Weld Overlay of Sealing Surfaces on Bellows Shutoff Valve Bodies

Literature Overview

This 1998 study from Dalian High-Pressure Valve Factory addresses a critical engineering challenge in high-pressure valve manufacturing: achieving reliable metallurgical bonding and dimensional accuracy when applying overlay welds to the sealing faces of bellows shutoff valve bodies. Bellows shutoff valves are widely used in high-pressure natural gas, hydrogen service, and refinery applications where leakage prevention is paramount. The bellows element isolates the stem from the process fluid, but the sealing faces between the valve body and trim require hardfacing or overlay welds to resist erosion, corrosion, and galling under extreme pressure and temperature conditions. The authors investigated the process parameters, consumable selection, and quality control measures necessary to produce defect-free overlay layers on the complex geometry of bellows valve bodies.

Core Technical Content

Base Metal and Overlay Material Selection

Bellows shutoff valve bodies are typically fabricated from carbon steel or low-alloy steel castings such as ASTM A216 WCB or WCC, sometimes upgraded to 9Cr-1Mo (ASTM A182 F91) for elevated temperature service. The sealing face overlay materials selected in this study include austenitic stainless steel grades (304, 316), nickel-based alloys (Inconel 625, Monel 400), and hardfacing alloys containing chromium carbide or tungsten carbide. The selection depends on the service medium, operating temperature, and pressure differential across the valve.

Parameter	Typical Specification
Base material	ASTM A216 WCB / WCC, or A182 F91
Overlay material	316 SS, Inconel 625, Stellite 6
Overlay thickness	1.5–3.0 mm
Number of passes	2–3
Welding process	GTAW (TIG) or SAW
Preheat temperature	150–250 °C
Interpass temperature	≤ 250 °C
Post-weld treatment	Solution annealing or PWHT per ASME VIII

Process Parameters and Welding Sequence

The bellows valve body geometry presents significant challenges for overlay welding due to restricted access, thermal distortion sensitivity, and the need to maintain tight dimensional tolerances on the sealing surface. The recommended welding sequence involves the following steps:

Surface preparation: mechanical grinding of the sealing face to remove scale, oxide, and contamination to a minimum surface roughness of Ra 6.3 μm.
Preheating: uniform preheat to 150–250 °C using induction heating or gas torch to reduce residual stress and minimize cracking risk, particularly when welding onto low-alloy steel with carbon equivalent (CE) exceeding 0.4%.
First pass (bonding pass): a thin root pass (0.5–1.0 mm) using a compatible filler metal that ensures good metallurgical bonding without excessive dilution from the base metal.
Subsequent passes: building up the required overlay thickness with appropriate filler metal, maintaining interpass temperature below 250 °C to avoid grain coarsening and reduce residual stress.
Final grinding: precision grinding of the overlay surface to achieve the required sealing flatness (typically ≤ 0.02 mm per 25 mm) and surface finish.

Key Process Windows

Process Parameter	Recommended Range	Rationale
Arc voltage	12–18 V (GTAW)	Controls bead width and penetration
Welding current	80–160 A (GTAW)	Adequate for 1.5–3 mm overlay
Travel speed	30–60 mm/min	Balances deposition rate and cooling rate
Shielding gas flow	15–20 L/min (Ar)	Prevents oxidation of overlay
Preheat	150–250 °C	Reduces HAZ hardness and cracking
Cooling rate	≤ 5 °C/s	Prevents martensitic transformation in HAZ

Defect Analysis and Countermeasures

Common Defects Encountered

Based on the study findings and engineering experience, the following defects are most frequently observed in bellows valve body overlay welding:

Defect Type	Root Cause	Countermeasure
Cracking (hot/cold)	High CE base metal, excessive restraint	Increase preheat, reduce interpass temp, use low-hydrogen filler
Porosity	Contaminated surface, insufficient shielding	Thorough cleaning, verify gas flow, back-purge
Undercut	Excessive current or travel speed	Reduce current, adjust torch angle
Excessive dilution	Thin first pass, high penetration	Use lower current for bonding pass, consider surfacing compound
Distortion	Asymmetric heat input	Balanced welding sequence, use backing bars
Insufficient bond	Surface contamination, inadequate penetration	Improve surface prep, increase current for first pass

Metallurgical Considerations

The dilution between the base metal and overlay material is a critical factor in determining the final properties of the overlay layer. For a two-pass overlay on carbon steel with 316 SS filler, the dilution in the first pass can reach 40–60%, resulting in a ferrite-austenite microstructure that may not provide the desired corrosion resistance. The second pass dilution typically drops to 15–25%, producing a predominantly austenitic microstructure. For nickel-based overlay materials such as Inconel 625, the dilution effect is less critical because the alloy maintains its properties even with significant iron dilution, but excessive dilution (>40%) can reduce the nickel content below the threshold required for resistance to pitting and crevice corrosion.

Integration with Engineering Practice

In high-pressure valve manufacturing, the bellows shutoff valve body overlay process must be qualified according to ASME Section IX or NB/T 47014 procedures. The qualified welding procedure specification (WPS) must define the essential variables including base metal thickness range, filler metal type and diameter, preheat and interpass temperature, travel speed, and post-weld treatment. Production welders must hold valid certifications within the qualified ranges.

Non-destructive testing (NDT) is essential for ensuring overlay quality. The recommended NDT sequence includes:

Visual inspection (VT) of all welds for surface defects.
Magnetic particle testing (MT) or penetrant testing (PT) of the overlay surface to detect cracks, porosity, and lack of fusion.
Ultrasonic testing (UT) at the overlay-base metal interface to verify bond integrity and detect subsurface defects.
Hardness testing of the overlay surface to confirm material composition and detect excessive dilution.
Hydrostatic pressure testing of the completed valve to verify sealing performance.

A typical acceptance criterion for the overlay layer includes: no cracks, no porosity exceeding 0.5% area fraction, hardness within the specified range (e.g., 180–250 HV for 316 SS overlay), and complete bond with no lack of fusion detectable by UT.

Study Insights and Implications

The study from Dalian High-Pressure Valve Factory demonstrates the systematic approach required for overlay welding on complex valve geometries. Several key insights emerge from the analysis:

The bellows valve body geometry demands careful thermal management to prevent distortion that would compromise the bellows-to-body fit.
Multi-pass overlay with controlled dilution is essential for achieving both metallurgical bonding and surface properties.
The qualification of welding procedures must account for the specific geometry and accessibility constraints of bellows valve bodies, which differ significantly from flat plate overlay qualification tests.
Post-weld grinding to achieve sealing surface flatness is a critical final operation that must be performed with precision tooling to avoid removing the entire overlay layer.

For engineers working in valve manufacturing or pressure equipment fabrication, this study provides a valuable reference for developing overlay welding procedures on complex geometries. The principles of thermal management, dilution control, and multi-pass welding strategy are directly transferable to other applications such as heat exchanger tube sheet overlay, reactor vessel head cladding, and pump casing repair. The emphasis on procedure qualification and NDT verification aligns with modern quality assurance requirements under ASME and NB standards.