Surfacing Technology for the Inner Wall of Flange Forging Blanks

Industrial Background and Technical Challenge

The paper by Chen Sunyi, published in Hot Working Technology (Vol. 34, No. 5, 2005, pp. 67-68), addresses a practical manufacturing challenge in the production of composite flanges with stainless steel inner wall surfacing. The research was conducted at the Maoming Petrochemical Machinery Factory, Sinopec. The specific problem addressed is the deformation of flange forging blanks during the inner wall surfacing process, which affects both the dimensional accuracy of the final product and the ultrasonic testing (UT) acceptance of the blank.

Composite flanges with stainless steel inner wall surfacing are widely used in petrochemical and chemical processing industries where the combination of a tough carbon steel or alloy steel outer body with a corrosion-resistant stainless steel inner lining provides an optimal balance of mechanical strength, corrosion resistance, and cost. The surfacing process deposits a layer of stainless steel onto the inner wall of a forged flange blank, creating a bonded composite structure. However, the thermal input from the surfacing process can cause significant deformation of the forging blank, particularly in the thin-walled regions and at geometric discontinuities such as the transition from the cylindrical body to the flange face.

Blank Design Optimization

The traditional approach to manufacturing composite flanges uses a simple ring-shaped forging blank, which is machined to the final flange geometry after surfacing. However, this approach has several disadvantages: the large amount of machining required increases manufacturing cost, the deep cavities created by machining can trap surfacing spatter and slag, and the thin remaining walls are more susceptible to deformation during surfacing.

The research proposes two alternative blank designs that are more economical than the traditional ring blank: a profile-matched blank (imitation blank) and a column-neck blank. The profile-matched blank is forged to approximate the final flange geometry, reducing the amount of machining required and providing a more uniform wall thickness that is less prone to deformation. The column-neck blank features a cylindrical neck section that provides additional material support during surfacing, reducing the tendency for the flange face to warp.

The profile-matched blank was selected as the preferred design for the subsequent deformation control study, as it offered the best combination of cost efficiency, deformation resistance, and UT accessibility. The column-neck blank was also evaluated but was found to have limitations in terms of UT accessibility due to the complex geometry of the neck region.

Deformation Control Measures

The research proposes four preventive measures and one corrective measure to control deformation during inner wall surfacing of the profile-matched blank. These measures are based on the understanding of the deformation mechanisms involved in the surfacing process, which include thermal expansion and contraction, residual stress development, and plastic deformation of the heated material.

The four preventive measures are as follows:

1. Optimized surfacing sequence: The surfacing passes are sequenced to distribute the thermal input symmetrically around the circumference of the blank. This ensures that the thermal expansion is balanced and does not create a net bending moment that would cause warping. The sequence is planned to start at the center of each pass and work outward, minimizing the differential heating between adjacent regions.
2. Controlled thermal input: The surfacing parameters (current, voltage, travel speed) are optimized to minimize the total heat input while maintaining adequate penetration and fusion. Lower heat input reduces the temperature gradient in the blank and thereby reduces the thermal stress that drives deformation. The parameters are selected based on the wall thickness and the required surfacing layer thickness.
3. Mechanical restraint: The blank is clamped or supported during surfacing to prevent free movement. The restraint is applied at locations that do not interfere with the surfacing process and does not create stress concentrations that could cause cracking. The restraint force is sufficient to prevent gross deformation but not so high as to cause plastic deformation of the blank.
4. Preheating and controlled cooling: The blank is preheated to a moderate temperature before surfacing to reduce the temperature gradient between the hot surfacing zone and the cold base material. After surfacing, the blank is cooled slowly to minimize thermal shock and residual stress. The preheating temperature and cooling rate are determined based on the material properties and the geometry of the blank.

The corrective measure involves post-surfacing deformation correction, which is performed if the blank exceeds the dimensional tolerance after surfacing. This is typically accomplished by mechanical straightening or pressing, which is performed at elevated temperature to reduce the force required and minimize the risk of cracking.

Quality Control and Ultrasonic Testing Considerations

The ultrasonic testing of the surfacing layer is a critical quality control step that verifies the absence of defects such as lack of fusion, porosity, and cracks at the interface between the surfacing layer and the base material. The blank design and surfacing process must be compatible with UT requirements, which means that the geometry must allow access for UT transducers and that the surfacing layer must have a smooth, uniform surface that does not interfere with UT signal propagation.

The profile-matched blank was found to provide good UT accessibility because the geometry closely approximates the final flange shape, allowing UT transducers to be positioned at the correct angle for reliable inspection. The traditional ring blank, with its deep machined cavities, presented challenges for UT due to the difficulty of positioning transducers and the interference from the machined surfaces.

The surfacing process parameters must also be controlled to minimize the formation of UT-relevant defects. Excessive heat input can cause burn-through and slag inclusion, while insufficient heat input can cause lack of fusion. The surfacing sequence and mechanical restraint measures also affect the residual stress distribution, which can influence the formation of stress-related defects such as cracking.

Engineering Practice and Cost-Benefit Analysis

The research demonstrates that the proposed blank design and deformation control measures result in significant practical benefits. The profile-matched blank reduces machining time and material waste, lowering the manufacturing cost. The deformation control measures reduce the need for post-surfacing correction and rework, improving the first-pass yield rate. The combination of these measures results in a more economical and reliable manufacturing process for composite flanges.

From a PDCA perspective, the research follows a systematic approach to process improvement. The "Plan" phase identified the deformation problem and proposed alternative blank designs. The "Do" phase implemented the proposed measures in production trials. The "Check" phase evaluated the effectiveness of the measures through dimensional measurement and UT inspection. The "Act" phase refined the measures based on the trial results and established standard operating procedures for production.

The cost-benefit analysis of the proposed approach shows that the initial investment in tooling for the profile-matched blank forging is offset by the savings in machining time, reduced scrap rate, and lower rework costs. The deformation control measures require additional labor for clamping and preheating, but these costs are minimal compared to the cost of rework or scrap. Overall, the proposed approach results in a net cost reduction and quality improvement.

Study Insights and Independent Reflection

The research by Chen Sunyi addresses a practical manufacturing problem that is encountered in the production of composite flanges for petrochemical applications. The systematic approach to solving the problem—through blank design optimization, process parameter control, and quality assurance—is a model for engineering problem-solving in surfacing technology. The emphasis on cost-effectiveness is particularly relevant in the petrochemical industry, where large volumes of flanges are required at competitive prices.

One aspect that deserves further consideration is the long-term performance of the surfacing layer under service conditions. The research focuses on the manufacturing process and does not address the corrosion resistance, mechanical properties, or fatigue behavior of the composite flange in service. For petrochemical applications, where the flanges are exposed to aggressive chemical environments and cyclic loading, the long-term performance of the surfacing layer is critical. Future work should extend the investigation to include service simulation testing, such as corrosion testing, fatigue testing, and pressure cycling testing.

Another consideration is the scalability of the proposed approach to different flange sizes and specifications. The research appears to focus on a specific flange size and specification, and the deformation control measures may need to be adapted for different geometries. A parametric study of the deformation behavior as a function of flange diameter, wall thickness, and surfacing layer thickness would provide more comprehensive guidance for process design.

Conclusion

The research by Chen Sunyi presents a practical and cost-effective solution for the inner wall surfacing of flange forging blanks. By proposing alternative blank designs and a comprehensive set of deformation control measures, the study addresses the key challenges of dimensional accuracy and UT acceptance in composite flange manufacturing. The profile-matched blank combined with optimized surfacing sequence, controlled thermal input, mechanical restraint, and preheating provides a robust manufacturing process that reduces cost and improves quality. This work contributes valuable practical experience to the field of surfacing technology in the petrochemical industry and demonstrates the importance of integrating design, process, and quality control considerations in surfacing-based manufacturing.