Zhuojin Pipe Fitting Co., LtdOverlay Welding Process for Stellite 6 on 16MnR Steel
Literature Overview
The technical paper by Bo Liyan, Guo Xiaochun, Zhang Xianlong, and Qiu Haiping from Daqing Oilfield Limited Liability Company and Daqing Petroleum Administration Bureau was published in Welding Technology (焊接技术, 2008, Vol. 37, Issue 6, pp. 24-26) under ISSN 1002-025X. This study focuses on the weldability analysis and overlay welding process development for depositing Stellite 6 cobalt-based alloy onto 16MnR low-alloy pressure vessel steel. The classification code is TG455, and the keywords include Stellite 6 alloy steel, welding, and crack prevention.
Material Characteristics and Weldability Challenges
Base Metal: 16MnR
16MnR is a low-alloy high-strength steel widely used for pressure vessels and boilers in China. Its key characteristics include:
| Property | Value |
|---|---|
| Carbon content | ≤ 0.20 wt% |
| Manganese content | 1.20-1.60 wt% |
| Yield strength | ≥ 345 MPa |
| Tensile strength | 510-660 MPa |
| Carbon equivalent | 0.35-0.45% |
| Typical application | Pressure vessels, boilers, heat exchangers |
The relatively low carbon equivalent of 16MnR provides good weldability, but the manganese content contributes to the formation of hard phases in the HAZ, which can be a concern when welding with dissimilar materials.
Overlay Material: Stellite 6
Stellite 6 is a cobalt-chromium-tungsten alloy (Co-Cr-W type) known for its exceptional wear resistance, corrosion resistance, and high-temperature strength. Its composition includes approximately:
| Element | Content (wt%) |
|---|---|
| Cobalt (Co) | ~61 |
| Chromium (Cr) | ~28 |
| Tungsten (W) | ~4 |
| Molybdenum (Mo) | ~1 |
| Carbon (C) | ~1.2 |
| Iron (Fe) | Balance |
Dissimilar Welding Challenges
The overlay welding of Stellite 6 onto 16MnR presents several metallurgical challenges:
- High dilution: The low melting point of the cobalt alloy relative to the steel substrate leads to significant dilution of the overlay layer by the base metal, reducing the concentration of Co, Cr, and W in the weld metal and potentially degrading the wear and corrosion resistance.
- Cracking susceptibility: Stellite 6 is inherently prone to hot cracking due to the formation of low-melting-point intermetallic phases (such as Co3W and Co3Mo) that segregate to grain boundaries during solidification. The dilution by iron from the base metal exacerbates this tendency.
- Residual stress: The large difference in thermal expansion coefficients between Stellite 6 (approximately 13.5 × 10⁻⁶/°C) and 16MnR (approximately 12.5 × 10⁻⁶/°C) generates significant thermal stresses during cooling, which can promote both hot cracking and cold cracking.
- Hardness mismatch: Stellite 6 has a hardness of approximately 35-40 HRC, while 16MnR has a hardness of approximately 20-25 HRC. This mismatch can lead to stress concentration at the interface.
Welding Process Qualification and Optimization
The authors conducted a systematic welding process qualification to determine the optimal overlay welding parameters. The key process variables and their optimization criteria are summarized below:
| Process Parameter | Optimized Range | Rationale |
|---|---|---|
| Welding process | GTAW (TIG) | Low heat input, low dilution, clean weld |
| Shielding gas | Argon (99.99%) | Inert atmosphere, no oxidation |
| Current type | DCEN | Deep penetration, stable arc |
| Current | 80-120 A | Controlled heat input |
| Travel speed | 5-8 cm/min | Adequate fusion, minimal dilution |
| Preheat temperature | 100-150°C | Reduce thermal stress, prevent cold cracking |
| Interpass temperature | < 150°C | Control heat accumulation |
| Electrode/wire diameter | 2.4-3.2 mm | Suitable for multi-layer build-up |
| Layer thickness | 1.5-2.5 mm | Control dilution between layers |
Crack Prevention Measures
The primary focus of the study was on preventing cracking in the Stellite 6 overlay weld. The following measures were implemented:
- Multi-layer overlay welding: Instead of a single thick layer, the Stellite 6 was deposited in multiple thin layers (typically 2-3 layers). The first layer acts as a transition layer with higher dilution, while subsequent layers have progressively lower dilution, ensuring the final overlay layer has adequate Co, Cr, and W content.
- Controlled heat input: Using GTAW with relatively low current and moderate travel speed minimizes the thermal cycle severity, reducing the tendency for hot cracking.
- Preheating: A moderate preheat of 100-150°C was applied to reduce the thermal gradient and cooling rate, which helps prevent both cold cracking in the HAZ and hot cracking in the weld metal.
- Post-weld heat treatment: A stress relief treatment at 500-600°C was applied to reduce residual stresses and improve the toughness of the overlay weld.
- Welding sequence optimization: The welding sequence was designed to minimize restraint and allow for thermal expansion during welding.
Quality Verification and Results
The overlay weld was verified through the following non-destructive and destructive testing methods:
- Visual inspection: Uniform bead profile, no surface cracks, proper geometric build-up
- Penetrant testing (PT): No surface-breaking cracks detected in the overlay weld
- Radiographic testing (RT): No internal defects such as porosity or lack of fusion
- Hardness testing: Hardness profile showing a gradual transition from base metal (20-25 HRC) through the dilution zone to the final overlay layer (30-35 HRC)
- Metallographic examination: Sound microstructure, no cracking at the interface or within the overlay layers
Engineering Practice Implications
This study is directly applicable to the repair and enhancement of pressure vessel components in the petroleum and chemical industries. Common applications include:
- Wear-resistant overlay on valve bodies and valve seats
- Corrosion-resistant lining on heat exchanger tube sheets
- Hardfacing on pump impellers and wear rings
- Repair of eroded surfaces on pressure vessel internals
The key engineering lesson is that the successful overlay welding of Stellite 6 onto low-alloy steel requires a careful balance between dilution control and crack prevention. The multi-layer approach with GTAW provides the best combination of low dilution and crack resistance, making it the preferred method for high-quality Stellite 6 overlay applications.
Study Reflection
This paper provides a practical and well-documented approach to dissimilar metal overlay welding, a common challenge in industrial equipment maintenance and enhancement. The emphasis on crack prevention through process optimization rather than material modification is particularly relevant for field repair applications where the base metal composition cannot be changed. The systematic approach of welding process qualification followed by multi-method quality verification exemplifies best practices in welding engineering. For engineers working in the petroleum and chemical industries, this study offers a reliable methodology for implementing Stellite 6 overlay welding on 16MnR pressure vessel components.