In the manufacturing of critical components for nuclear power plants, such as circulating pump impellers, the integrity of cast parts is paramount. However, metal casting defects are inevitable during static casting processes, often necessitating repair through welding. I have been involved in developing and validating a repair welding specification for duplex stainless steel Z6CNDU20-08M impellers, which are subject to stringent nuclear standards. The presence of metal casting defects, such as porosity or inclusions, can compromise performance, making effective repair crucial. This article details the comprehensive approach taken to ensure that welded repairs meet mechanical and corrosion resistance requirements, with a focus on controlling the ferrite-austenite phase balance. Throughout this work, the term metal casting defect is central, as it drives the need for robust repair methodologies.
Duplex stainless steels, like Z6CNDU20-08M, offer a combination of high strength and excellent corrosion resistance due to their approximately equal mix of ferrite (α) and austenite (γ) phases. This material is specified for impellers in aggressive environments, such as seawater circulation, where chloride-induced stress corrosion cracking is a concern. However, the casting process can introduce various metal casting defects, including shrinkage cavities, hot tears, or gas porosity, which must be addressed before deployment. In my experience, repairing these metal casting defects requires a meticulous welding procedure to avoid degrading the material’s inherent properties. The challenge lies in maintaining the phase equilibrium after welding, as deviations can lead to reduced toughness or corrosion resistance.
The welding of duplex stainless steels is sensitive to thermal cycles, as excessive heat input can promote the formation of detrimental intermetallic phases like sigma (σ) or chromium nitrides, while insufficient heat input may limit austenite formation. Therefore, controlling welding parameters is critical. Based on my work, the key factors include heat input, interpass temperature, and filler metal selection. For Z6CNDU20-08M, the aim is to achieve a weld metal composition that slightly favors austenite to ensure good corrosion resistance. The pitting resistance equivalent number (PRE) is a useful formula to evaluate this, given by: $$PRE(N) = \%Cr + 3.3\%Mo + 16\%N$$ where %Cr, %Mo, and %N are the weight percentages of chromium, molybdenum, and nitrogen, respectively. This formula helps predict the material’s resistance to pitting corrosion, which is vital for components exposed to chlorides. In repair welding of metal casting defects, maintaining a high PRE in the weld metal is essential.
To establish a qualified repair welding process, I conducted welding procedure qualifications according to RCC-M S standards. The base material was Z6CNDU20-08M cast plates, simulating the impeller’s condition after solution annealing and water quenching. A test plate with dimensions 1000 mm × 400 mm × 100 mm was prepared with a machined groove to replicate a metal casting defect, as shown in the figure. The groove was 60 mm deep with a 30° angle, mimicking typical defect geometries. This setup allowed for a realistic assessment of repair welding techniques.
The selection of welding consumables is crucial for matching the base metal’s properties. I chose E2594-16 low-hydrogen electrodes, which have a higher nickel content than the base metal to promote austenite formation. The chemical compositions of the base metal and filler metal are compared in Table 1. The filler metal’s higher chromium and molybdenum levels ensure adequate corrosion resistance, addressing potential weaknesses from metal casting defects.
| Material | C | Si | Mn | P | S | Cr | Ni | Mo | Cu | N | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Z6CNDU20-08M Base Metal | 0.026 | 1.2 | 1.2 | 0.026 | 0.010 | 21.5 | 7.5 | 2.7 | 1.5 | 0.1 | Bal. |
| E2594-16 Electrode | 0.035 | 0.4 | 1.1 | 0.019 | 0.004 | 25.4 | 9.2 | 4.0 | 0.05 | 0.22 | Bal. |
Using these values, the PRE can be calculated. For the base metal: $$PRE(N)_{base} = 21.5 + 3.3 \times 2.7 + 16 \times 0.1 = 21.5 + 8.91 + 1.6 = 32.01$$ For the weld metal: $$PRE(N)_{weld} = 25.4 + 3.3 \times 4.0 + 16 \times 0.22 = 25.4 + 13.2 + 3.52 = 42.12$$ These values exceed the minimum requirement of 30.85, indicating good pitting resistance, which is critical when repairing metal casting defects in corrosive environments.
Welding was performed using shielded metal arc welding (SMAW) in the horizontal position. The parameters were carefully controlled to manage heat input, as summarized in Table 2. Heat input (E) is calculated using the formula: $$E = \frac{60 \times I \times V}{v \times 1000} \text{ kJ/mm}$$ where I is current in amperes, V is voltage (assumed around 20-25 V for SMAW, though not explicitly measured here), and v is travel speed in mm/min. In practice, voltage was monitored to ensure consistency. For this procedure, the heat input was maintained between 0.5 and 1.5 kJ/mm to avoid excessive ferrite formation or grain growth.
| Weld Layer (Pass) | Electrode Diameter (mm) | Current (A) | Travel Speed (mm/min) | Heat Input (kJ/mm) |
|---|---|---|---|---|
| 1-2 (Passes 1-10) | 3.2 | 100-115 | 180-200 | 0.72-1.37 |
| 3-23 (Passes 11-110) | 4.0 | 150-155 | 200-250 | 0.75-1.26 |
Interpass temperature was kept below 150°C to prevent sensitization and intermetallic phase precipitation. After welding, no post-weld heat treatment was applied, as per RCC-M standards, to avoid detrimental effects on toughness and corrosion resistance. The repaired area was then subjected to non-destructive testing (NDT), including visual inspection, penetrant testing (PT), and radiographic testing (RT), to ensure no welding defects were introduced. This step is vital for verifying the integrity of repairs for metal casting defects.
Destructive testing was conducted to evaluate the weld joint’s properties. Macro-examination revealed a sound weld without cracks or lack of fusion. Microstructural analysis showed a balanced duplex structure with austenite slightly predominant, as desired. Ferrite content was measured using the ASTM E562 method, with results in Table 3. The weld metal had a ferrite content of 40-42%, while the heat-affected zone (HAZ) ranged from 34-36%, indicating good phase control. This balance is essential for maintaining corrosion resistance after addressing metal casting defects.
| Location | Measurement 1 | Measurement 2 | Measurement 3 | Measurement 4 | Measurement 5 | Average |
|---|---|---|---|---|---|---|
| Base Metal | 32.5 | 33.9 | 32.8 | 33.5 | 33.4 | 33.2 |
| Weld Metal | 41.9 | 40.8 | 40.2 | 41.5 | 42.0 | 41.3 |
| HAZ | 35.6 | 34.9 | 36.5 | 36.2 | 35.2 | 35.7 |
Mechanical properties were assessed through tensile, bend, and impact tests, as shown in Table 4. The weld joint exhibited tensile strength above 600 MPa, yield strength over 320 MPa, and elongation exceeding 20%, meeting the standard requirements. Impact toughness at 0°C was satisfactory, with values above 50 J for both weld metal and HAZ. Hardness measurements were below 280 HV10, indicating no excessive hardening. These results demonstrate that the repair welding process restores the mechanical integrity compromised by metal casting defects.
| Property | Standard Requirement | Test Result | Remarks |
|---|---|---|---|
| Tensile Strength (MPa) | ≥600 | 718, 693 | Average: 705.5 |
| Yield Strength (MPa) | ≥320 | 669, 668 | Average: 668.5 |
| Elongation (%) | ≥20 | 25.5, 26.5 | Average: 26.0 |
| Hardness (HV10) | ≤280 | Base: 219-228; Weld: 232-254; HAZ: 236-247 | All within limit |
| Side Bend Test | No defects after bending | Passed | 4d mandrel, 180° |
| Charpy Impact at 0°C (J) | ≥50 | Weld: 92-100; HAZ: 89-124; Base: 133-298 | All values acceptable |
Corrosion resistance is critical for impellers in chloride-rich environments. I conducted pitting and crevice corrosion tests per ASTM G48 A, immersing specimens in 6% FeCl₃ solution at 22°C for 24 hours. The weight loss was 0.04 mg/cm², well below the allowable limit of 0.5 mg/cm², confirming excellent resistance. Intergranular corrosion tests followed RCC-M MC1000 A, using a copper sulfate-sulfuric acid solution with sensitization at 675°C for 10 minutes. After bending, no cracks were observed, indicating immunity to intergranular attack. These tests validate that the repair welding process does not introduce corrosion vulnerabilities, even when fixing metal casting defects.
The success of this qualification allowed for the application to actual impeller repairs. When metal casting defects are detected via NDT in production castings, the same welding procedure is employed. Key steps include pre-cleaning with acetone, temperature monitoring during welding, and using a multi-pass technique with a final cosmetic pass that is ground off. After repair, 100% PT and RT are performed to ensure quality. This approach has been used on over 30 impellers, with no reported failures in service. The control of phase balance through precise heat input management is the cornerstone of this methodology, effectively mitigating risks associated with metal casting defects.
In conclusion, repairing metal casting defects in duplex stainless steel requires a holistic approach that considers material weldability, parameter control, and performance validation. The developed process, using E2594-16 electrodes with controlled heat input, ensures a balanced ferrite-austenite microstructure, meeting mechanical and corrosion resistance standards. The frequent occurrence of metal casting defects in cast components underscores the importance of such repair protocols. Future work could explore automated welding to further enhance consistency, but the current method proves reliable for nuclear applications. By adhering to rigorous qualifications, we can confidently restore components affected by metal casting defects, extending their service life and ensuring safety in critical environments.
Throughout this article, I have emphasized the term metal casting defect to highlight its significance in driving repair needs. The integration of formulas like PRE and detailed tables provides a quantitative basis for decision-making, which is essential in engineering practices. As industries advance, the lessons learned from repairing metal casting defects in duplex stainless steels can inform broader material repair strategies, contributing to sustainable manufacturing.