1. What is the ERW process, and why is it significant for manufacturing Inconel 690 pipes compared to seamless methods?
ERW stands for Electric Resistance Welding. It is a high-speed process where a strip of metal (skelp) is formed by rolls into a cylindrical shape. The critical step occurs when the open edges are brought together under pressure; an electric current is passed through them, generating intense heat due to resistance at the point of contact. This heat fuses the edges together, creating a solid-phase weld without the use of filler metal.
The significance for Inconel 690 lies in cost and production efficiency:
Cost-Effectiveness: The ERW process is generally faster and requires less energy than extruding or piercing a solid billet to create a seamless pipe. This results in a lower final cost per foot of pipe.
Dimensional Consistency: ERW pipes exhibit excellent dimensional consistency, particularly in wall thickness concentricity, and can be produced in long continuous lengths.
Application-Specific Suitability: For many applications involving lower pressures, corrosive fluids, or structural components, the properties of an ERW Inconel 690 pipe are entirely sufficient. It provides the same base metal corrosion resistance as a seamless pipe at a more economical price point, making projects more feasible.
2. How is the weld integrity of an Inconel 690 ERW pipe ensured, and why is this critical for its performance in corrosive service?
Ensuring the integrity of the longitudinal weld is the most critical aspect of manufacturing ERW pipe, especially for a high-performance alloy like Inconel 690. The weld must be as robust as the parent metal. This is achieved through a multi-step process:
In-Process Controls: Precise control of the welding current, pressure, and speed is essential to create a clean, continuous weld nugget free of contaminants.
Post-Weld Heat Treatment (PWHT): The welded pipe undergoes a solution annealing heat treatment (typically at 1600-1850°F / 870-1010°C). This serves two vital functions: it dissolves any chromium carbides that may have precipitated in the heat-affected zone (HAZ), restoring corrosion resistance, and it normalizes the microstructure across the weld, parent metal, and HAZ into a uniform, recrystallized grain structure.
Non-Destructive Testing (NDT): 100% of the weld length is inspected using advanced methods. Eddy Current Testing is commonly used for initial screening. For critical applications, Ultrasonic Testing (UT) is employed to detect any subsurface imperfections, and Radiographic Testing (RT) may be used to provide a visual record of weld quality.
This rigorous assurance is critical because a flawed weld could become a site for preferential corrosion, cracking, or failure, negating the superb corrosion resistance of the Inconel 690 base metal.
3. In which specific applications would an Inconel 690 ERW pipe be specified over a seamless pipe?
An engineer would specify an Inconel 690 ERW pipe in applications where its cost advantage is significant and the service conditions are well within its performance capabilities. Key applications include:
Chemical Process Industry (CPI) Transfer Lines: For conveying aggressive chemicals (e.g., nitric acid, nitrates) at moderate temperatures and pressures where the superb corrosion resistance of Inconel 690 is required, but the extreme pressure capability of a seamless pipe is not.
Pollution Control and Scrubber Systems: Ducting and piping in flue gas desulfurization (FGD) and other scrubber systems that handle wet, corrosive gases and acidic condensates. The large diameters often needed are more economical to produce via ERW.
Heat Recovery Systems: As tubing for condensing heat exchangers and other waste heat recovery units where the environment is corrosive but not at ultra-high pressure.
Structural and Support Applications: In nuclear or chemical plants, Inconel 690 ERW pipe can be used for instrumentation tubing, sheathting, and structural supports within corrosive environments where the primary requirement is material compatibility rather than pressure containment.
4. What are the limitations of the ERW process for Inconel 690, and when must a seamless pipe be chosen instead?
Despite its advantages, the ERW process has inherent limitations that dictate when a seamless pipe is mandatory:
High-Pressure Service: Seamless pipe is required for high-pressure applications, typically governed by codes like ASME B31.3 for process piping. The presence of a longitudinal weld is considered a potential weakness under cyclic high pressure, even if it is fully tested. Critical systems in nuclear power plants (e.g., primary coolant lines) will always use seamless pipe for this reason.
Extreme Temperature and Creep Service: In services involving sustained high temperatures and creep (gradual deformation under stress), the homogeneous structure of a seamless pipe is preferred. The microstructural variation across an ERW weld, even after heat treatment, can be a concern under these most demanding conditions.
Weldability Concerns for Fabrication: When the pipe itself will be extensively welded during installation, especially with branch connections (e.g., welding nozzles directly onto the pipe body), a seamless pipe provides more flexibility for weld placement without overlapping the ERW weld with a new fabrication weld, which can complicate stress patterns and heat treatment.
5. How does the technical requirements for Inconel 690 ERW Pipe differ between ASTM B163 and ASTM B775?
The choice of ASTM standard defines the technical requirements and intended use.
ASTM B163: This standard is for seamless nickel and nickel-alloy pipe and tube. It would not be used for an ERW product. Specifying B163 implies a requirement for a seamless manufacturing process.
ASTM B775: This is the standard for welded nickel and nickel-alloy pipe. It is the correct specification for ERW Inconel 690 pipe. It covers the chemical, mechanical, and dimensional requirements specific to the welded product form, including provisions for the weld seam, necessary heat treatment, and non-destructive testing. Therefore, a procurement specification for Inconel 690 ERW pipe must explicitly call out ASTM B775 to ensure the manufacturer supplies a welded product that meets all the required quality and testing protocols for that specific form.
