Elsevier

Materials & Design

Volume 38, June 2012, Pages 38-46
Materials & Design

Joint properties and their improvement of AISI 310S austenitic stainless steel thin walled circular pipe friction welded joint

https://doi.org/10.1016/j.matdes.2012.02.006Get rights and content

Abstract

This paper describes the joint properties and their improvement in thin walled circular pipe friction welded joint for an AISI 310S austenitic stainless steel. Pipes were welded with the combination of the same thickness and outer diameter by a continuous drive friction welding machine that has an electromagnetic clutch. Then, when the clutch was released, the relative speed between both specimens instantly decreased to zero. When the joint with a pipe thickness of 1.50 mm was made at a friction pressure of 120 MPa, the joining could be successfully achieved and that had 100% efficiency with the base metal fracture. However, the joining became difficult with decreasing pipe thickness, and it was not successful at a pipe thickness of 0.50 mm. On the other hand, when the joint with a pipe thickness of 0.50 mm was made at a friction pressure of 30 MPa, the joining could be successfully achieved, although that did not have 100% efficiency. Then, when the joint was made under a friction time of 0.6 s, i.e. the friction torque reached just after the initial peak, and a forge pressure of 60 MPa, it had 100% efficiency with the base metal fracture. However, when that was made with high forge pressure such as 120 MPa, the joining could not be achieved because the adjacent region of the weld interface had heavy buckling. To obtain the successful joining and 100% joint efficiency with the base metal fracture for the thin walled circular pipe, the joint should be made with opportune friction welding condition as follows: low friction pressure, a friction time of just after the initial peak of the friction torque, and a forge pressure of double value of a friction pressure.

Highlights

Friction welding method with braking deformation reducing from joint was developed. ► The thin walled circular pipe stainless steel joint did not have heavily deformation. ► The joint had 100% efficiency with the base metal fracture. ► That is, the joining by this friction welding method could be successfully achieved.

Introduction

The circular shape of the pipe or tube (referred to as circular pipe) is mainly used for the piping components of transport equipment for liquid or gas. They are widely used as the various components or parts of a plant, pipeline, vacuum vessel, and so on. Moreover, they are also used as the cases for various sensors or actuators. To use such structures, it is necessary to join them using the pipe shape. Generally, the joint of a pipe shape with metallic materials including non-ferrous metals is made by fusion welding, such as shielded metal arc welding, tungsten inert gas (TIG) welding, and laser welding. However, it is not convenient to use fusion welding for pipe shape, because that has some problems for actual welding construction such as sound weld bead formation [1]. In particular, it is difficult to weld thin pipe such as 1 mm or less, since the joint properties will be dependent on operator skill or working conditions [2]. Furthermore, those problems are remarkably generated in the welding of stainless steel, because that material is easily distorted largely by the welding heat input due to its low thermal conductivity [3]. Hence, it is strongly necessary to accurately establish a joining method to mass-produce joints of thin pipe with high reliability.

Friction welding is well known amongst solid state joining methods. This method is very useful for the joining of dissimilar materials, and the welding process is easily automated. Also, this welding method has several advantages over fusion welding, e.g., high energy efficiency, narrower heat affected zone (HAZ), and low welding cost [4], [5]. In particular, friction welding is able to easily produce joints with high reliability; it is widely used in the automobile industry and applied to fabricate important parts such as drive shafts, and engine valves. Moreover, this welding method can also provide the circular pipe joint [6], [7]. For example, Eberhard et al. [8] showed the results of the steel circular pipe joint, which had the wall thickness (referred to as pipe thickness) of 12.7 mm. Ogawa et al. [9] showed the results of the steel circular pipe joint, which had the pipe thickness of 3 mm with a relatively good joint strength. Kawai et al. [10] and Ohkubo et al. [11] showed the results of the circular pipe joint that had the combination of the steel and aluminium alloy with the pipe thickness of 3 mm. In those ways, some researchers have reported that the joining of the circular pipe could be successfully achieved and a relatively good joint was obtained. However, research on the friction welding of circular pipe has been slight in comparison with that on circular solid rods. Furthermore, those investigations were carried out using relatively thick pipe thickness that was greater than 3 mm, i.e. the ratio of the pipe thickness to the outside diameter of the pipe was larger than 0.1. Thus, it is necessary to clarify the joint properties of friction welds of the thin walled circular pipe such as that ratio of 0.1 or small. On the other hand, Tsuchiya et al. [12] and Shin et al. [13] showed the results of the circular pipe joint that had the pipe thickness of 1 mm. However, the details of the joining phenomena of the thin walled circular pipe friction welding were not clarified. Moreover, the result of the thin walled circular pipe joint, which had its ratio smaller than 0.1, have scarcely been investigated, because it is able to estimate that the joining is very difficult with decreasing the pipe thickness. In particular, an expansion of the usage of the thin walled circular pipe such as stainless steel is expected to accompany the development of new materials with high functionality because high performance is required in use of those pipe components for energy and resources savings. Therefore, to adapt the friction welding method for thin walled circular pipe with a ratio of 0.1 or less is more strongly required.

In previous works [14], [15], the authors clarified the joining mechanism during the friction welding process for similar material joints of circular solid rods. Then, the authors showed that the friction welded joints of several steels had 100% joint efficiency using only the first stage (close to the initial peak) of the friction process without adding forge pressure. In this case, those joints were made by a continuous drive friction welding machine that has an electromagnetic clutch. Then, when the clutch was released, the relative speed between both specimens instantly decreased to zero. This new welding technique has several advantages over the conventional friction welding method (referred to as conventional method), e.g., less axial shortening (burn-off) and less flash (burr or collar). In particular, this welding method did not have the final peak torque of the friction torque curve during the welding process, which will be described later.

This study presents further detail for the friction welded joint of thin walled circular austenitic stainless steel pipe and discusses improvements of a joining method to obtain high joint tensile strength. In this report, the authors present the tensile strengths of a welded joint with thin walled circular pipe of various thicknesses, under various friction welding conditions. The authors also show the tensile strengths of the joint created with the conventional method in contrast to those created with the friction welding technique developed by the authors. Furthermore, the authors propose that this friction welding technique is suitable for welding of thin walled circular austenitic stainless steel pipe.

Section snippets

Experimental procedure

The material used was type 310S austenitic stainless steel with American Iron and Steel Institute (AISI) standard (AISI 310S) in 16 mm diameter rod. Two kinds of stainless steels with slightly different tensile properties were used for this experiment because they were purchased at different times. One of the chemical compositions was 0.01C–0.27Si–1.08Mn–0.027P–0.001S–19.20Ni–24.24Cr in mass%, and the other was 0.01C–0.18Si–1.04Mn–0.031P–0.001S–19.01Ni–24.09Cr in mass%. Their ultimate tensile

Results of continuous drive friction welding

Fig. 2 shows an example of the joint appearance and its cross-section made by the conventional method. In this case, the joint was made by a direct drive friction welding machine with using a pipe thickness of 0.80 mm (t/OD = 0.062), and the friction welding condition was set to the following values: a friction pressure of 120 MPa, a friction time of 0.3 s, a forge pressure of 120 MPa, and a braking time of about 1.0 s. The outer flash was not uniformly generated to the radial direction at the weld

Suggestion of new friction welding technique

Based on the results of the conventional method, it is necessary to reduce the braking deformation during the rotation stop from the joint to obtain successful joining of thin walled circular pipe. As described in the introduction section, the authors suggested the new friction welding technique, and the steel or aluminium joint made by this technique had less axial shortening and less flash which was compared with the conventional method. Fig. 4 shows the general view of a part of the friction

Joint tensile properties at various pipes

Fig. 6 shows the cross-section of the joint in various pipe thicknesses at a friction pressure of 120 MPa. In this case, friction time was set to 0.3 s, and forge pressure was applied at an identical friction pressure, i.e. 120 MPa. When the joint was made by using a pipe thickness of 1.50 mm (t/OD = 0.111), a cross-section of this joint showed that it was successfully joined, and the outer and inner flashes were almost uniformly generated to the radial direction at the weld interface (Fig. 6a). When

Results of low friction pressure

To obtain successful joining for a pipe thickness of 0.50 mm (t/OD = 0.040), the joint was made by a friction pressure of 30 MPa because it was considered that the deformation during the friction process was decreased at low friction pressure. Fig. 9 shows an example of the structure of the joint at a friction time of 0.6 s. In this case, a forge pressure was applied at an identical friction pressure, i.e. 30 MPa. The joining could be successfully achieved, and the flash of the joint was almost

Conclusions

This paper described the joint properties and their improvement in an AISI 310S austenitic stainless steel thin walled circular pipe friction welded joint. The thin walled circular pipe had various ratios that were defined by the pipe thickness (t) and its outer diameter (OD), and those were welded with the combination of the same thickness and outer diameter by a continuous drive friction welding machine that has an electromagnetic clutch. The following conclusions are provided.

  • (1)

    When the joint

Acknowledgements

This research was partially supported by the coordinated research program of Toshiba Corporation Power Systems Company during 2006–2008 years period, and the authors would like to thank Dr. Yumiko Tsuchiya, Dr. Norihisa Saitoh, and Dr. Masashi Takahashi from that company for their support. We also wish to thank the staff members of the Machine and Workshop Engineering at the Graduate School of Engineering, University of Hyogo.

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