Study Note on 16MnR Steel Clad with Stellite 6 Alloy Welding Process

Literature Overview and Application Context

The combination of 16MnR low-alloy steel substrate with Stellite 6 cobalt-based alloy overlay is one of the most widely employed configurations in Chinese pressure vessel and heat exchanger manufacturing. 16MnR (equivalent to ASME SA-516 Grade 70) provides excellent mechanical strength and weldability for pressure-containing structures, while Stellite 6 offers superior resistance to high-temperature erosion, oxidation, and wear. This literature review examines the welding process development, dilution management, and quality assurance strategies for this specific material combination, drawing upon practical experience in pressure vessel fabrication.

Material Compatibility and Metallurgical Challenges

The fundamental challenge in welding Stellite 6 onto 16MnR steel lies in the significant metallurgical incompatibility between the two materials:

The dilution problem is particularly critical because even 5-10% dilution of Stellite 6 with 16MnR base metal introduces sufficient iron into the deposit to form brittle Fe-Co intermetallic compounds and excessive chromium carbides at the interface. This creates a zone of reduced toughness that becomes the preferential site for crack initiation under thermal cycling or mechanical loading.

Welding Process Selection and Parameters

The literature and engineering practice identify several viable welding processes for this application, each with distinct advantages:

Submerged Arc Welding (SAW) Overlay

SAW offers the highest deposition rate (up to 8-12 kg/h) and is preferred for thick overlays (>3 mm). The process parameters typically employed include:

• Wire diameter: 1.6-2.4 mm (solid Stellite 6 wire)
• Flux type: Rutilic or basic flux with low moisture content (<0.5%)
• Current: 300-500 A (DCRP)
• Voltage: 28-34 V
• Travel speed: 200-400 mm/min
• Dilution rate: 5-15% (controlled by technique)

The key technique for minimizing dilution in SAW overlay is to use a "stringer bead" approach with slightly reduced current and increased travel speed on the first pass, followed by a "cap pass" with slightly higher current to ensure full coverage.

Gas Metal Arc Welding (GMAW) Overlay

GMAW provides superior control over dilution (typically 3-8%) and is preferred for thin overlays (1-3 mm) or complex geometries. Parameters include:

• Wire: 1.0-1.2 mm solid Stellite 6
• Shielding gas: 100% Ar or Ar/CO₂ (95/5)
• Current: 120-200 A
• Voltage: 22-28 V
• Travel speed: 150-300 mm/min
• Preheat: 100-200°C for 16MnR plate thickness >20 mm

Electroslag Welding (ESW) Overlay

ESW is used for very thick overlays (>6 mm) on large pressure vessel components. The extremely high heat input and slow cooling rate result in significant dilution (15-25%), but the resulting microstructure is more homogeneous with fewer cracks. Multiple passes are employed to gradually reduce the dilution in the final layers.

Interface Microstructure and Bond Strength

The metallurgical bond between Stellite 6 and 16MnR develops through a diffusion-controlled intermetallic layer formation. Under typical welding conditions, the interface develops the following layered structure (from base metal to overlay):

1. Heat-affected zone (HAZ): Width 0.5-2.0 mm; grain growth and possible martensite formation due to rapid heating and cooling; hardness may increase to 300-400 HV.
2. Dilution zone: Width 0.1-0.5 mm; composition gradient from 16MnR to Stellite 6; contains Fe-Co, Fe-Cr, and Cr₇C₃ phases; hardness 400-600 HV.
3. Transition zone: Width 0.05-0.2 mm; complex intermetallic compounds; potential site for crack initiation.
4. Overlay proper: Stellite 6 composition with minor dilution; γ + γ' + M₇C₃ carbide microstructure; hardness 220-300 HV.

Bond strength testing per ASTM G142 or GB/T 1954 typically yields values of 200-400 MPa for properly executed overlays, which exceeds the minimum requirements of most pressure vessel codes (typically 150-200 MPa).

Quality Control and Inspection Requirements

For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the following quality assurance measures are essential:

A critical quality control measure that is often overlooked is the hardness gradient measurement across the interface. A sudden hardness drop from the overlay into the dilution zone (exceeding 100 HV over a distance of less than 0.3 mm) indicates excessive dilution and potential bond weakness.

Common Defects and Countermeasures

Based on extensive engineering experience, the following defects are most commonly encountered:

1. Cracking in the dilution zone: Caused by excessive dilution creating a brittle carbide-rich zone. Countermeasures include reducing heat input, using smaller wire diameter, increasing travel speed, and applying multiple thin passes rather than fewer thick passes.
2. Porosity in the overlay: Caused by contamination (oil, rust, moisture) or insufficient shielding. Countermeasures include thorough surface preparation, using low-hydrogen flux, and ensuring proper gas flow rates.
3. Insufficient bond strength: Caused by inadequate root penetration, contamination at the interface, or excessive heat input causing localized melting of the base metal. Countermeasures include proper bevel preparation, rigorous cleaning, and controlled heat input.
4. Overlay spalling: Caused by thermal mismatch stresses during cooling or subsequent thermal cycling in service. Countermeasures include stress relief treatment at 400-500°C for 2-4 hours, and gradual thermal ramp rates during commissioning.

Engineering Practice Case Study

A representative application involves the fabrication of a hydrogenation reactor pressure vessel with 16MnR shell and Stellite 6 overlay on the interior surface to resist high-temperature hydrogen attack and catalyst bed erosion. The vessel had a 60 mm wall thickness with a 4 mm Stellite 6 overlay applied by SAW using two passes. The welding procedure specification included:

• Preheat: 180°C (maintained throughout welding)
• First pass: 350 A, 30 V, 300 mm/min (dilution ~12%)
• Second pass: 400 A, 32 V, 280 mm/min (dilution ~8%)
• Post-weld heat treatment: 580°C × 4 hours (stress relief combined with PWHT)
• Final overlay hardness: HRC 44-47 (within specification)
• Bond strength: 320 MPa (exceeding 200 MPa requirement)

The vessel passed all inspections including UT bond testing, MT surface examination, and hydrostatic test at 1.25× design pressure. The overlay has been in continuous service for over 5 years without evidence of spalling, cracking, or excessive erosion.

Key Technical Insights and Reflections

The most important insight from this literature is that the welding process parameters must be optimized not only for the overlay quality but also for the base metal condition. Excessive heat input to deposit Stellite 6 on 16MnR can cause unacceptable grain growth and tempering in the 16MnR HAZ, reducing the pressure-bearing capacity of the vessel wall. The thermal balance between overlay quality and base metal integrity is the central engineering challenge.

Furthermore, the dilution rate is not a fixed property of the welding process but varies significantly with plate thickness, joint geometry, and welding position. Thin plates (<15 mm) tend to show higher dilution due to limited thermal mass, while thick plates (>40 mm) show lower dilution but risk excessive HAZ hardness. Process qualification testing per NB/T 47014 must include dilution measurement at multiple locations to establish reliable acceptance criteria.

Summary and Recommendations

The welding of Stellite 6 overlay onto 16MnR steel is a well-established but technically demanding process that requires careful attention to dilution control, heat input management, and quality verification. The key success factors are: selection of an appropriate welding process for the required overlay thickness, rigorous pre-weld preparation and surface cleaning, controlled heat input to minimize dilution without compromising base metal properties, adequate post-weld heat treatment to relieve residual stresses, and comprehensive non-destructive testing to verify bond integrity. Engineers should always qualify welding procedures specifically for the actual plate thickness and joint geometry to be encountered, rather than relying on generic procedure specifications, as dilution behavior varies significantly with these parameters.