The manufacturing of high-power steam turbines demands components capable of withstanding extreme service conditions of elevated temperature and pressure. Critical elements such as cylinders, valves, and steam chests are frequently produced from low-alloy steel castings. Materials like ZG15Cr2Mo1 and ZG17Cr2Mo1 are selected for their excellent resistance to creep and other high-temperature degradation mechanisms inherent to steam turbine operation. However, the production of these large, complex steel casting components is not without challenges. Defects such as shrinkage porosity, cracks, or inclusions can originate during the solidification and cooling phases of the casting process. Furthermore, during the final precision machining stages, operator errors can sometimes lead to dimensional deviations, rendering a near-finished part unusable.
When such flaws are detected in a finished or nearly finished steel casting, repair by welding is often the most economical and time-efficient solution compared to scrapping and recasting the entire component. The standard repair methodology depends heavily on the condition of the part. If the component can undergo a full or localized post-weld heat treatment (PWHT) to restore the microstructure and mechanical properties of the heat-affected zone (HAZ), a weld repair using a filler metal matching the base material composition is typically performed. However, a significant dilemma arises in the context of precision-machined components. Many large steel casting assemblies have tight dimensional tolerances and complex geometries. Subjecting such a part to PWHT, which involves heating to temperatures often exceeding 700°C, poses a high risk of distortion, potentially negating all previous machining work and making the part irrecoverable.
In scenarios where PWHT is not feasible, the repair strategy must shift. The primary goal is to use a welding filler material that will produce a sound, crack-resistant weld joint without the requirement for subsequent PWHT. This is where nickel-based alloys, specifically those conforming to specifications like ERNiCr-3 (AWS A5.14), become indispensable. These alloys offer superior crack resistance in the as-welded condition due to their austenitic microstructure, which is far less susceptible to hydrogen-induced cold cracking compared to the martensitic or bainitic microstructures that can form in the HAZ of low-alloy steel casting materials upon rapid cooling. Traditionally, the application of these nickel-based alloys for repair has been executed via manual processes: Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc Welding (SMAW). While these methods offer excellent control and are suitable for small repairs, their deposition rates are inherently low. This makes them economically and temporally prohibitive for repairing extensive defects or large volumetric build-ups on major steel casting components.
This research gap presents a clear opportunity: to develop and qualify a high-deposition rate welding process utilizing ERNiCr-3 filler metal for the repair of low-alloy steel castings. Gas Metal Arc Welding (GMAW), a semi-automatic or automatic process, offers a compelling solution. It provides a significantly higher deposition rate and better overall productivity compared to GTAW or SMAW. While GMAW with nickel-based wires is established in other industries (e.g., cladding in petrochemical vessels), its application for structural repair of critical power generation steel castings is less documented and requires rigorous qualification. The objective of this work is to systematically develop, test, and qualify a GMAW procedure using ERNiCr-3 solid wire for welding on low-alloy creep-resistant steel plate, serving as a surrogate for common steel casting grades. The qualified procedure is then applied to a real-world industrial repair case, demonstrating its viability for improving efficiency in steel casting reclamation.
Fundamentals of Welding Low-Alloy Steel Castings with Nickel-Based Fillers
The successful repair of a low-alloy steel casting with a dissimilar nickel-based filler metal hinges on understanding and controlling several key metallurgical and procedural factors. The base materials, such as 12Cr2Mo1R or its cast equivalent ZG15Cr2Mo1, are strengthened by alloying elements like chromium and molybdenum. While these provide high-temperature strength, they also increase the hardenability of the steel. The Carbon Equivalent (CE) is a useful empirical formula to assess this tendency. A common IIW formula is:
$$ CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15} $$
For a typical 12Cr2Mo1 composition, the CE can exceed 1.0, indicating very high hardenability. During welding, the region adjacent to the weld (the HAZ) is heated into the austenite phase region and subsequently cools rapidly. Without proper control, this can lead to the formation of hard, brittle microstructures like martensite, which are highly susceptible to cold cracking, especially in the presence of diffusible hydrogen from the welding atmosphere or contaminants.
Nickel-based filler metals like ERNiCr-3 provide a solution. Their primary alloying elements are nickel and chromium, with additions of niobium (for microstructural stabilization) and manganese (for sulfur control). The weld metal solidifies as a fully austenitic structure. This austenitic matrix has high solubility for hydrogen and excellent ductility, dramatically reducing the risk of hydrogen cracking in the weld metal itself. Furthermore, the weld’s inherent strength and ductility can accommodate strains from the contracting HAZ. Crucially, the austenitic microstructure is stable and does not require a specific transformation-heat treatment, making PWHT optional rather than mandatory. However, to prevent cracking in the HAZ of the steel casting, preheat and strict interpass temperature control remain essential to slow the cooling rate and allow hydrogen to diffuse away.
The transition from manual processes to GMAW for this application introduces new procedural considerations. GMAW operates with higher currents and travel speeds, leading to different thermal cycles. The process parameters—current (I), voltage (V), and travel speed (v)—directly determine the heat input per unit length of weld, a critical parameter defined as:
$$ Q = \frac{\eta \cdot I \cdot V}{v} $$
where \( Q \) is the heat input (J/mm or kJ/cm), \( \eta \) is the arc efficiency (approximately 0.8 for GMAW with argon shielding), \( I \) is the welding current (A), \( V \) is the arc voltage (V), and \( v \) is the travel speed (mm/s). Excessive heat input can lead to excessive dilution of the steel casting base metal into the nickel-based weld pool, altering the weld metal composition and potentially compromising its corrosion and mechanical properties. Conversely, insufficient heat input may cause lack of fusion defects. Therefore, a systematic procedure qualification test is imperative to establish a parameter window that produces sound, property-conforming welds.
Procedure Qualification Test according to ASME IX
To ensure the reliability and repeatability of the welding process, the development was structured as a formal Welding Procedure Qualification Record (WPQR) following the guidelines of the ASME Boiler and Pressure Vessel Code, Section IX. This standard provides a framework to demonstrate that a proposed welding procedure is capable of producing joints meeting prescribed mechanical property requirements.
Materials and Experimental Setup
A hot-rolled plate of 12Cr2Mo1R steel, with a thickness of 32 mm, was selected as the base metal. This material is metallurgically representative of common low-alloy steel casting grades like ZG15Cr2Mo1 and falls within the same P-Number grouping (P5A) in ASME IX, allowing for a qualified procedure on plate to be applied to castings of the same P-Number. The plate was in a normalized and tempered condition. The filler metal was a 1.2 mm diameter CHM-NiCr-3 solid wire, which conforms to the ERNiCr-3 specification. The shielding gas was high-purity (99.995%) argon.
The chemical compositions of the base metal and the welding wire are detailed in Table 1.
| Table 1: Chemical Composition of Base Metal and Filler Wire (wt.%) | ||
|---|---|---|
| Element | 12Cr2Mo1R Base Metal | ERNiCr-3 Filler Wire |
| C | 0.13 | 0.031 |
| Si | 0.04 | 0.050 |
| Mn | 0.53 | 3.04 |
| P | 0.006 | 0.004 |
| S | 0.0013 | 0.004 |
| Cr | 2.45 | 20.28 |
| Mo | 1.06 | – |
| Ni | 0.04 | Bal. |
| Ti | – | 0.55 |
| Nb | – | 2.47 |
| Fe | Bal. | 1.15 |
A single-V groove joint design was prepared with a 60° included angle and a root face of 2 mm. A root gap of 4-5 mm was maintained, and a steel backing strip of the same base material was used to support the root pass. The backing strip was to be removed by machining after welding. The joint preparation is illustrated schematically below.
Prior to welding, the groove faces and adjacent areas (approximately 75 mm on either side) were meticulously cleaned by grinding to remove all scale, rust, and potential contaminants, followed by degreasing with acetone.
Welding Process and Parameters
The welding was performed in the flat (1G) position using a digitally controlled inverter GMAW power source. A dedicated welding program for nickel-based solid wires was utilized to ensure stable arc characteristics. A critical aspect of the procedure was thermal management. Based on the high carbon equivalent of the base metal and established practices for nickel-based repair of steel castings, a preheat temperature of a minimum 80°C was applied using oxy-fuel torches. This preheat, along with a controlled maximum interpass temperature, is vital to prevent the formation of hard, crack-susceptible microstructures in the HAZ. The maximum interpass temperature was limited to 210°C to avoid excessive heat buildup, which could degrade the mechanical properties of both the HAZ and the previously deposited weld metal.
The weld was completed in 20 passes. The welding parameters for the root/filling passes and the final capping passes were recorded and are summarized in Table 2. The heat input was calculated using the formula provided earlier.
| Table 2: Welding Procedure Parameters | ||||
|---|---|---|---|---|
| Pass Group | Welding Current, I (A) | Arc Voltage, V (V) | Travel Speed, v (cm/min) | Heat Input, Q (kJ/cm) |
| Root & Filling Passes (1-5) | 186 – 230 | 26.0 – 27.6 | 16.0 – 28.2 | 10.5 – 23.8 |
| Cap Passes (6-20) | 220 – 242 | 28.6 – 30.3 | 16.2 – 31.6 | 12.7 – 26.5 |
After welding, the test coupon was allowed to cool slowly in still air to room temperature. No post-weld heat treatment was applied. The backing strip was then removed by machining, and the weld surface was ground flush for examination.
Results of Procedure Qualification Tests
Non-Destructive Examination (NDE)
The completed weld was subjected to standard NDE methods. Liquid Penetrant Testing (PT) of the weld surface revealed no surface-breaking defects such as cracks or porosity. Subsequently, full-volume Radiographic Testing (RT) was performed. The radiographic film was evaluated according to ASME Section IX acceptance criteria and showed no evidence of internal defects like porosity, slag inclusions, or lack of fusion. The weld was deemed sound by NDE standards, allowing for the coupon to be sectioned for destructive mechanical testing.
Macro- and Microstructural Examination
A transverse cross-section of the weld was polished and etched to reveal the macrostructure. The examination showed full penetration, a symmetrical weld profile, and no visible defects such as cracks, lack of sidewall fusion, or excessive porosity. The different weld passes were clearly demarcated.
Metallographic samples were extracted for microscopic analysis. The key observations are summarized below:
- Base Metal (12Cr2Mo1R): The microstructure consisted of tempered bainite with some ferrite, characteristic of a normalized and tempered low-alloy steel.
- Heat-Affected Zone (HAZ): A gradient of microstructures was observed. The coarse-grained region near the fusion line exhibited a mixture of bainite and martensite. Further away, the microstructure transitioned to fine-grained bainite and finally to the unaffected base metal. No continuous network of brittle phases or micro-cracks was detected, confirming the effectiveness of the preheat and interpass temperature controls in preventing HAZ cold cracking in this simulated steel casting repair scenario.
- Weld Metal (ERNiCr-3): The microstructure was fully austenitic, with a typical dendritic solidification structure. Secondary phases, likely carbides and Laves phases rich in niobium, were observed in the interdendritic regions, which is normal for this alloy in the as-welded condition.
Mechanical Property Testing
Transverse tensile, side-bend, and Charpy V-notch impact tests were conducted to qualify the mechanical integrity of the welded joint according to ASME IX.
1. Transverse Tensile Test: All specimens fractured in the base metal, away from the weld and HAZ. The measured tensile strengths significantly exceeded the specified minimum tensile strength (SMTS) of the 12Cr2Mo1R base metal (520 MPa). This confirms that the weld joint, with its nickel-based filler, possesses adequate strength. The results are presented in Table 3.
| Table 3: Room Temperature Transverse Tensile Test Results | ||
|---|---|---|
| Specimen ID | Location Sampled | Tensile Strength (MPa) |
| 1 | Weld Cap Region | 611 |
| 2 | Weld Root Region | 615 |
| 3 | Weld Cap Region | 605 |
| 4 | Weld Root Region | 611 |
2. Guided Bend Test: Four full-thickness side-bend specimens were tested to a 180° bend around a 40 mm diameter former. After bending, the convex (tension) surfaces were inspected. No open discontinuities exceeding 3 mm in length were found in either the weld metal or the HAZ, demonstrating excellent ductility of the dissimilar metal joint.
3. Charpy V-Notch Impact Test: Impact toughness is a critical property for power plant components subject to thermal stresses. Specimens were notched in the weld metal center and in the HAZ (fusion line +2 mm). Tests were conducted at room temperature (20°C). The results, detailed in Table 4, show remarkably high impact energy absorption for both locations. All values far surpass the typical acceptance criterion of 47 J for the base material, indicating that the weld joint maintains excellent toughness in the as-welded condition.
| Table 4: Charpy V-Notch Impact Test Results at 20°C | ||||
|---|---|---|---|---|
| Specimen Set | Notch Location | Impact Energy (J) | Average (J) | |
| 5 | Weld Metal (Top) | 230, 220, 217 | 222 | |
| 6 | Weld Metal (Root) | 244, 252, 258 | 251 | |
| 7 | HAZ (Top) | 233, 219, 243 | 231 | |
| 8 | HAZ (Root) | 235, 226, 239 | 233 | |
4. Elevated Temperature Tensile Test: To simulate service conditions, tensile tests were also performed at 500°C. As shown in Table 5, all specimens again failed in the base metal, and the yield strengths measured were well above the expected hot yield strength of the base material at this temperature, validating the joint’s suitability for high-temperature service typical of a steam turbine steel casting.
| Table 5: Elevated Temperature (500°C) Tensile Test Results | ||
|---|---|---|
| Specimen ID | Location Sampled | Yield Strength at 500°C (MPa) |
| 9 | Weld Cap Region | 317 |
| 10 | Weld Root Region | 337 |
| 11 | Weld Cap Region | 311 |
| 12 | Weld Root Region | 342 |
The comprehensive mechanical testing confirmed that the GMAW procedure using ERNiCr-3 wire produced a welded joint on 12Cr2Mo1R plate with properties fully compliant with ASME IX requirements and suitable for the intended repair application on P5A group steel castings.
Industrial Application and Critical Considerations for Steel Casting Repair
With the welding procedure successfully qualified, the next step was its implementation in an actual production environment for repairing a defective steel casting. The transition from a controlled test plate to a complex, valuable component necessitates stringent adherence to the qualified procedure and attention to practical details.
Key Application Guidelines
When applying this GMAW process for repairing a steel casting, several factors must be meticulously controlled:
- Surface Preparation: The repair area on the steel casting must be thoroughly cleaned. All defective material must be removed by grinding or machining until sound metal is revealed. The groove must be free of oxides, paint, oil, and moisture. A bright metallic finish is mandatory to prevent porosity and ensure weld metal purity.
- Thermal Management: Strict control of preheat and interpass temperature is non-negotiable. For P5A group steel castings like ZG15Cr2Mo1, a minimum preheat of 80°C must be uniformly applied and maintained. The interpass temperature must be monitored continuously and kept below 210°C. Exceeding this limit can lead to excessive grain growth in the HAZ and reduced toughness.
- Process Stability: A modern, digitally controlled GMAW power source capable of stable operation with nickel-based wires is essential. The shielding gas flow rate must be sufficient to protect the weld pool from atmospheric contamination, especially in drafty workshop environments. Proper gun angle and technique are required to ensure sidewall fusion and avoid lack-of-fusion defects, as nickel alloy weld metal is more viscous and has lower penetration than steel weld metal.
- Heat Input Control: The welding parameters (current, voltage, travel speed) must be maintained within the qualified range to control dilution and microstructure. The heat input \( Q \) should be calculated and logged for critical repairs.
- Welding Position & Distortion Control: Whenever possible, the repair should be positioned for flat (PA) welding to ensure optimal gas coverage and ease of deposition. For large repairs on massive steel castings like turbine cylinders, distortion control is paramount. Techniques such as strategic sequencing (e.g., back-step or skip welding), the use of strong-backs or clamps, and even assembling mating halves (e.g., bolting the upper and lower halves of a cylinder together) can be employed to minimize distortion and preserve the casting’s dimensional integrity.
Case Study: Repair of a High-Pressure Inner Casing
The qualified procedure was deployed to salvage a high-pressure inner casing manufactured from ZG15Cr2Mo1. During final machining, a dimensional error was made, resulting in an undersized groove for the stationary blading (the “stationary blade slot”). The error was significant enough that the already-machined blades could not be fitted, and the project timeline precluded manufacturing a new casting.
The repair strategy was as follows:
- Joint Preparation: The undersized area was machined to create a clean, uniform groove. The mating casing half was bolted on to provide structural restraint against welding distortion.
The successful execution of this repair demonstrated the tangible benefits of the GMAW process for steel casting reclamation: a substantial increase in deposition rate (leading to shorter repair times), excellent as-welded mechanical properties negating the need for PWHT, and minimal distortion due to controlled heat input and proper fixturing. This approach transforms a potentially catastrophic and schedule-breaking defect into a manageable in-house repair operation.
Conclusion and Outlook
This research and application work has conclusively demonstrated the feasibility and significant advantages of employing Gas Metal Arc Welding (GMAW) with ERNiCr-3 nickel-based solid wire for the repair of low-alloy creep-resistant steel castings. The systematic procedure qualification, performed according to ASME Section IX on 12Cr2Mo1R plate (P-No. P5A), yielded the following key outcomes:
- Robust Welding Procedure: A qualified GMAW procedure was established, specifying key parameters including a minimum preheat of 80°C, a maximum interpass temperature of 210°C, and a controlled heat input range of approximately 10-27 kJ/cm. This procedure effectively prevents hydrogen-induced cold cracking in the heat-affected zone of the steel casting.
- Excellent Mechanical Properties: The dissimilar metal weld joint exhibited superior mechanical properties in the as-welded condition. All tensile specimens failed in the base metal, confirming weld strength. Bend tests demonstrated high ductility. Most notably, Charpy impact tests revealed exceptionally high toughness (＞200 J average) at room temperature for both weld metal and HAZ locations, far exceeding standard requirements.
- Successful Industrial Implementation: The procedure was successfully applied to repair a major high-pressure turbine casing steel casting. The use of GMAW significantly improved deposition efficiency compared to traditional manual methods, reduced overall repair time, controlled distortion, and produced a sound, machineable weld without the need for post-weld heat treatment.
The methodology developed here provides a reliable template for extending this repair technology to other grades of steel castings. Future work could involve qualifying similar procedures for other P-Number groups, such as high-chromium martensitic steels (e.g., P91 castings), further expanding the toolbox for efficient steel casting salvage. The integration of more advanced automation, such as robotic GMAW or automated wire-feeding systems, could offer even greater consistency, quality, and productivity for large-scale steel casting repairs in the future. This approach represents a significant step forward in sustainable manufacturing, reducing waste and lead times by enabling the reliable and efficient reclamation of high-value power generation components.