Welding Process for Overlay Welding Stellite 6 on 16MnR Steel

Literature Overview

The study by Bo, Guo, Zhang, and Qiu from Daqing Oilfield addresses the overlay welding of Stellite 6 cobalt-based alloy onto 16MnR pressure vessel steel. Published in Welding Technology (2008, Vol. 37, Issue 6, pp. 24-26), this work tackles a classic dissimilar material welding challenge where a hardfacing alloy with high alloy content is deposited onto a low-alloy steel substrate. The application context is oilfield equipment repair, where components such as valve seats, pump impellers, and pipeline fittings require erosion and corrosion resistance that exceeds the capabilities of the base material.

Material Compatibility Analysis

The dissimilar material combination presents several metallurgical challenges:

Property	16MnR (Base Metal)	Stellite 6 (Overlay)
Carbon content	0.12-0.20 wt%	0.35-0.45 wt%
Chromium	0.30-0.60 wt%	21-23 wt%
Cobalt	0.02-0.10 wt%	53-59 wt%
Molybdenum	0.008-0.030 wt%	2.5-3.0 wt%
Thermal expansion coefficient	12 × 10⁻⁶/°C	14.5 × 10⁻⁶/°C
Thermal conductivity	45 W/(m·K)	11 W/(m·K)
Hardness	120-160 HV	380-450 HV
Dilution tendency	Low	High (50-80% dilution in first pass)

The significant differences in thermal expansion coefficient and thermal conductivity create substantial residual stresses at the interface. The high dilution rate in the first overlay pass—where Stellite 6 melts into the 16MnR base metal—produces a transition layer with unpredictable composition and properties. This dilution layer is the primary site for cracking during cooling.

Weldability Challenges and Solutions

Cold Cracking Prevention

The primary defect risk is cold cracking in the dilution zone, caused by:

Martensite formation in the high-carbon, high-alloy dilution layer
High residual tensile stress from differential thermal contraction
Hydrogen embrittlement from the welding process

The authors' strategy for cold crack prevention included:

Preheating at 200-250°C: Reduces cooling rate and promotes hydrogen diffusion.
Low-hydrogen process selection: TIG (GTAW) welding was selected for the first pass to minimize hydrogen pickup, followed by MIG (GMAW) or SAW for subsequent passes.
Rapid transition to full alloy composition: Multiple thin passes were used to quickly build up the overlay thickness before significant dilution could occur, reducing the volume of the problematic dilution zone.
Post-weld stress relief: Heat treatment at 700-750°C to relieve residual stresses and temper any retained martensite.

Process Parameters

Parameter	First Pass (TIG)	Subsequent Passes (SAW/MIG)
Current	120-160 A	350-450 A
Voltage	12-16 V	28-35 V
Travel speed	80-120 mm/min	200-300 mm/min
Wire diameter	2.4 mm	3.2 mm
Shielding gas	Argon	Ar + 2% O2 (MIG) or flux (SAW)
Interpass temperature	150-200°C	200-250°C
Layer thickness	1.5-2.0 mm	3.0-4.0 mm

Interface Metallurgy

The dilution zone metallurgy is critical to the long-term performance of the overlay. In the first pass, the dilution ratio typically ranges from 50-80% base metal. This creates a zone with intermediate alloy content where:

Chromium concentration drops to 5-12 wt%, below the threshold for full austenitic or carbide-strengthened structure
The microstructure transitions from ferritic-pearlitic (16MnR) through a mixed zone to the fully austenitic with M7C3 carbides (Stellite 6)
Microcracks may initiate at the interface due to thermal mismatch and compositional gradients

The authors addressed this by ensuring that the first pass was deposited with controlled dilution through careful joint preparation—a shallow groove was machined to limit the base metal contribution—and by using a transition filler (such as a medium-alloy cobalt-based alloy) for the first pass before switching to full Stellite 6.

Quality Control and Verification

Test Method	Acceptance Criteria	Purpose
Visual inspection	No cracks, undercut, or excessive spatter	Surface integrity
Penetrant testing (PT)	No linear indications	Surface-breaking cracks
Magnetic particle testing (MT)	No indications	Surface/subsurface defects
Hardness test (HV)	350-450 HV in overlay	Confirm Stellite 6 properties
Dilution analysis (optical emission)	<50% base metal in first pass	Interface control
Cross-sectional metallography	No cracks at interface	Bond integrity

Engineering Practice Considerations

In oilfield service environments, Stellite 6 overlays are commonly applied to:

Drill pipe tool joints for erosion resistance
Valve trim for sealing surface durability
Pump impellers for cavitation and erosion resistance
Choke valves for high-pressure, high-temperature service

The critical engineering decision is the selection between TIG and SAW for the overlay process. TIG provides superior control of the first pass dilution and is preferred for thin overlays or precision components, while SAW offers higher deposition rates for thicker overlays on large components. A hybrid approach—TIG for the first pass and SAW for the remaining passes—is often the optimal solution for production efficiency.

A common field problem is the use of excessive preheating, which can cause grain growth in the 16MnR base metal and reduce its toughness. The preheat temperature should be carefully controlled to the minimum effective value (200°C rather than 300°C) to balance crack prevention against base metal degradation.

Summary

The overlay welding of Stellite 6 on 16MnR steel requires careful management of dilution, residual stress, and hydrogen control to prevent cracking in the dilution zone. The multi-pass approach with TIG for the first pass and SAW for subsequent passes, combined with moderate preheating and post-weld stress relief, provides a reliable process window. This literature serves as a practical reference for engineers applying cobalt-based hardfacing alloys to low-alloy steel components in demanding service conditions.