Weld Overlay of Flow Nozzles and Orifice Plates for Energy Equipment

Application Context and Technical Challenges

This 1991 paper by Pei Boyuan from Changzhou Energy Equipment General Factory addresses a specialized but important application of weld overlay technology: the cladding of flow nozzles and orifice plates used in flow measurement systems within energy equipment such as boilers, pressure vessels, and process piping. Flow nozzles are precision-machined components that create a controlled pressure drop to enable accurate flow measurement. They are typically installed in process piping and are subjected to erosive flow conditions, particularly in high-velocity steam, gas, or liquid service. The challenge is to apply a corrosion and erosion resistant overlay layer while maintaining the dimensional accuracy of the orifice bore, which directly determines the accuracy of the flow measurement.

The technical difficulty of this application lies in the contradiction between the requirements of weld overlay and the precision metrology requirements of flow measurement devices. Conventional weld overlay processes introduce significant heat input, which can cause distortion of thin-walled or precision components. The overlay layer must be thick enough to provide adequate service life against erosion and corrosion, yet thin enough to avoid excessive distortion of the orifice geometry. Additionally, the overlay material must be compatible with the base material and the process fluid, while the weld metal must not introduce impurities that could affect flow meter calibration.

Material Selection and Overlay Process Design

The selection of overlay material for flow nozzles depends on the service environment. In boiler steam systems, where the primary degradation mechanism is high-temperature oxidation and erosion by entrained fly ash particles, austenitic stainless steel overlays such as 310 or 310H are commonly specified. In process applications involving corrosive media, overlay materials may include nickel-based alloys such as Hastelloy C-276 or Alloy 625.

The overlay process selected for this application is typically gas tungsten arc welding (GTAW) or gas metal arc welding (GMAW) with precise parameter control. GTAW is preferred for thin overlay layers (1-3 mm) because it provides excellent arc stability and minimal heat input. The key process parameters include:

A critical aspect of the process design is the sequence of operations. The overlay must be applied before final machining of the orifice bore, but the heat input must be limited to prevent distortion of the nozzle body. The recommended approach is to apply the overlay in multiple thin passes (0.5-1.0 mm per pass), with careful monitoring of the accumulated distortion. Post-overlay machining of the orifice bore to final dimensions is mandatory, as weld overlay alone cannot achieve the tight tolerances required for flow measurement accuracy.

Distortion Control and Quality Assurance

The distortion control strategy is the most technically demanding aspect of this application. The authors emphasize several measures:

1. Symmetric welding sequence: Overlay passes should be applied in a symmetric pattern around the nozzle axis to balance thermal contraction forces.
2. Low heat input: The heat input per unit length should be kept below 1.5 kJ/mm for GTAW and below 2.0 kJ/mm for GMAW to minimize thermal distortion.
3. Interpass temperature control: The interpass temperature should not exceed 150 °C to prevent softening of previously deposited layers and to limit cumulative thermal strain.
4. Post-weld stress relief: A low-temperature stress relief treatment at 300-400 °C for 2 hours can reduce residual stresses without significantly affecting the overlay properties.

Quality assurance for overlay-applied flow nozzles requires a combination of non-destructive testing and dimensional verification. Penetrant testing (PT) is used to detect surface cracks and porosity in the overlay layer. Ultrasonic testing (UT) may be employed to verify bond strength at the overlay-base interface. The orifice bore must be measured with precision gauges to verify that the diameter is within the specified tolerance (typically ±0.1 mm or tighter, depending on the accuracy class of the flow meter).

Engineering Practice and Lessons Learned

In practical applications, several challenges have been encountered that are not always addressed in the original literature. The most common issue is insufficient overlay thickness, which leads to premature failure due to erosion through the overlay layer. The recommended minimum overlay thickness is 3 mm for steam service and 4 mm for erosive liquid service, with a surface finish requirement of Ra 1.6 μm or better after machining.

Another practical concern is the interaction between the overlay layer and the flow meter calibration. The presence of the overlay layer changes the bore diameter and surface roughness, which can affect the discharge coefficient used in flow calculations. It is therefore essential to recalibrate the flow meter after the overlay is applied and machined to final dimensions.

The study also notes that the overlay material should be selected with consideration for the expected service life. For boiler applications where nozzle replacement is scheduled at 5-10 year intervals, a 3-4 mm overlay of 310 stainless steel provides adequate protection. For more severe service conditions, thicker overlays of nickel-based alloys may be required, but the increased thickness must be balanced against the risk of distortion.

Summary and Professional Reflection

This publication provides a practical and focused treatment of a specialized cladding application that is often overlooked in general welding literature. The key insight is that weld overlay of precision components requires a different approach than overlay of bulk components. The process parameters, distortion control strategies, and quality assurance requirements must all be tailored to the specific geometric and functional requirements of the component. For engineers working on flow measurement devices, the practical guidance provided in this paper regarding overlay thickness, material selection, and post-overlay machining is directly applicable and saves significant time in process development. The emphasis on low heat input and symmetric welding sequences is particularly valuable for preventing distortion in thin-walled components.