Manual Surfacing of Cobalt-Based Alloy on High-Pressure Valves

Literature Overview

This paper by Wen Xiangjun, published in Petrochemical Equipment (Vol. 18, No. 1, 1989, p. 47), addresses a critical maintenance challenge in petrochemical plants: the manual surfacing of cobalt-based alloys on high-temperature, high-pressure valve sealing surfaces. The work is particularly notable for its analysis of cracking mechanisms in cobalt-based alloy surfacing and the development of modified welding procedures to prevent weld defects.

Material and Application Context

Cobalt-Based Alloy Properties

Cobalt-based alloys (such as Stellite 6, Stellite 21, and equivalent Chinese grades like D241, D269) are widely used for valve sealing surfaces due to their exceptional combination of:

Property	Value/Range
Hardness (as-welded)	35-45 HRC
Hardness (after annealing)	30-38 HRC
Creep resistance	Excellent to 900°C
Thermal fatigue resistance	Excellent
Oxidation resistance	Good to 1000°C
Wear resistance	Excellent
Thermal conductivity	10-15 W/(m·K)

The low thermal conductivity of cobalt-based alloys is both an advantage (maintaining high-temperature hardness) and a challenge (creating steep thermal gradients during welding).

Valve Application Requirements

High-pressure valves in petrochemical service operate under extreme conditions:

Temperature: 300-600°C
Pressure: 10-42 MPa
Service media: Hydrocarbons, hydrogen, sour gas, steam
Cycle loading: Frequent open-close operations causing thermal fatigue

The valve sealing surfaces must maintain dimensional accuracy and hardness under these conditions, making cobalt-based alloy surfacing a critical maintenance technology.

Cracking Analysis and Prevention

Conventional Approach and Its Limitations

The conventional welding procedure for cobalt-based alloy surfacing involves:

Preheating to 300-400°C.
Interpass temperature control at 200-300°C.
Post-weld high-temperature annealing (900-1000°C) with slow cooling.

However, for valve components with high rigidity (thick walls, complex geometry), this conventional approach often results in:

Cold cracking: Caused by hydrogen embrittlement combined with high residual stresses in the rigid component.
Hot cracking (solidification cracking): Caused by the high liquid range of cobalt-based alloys and the restraint imposed by the rigid valve body.

Cracking Mechanism Analysis

The paper provides a systematic analysis of the cracking mechanisms:

Cold cracking occurs when:

The weld metal contains sufficient hydrogen from the atmosphere or consumable.
The base material has a martensitic or high-strength microstructure.
The residual stress exceeds the material's tensile strength.

In cobalt-based alloy surfacing on high-pressure valves, the combination of low thermal conductivity (creating high thermal gradients), high residual stress (due to component rigidity), and potential hydrogen absorption creates a highly susceptible condition for cold cracking.

Hot cracking occurs when:

The weld metal has a wide freezing range with low ductility in the solidification temperature range.
The component restraint prevents shrinkage strain accommodation.
Sulfur or phosphor segregation at grain boundaries reduces boundary strength.

Cobalt-based alloys are inherently susceptible to hot cracking due to their solidification behavior, and the rigid valve body exacerbates this tendency.

Modified Welding Procedure

Based on the cracking analysis, the paper proposes a modified procedure:

Parameter	Conventional	Modified
Preheat temperature	300-400°C	400-500°C
Interpass temperature	200-300°C	300-400°C
Welding technique	Standard SMAW	Low heat input, short arcs
Post-weld treatment	High-temp annealing	Stress relief at 600-700°C
Cooling rate	Slow furnace cool	Controlled rate, avoid rapid cool

The key modifications include:

Increased preheating and interpass temperatures: Reducing the thermal gradient and slowing the cooling rate to minimize residual stress and avoid brittle microstructures.
Low heat input welding: Using short arcs and frequent stops to minimize the volume of metal affected by the thermal cycle, reducing the risk of cracking.
Modified post-weld treatment: Instead of high-temperature annealing (which may cause grain coarsening and loss of hardness), a lower-temperature stress relief treatment at 600-700°C reduces residual stresses while preserving the alloy's wear resistance properties.
Weld geometry optimization: Using shallow, wide beads rather than deep, narrow beads to reduce stress concentration and improve strain accommodation.

Quality Control and Inspection

For cobalt-based alloy surfacing on critical valve components, the following inspection regime is recommended:

Visual inspection: Check for surface cracks, porosity, undercut, and bead uniformity.
Penetrant testing (PT): Detect surface-breaking cracks and defects.
Magnetic particle testing (MT): If applicable to the ferromagnetic base material, detect subsurface cracks.
Hardness testing: Verify overlay hardness in the range of 35-45 HRC (as-welded) or 30-38 HRC (after treatment).
Dimensional inspection: Verify overlay thickness and surface flatness to meet valve sealing requirements.

Engineering Practice Considerations

The modified procedure described in this paper has been successfully applied to high-pressure valve repair in petrochemical plants. The approach is particularly valuable for:

Globe valves with seat rings requiring cobalt-based alloy overlay.
Gate valves with hardened sealing surfaces.
Check valves with wear-resistant seat surfaces.

The key engineering insight is that the conventional approach to cobalt-based alloy surfacing—designed for less rigid components—must be modified for thick-walled, high-rigidity valve bodies. The increased preheating and interpass temperatures, combined with the modified post-weld treatment, effectively reduce cracking risk while maintaining the required surface properties.

Key Reflections

This 1989 paper remains highly relevant to modern practice, as the fundamental metallurgical challenges of cobalt-based alloy surfacing have not changed. The systematic approach to cracking analysis—identifying the mechanism, understanding the contributing factors, and developing targeted countermeasures—is a model for welding problem-solving. Engineers working on similar applications today should adopt this analytical framework rather than relying solely on empirical trial-and-error approaches. The paper also highlights the importance of understanding component geometry and rigidity when designing welding procedures, a consideration that is often overlooked in standard procedure development.