CLADDING TECHNOLOGY SHANXI CO., LTDStellite Alloy Weld Overlay Repair of 15CrMo Forged Steel Gate Valve
Literature Overview
This study addresses the practical challenge of repairing a gate valve body fabricated from 15CrMo forged steel using Stellite alloy overlay welding. The valve operates in high-temperature and high-pressure service, where erosion and galling at the seat and guide surfaces progressively degrade sealing integrity. The authors propose a systematic repair methodology covering substrate preparation, filler selection, welding sequence design, and post-weld treatment. The core contribution lies in demonstrating how dilution control and heat input management can be achieved on a thick-walled forged component without inducing detrimental microstructural changes in the base metal.
Core Technical Points
Substrate Characterization
15CrMo is a normalized or quenched-and-tempered low-alloy steel containing approximately 0.12–0.18% C, 0.80–1.10% Cr, and 0.25–0.30% Mo. Its typical hardness after tempering at 650–700°C ranges from 180 to 240 HBW. The carbon equivalent (Ceq) calculated per IIW formula is approximately 0.38%, placing it in a moderate carbon equivalent category. This means the heat-affected zone (HAZ) is susceptible to hardening and potential cracking if the welding process is not properly controlled. The prior austenite grain size and tempering precipitate distribution are critical factors governing the susceptibility to hydrogen-assisted cracking and temper embrittlement.
Filler Metal Selection
The study evaluates Stellite 6 (UNS S66200, Co-28Cr-6W-6Mo-1Si-1Fe balance) and Stellite 21 (UNS S66000, Co-25Cr-12W-12Mo-1.5Si-1.5Fe balance) as candidate overlay materials.
| Parameter | Stellite 6 | Stellite 21 |
|---|---|---|
| Co content (wt%) | ~63 | ~48 |
| Cr content (wt%) | 27–32 | 23–27 |
| W content (wt%) | 5.5–7.5 | 11–13 |
| Mo content (wt%) | 5.5–7.5 | 11–13 |
| Hardness (as-cast, HBW) | 360–420 | 360–400 |
| Max service temperature (°C) | 900 | 870 |
| Dilution tolerance (base metal %) | Up to 30% | Up to 40% |
Stellite 21 was ultimately selected for the repair because its higher W and Mo content provides better resistance to dilution-induced softening when interfacing with the 15CrMo base. The presence of primary carbides (MC and M7C3 type) in the as-welded microstructure contributes to both wear resistance and thermal stability.
Welding Process Parameters
The repair was executed using gas metal arc welding (GMAW) with a Stellite 6 or Stellite 21 solid wire, protected by a mixed shielding gas of 98% Ar + 2% CO2.
| Parameter | Value |
|---|---|
| Wire diameter | 1.2 mm |
| Current | 160–180 A |
| Voltage | 22–25 V |
| Travel speed | 8–12 cm/min |
| Heat input | 0.8–1.2 kJ/mm |
| Preheat temperature | 150–200°C |
| Interpass temperature | ≤ 250°C |
| Post-weld heat treatment | 650°C × 2 h (furnace, air cooling) |
The preheat temperature of 150–200°C serves a dual purpose: it reduces the cooling rate in the HAZ to below 20°C/s, thereby limiting the formation of martensitic phases, and it minimizes the risk of hydrogen-induced cracking. The interpass temperature was maintained below 250°C to avoid excessive grain growth in the Stellite overlay and to prevent over-tempering of the 15CrMo base metal.
Dilution Control Strategy
Dilution is the single most critical factor governing the final properties of the overlay. In the first pass, dilution from the 15CrMo base metal into the Stellite melt pool can reach 35–50%, which substantially reduces the hardness and corrosion resistance of the overlay. The study employs a three-pass strategy:
- First pass: A low-dilution approach using short arc length, low current, and rapid travel speed. The goal is to establish a metallurgical bond while minimizing base metal melting. Dilution is typically 35–45%.
- Second pass: Applied over the first pass with slightly higher current. The dilution drops to 10–20% because the previous pass already contains a significant fraction of Stellite alloy.
- Third pass: Final finishing pass with full alloy composition. Dilution is below 5%, ensuring the as-welded hardness reaches the target range of 380–420 HBW.
The resulting hardness profile across the overlay thickness shows a gradient from approximately 260 HBW at the interface to 400 HBW at the surface, which is acceptable for gate valve seat applications where the critical wear zone is the functional surface.
Microstructural Analysis and Defect Assessment
Interface Microstructure
Metallographic examination of the overlay-base interface reveals a decarburized zone in the 15CrMo base metal extending approximately 0.3–0.5 mm from the fusion line. This zone exhibits a ferrite-pearlite microstructure with reduced hardness (160–180 HBW compared to the base 200–220 HBW). The decarburization occurs because carbon from the base metal diffuses into the molten Stellite pool during welding. While this softening zone is undesirable, it is manageable if the overlay is designed to carry the functional load and the base metal retains sufficient structural integrity.
The fusion zone microstructure transitions from ferrite-pearlite in the base through a mixed region containing coarse austenite and martensite to the Stellite dendritic structure with interdendritic carbides. The presence of residual austenite in the transition zone is beneficial because it provides some toughness and reduces the risk of cracking during subsequent thermal cycling.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in HAZ | Excessive cooling rate, hydrogen pickup | Increase preheat, use low-hydrogen consumables, control interpass temperature |
| Cracking in overlay | Thermal stress from high thermal expansion mismatch | Multi-pass welding, post-weld stress relief, use of ductile intermediate layer |
| Porosity | Gas entrapment, insufficient shielding | Increase shielding gas flow, clean surface, use dry flux/wire |
| Excessive dilution | High heat input, long arc length | Reduce current, shorten arc, increase travel speed, use multi-pass technique |
| Undercut | Excessive current, improper gun angle | Adjust parameters, use proper weave pattern |
Post-Weld Heat Treatment
A post-weld stress relief treatment at 650°C for 2 hours in a furnace followed by air cooling is essential. This treatment serves to:
- Relieve residual welding stresses that can reach 200–400 MPa in the overlay and HAZ
- Temper any martensitic phases formed in the HAZ during welding
- Stabilize the microstructure of the Stellite overlay by promoting carbide coarsening
- Reduce the risk of delayed cracking
However, the PWHT temperature must not exceed 700°C to avoid over-tempering of the 15CrMo base metal, which would reduce its yield strength below the required minimum. The cooling rate from the PWHT temperature should be controlled to avoid re-hardening of the HAZ.
Engineering Practice Integration
Application Context
The gate valve in question is installed in a hydrocracking unit where it handles hot hydrocarbon streams at 380°C and 18 MPa. The original valve seat was machined from the same 15CrMo forging, but after approximately 3 years of service, erosion from hot fluid flow and galling during cycling had reduced the seat surface hardness to 180 HBW, resulting in seal leakage. The repair was performed during a scheduled turnaround with the valve removed from service.
Repair Procedure
The repair followed a rigorous procedure:
- Surface preparation: The worn seat surface was ground flat using a portable grinder with 60-grit aluminum oxide discs, followed by 120-grit finishing. The surface was cleaned with acetone to remove grinding debris and contaminants.
- Preheating: The valve body was preheated to 180°C using a propane torch, with temperature monitoring at three locations using a type-K thermocouple.
- Welding: Three passes of Stellite 21 were deposited using GMAW. The first pass was applied with a 1.2 mm wire at 160 A, 22 V, and 10 cm/min. The second and third passes used 170 A, 23 V, and 9 cm/min.
- Interpass inspection: After each pass, the surface was inspected using magnetic particle testing (MT) per ASTM E709 to detect any surface cracks or indications.
- Post-weld treatment: The valve was placed in a furnace for PWHT at 650°C × 2 h, followed by air cooling to below 100°C before removal.
- Final inspection: The repaired seat was machined to the final profile, and the overlay hardness was verified at five locations using a portable ultrasonic hardness tester. All readings were within the 380–410 HBW range.
Performance Verification
After reinstallation and return to service, the valve was monitored for 12 months. No seat leakage was detected, and the overlay surface showed only minimal wear after 8 months of continuous operation. This represents a significant improvement over the original 3-year service life of the unclad seat.
Key Reflections and Study Insights
The most valuable insight from this study is the recognition that overlay repair of low-alloy steels is fundamentally a dilution management problem. The mechanical and corrosion properties of the overlay are almost entirely determined by the dilution level, and achieving low dilution requires careful control of every welding parameter. The three-pass strategy, where each subsequent pass reduces dilution progressively, is a robust approach that can be applied to a wide range of overlay repair scenarios.
Another important lesson is the critical role of post-weld heat treatment. Without PWHT, the residual stresses in the overlay and HAZ can lead to cracking during subsequent thermal cycling, particularly in high-temperature service. The selection of PWHT temperature requires careful consideration of the base metal's tempering response and the overlay's carbide stability.
From a practical standpoint, the study demonstrates that Stellite overlay repair is a viable and cost-effective alternative to valve replacement for critical gate valves in high-temperature service. The repair restores the functional surface properties while preserving the structural integrity of the forged body. However, the study also highlights that the repair must be performed by experienced welders with access to proper preheat, PWHT, and NDT capabilities. Attempting such a repair without these resources would likely result in failure.