Zhuojin Pipe Fitting Co., LtdFactors Affecting Hardness of Cobalt-Based Alloy Overlay Welding Layers
Literature Overview
This paper, authored by Xie Fubiao from Suzhou Valve Factory and published in Valves (1994, No. 3, pp. 17–20), investigates the factors that affect the hardness of cobalt-based alloy overlay welding layers used for metal-to-metal sealing surfaces on valves. The paper examines the influence of welding method, dilution rate, overlay layer thickness, and base metal material on the hardness of the deposited overlay. The authors provide a detailed analysis of the primary factors and discuss the relationship between overlay hardness and dilution rate, as well as the relationship between overlay hardness and the iron content in the overlay layer.
Core Technical Analysis
Cobalt-Based Alloy Overlay Systems
Cobalt-based alloys are widely used in overlay welding applications for valve sealing surfaces due to their excellent combination of hardness, wear resistance, corrosion resistance, and red hardness. The common cobalt-based overlay systems include:
| Alloy System | Typical Composition | Hardness (HV) | Application |
|---|---|---|---|
| Stellite 6 | Co-6Cr-4W-4Mo-3Fe | 400–450 HV | Valve seats, pump impellers |
| Stellite 21 | Co-27Cr-1.5W | 200–250 HV | Corrosion-resistant applications |
| Stellite 25 | Co-14Cr-3.5W | 350–400 HV | Wear-resistant applications |
| Hastelloy X | Ni-15Cr-16Mo-2.5W | 250–300 HV | High-temperature applications |
| Custom Co-Cr-W | Co-20Cr-10W-5Mo | 450–550 HV | High-wear applications |
The hardness of the overlay layer is primarily determined by the composition of the deposited metal, which is influenced by the dilution rate and the base metal composition.
Dilution Rate and Hardness
The dilution rate is defined as the ratio of the base metal incorporated into the weld deposit relative to the total weld metal mass. The dilution rate directly affects the iron content in the overlay layer, which in turn affects the hardness. The relationship between dilution rate and hardness can be expressed as:
- Low dilution rate (< 10%): The overlay layer composition is close to the filler metal composition, resulting in high hardness (400–500 HV for typical cobalt-based alloys).
- Moderate dilution rate (10–30%): The iron content increases, leading to a moderate reduction in hardness (300–400 HV).
- High dilution rate (> 30%): The iron content is significantly increased, leading to a substantial reduction in hardness (< 300 HV) and potentially altering the phase composition of the overlay layer.
The paper provides quantitative data on the relationship between dilution rate and hardness, demonstrating that the hardness decreases linearly with increasing dilution rate for a given filler metal composition.
Welding Method and Its Influence
The welding method used for overlay welding significantly affects the dilution rate and, consequently, the hardness of the overlay layer. The following table compares the dilution rates and hardness values for different welding methods:
| Welding Method | Typical Dilution Rate | Overlay Hardness (HV) | Advantages |
|---|---|---|---|
| SMAW (Stick welding) | 20–40% | 300–400 | Equipment simplicity |
| GTAW (TIG welding) | 10–25% | 350–450 | Low dilution, good control |
| GMAW (MIG welding) | 15–30% | 350–420 | High deposition rate |
| Plasma Arc Welding | 10–20% | 400–500 | Low dilution, high efficiency |
| Oxy-fuel Flame Welding | 30–50% | 250–350 | Equipment simplicity |
The plasma arc welding process typically provides the lowest dilution rate and the highest overlay hardness due to its concentrated heat input and low penetration. The oxy-fuel flame welding process, on the other hand, provides the highest dilution rate and the lowest overlay hardness due to its diffuse heat input and high penetration.
Overlay Layer Thickness
The overlay layer thickness affects the hardness through the dilution rate profile. The first pass of overlay welding typically exhibits the highest dilution rate (30–50%), while subsequent passes show progressively lower dilution rates as the previous deposit becomes the new base metal. The hardness profile across the depth of the overlay layer is therefore non-uniform:
- First pass (closest to base metal): Highest dilution rate, lowest hardness.
- Intermediate passes: Moderate dilution rate, moderate hardness.
- Final pass (surface): Lowest dilution rate, highest hardness.
The paper discusses the importance of achieving a uniform hardness profile across the overlay layer thickness. This can be accomplished through careful control of the welding parameters and the number of passes. A minimum overlay thickness of 2–3 mm is typically required to ensure that the surface hardness is representative of the filler metal composition.
Base Metal Material
The base metal material affects the hardness of the overlay layer through the dilution rate and the chemical composition of the base metal. The following table summarizes the influence of different base metals on overlay hardness:
| Base Metal | Typical Composition | Dilution Effect | Overlay Hardness Impact |
|---|---|---|---|
| Carbon steel (A105) | Fe-0.2C-0.5Mn | High dilution (Fe content) | Significant hardness reduction |
| Low-alloy steel (A216 WCB) | Fe-0.2C-0.5Mn-0.5Cr-0.5Mo | Moderate dilution | Moderate hardness reduction |
| Austenitic stainless steel (304) | Fe-18Cr-8Ni | Moderate dilution (Ni, Cr) | Slight hardness reduction |
| Nickel-based alloy (Inconel 625) | Ni-22Cr-9Mo | Low dilution (Ni, Cr, Mo) | Minimal hardness reduction |
The base metal composition influences the dilution rate through the heat input and the melting behavior. Nickel-based base metals tend to have lower dilution rates due to their lower thermal conductivity and higher melting point, resulting in higher overlay hardness.
Hardness and Sealing Performance
The hardness of the cobalt-based overlay layer is directly related to the sealing performance of the valve. For metal-to-metal sealing surfaces, the overlay layer must possess sufficient hardness to resist deformation under sealing pressure while maintaining a smooth surface finish. The following table summarizes the relationship between overlay hardness and sealing performance:
| Overlay Hardness (HV) | Sealing Performance | Application |
|---|---|---|
| < 300 | Poor sealing, prone to deformation | Not suitable for metal-to-metal sealing |
| 300–400 | Good sealing, moderate wear resistance | Low-pressure valve applications |
| 400–500 | Excellent sealing, high wear resistance | High-pressure valve applications |
| > 500 | Excellent sealing, but may be brittle | Special high-wear applications |
The optimal hardness range for valve sealing surfaces is typically 400–500 HV, which provides a good balance between sealing performance and wear resistance.
Engineering Practice
In the manufacturing of valves with cobalt-based overlay sealing surfaces, the following practices are recommended:
- Pre-weld preparation: The valve seat surface must be thoroughly cleaned and ground to remove oxidation scales and contaminants. A shallow V-groove may be prepared to ensure full fusion of the first pass.
- Welding process selection: Plasma arc welding or TIG welding is preferred for low dilution and high hardness. The welding parameters must be carefully controlled to minimize dilution.
- Multi-pass overlay: At least 2–3 passes are required to achieve a uniform hardness profile. The first pass provides the metallurgical bond, while subsequent passes build up the wear-resistant layer.
- Post-weld machining: The overlay surface must be machined to the required dimensional accuracy and surface finish (typically Ra 0.4–0.8 μm) to ensure proper sealing.
- Hardness verification: The overlay hardness must be measured at multiple locations to ensure uniformity. The hardness should be within the specified range (typically 400–500 HV) at all measurement points.
Study Insights and Implications
This paper provides a detailed analysis of the factors that affect the hardness of cobalt-based alloy overlay welding layers, with a particular focus on the dilution rate and its relationship to the iron content in the overlay. The paper demonstrates that the hardness of the overlay layer is not an intrinsic property of the filler metal but is strongly influenced by the welding process and the base metal composition.
The key insight from this paper is that the dilution rate must be carefully controlled to achieve the desired overlay hardness. This requires a systematic approach to process optimization, involving the selection of the appropriate welding method, the control of welding parameters, and the verification of the overlay hardness through testing.
The paper also highlights the importance of considering the base metal material in the design of overlay welding processes. The base metal composition influences the dilution rate and, consequently, the overlay hardness. This must be taken into account when selecting the filler metal and the welding process for a specific application.