Zhuojin Pipe Fitting Co., LtdFactors Affecting Hardness of Cobalt-Based Alloy Overlay Weld Deposits
Literature Overview
The paper by Xie Fubiao from Suzhou Valve Factory, published in the Valve journal (Issue 3, 1994, pages 17-20), provides a detailed analysis of the factors that influence the hardness of cobalt-based alloy overlay weld deposits. Cobalt-based overlay materials are widely used in valve manufacturing, particularly for sealing surfaces in high-temperature and high-pressure applications where both wear resistance and metallurgical bonding to the base material are required. The hardness of the overlay deposit directly affects the sealing performance, wear life, and overall reliability of valve components. This paper systematically investigates how welding method, dilution rate, overlay thickness, and base material composition affect the resulting deposit hardness.
Core Technical Content and Key Findings
The author identifies four primary factors that influence cobalt-based overlay deposit hardness and provides detailed analysis for each factor. The systematic approach to investigating these factors makes this paper a valuable reference for engineers designing overlay welding processes for valve and similar applications.
Welding Method and Process Parameters
Different welding methods produce different heat input levels and cooling rates, which directly affect the microstructure and hardness of cobalt-based deposits:
| Welding Method | Typical Heat Input | Cooling Rate | Typical Hardness (HV) | Dilution Level |
|---|---|---|---|---|
| SMAW (stick) | High | Moderate | 350-450 | High (15-30%) |
| GTAW (TIG) | Low to moderate | Fast | 400-500 | Low (5-15%) |
| GMAW (MIG) | Moderate | Moderate | 380-480 | Moderate (10-20%) |
| Submerged arc | High | Slow | 300-400 | High (20-35%) |
| Plasma arc | Low | Very fast | 450-550 | Very low (3-10%) |
The welding method selection is critical because it determines the dilution rate, which in turn affects the final composition and hardness of the deposit. Methods with lower heat input such as TIG and plasma arc welding produce lower dilution rates, resulting in deposit compositions closer to the filler material and consequently higher hardness. Conversely, methods with higher heat input such as submerged arc welding produce higher dilution and lower hardness due to the incorporation of softer base metal into the deposit.
Dilution Rate and Hardness Relationship
The paper establishes a clear quantitative relationship between dilution rate and deposit hardness. As the dilution rate increases, the concentration of cobalt and other hardening alloying elements in the deposit decreases, leading to a reduction in hardness. This relationship is particularly important for valve applications where the sealing surface hardness must meet specific requirements to ensure proper contact pressure distribution and seal integrity.
The dilution rate can be estimated from the following considerations:
- Base metal melting volume relative to filler material volume.
- Weld bead geometry and penetration depth.
- Number of passes and their overlap.
- Welding parameters (current, voltage, travel speed).
The relationship between dilution rate and hardness follows a generally linear trend for cobalt-based systems, with hardness decreasing approximately 20-40 HV per 10 percentage point increase in dilution rate, depending on the specific alloy composition and base material.
Overlay Thickness Effects
The paper also addresses the effect of overlay layer thickness on hardness distribution. In multi-pass overlay welds, the hardness profile through the thickness is not uniform:
| Layer Position | Dilution Level | Typical Hardness | Composition Characteristic |
|---|---|---|---|
| First pass (near base) | Highest | Lowest | Highest Fe content |
| Second pass | High | Low-moderate | Moderate Fe content |
| Third pass | Moderate | Moderate | Lower Fe content |
| Subsequent passes | Low | Highest | Closest to filler composition |
This hardness gradient through the overlay thickness has important implications for valve sealing performance. The sealing surface is typically machined from the top of the overlay, where hardness is highest and most uniform. However, if the total overlay thickness is insufficient, the lower hardness near the fusion line can affect the overall structural integrity and may lead to premature failure through cracking or delamination.
Base Material Composition Effects
The base material composition directly affects the dilution characteristics and final deposit properties. Different valve body materials (carbon steel, low-alloy steel, stainless steel, duplex steel) have different melting behaviors and different interactions with the cobalt-based filler material:
| Base Material | Typical Fe Content in Deposit | Hardness Reduction | Special Considerations |
|---|---|---|---|
| Carbon steel (A105) | High | Significant (40-60 HV) | High dilution, lower hardness |
| Low-alloy steel (A216 WCB) | Moderate-high | Moderate (30-50 HV) | Mo and Cr affect deposit composition |
| Stainless steel (316) | Moderate | Moderate (25-40 HV) | Ni and Cr from base affect deposit |
| Duplex stainless | Moderate | Low-moderate (20-35 HV) | Good metallurgical compatibility |
Engineering Practice Integration
For valve manufacturers and engineers, this literature provides critical guidance on overlay welding process design for cobalt-based sealing surface applications:
- Welding method selection: TIG or plasma arc welding should be preferred for applications requiring high deposit hardness, as these methods produce lower dilution rates and more uniform composition.
- Multi-pass strategy: A minimum of 3-4 passes should be used to achieve adequate thickness with uniform hardness in the top layers, with the first 1-2 passes serving as transition layers.
- Dilution monitoring: Chemical analysis or hardness mapping should be performed on qualification coupons to verify that the production process achieves the required dilution rate and hardness level.
- Base material consideration: The base material composition must be accounted for in the overlay design, with appropriate filler material selection to compensate for expected dilution effects.
- Post-weld treatment: Controlled tempering may be required to achieve the target hardness level while relieving residual stresses, particularly for thick overlay deposits on thick-walled components.
The following table provides recommended process parameters for cobalt-based overlay welding on valve sealing surfaces:
| Parameter | Carbon Steel Base | Stainless Steel Base | Target Hardness |
|---|---|---|---|
| Welding method | TIG or plasma | TIG or plasma | 400-500 HV |
| Current | 100-150 A | 80-120 A | - |
| Travel speed | 150-250 mm/min | 150-250 mm/min | - |
| Number of passes | 4-6 | 4-6 | - |
| Layer thickness per pass | 0.5-1.0 mm | 0.5-1.0 mm | - |
| Total overlay thickness | 3-5 mm | 3-5 mm | - |
| Preheat | 150-200°C | 100-150°C | - |
| Interpass temperature | <250°C | <200°C | - |
Study Insights and Implications
The systematic investigation of hardness-affecting factors presented in this paper demonstrates the importance of understanding the fundamental metallurgical relationships in overlay welding process design. The dilution rate emerges as the most critical parameter because it directly determines the final composition of the deposit and consequently its hardness. This insight has practical implications for process development: rather than simply specifying a filler material, engineers must specify the entire welding process including method, parameters, and number of passes to ensure the target hardness is achieved.
The paper also highlights an important aspect of overlay welding quality control: the need to verify not just the surface hardness but the hardness profile through the entire overlay thickness. Surface hardness measurements alone may provide misleading results if the lower layers have significantly different hardness due to dilution effects. A comprehensive hardness map across the deposit cross-section provides a more complete picture of the overlay quality.
For modern valve manufacturing, these principles remain highly relevant as cobalt-based overlay materials continue to be widely used for sealing surfaces in demanding applications. The process development methodology described in this paper can be enhanced with modern analytical techniques such as EDS for composition mapping and micro-Vickers hardness profiling to provide more detailed and quantitative characterization of the overlay deposit properties.
Summary
This paper on cobalt-based overlay weld deposit hardness provides a comprehensive and practical analysis of the key factors affecting deposit hardness, with dilution rate emerging as the most critical parameter. The systematic investigation of welding method, dilution rate, overlay thickness, and base material effects offers engineers a clear framework for process design and quality control in cobalt-based overlay welding applications, particularly for valve sealing surfaces where hardness directly impacts functional performance and service life.