Welding Process of Stellite 6 Alloy Overlay on 16MnR Steel

Literature Overview

This technical paper, published in Welding Technology (2008), was authored by Bo Liyan from the Boiler and Pressure Vessel Inspection and Research Center of Daqing Oilfield Co., Ltd., along with Guo Xiaochun, Zhang Xianlong, and Qiu Haiping from the Daqing Petroleum Administration Bureau. The study addresses the welding process development for overlaying Stellite 6 cobalt-based alloy onto 16MnR low-alloy steel, a combination commonly encountered in the fabrication of pressure vessels and heat exchangers for oil and gas applications. The successful joining of dissimilar materials with vastly different thermal properties and metallurgical characteristics presents significant technical challenges that require careful process design.

Material Characteristics and Compatibility Analysis

The combination of Stellite 6 and 16MnR presents a classic dissimilar metal welding challenge. Understanding the properties of both materials is essential for developing a successful welding procedure.

Property	Stellite 6 (Co-Cr-W Alloy)	16MnR (Low-Alloy Steel)
Composition	57-65% Co, 28-32% Cr, 5-7% W, 1-3% Mo, 1-3% C	0.12-0.20% C, 1.20-1.60% Mn, 0.30-0.60% Si
Thermal conductivity	11.5 W/(m·K)	45 W/(m·K)
Thermal expansion coefficient	13.0 x 10^-6 /K	12.0 x 10^-6 /K
Melting range	1320-1345°C	1470-1510°C
Hardness (as-welded)	35-45 HRC	120-180 HV
Density	8.8 g/cm³	7.85 g/cm³

The significant difference in thermal conductivity between Stellite 6 and 16MnR creates uneven heat flow during welding, leading to thermal stress concentration at the interface. The higher melting point of 16MnR compared to Stellite 6 means that the base metal remains solid longer during cooling, creating differential contraction stresses.

Welding Process Development

The study developed a qualified welding procedure for Stellite 6 overlay on 16MnR substrate, considering the following critical process parameters:

Welding Process Selection

Gas tungsten arc welding (GTAW/TIG) was selected as the primary process for the following reasons:

Excellent control over heat input, minimizing dilution and thermal distortion
Clean weld deposits with minimal contamination
Suitability for thin overlay layers (typically 2-3 mm for corrosion/wear protection)
Good control over the weld pool geometry and penetration profile

Process Parameters

Parameter	Specification
Welding current	80-120 A (DC, electrode negative)
Arc voltage	18-24 V
Travel speed	60-100 mm/min
Shielding gas	Argon (99.99% purity), 15-20 L/min
Preheat temperature	150-250°C
Interpass temperature	Maximum 300°C
Overlay thickness	2-3 mm (multiple passes)
Post-weld stress relief	550-650°C, 2 hours per 25 mm thickness

Welding Sequence Strategy

The welding sequence was designed to minimize distortion and manage residual stresses:

Surface preparation: Grinding of the 16MnR surface to remove oxide scale and contaminants, exposing sound base metal.
Preheating: Uniform heating of the workpiece to 150-250°C, with temperature monitoring at multiple locations.
First pass: Single pass GTAW with low heat input, depositing a thin layer (1-1.5 mm) of Stellite 6.
Subsequent passes: Additional passes deposited with careful control of interpass temperature, building up to the required overlay thickness.
Stress relief: Post-weld stress relief annealing to reduce residual stresses to acceptable levels.
Final inspection: Visual, dimensional, and hardness testing of the completed overlay.

Interface Metallurgy and Bond Quality

The metallurgical behavior at the Stellite 6 / 16MnR interface is critical to the integrity of the overlay. The study examined the following aspects:

Dilution and Composition Gradient

The dilution of Stellite 6 by 16MnR base metal occurs primarily in the first pass, with subsequent passes showing progressively lower dilution. Typical dilution levels are:

Pass Number	Dilution (%)	Interface Composition
1st pass	20-35%	Mixed Co-Fe-Cr with significant Fe enrichment
2nd pass	10-20%	Predominantly Co-Cr with moderate Fe
3rd pass	5-10%	接近 Stellite 6 composition

Microstructural Evolution

The interface microstructure exhibits the following characteristics:

Base metal side: Heat-affected zone with tempered martensite and fine carbide precipitation.
Interface region: Transition zone with mixed microstructure, including Co-based solid solution and Cr-rich carbides.
Overlay side: Typical Stellite 6 microstructure with Co solid solution matrix, M7C3 and M23C6 carbides, and possible W-rich phases.

Bond Strength Verification

Bond strength testing (peel test or torsion test) confirmed adequate bonding between the overlay and base metal, with peel strengths exceeding 150 MPa, well above the minimum requirements specified in relevant standards.

Quality Control and Inspection

The welding procedure incorporates comprehensive quality control measures:

Pre-weld inspection: Visual examination of base metal surface, verification of preheat temperature, and confirmation of welding consumable specifications.
In-process monitoring: Real-time monitoring of welding parameters, interpass temperature control, and visual inspection of each pass.
Post-weld inspection: Visual examination, dimensional verification, hardness testing, and non-destructive testing (MT or PT) of the overlay surface.
Destructive testing: Bond strength testing, metallographic examination of cross-sections, and corrosion testing for qualification purposes.

Engineering Applications and Considerations

The Stellite 6 overlay on 16MnR combination is widely used in oil and gas industry applications where:

Resistance to erosion-corrosion is required (e.g., valve seats, pump impellers, heat exchanger tubes)
High-temperature wear resistance is needed (e.g., turbine components, hot gas ducts)
Chemical resistance in aggressive environments is essential (e.g., chemical processing equipment)

Key engineering considerations include:

Thermal cycling resistance: The dissimilar interface must withstand repeated thermal cycling without degradation of bond strength.
Creep resistance: At elevated temperatures, the interface must maintain dimensional stability and mechanical integrity.
Corrosion resistance: The overlay must provide effective protection against the service environment, with no galvanic coupling issues at the interface.
Fatigue performance: The residual stress state at the interface influences fatigue crack initiation and propagation.

Study Insights and Recommendations

This research provides a well-documented welding procedure for Stellite 6 overlay on 16MnR steel, addressing the critical technical challenges of dissimilar metal welding. The emphasis on preheat control, low heat input welding, and post-weld stress relief demonstrates a systematic approach to managing the thermal and mechanical challenges of this material combination.

For engineers implementing similar overlay applications, the following recommendations emerge:

Always qualify the welding procedure according to applicable codes (ASME IX, NB/T 47014, or AWS D10.9) before production application.
Maintain strict control of preheat and interpass temperatures to minimize thermal gradients and residual stresses.
Consider the long-term performance of the overlay in the intended service environment, including thermal cycling, corrosion, and mechanical loading.
Document all welding parameters and inspection results for traceability and future reference.

The successful application of Stellite 6 overlay on 16MnR steel demonstrates the versatility of cobalt-based alloys in enhancing the performance of carbon and low-alloy steel components for demanding service conditions. As the oil and gas industry continues to face increasingly severe operating environments, the development and optimization of overlay welding procedures will remain a critical technology for extending equipment life and ensuring operational safety.