1. Introduction
Land resource reserves continue to decrease as the consumption of natural resources increases in various fields. The mining environment of natural resources has gradually expanded from land to abyss or severely cold regions. Therefore, there are continuously increasing demands for special alloy pipes to be adopted in transport steel pipes and marine structures [
1]. Among such pipes, lined and clad pipes suitable for high-pressure environments with corrosion and heat resistance have been the focus of active research. These pipes are special alloy pipes with excellent corrosion resistance made by bonding different materials, i.e., a corrosion-resistant alloy (CRA) and steel for the inner and outer pipes, respectively. The difference between the two pipes lies in the method of bonding their constituent materials. Lined pipes use mechanical bonding, while clad pipes adopt metallurgical bonding [
2]. The former is applicable to relatively more fields, as its manufacturing equipment is less expensive than that of the latter and because it enables lightweight design [
3].
The hydroforming process is used to fabricate lined pipes using a die with specific geometry, while the outer pipe is mechanically bonded with the inner pipe, which is subjected to plastic deformation by applying hydraulic pressure using its elastic recovery force. However, for the bonding method via the hydroforming process, bonding occurs only within the elastic section of the outer pipe material. It is also difficult to constantly apply high hydraulic pressure to the inner side during the production of long pipes or pipes with a large diameter because this may decrease the bonding strength between the different materials. The application of the cold drawing process can be considered instead of the hydroforming process to address the limitations of the bonding strength during the production of lined pipes. This method facilitates the application of high pressure to each pipe in a relatively uniform manner. Studies have also been actively conducted on producing curved pipes by applying the bending forming process to the lined pipes fabricated to construct a transport system. Christopher et al. conducted research on the method of optimizing the design of simplified tool geometry based on the contact normal stress distribution of dies according to the bending radius of the curved pipe in the rotary draw bending process (RDBP) [
4]. Andrea et al. proposed an analytical approach to predict the stress–strain distribution of the pipe cross section, including the spring-back phenomenon as thermodynamic process parameters in the RDBP [
5]. Safdarian investigated the effects of the RDBP parameters on the fracture, wrinkling, and ovality of piping via experiments and numerical analysis [
6]. Ruan et al. examined the applicability of the hydroforming process, considering the RDBP using numerical and experimental methods to produce an ultra-small bending radius elbow with STS304 [
7]. Kong et al. studied the deformation behavior and the failure in bending-bulging forming which is good at fabricating elbow parts with small relative bending radius [
8].
In addition, research on the direct extrusion bending process has been actively conducted in recent years. Shiraishi et al. predicted the curvatures of the rectangular bars and tubes with flanges by combining the fundamental bending properties of the process obtained in extrusion of the rectangular bars with the shape effect of the bars and tubes [
9]. Zhou et al. considered an analytical model for quantitatively describing the bending behavior of aluminum profiles produced in a novel extrusion process and proposed a novel method for directly forming curved profiles/sections from billets in one extrusion operation using two opposing punches [
10,
11].
For lined pipes, the inner pipe, made of CRA, is designed to have a smaller wall thickness than the outer pipe made of steel, although its strength is generally lower. Although the inner pipe produced using the RBDP has usually lower strength than the outer pipe made of steel in lined pipes, it is designed to have a smaller wall thickness. Hence, lined pipes have inherent issues, such as the buckling and wrinkling that occur in the inner pipe with relatively weaker strength, the separation of the inner and outer pipes during the metal forming due to the thickness difference, and the disparity between the centers of the bending radii of the pipes after forming.
This study proposes the cut-forging-joint process (CFJP), a novel bi-metal curved pipe forming method that introduced the forging process to minimize the difference in thickness between the processed curved pipes. Although the existing RDBP performs forming in order of bending–cutting–joining, the CFJP is performed in order of cutting–forging–joining. This approach can minimize the amount of discarded material generated by cutting a part after bending the manufactured curved pipe in the RDBP and secure the final geometry with a uniform wall thickness [
12]. It can also control the load and set the machining allowance through the initial material to perform forging, including the design of the mandrel’s dimensions [
13]. In this study, the CFJP was applied as a method for manufacturing double-curved pipes, while the formability and dimensional precision were analyzed according to the process parameters to secure the reliability of the process design.
Figure 1 shows a schematic diagram of the overall process procedure. The initial material and geometry of the die suitable for the manufacturing of targeted short radius elbows (SR) in accordance with the curved pipe standards were designed with appropriateness based on the dimensional precision of the final prototype by performing finite element simulation beforehand [
14,
15]. The possibility of forming bi-metal curved pipes within the tolerance of standards was verified via comparison with the prototype fabricated using the actual double curved pipe forming experiment.
3. Results
Relative to the forging simulation results, the deformation geometry depending on the setting of forming conditions was secured, as illustrated in
Figure 5.
Figure 5a presents the deformation of the single pipe condition made of the DSS without mandrel. Compared to the result of using the cut lined pipe as the initial billets, it was shown that the overall thickness of
Figure 5a increased more than
Figure 5b. The measured average thicknesses of the top and bottom parts were 4.51 mm and 6.32 mm, respectively, on the single pipe. As bending deformation occurred at the forming point, the thickness of the top end tended to increase more than other parts owing to the load acting at the top increasing by up to 6.38 mm. The central part of the bottom thickness was also bent more than necessary owing to insufficient internal pressure, and the thickness increased by up to 6.63 mm. Compared to the target overall length, the bottom length exceeded 53.74 mm for the single pipe and 54.27 mm for the double pipe, thereby verifying that the design requires the bottom part of the initial material to be cut off.
The deformation of bi-metal pipes condition shown in
Figure 5b comprising the outer pipe made of APIX 5L X65 and the inner pipe made of DSS without a mandrel. The average top thicknesses were calculated to be 2.89 mm and 2.12 mm for the outer and inner pipes, respectively, while the average bottom thicknesses were 3.19 mm and 2.27 mm. These values demonstrate that the thickness sensitivity is higher for the forming of the lined pipe than the single pipe owing to the difference in mechanical properties between the outer and inner pipes. The maximum forming load of single pipe and lined pipe was calculated to be 45.8 tons and 50.7 tons, respectively, based on when the punch moved by 190 mm, when mandrel is used, it is thought the load will rise further due to frictional force than before. These results imply that the forming load significantly impacts the thickness accuracy of the curved pipe, making it necessary to apply a method of securing dimensional precision via the appropriate dispersion of the load. Therefore, the necessity of designing and introducing the mandrel for the type of each pipe models was also confirmed to secure the roundness of the formed product, minimize the thickness change of the top and bottom parts, and obtain uniform thickness distribution.
The bottom geometry of the formed products according to the cutting angle of the initial billets demonstrated that the tip of the top-right part was bent by up to 5.8 mm to the left when pressurized with the punch. While the right part was not cut, deformation on the top right part occurred more as the length ratio decreased. In addition, for the left part of the initial material, the geometry of the bottom left side was curved upward as forming progressed. Finally, a cutting length of at least 13.1 mm was required from the bottom-left part to the cutting limit line. In addition, the required cutting line increased proportionally as the length ratio increased. These results indicate that the initial billets must be designed with the consideration that the bottom part of the final prototype contained the machining allowance that enables a cut of at least 13.1 mm. In addition, its top right part should allow a cut of 5.8 mm when pressurized, regardless of the right part’s geometry after designating the cutting angle on the left side. These are the results obtained based on the billets that do not involve cutting on the right side. The values must be modified if the cutting of the right side is assumed. When the machining allowance of up to 20 mm is considered for top and bottom lengths of 190 mm and 50 mm, respectively—which are dimensions calculated using the geometrical method to secure stable machining allowance and produce a curved pipe with the target dimensions—it is possible to consider 190–210 mm and 50–70 mm for the top and bottom lengths, respectively. When the top length of the initial billets was set to 200 mm, its bottom length and cutting angles on both sides were set as design variables with the main purpose of material loss minimization according to the objective of this study. The bottom length that can minimize the required cutting length was determined to be 60–70 mm, as shown in
Figure 6 and
Table 4. These values were considered assuming that the maximum loss length is 5 mm during additional face milling after the forging process.
The minimum forming load was calculated to be 87 tons at a bottom length of 60 mm, while the maximum forming load was 97 tons at a bottom length of 100 mm. Consequently, appropriate mandrel geometry was selected for controlling the loads acting on the top and bottom parts, and the proper forming process design for bi-metal curved pipe was realized using the top and bottom lengths and tolerances. In all designed cases, the outside diameter measured by simulation was the equal to 88.9 mm, and it was confirmed that the error rate was calculated to be within 0.22%, compared to the average diameter of the manufactured product that had undergone the actual forging test, which was 89.1 mm. The overall average thickness of the product was 4.45 mm for the experimental values and was 4.429 mm for the analysis values. The overall thickness showed a tendency to increase after forging compared to the initial billets. For the same top length, the change rate of the outer thickness according to the bottom length did not show a specific trend, which means that the standard for predicting the thickness of the outer and inner pipe cannot be determined solely by the top and bottom length of the initial material. The difference in thickness change between the experimental and the analysis values using forming simulation program is thought to be caused by the fact that the actual inner space of dies is wider than the space of the finite element model used in the forming simulation.
By calculating the volume of the bi-metal curved pipe with increased thickness through the forging process and considering the volume of material loss caused by cutting, processing allowance and tolerance setting range can be determined. The total thickness during forming increases similarly overall for the initial billets designed in this study. Thus, it is assumed this is the standard loss volume that occurs primarily. In addition, the required cut length generated depending on the design shape of the initial billets and the sum of the secondary loss volume and the primary loss volume should be considered together. However, additional cutting related to the primary and secondary loss volume may not be necessary in some cases according to shape results. If it is arbitrarily assumed that about half of the primary loss volume at the bottom end of the manufactured product is lost in a symmetrical shape, the required cutting length according to the shape of the initial material will be different, but in the end, the total cutting volume due to cutting is almost the same. This indicates that the increased thickness and loss volume of initial billets can be controlled by the mechanical properties and cutting shape of initial billets for targeted bi-metal curved pipes.
Author Contributions
Project administration, J.P.; supervision, S.K. and J.P.; formal analysis, S.H. and D.C.; literature review, J.P., S.K., S.H. and D.C. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean Government (MOTIE) (P0002092, The Competency Development Program for Industry Specialist).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Karan, S. Dissimilar welding between piping and valves in the offshore oil and gas industry. Weld. Int. 2021, 35, 34–44. [Google Scholar]
- Lin, Y.; Stelios, K. Hydraulic expansion of lined pipe for offshore pipeline applications. Appl. Ocean Res. 2021, 108, 102523. [Google Scholar]
- Lin, Y.; Xiao, J.; Han, B.; Wang, X. Microstructure and mechanical properties of welded joints of L415/316L bimetal composite pipe using post internal-welding process. Int. J. Press. Vessels Pip. 2020, 179, 104026. [Google Scholar]
- Christopher, H.; Rainer, S.; Bernd, E. Rotary-draw-bending using tools with reduced geometries. Procedia Manuf. 2018, 15, 804–811. [Google Scholar]
- Andrea, G.; Enrico, S.; Stefania, B. Insights on tube rotary draw bending with superimposed localized thermal field. CIRP J. Manuf. Sci. Technol. 2021, 33, 30–41. [Google Scholar]
- Safdarian, R. Investigation of tube fracture in the rotary draw bending process using experimental and numerical methods. Int. J. Mater. Form. 2020, 13, 493–516. [Google Scholar] [CrossRef]
- Ruan, S.; Lang, L.; Ge, Y. Hydroforming process for an ultrasmall bending radius elbow. Adv. Mater. Sci. Eng. 2018, 3, 7634708. [Google Scholar] [CrossRef]
- Kong, D.; Lang, L.; Ruan, S.; Sun, Z.; Zhang, C. A novel hydroforming approach in manufacturing thin-walled elbow parts with small bending radius. Int. J. Adv. Manuf. Technol. 2017, 90, 1579–1591. [Google Scholar] [CrossRef]
- Shiraishi, M.; Nikawa, M.; Goto, Y. An investigation of the curvature of bars and tubes extruded through inclined dies. Int. J. Mach. Tools Manuf. 2003, 43, 1571–1578. [Google Scholar] [CrossRef]
- Zhou, W.; Xi, Z. Bending behaviour analysis of aluminium profiles in differential velocity sideways extrusion using a general flow field model. Metals 2022, 12, 877. [Google Scholar] [CrossRef]
- Zhou, W.; Lin, J.; Dean, T.; Wang, L. Feasibility studies of a novel extrusion process for curved profiles: Experimentation and modelling. Int. J. Mach. Tools Manuf. 2018, 126, 27–43. [Google Scholar] [CrossRef]
- Gaochao, Y. Development of a cold stamping process for forming single-welded elbows. International Journal of Advanced Manufacturing Technology. Int. J. Adv. Manuf. Technol. 2017, 88, 1911–1921. [Google Scholar]
- Han, S.W.; Oh, I.Y.; Woo, Y.Y.; Ra, J.H.; Moon, Y.H. Tubular blank design to fabricate an elbow tube by a push-bending process. J. Mater. Process. Technol. 2018, 260, 112–122. [Google Scholar]
- ASME B16.9; An American National Standard. Factory-Made Wrought Buttwelding Fittings. American Society of Mechanical Engineers: New York, NY, USA, 2018; pp. 1–35.
- ASME B36.10; An American National Standard. Welded and Seamless Wrought Steel Pipe. American Society of Mechanical Engineers: New York, NY, USA, 2019; pp. 1–18.
- Zhang, Q.; Ma, S.; Jing, T. Mechanical properties of a thermally-aged cast duplex stainless steel by in situ tensile test at the service. Metals 2019, 9, 317. [Google Scholar] [CrossRef]
- Jeffrey, A.; Andres, G.; Arnaldo, Z.; David, W.T.; Barbara, F.R.; Yoshiki, O.; Keith, M. Corrosion behavior of 2205 duplex stainless steel. Am. J. Orthod. Dentofac. Orthop. 1997, 112, 69–79. [Google Scholar]
- Marek, H.; Jacek, Z. Possibilities of application measurement techniques in hot die forging processes. Measurement 2017, 110, 284–295. [Google Scholar]
- Sorokin, A.A.; Zakirov, D.M.; Semenov, V.V.; Moskvina, T.P. Steels for making standard parts using cold die forging. Metallurgist 2021, 64, 1043–1045. [Google Scholar] [CrossRef]
- Vazouras, P.; Karamanos, S.A. Structural behavior of buried pipe bends and their effect on pipeline response in fault crossing areas. Bull. Earthq. Eng. 2017, 15, 4999–5024. [Google Scholar] [CrossRef]
- Kim, Y.J.; Oh, C.S. Effect of attached straight pipes on finite element limit analysis of elbow. Int. J. Press. Vessels Pip. 2007, 83, 177–184. [Google Scholar] [CrossRef]
- Guo, W.; Wei, W.; Xu, Y.; El-Aty, A.A.; Liu, H.; Wang, H.; Luo, X.; Tao, J. Wall thickness distribution of Cu–Al bimetallic tube based on free bending process. Int. J. Mech. Sci. 2019, 150, 12–19. [Google Scholar] [CrossRef]
| Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |
© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).