How To Control Welded Pipe

Transcription

1 INTEGRATED SYSTEM FOR PROCESS CONTROL OF HIGH FREQUENCY ELECTRIC RESISTANCE WELDED STEEL PIPE Esteban Baralla Siat S.A. Tenaris Group Guatemala 3400, (B1822AXZ) Valentín Alsina, Buenos Aires, Argentina Claudio Tommasi Siat S.A. Tenaris Group Guatemala 3400, (B1822AXZ) Valentín Alsina, Buenos Aires, Argentina Abstract. A control system that integrates on-line multichannel ultrasonic inspection and welding parameter monitoring was implemented at Siat s high frequency electric resistance welded (HF-ERW) pipe facilities. The system was developed to correlate on real time basis the welding conditions (welding variables) and the ultrasonic weld indications in order to narrow the process control limits and improve the quality and reliability of the product. Most of Siat s HF-ERW pipes are produced according to API 5L standard plus supplementary client requirements and destined for conveying gas and oil at high pressures. During last years conditions of use of HF-ERW pipes have become increasingly severe: higher pressures, low temperature environments, offshore applications, sour service and high toughness requirements. Consequently customer specifications are more stringent regarding pipe production and quality and process control. The control system was conceived to satisfy client specifications and as a tool to establish relationships among weld ultrasonic indications and welding variables with the aid of metallographic examination. Some cases are shown where the integrated control system proves to be a powerful tool to prevent the occurrence of weld defects, to assist its early detection or to set up the optimum operative window. Keywords. ultrasonic test, high frequency welding, electric resistance welding, process control, steel pipes. 1. Introduction High frequency electric resistance welding (HF-ERW) is one of the most common methods for production of high quality welded carbon steel pipes for conveying gas and oil at high pressures. The widening of the field of application of HF-ERW pipes into more severe environments has been possible due to the progress in the areas of steel making, hot coil rolling and pipe making technology. To keep up with this evolution quality and process control equipment and procedures have also been updated. Most of these pipes are produced according to API 5L Standard plus client supplementary requirements. A typical quality plan shall conform to the following inspections and tests: - Chemical analyses. - Mechanical tests of weld seam and base metal (tensile test, Charpy V-Notch test, flattening test). - Metallographic examination. - Hydrostatic test. - Ultrasonic test of the weld area. - Visual inspection. - Dimensional control. - First day qualification tests. The results of these activities are always recorded pipe by pipe on a computerized system, but many times it is also required the monitoring of process variables in order to get full traceability. The new control system was developed to give an answer to the outstanding quality requirements for HF-ERW pipes. By correlating on real time the welding variables with the results of the on-line ultrasonic inspection it is possible to establish and maintain the optimum welding conditions to get a reliable product. This report describes the features of the integrated control system. 02 a 06 de Junho de 2003 / June 2 to Rio de Janeiro - RJ - Brasil

2 2. Description of the integrated control system 2.1. Production of high frequency electric resistance welded pipes Siat produces high frequency electric resistance welded pipes from 114 mm (4 ½ inch) to 610 mm (24 inch) outside diameter. Figure (1) illustrates the production sequence. Figure 1. Flowchart of high frequency electric resistance welding process. Hot rolled coils loaded into the mill are uncoiled and flattened by a leveler. The leading end of the strip, usually of irregular shape, is cut off. If required by the specification, the coil is inspected by ultrasonic test to detect laminations. As the coil proceeds down the line a rotary side trimmer shears the edges to get an accurate width. The skelp goes into the forming mill where it is gradually cold formed to an oval shape. In the forming process the attention is centered on the reduction of edge strains and prevention of edge fluctuation, since it is a key point to assure a stable weld. To make the weld the edges that converge to form a V are continuously heated by the pass of a high frequency current (300 to 400 KHz) and are immediately forged together by a squeeze roll assembly. The material extruded on the inside and outside weld surfaces is removed by scarfing while still hot. Afterwards, the weld seam and heat affected zone undergo a normalizing heat treatment to get a fine grain microstructure of similar characteristics to the base metal. The heat treatment consists in heating by induction the weld area to achieve complete austenization follow by air cooling until the temperature is below 350 C. Downstream, the welded coil goes through the sizing mill to obtain the proper round shape and straightness. After the pipe exits the sizing section 100 % of the weld area is inspected on-line by an automatic ultrasonic equipment. Then the pipes are cut to the required length by the flying cut off and continue to the finishing floor. Every joint is subjected to the following operations and inspections in accordance to the ruling specification: pipe ends beveling, hydrostatic test, bevel ultrasonic inspection to detect laminations, weld ends manual ultrasonic

3 inspection, final automatic ultrasonic inspection of the weld, visual and dimensional control, pipe marking and varnishing or coating depending on client requirements. Finally finished pipes are storage at the pipe yard. The high frequency welding, the weld heat treatment and the on-line weld ultrasonic inspection, highlighted in Fig. (1), are the processes linked by the integrated control system High frequency electric resistance welding process The high frequency current is transmitted by sliding contacts as seen in Fig. (2). The current travels up and down the Vee heating the edges. The squeeze rolls supply the mechanical pressure necessary to forge the edges and produce the weld. There is not addition of filler metal. Figure 2. High frequency electric resistance welding. Figure (3) shows macrographs of the weld area (a) immediately after the squeeze roll centerline: the white line in the center is the weld line, typical of high frequency welding, the sand clock shaped region is the heat affected zone and the black cavities on the outside and inside beads contains the oxides and non-metallic inclusions expelled by forging; (b) in the final condition, after the outside and inside beads have been trimmed and the weld and heat affected zone normalized. a b Figure 3. Macrographs of the weld area: a) Immediately after squeeze rolls, b) final condition after heat treatment. (Pipe size: outside diameter 610 mm, wall thickness 11.9 mm, steel grade API 5L X70) Process control methods for high frequency welding As explained below, different methods are being used by pipe manufacturers for process control purposes Manual control The welding power is adjusted manually by the operator, based on the brightness of the welding zone at the convergence Vee. The method is strongly dependant on the operator judgement and it is not possible to keep the welding variables within narrow limits.

4 Automatic control Several methods or combinations thereof are in use. All of them are based on the automatic adjustment of the welding power in order to keep a target parameter within the preset range. Next there is a brief description of the most common ones. - Heat input control: The heat input is defined as a function of the welding power, the welding speed and the material wall thickness. The target is to maintain the heat input within the desired limits. - Welding temperature control: The welding temperature, measured at the convergence Vee, is a function of the heat input and the electric impedance of the Vee. In this case the welding temperature must be kept within the preset range (Terada et al, 1981; Haga et al, 1985; Mihara et al, 1986). - Control of the welding current frequency: The optimum welding condition may be correlated to the variation of the electric frequency of the welding current. Therefore, the objective is to keep the variation of the welding current frequency within the specified limits (Haga et al, 1985; Watanabe et al, 1986). - Control of the shape of the outside weld bead: It is possible to determine the bead shapes associated to the optimum welding condition. Then the objective is to keep the actual bead shape within the allowable range (Mihara et al, 1986) Integrated control system To establish a control system two questions must be solved. The first one is to determine the optimum range for the welding variables, the second is to keep them within the optimum range. The heat input and the welding temperature control methods have been used successfully at Siat to deal with the second question since the early nineties. The objective of the integrated control system is to give an accurate answer to the first subject. Therefore the welding variables (causes) are related to the results of the ultrasonic inspection (effects) on real time basis as shown in Fig. (4). Welding Variables High Frequency Welding Process Ultrasonic Inspection Results Operative Window Optimization Figure 4. Schematic diagram of the integrated control system. Any indication of the ultrasonic equipment is analyzed against the welding parameters at that exact location. Furthermore, the type and size of the defect or indication may be determined with the aid of metallographic examination, using samples similar to that of Fig. (3.b). Through this feedback procedure it is possible to establish a narrow optimum working range for any of the welding variables On-line weld ultrasonic inspection Siat has been using the ultrasonic test for the weld seam since the middle sixties according to API 5L requirements. At the present time, the on-line ultrasonic equipment is located upstream of the flying cut off, as shown in Fig. (1). Thus 100 % of the weld is continuously inspected along the complete coil, before it is cut to the required joint lengths. The ultrasonic equipment is Krautkramer USIP 20 with 8 channels and 8 cycles. The weld inspection is performed with four 70 degree shear wave probes, or more if required, each of them dedicated to the detection of longitudinal defects by pulse echo mode. Additionally, coupling is controlled between opposite probes by through transmission mode. The amplitude signals from each of the four channels dedicated to defect detection are addressed to four analog outputs and recorded by the integrated control system as a function of the coil position. Whenever the amplitude signal of any of the channels exceeds the corresponding gate threshold an alarm is triggered, the pipe is automatically paint marked at the exact location of the indication an this event is also recorded by the integrated control system. The typical automatic ultrasonic procedure is summarized in Tab. (1).

5 Table 1. Procedure for on-line weld ultrasonic inspection. Item Specification Remarks Equipment Krautkramer USIP 20 8 channels / 8 cycles Probe type Krautkramer W70Z4N Probe frequency: 4 MHz Beam angle: 70 degree Probe arrangement 4 probes, 2 on each side of the weld. Distance (d) from the probe center to the weld centerline Weld Line is set according to the pipe diameter and wall thickness. d Cycle configuration Reference standard Cycle Probe A Mill speed 30 m/min maximum Pulse repetition frequency 1500 to 2000 Hz per cycle Reference signal Cycles 1 to 4: 80 % TSH Cycles 5 and 6: 100 % TSH Gate threshold B C D T R T R T R T R T - - R - - T - - R - - Cycles 1 to 4: 64 % TSH set for coincidence mode Cycles 5 and 6: 50 % TSH set for anticoincidence mode 4 cycles in Pulse / Echo mode for longitudinal defect detection. 2 cycles in Through Transmission mode for coupling control 3.2 mm drilled hole (one on the weld centerline and two 6 mm each side of the weld) and N10 notch (one external and one internal) Depending on distance d Based on 3.2 mm drilled hole. Gain control adjustable in 0.1 db steps. One gate for each cycle. The gate threshold is adjustable in 0.5 % increments and the amplitude evaluation resolution is 0.4 % TSH. The gate start and width are set with the 3.2 mm drilled holes at both sides of the weld. Defect alarm Automatic paint marking and visual alarm Triggered through power outputs. Loss of coupling alarm Visual alarm Triggered through digital outputs Record of signal amplitude For cycles 1 to 4 One analog output per cycle, linked to the integrated control system. The indications of the automatic equipment are checked by manual ultrasonic inspection, calibrated with the same reference standard, in order to evaluate whether they are within the acceptance limits of the applicable specifications or not. Whenever is necessary, samples containing defects are cut and sent to the metallographic laboratory for further analysis in order to diagnose the type and characteristics of the defects.

6 2.6. Examples The following examples correspond to the production of pipes of outside diameter 168 mm, wall thickness 7.1 mm, grade API 5L X42 and are brought out to give a brief overview of the capabilities and limitations of the integrated control system. They help to understand the relationships among the welding conditions, the ultrasonic test results and the type of defects. Example 1: Figure (5) is a photograph of a sample with a defect known as cold weld (API Standard 5T1, 1996), as it is seen after a nick break test. The darkest areas are zones with lack of fusion, while the adjacent metal presents brittle fracture due to inadequate bonding strength. Figure 5. Example 1. Nick break specimen with a cold weld. Figure (6) is a hardcopy of the on-screen presentation for the ultrasonic signal amplitudes and the welding variables for the coil section where the defect of Fig. (5) was found. Ultrasonic indication Signal amplitudes (% TSH) Coil length (m) Welding power (KW) Drop of welding variables Welding speed (m/min) Welding Voltage and Amperage (%) Figure 6. Example1. Records of ultrasonic test results and welding conditions.

7 The upper graph plots the signal amplitudes of each of the four channels used for ultrasonic inspection as a function of the coil length. The header includes information to assure the traceability to the final product, as the production order number, the coil number, the location of the pipe within the coil, the pipe length after being cut by the flying cut off, and the time. The lower graph presents the welding variables also related to the coil length. The cause of the defect is the low heat input condition that results from the sudden drop of the welding power, while the welding speed remains without any significant variation. The ultrasonic graph puts in evidence that the ultrasonic equipment picked up the defect, and triggered the paint marking alarm. This example shows a clearly defined correlation among the elements that form part of the integrated control system and it outlines the basis used for optimization of the working range for the welding variables for different pipe sizes and grades produced during last year since the integrated control system was put into operation. Example 2: Is illustrated by Fig. (7) and Fig. (8). Figure 7. Example 2. Nick break test specimen with lack of fusion. Figure 8. Example 2. Nick break test specimen with the presence of oxides. Both defects were detected by the ultrasonic inspection, but there were not significant variations of the welding parameters. Probably the frequency for data collection of the integrated control system is not enough to keep up with rapid changes in welding variables. The welding speed for this kind of pipe is 14 m/min (230 mm/sec) and the frequency for data collection is 10 Hz. This means an average of one sample every 23 mm, while the defects of Fig. (7) and Fig. (8) have an extension in the longitudinal direction of scarcely 2 mm. Otherwise the defects may arise from foreign causes to the electrical behavior. Further investigation is necessary to adequate the system. Example 3: Shows the results of an experience developed to evaluate the consequences of a poor edge preparation. One of the edges of the coil was intentionally damaged. As welding proceeded there was no change in the welding variables, but the ultrasonic inspection disclosed the defect of Fig. (9). Figure 9. Example 3. Nick break specimen of an edge damaged before welding. It comes out that this kind of defects and some others originated in intrinsic problems of the base material fall outside the scope of the integrated control system. Even though the system is powerful to correlate on real time basis welding conditions and ultrasonic test results, there is still work to do to improve its performance.

8 2.7. Conclusions The integrated control system has been working reliably for almost one year and allows to relate accurately the results of the ultrasonic inspection with the welding conditions. It has proved good efficiency for correlating welding variable deviations to ultrasonic indications being an useful tool to adjust the limits of the operative window for process control. The data acquisition frequency of the system seems not to be capable of recording rapid variations of the welding variables lasting few tenths of a second. This assumption is based in the existence of minor defects detected by ultrasonic inspection, without any significant variations in the welding conditions. Some types of defects are out of the scope the integrated control system like those caused by edge damages or problems inherent to base material, as non metallic inclusions. Finally, even though the integrated control system has been helpful to improve the welding conditions and consequently pipe quality, there is still work to do to extend its field of application Work to be done Investigate the causes of minor defects detected by the ultrasonic test and not explained by deviations in the welding conditions. To deal with this situation the following alternatives are being analyzed: - To increase the frequency for data collection from 10 samples per second that is being used to approximately 100 per second. - To add new variables to the system or combinations of the ones that are being used. - To analyze other causes not related to welding variables and that are out of the scope of the integrated control system. 3. References American Petroleum Institute, 1996, Standard on Imperfection Terminology, API Standard 5T1, Tenth Edition, November Haga, H., Mizuhashi, N., Tanaka, N., Yamada, M., Watanabe, Y. and Hosooka, A., 1985, Automatic Heat Input Control in Production Welding of Médium-Diameter ERW Pipe, Nippon Steel Technical Report N 26 July 1985, pp Mihara, Y., Suzuki, K., Ohkawa, T., Harada, N., Komine, I. and Ishiro, S., 1986, A New Automatic Heat Input Control for Production of Electric Resistance Welded Pipe, Transactions ISIJ, Vol. 26, 1986, pp Terada, T., Yutaka, H., Otano, T., Yoshimoto, Y., Takizawa, S., Fukai, M., Onishi, F. and Yokoyama, E., 1981, Manufacturing of New 26-inch ERW High-Test Line Pipe, Kawasaki Steel Technical Report N 4 December 1981, pp Watanabe, N., Funaki, M., Sanmiya, S., Kosuge, N., Haga, H. and Mizuhashi, N., 1986, An Automatic Power Input Control System in High Frequency Electric Resistance Welding, Transactions ISIJ, Vol. 26, 1986, pp