Weld Overlay of Nickel Alloy 625 on Tube Sheets Process Analysis

Literature Overview

This study note addresses a 2016 technical contribution by Wang Na and Ji Qiang from Fushun Chemical Machinery Equipment Manufacturing Co., Ltd., focused on the weld overlay of Nickel-based Alloy 625 (Inconel 625) onto tube sheets. Tube sheets are critical components in heat exchangers, particularly those operating in aggressive chemical environments such as chlorinated media, acidic solutions, and high-temperature oxidation conditions. The selection of Alloy 625 for overlay is driven by its outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking, as well as its excellent mechanical properties at elevated temperatures.

Core Technical Content and Process Parameters

The fundamental challenge in overlaying Alloy 625 on carbon steel or low-alloy steel tube sheets lies in achieving adequate metallurgical bonding while minimizing dilution of the base metal into the overlay layer. Excessive dilution reduces the corrosion resistance of the overlay by introducing carbon, manganese, and other ferritic elements that compromise the austenitic microstructure of Alloy 625.

The following table summarizes typical process parameters and considerations for Alloy 625 tube sheet overlay:

Parameter	Typical Range	Notes
Welding method	GTAW (TIG) or SAW	GTAW preferred for thin sections and precision; SAW for thicker deposits
Base metal	Carbon steel (Q345R, SA-516 Gr.70)	Preheating 150–250 °C typically required
Wire/filler	ERNiCrMo-3 (Alloy 625 equivalent)	Low carbon grade to avoid sensitization
Wire diameter	1.6–3.2 mm	GTAW uses smaller diameters; SAW uses larger
Shielding gas	Pure Ar or Ar/He mix	Flow rate 15–25 L/min for GTAW
Heat input	0.5–2.5 kJ/mm	Lower for GTAW; higher for SAW
Overlay thickness	3–6 mm typical	Must exceed maximum weld bead height per ASME IX
Interpass temperature	≤250 °C	Prevents hot cracking in the overlay

Welding Process Selection

The choice between GTAW and SAW is dictated by the geometry of the tube sheet, the required overlay thickness, and production volume requirements. For single-pass or two-pass overlay on relatively flat tube sheets, GTAW offers superior control over heat input and dilution, producing a cleaner, more uniform deposit with minimal spatter. However, for large-diameter tube sheets requiring substantial overlay thickness (typically greater than 4 mm), SAW provides significantly higher deposition rates and better productivity.

A critical process consideration is the direction of welding relative to the tube sheet orientation. Welding should generally proceed in a direction that avoids excessive thermal stress concentration near the tubesheet-to-shell weld junction. Multi-pass strategies are employed where the first pass serves as a transition layer and subsequent passes build the required overlay thickness.

Metallurgical Considerations and Dilution Control

Dilution is the single most important variable governing the performance of Alloy 625 overlays. The carbon content of the final overlay must remain below 0.10 wt% to prevent chromium carbide precipitation at grain boundaries, which would compromise corrosion resistance. When overlaying Alloy 625 on carbon steel, dilution rates of 15–25% are common in the first pass, decreasing to 5–10% in subsequent passes.

The microstructure of the overlay is ideally fully austenitic with some delta ferrite (3–8%) to suppress hot cracking. Excessive delta ferrite above 15% can reduce ductility and increase susceptibility to intergranular corrosion. Metallographic examination using a Lebel ferrite meter is recommended to verify the ferrite content in production.

Common defects encountered during Alloy 625 tube sheet overlay include:

Defect Type	Root Cause	Countermeasure
Hot cracking	Excessive delta ferrite, high sulfur/phosphorus in base metal	Preheat base metal, use low-sulfur filler, control interpass temperature
Excessive dilution	High heat input, single-pass strategy	Reduce heat input, use multi-pass with lower current
Cracking in overlay	Rapid cooling, hydrogen embrittlement	Post-weld heating to 300 °C for 30 min, use low-hydrogen consumables
Lack of fusion	Insufficient heat input, poor surface preparation	Increase preheat, clean surfaces, verify electrode angle

Engineering Practice and Quality Assurance

In engineering practice, the quality of Alloy 625 tube sheet overlays is verified through a combination of non-destructive testing (NDT) and destructive testing. Penetrant testing (PT) or magnetic particle testing (MT) is applied to the overlay surface to detect surface cracks, while ultrasonic testing (UT) or radiographic testing (RT) is used to assess bond quality and subsurface defects.

Mechanical property testing includes tensile testing of weld coupons, hardness measurement (typically 220–280 HV for Alloy 625 overlay), and intergranular corrosion testing per ASTM A263/A264 requirements. The overlay thickness is verified by ultrasonic thickness measurement or by macrographic sectioning at qualified locations.

From a standards perspective, the welding procedure must be qualified per ASME Section IX or NB/T 47014, with supplementary essential variables including overlay thickness, heat input, and interpass temperature. The welder qualification must also cover the specific overlay geometry, as tube sheet overlays present unique positional challenges.

Study Insights and Reflections

The Fushun Chemical Machinery study highlights a practical, production-oriented approach to Alloy 625 tube sheet overlay that is directly applicable to heat exchanger manufacturing in the petrochemical industry. One key insight is that the economics of overlay production are heavily influenced by the choice of welding process: GTAW provides superior quality but at significantly lower deposition rates, while SAW offers productivity at the cost of slightly higher dilution.

A critical reflection is the importance of pre-weld consumable control. The moisture content of flux-cored or solid wire must be rigorously controlled, as hydrogen absorption from contaminated consumables is a primary cause of overlay cracking in Alloy 625 deposits. Additionally, the study underscores the value of standardized welding procedure specifications (WPS) that incorporate dilution monitoring as a routine quality checkpoint, rather than relying solely on post-weld testing.

For engineers designing and fabricating heat exchangers with Alloy 625 tube sheet overlays, the key takeaway is that successful overlay is not merely a matter of selecting the correct filler metal but requires a holistic approach encompassing base metal preparation, thermal management, dilution control, and comprehensive quality verification. The integration of these factors into a systematic quality management framework is essential for achieving reliable, long-term service performance in aggressive chemical environments.