Rail Gas Pressure Welding Process without Using Expert Skills

Transcription

1 PAPER Rail Gas Pressure Welding Process without Using Expert Skills Ryuichi YAMAMOTO Mitsumasa TATSUMI Rail Welding Laboratory, Track Technology Division Yota KAKIZAKI East Japan Railway Company (Former RTRI) Gas pressure welding is commonly used for rail welding in Japan. This technique however requires specialist skills for the rail-end surface grinding stage and the rail heating process using a burner. In anticipation of a decrease in the number of skilled welders, a new simplified and standardized gas welding method with minimal requirement for expert training has therefore been studied and developed. This paper describes the newly developed gas pressure welding process, and presents the performance test results of the gas pressure welds prepared according to this process. Keywords: rail, gas pressure welding, rail-end surface grinding, burner swing, deskilling 1. Introduction In Japan, four welding processes have been generally adopted for producing continuous welded rail: flash welding (FW), gas pressure welding (GPW), enclosed arc welding (EAW), and aluminothermic welding (TW). Figure 1 shows the current application ratio of each welding process. Gas pressure welds account for approximately 30% of all welding processes. GPW however requires specialist skills for the rail-end surface grinding stage and the rail heating stage using a burner. As the number of well-trained welders is expected to decrease in the future, it is necessary to deskill the GPW process. As a consequence, possible steps for simplification and mechanization of GPW were examined. This paper describes the results of those investigations. Flash 24% Enclosed arc 7% Total number 64,000 Gas pressure 31% Aluminothermic 38% Fig. 1 Application ratio of each welding process in 2014 (JR Group) 2. Investigation into the simplification of the railend surface grinding process for welding of newly manufactured rails technique. Therefore, a non-trained welder may actually damage the flatness of the rail-end surface. It should be noted that GPW is almost always applied to first stage welding or second stage welding, where the majority of welded rails are newly manufactured rails. The end surfaces of newly manufactured rails are generally flat enough to be welded by GPW without grinding, because manufacturers cut the rails with great accuracy. Therefore, the flatness of the end surfaces of newly manufactured rails was examined and the degree of flatness was compared with the end surfaces of rails which had been ground with a special grinder, in order to examine whether the grinding of rail-end surfaces on newly manufactured rails could be omitted or not. 2.1 Device for measuring flatness of rail-end surfaces Although the flatness of rail-end surfaces of rails is examined with a square in the conventional GPW process, a special device was built to gauge the flatness of the railend surface of JIS 60 kg rails in order to obtain more quantitative information about the flatness for the purposes of this study. Figure 2 shows the measuring process and structure of the measurement device. There are 11 holes (reference points) on the reference plate which is perpendicular to the rail longitudinal axis. After measuring the distance between the outer surface of the reference plate and the rail-end surface at each reference point using the depth gauge, the flatness is evaluated through the difference in measured values at respective reference points. The flatness in three regions was examined in particular, namely the vertical region (hereinafter 2-10 which means, difference between the measured value at reference point 2 and at reference point 10), horizontal region of the rail head (4-5), and horizontal region of the rail base (9-11). A special grinder is used for rail-end surface grinding in the conventional GPW process, which requires a special 223

2 Balance weight Depth gauge Reference plate (a) Measurement process Angle adjustment knob Reference plate (b) Structure of the device Fig. 2 Device for measuring the flatness of rail-end surfaces Reference plate The flatness of newly manufactured rail-end surfaces and ground rail-end surfaces Figure 3 compares the flatness of newly manufactured rail-end surfaces with ground rail-end surfaces. 54 newly manufactured rail samples, and 31 ground rail-end surface samples were used. In Fig. 3, the flatness index is plotted on the vertical axis which indicates the difference in elevation of the respective reference points. The absolute values of the flatness index for newly manufactured rail-end surfaces varied less than those for ground rail-end surfaces. This demonstrates that the newly manufactured rail-end surfaces are already flat enough for GPW once any rust has been removed with a smallscale grinder. Flatness index (mm) Newly manufactured rail (N=54) End surface grinding rail (N=31) Vertical region Horizontal region Horizontal region of the head of the base Fig. 3 Flatness of newly manufactured rail-end surfaces and ground rail-end surfaces Examination of heating burner automatic swing device 3.1 Heating process in the conventional method As mentioned above, GPW process requires specialist skills for the heating process using a heating burner. The interface between butting sections must be intensively heated to obtain a good quality weld; this part of the process can sometimes lead to overheating of the rail surface. The heating burner therefore needs to be swung during the process to avoid overheating, as shown in Fig. 4. There are however no manuals on how to swing the burner, therefore the technique depends on each welder s intuitive skill. This situation could lead to a fluctuation in the quality of the gas pressure weld. Therefore, the following two items were examined in order to deskill the heating process. - Standardization of heating burner swing pattern - Development of an automatic heating burner swing device Burner Weld interface Fig. 4 ing of heating burner 3.2 Influence of the swing pattern on the quality of the gas pressure weld The gas pressure welds produced under the various welding conditions were evaluated to examine the influence of the swing pattern on the quality of the gas pressure weld. Figure 5 shows the test piece being produced, and Table 1 shows the welding conditions. The start time of the burner swing means the time from when compressive deformation begins. 224

3 Table 1 Production conditions of each test piece Gas flow Oxygen 100 (./min) Acetylene 103 ~ 107 Upset force (kn) 170 Upset length (mm) 24 start time (sec)* 0 ~ 60 amplitude (mm) 20 ~ 40 speed (mm/sec) 13 ~ 25 * Time from when compressive deformation begins Fig. 5 Production of a test piece Rapture load (MN) width:30mm, speed: 20mm/sec HU criterion HD criterion Time from the start timing of compressive deformation (sec) (a) Rupture load HU HD Fig. 6 Influence of the swing start time on the static strength property of welds Deflection (mm) width:30mm, speed :20mm/sec HU HD HU criterion HD criterion Time from the start timing of compressive deformation (sec) (b) Deflection Figure 6 shows the results of the static bending tests which were performed on gas pressure welds produced under the different conditions. JIS 60 kg standard carbon rails were used in this test. It is known that rail surfaces start to melt 80 sec after compressive deformation begins. Therefore, the upper limit for swinging to begin was set at 60 secs, to offer some margin against the melting of the rail surface. There was no correlation between the rupture load at which the gas pressure welds were broken in the head up (HU) position and the swing start time. A correlation was observed however, between the rupture load at which the gas pressure welds were broken in the head down (HD) position and the swing start time; the rupture load tended to be lower as the swing start time decreased. These results showed that gas pressure welds, strong enough to be put into service, could be produced when the swing start time was set at 60 sec, with a swing width of 30 mm and swing speed of 20 mm/sec. The ratio of heat dispersion to the periphery increases when the swing start time is advanced. As a result, it is considered that the quality of the weld drops due to a decrease in heat input to the welded part. There was correlation between the rupture load of the test pieces in HD breaking mode, in which the rail head area was tensioned, and the swing start time in this test. The time needed to heat the welded part to the sufficient temperature increased as the volume of material to be heated increased. Therefore, it is considered that quality can diminish easily in the rail head area which has a larger volume with the swing start time is advanced. It was also found that the burner swing width should be as narrow as possible to reduce the amount of heat dispersion to the periphery. The above results indicate that delaying the swing start time and narrowing the swing width of the burner are required to achieve sound welds while preventing excessive melting of the rail surface. Rail surface melting was then observed under the various welding conditions during the heating process. Table 2 shows the melting state of each test piece. Results showed that a 20 mm swing width does Table 2 Melting state of rail surface in each swing pattern pattern start time (sec) width (mm) speed (mm) Melting state of rail surface No melting 20 No melting 15 Melting around the jowl (40sec after swing start time) Melting of rail bottom (just before the end of the heating process) 225

4 not cause surface melting, whereas a 15 mm swing width caused surface melting when the swing start time was set at 60 sec. 3.3 Examination of swing pattern for practical use Standardization of the burner swing pattern was examined based on the above results. As a result, a suitable swing pattern for the burner was proposed, which ensures sound welds, and avoids overheating of the rail surface during the heating process. Table 3 shows the proposed swing pattern for welding a JIS 60 kg standard carbon rail. Table 3 Proposed swing pattern for the heating burner Proposed process Conventional process Automatic Manual mode pattern Start time*: 60 sec Amplitude : 20 mm Speed Judgment by welder 3.4 Development of automatic heating burner swing device As mentioned before, a swing pattern for the heating burner was proposed, which could be applied in practice. While standardization of the joint welding process could reduce fluctuation in the quality of welds, possibility of fluctuation would still exist while the operation was carried out manually. Therefore, with a view to deskilling the process completely, an automatic heating burner swing device was examined, which would be capable of reproducing the proposed swing pattern automatically. As a result, an automatic heating burner swing device for TGP-V type gas pressure welding machine was developed. Figure 7 shows the appearance and the structure of this device. The heating burner is connected with a coupling rod to the swing block, which is reciprocated by means of a ball screw driving mechanism. This device also has the mechanism to make the heating burner follow the movement of the weld interface. : 20 mm/sec * Time from when compressive deformation begins Mechanical part of swing Joint bar Screw axis Gas pressure welding machine part (b) Side view Screw axis Ball Burner Automatic swing device (a) Mounting state on GPW machine Motor (c) Structure of swing mechanical part Fig. 7 Automatic swing device of heating burner Table 4 Proposed GPW process Rail-end surface grinding Proposed process Conventional process Small-scale grinder used to Special grinding machine remove rust only required for grinding Gas flow Oxygen 100 (./min) Acetylene 105 Upset force (kn) 170 Upset length (mm) 24 Burner swing mode pattern Automatic Manual Start time* : 60 sec Amplitude : 20 mm Judgment by welder Speed : 20 mm/sec * Time from when compressive deformation begins 226

5 4. Gas pressure welding process without using expert skills Based on the above mentioned results, a new gas pressure welding process for rails was proposed, which does not require specialist knowledge or skills for execution. Table 4 compares the specifications of the proposed process and the conventional process. In the proposed process, GPW execution is performed after simply removing any rust on the rail-end surfaces with a small-scale grinder, and the heating burner is swung in the automatic mode. Rapture load (MN) 4.1 Proposed process ＨＵ criterion ＨＤ criterion JIS 60kg standard carbon rail ＨＵ ＨＤ Deflection (mm) 100 Fig. 9 Results of the static bending test 4.2 Evaluation of applicability of the proposed process The joints welded using the proposed process were evaluated in order to verify the applicability of the proposed process Production of test welds 8 specimens, each a 1.5 m long JIS 60 kg standard carbon rail, were prepared using the automatic heating burner swing device, and welded according to the conditions shown in Table 3. Figure 8 shows the GPW being carried out with the automatic heating burner swing device. No problems were encountered in terms of workability of the GPW method. (a) Sample-1 HU) (b) Sample-2 HD) Fig. 10 Examples of fracture surfaces Test piece Automatic swing device Inner span 150 mm Heating bumer Fig. 11 Bending fatigue test set up Fig. 8 GPW carried with the automatic heating burner swing device Results of test weld evaluations In accordance with instructions on non-destructive inspection of welded rails magnetic particle tests were performed on all the specimens, which were found to be sound and did not exhibit any linear indications. Figure 9 shows the results of the static bending tests carried out on the specimens supported at two points located 1 m apart and a load applied at the center. In this figure, HU indicates the rail base tension condition and HD indicates the rail head tension condition. The rupture load and deflection were higher than those stipulated in JR specification for all the specimens. The condition of the surfaces of the fractures in the tested welds were good, as shown in Fig. 10. Table 5 Result of the bending fatigue test Stress (N/mm2) Min Max Range 2σ Number of cycles ( 106) 2.00 Breakage None A four-point bending fatigue test was performed in the HU position to generate tensile stress in the rail base area of a specimen supported on a span of 1.3 m (inner span: 0.15 m), by applying a pulsating stress of a minimum of 30 N/mm2 up to two million times. Figure 11 shows the bending fatigue test set up. Table 5 shows the test results. Results show that the specimen had the same fatigue strength of 320 N/mm 2 (stress range) as the welded joint made with the conventional process. 227

6 4.3 Applicability of proposed process Hardness tests and micro-structure tests were carried on the one remaining test weld. Neither test revealed any abnormal indications. According to the above results of respective evaluation tests therefore, it is considered that gas pressure welds produced with the proposed process could be introduced for practical use. 5. Conclusions In anticipation of a decrease in the number of skilled welders, a new simplified and standardized gas welding method with minimal requirement of specialist skills was studied. As a result, a newly developed gas pressure welding process was proposed, which simplifies the rail-end surface grinding process and automates heating burner swinging. The gas pressure welds produced with this new GPW method on JIS 60 kg standard carbon rail were evaluated. Results confirmed that the performance of the new welding process was the same as the conventional welding process. References [1] Oishibashi, H., and Ueyama, K., Investigation for Quality Estimation Mechanism of Gas Pressure Welds, Journal of the Japan Welding Society,Vol.14, No.2, pp , 1996 (in Japanese). Authors Ryuichi YAMAMOTO, Dr. Eng. Senior Chief Researcher, Laboratory Head, Rail Welding Laboratory, Track Technology Division Research Areas: Rail Welding Yota KAKIZAKI East Japan Railway Company (Former RTRI) Research Areas: Track Technology Mitsumasa TATSUMI Senior Chief Researcher, Rail Welding Laboratory, Track Technology Division Research Areas: Rail Welding, Nondestructive Inspection 228