multiple passes to build up the required overlay thickness, which ranges from 3 to 10 mm depending on the application. The process selection depends on the tube sheet diameter, cladding thickness, and production requirements. For large-diameter tube sheets (above 1000 mm), submerged arc welding (SAW) or electroslag welding (ESW) is preferred for the base passes due to high deposition rates, while GTAW is used for the cap pass to ensure surface quality. For smaller tube sheets or high-quality requirements, GTAW or plasma transferred arc (PTA) welding may be used for all passes.

The welding parameters for the base passes and cap pass are optimized separately to balance deposition rate and quality. For the base passes using SAW, the current is typically 350-500 A, voltage 28-35 V, and travel speed 150-250 mm/min. For the GTAW cap pass, the current is 120-160 A, voltage 18-22 V, and travel speed 80-120 mm/min. The wire feed rate is synchronized with the travel speed to maintain a consistent bead profile.

The interpass temperature is a critical parameter that must be controlled to prevent cracking and maintain the mechanical properties of the previous pass. For Inconel 625 cladding, the interpass temperature should be maintained below 250 degrees C, and a maximum of three passes should be deposited before allowing the temperature to cool below 150 degrees C.

Quality Control and Inspection

The quality control program for tube sheet cladding includes incoming material inspection, in-process monitoring, and post-weld inspection. The incoming material inspection verifies the chemical composition, mechanical properties, and certification of the Inconel 625 wire or strip electrode. The wire composition must conform to ASTM B335 or the equivalent specification, with nickel content above 58 wt%, chromium above 20 wt%, and molybdenum above 8 wt%.

In-process monitoring includes continuous recording of welding parameters (current, voltage, travel speed, wire feed rate) and periodic visual inspection of the weld bead profile. The welding operator must be certified in accordance with ASME IX or the equivalent national qualification standard, and the welding procedure qualification must be performed in accordance with NB/T 47014 or ASME IX.