REAL TIME DEFECT DETECTION IN WELDS BY ULTRASONIC MEANS In fulfilment of the requirements for the degree of Doctor of philosophy. By Yicheng Lu B.Sc. M.Sc. MeWS December 1992. Department of Materials Technology, BruneI University, Uxbridge, Middlesex UB8 3PH To: All those who helped or attempted to help me in the past. ABSTRACT A computer controlled weld quality assurance system has been developed to detect weld defects ultrasonically whilst welding is in progress. This system, including a flash analogue to digital converter and built-in memories to store sampled data, a peak characters extractor and a welding process controller, enabled welding processes to be controlled automatically and welding defects to be detected concurrently with welding. In this way, the weld quality could be satisfactorily assured if no defect was detected and the welding cost was minimised either through avoiding similar defects to occur or by stopping the welding process if repaIr was necessary. This work demonstrated that the high temperature field around the weld pool was the major source of difficulties and unreliabilities in defect detection during welding and, had to be taken into account in welding control by ultrasonic means. The high temperatures not only influence ultrasonic characteristic parameters which are the defect judgement and assessment criterion, but also introduce noise into signals. The signal averaging technique and statistical analysis based on B-scan data have proved their feasibility to increase 'signal to noise ratio' effectively and to judge or assess weld defects. The hardware and the software for the system is explained in this work. By using this system, real-time 'A-scan' signals on screen display, and, A-scan, B-scan or three dimensional results can be printed on paper, or stored on disks, and, as a result, weld quality could be fully computerized. Page Cl CONTENTS 1. INTRODUCTION ......................................... 1 2. LITERATURE SURVEY ................................... 3 2.1 ARC WELDING QUALITY CONTROL. ................. 3 2.1.1 Development of welding defects. 3 2.1.2 Automatic control of weld defects. ................ 4 2. 1.2. 1Seam tracking . .. 5 2.1.2.2Weld penetration and weld pool size control. .. 9 2.2 WELD INSPECTION. ................................ 11 2.2.1 General Survey ............................... 12 2.2.2 Ultrasonic inspection of welds ................... 13 2.2.3 Austenitic weld inspection. ...................... 16 2.2.3. 1Difficulties in austenitic weld inspection. ...... 16 2.2.3.2 Techniques to improve austenitic weld inspection. ............................ 17 2.3 AUTOMATIC WELD DEFECT DETECTION DURING WELDING. ..................................... 21 2.3. 1 Significance. ................................. 21 2.3.2 Difficulties of defect detection during welding. .... .. 22 2.3.2.1 Temperature. .......................... 22 2.3.2.2Coupling problems in automatic scanning. 26 2.3.3 On-line weld inspection ......................... 28 2.4 APPLICATION OF MICRO-COMPUTER IN WELD DEFECT DETECTION ............................ 30 3. COMPUTER CONTROLLED EXPERIMENT SYSTEM .......... 32 3.1 SYSTEM INTRODUCTION. ........................... 32 3.1.1 Ultrasonic system. ............................. 33 3.1. 2 Welding system. .............................. 33 Contents C2 3.2 GENERAL CONSIDERATIONS. ....................... 34 3.2.1Time and amplitude ............................ 35 3.2.2 Time window. ................................ 39 3.2.3 Signal width ................................. 40 3.3 HARDWARE. ...................................... 41 3.3.1 Interface. .................................... 41 3 ..3 2F' unctIon contro I CIrCUIt.. .. 42 3.3.2.1Synchronous circuit. ..................... 42 3.3.2.2Time gate circuit. ....................... 44 3.3.2.3 Peak detector. ......................... 45 3.3.3 Flash analogue to digital converter. ................ 47 3.3.4 Welding system control circuit. ................... 51 3. 4 SOFTWARE. ....................................... 53 3.4.1 Introduction. ................................. 53 3.4.2 Programming techniques. ........................ 57 3.4.3 Software functions. ............................ 62 4. EXPERIMENTAL TECHNIQUES AND RESULTS .............. 64 4.1 SYSTEM TESTING. ................................. 64 4.1 . 1 Calibration .................................. 64 4.1.2 Amplitude versus signal width. .. 67 4.1.3 Datum point for measuring tIme. .................. 69 4.2 EXPERIMENTAL SIMULATION. ...................... 70 4.3 HIGH TEMPERATURE EFFECTS ON RESULTS. ......... 74 4.3.1 Arrangement. ................................ 75 4.3.2 Signal time change. ............................ 76 4.3.3 Signal amplitude change. ........................ 79 4.4 SIGNAL PROCESSING AND ANALYSING. .............. 81 4.5 ON-LINE WELD DEFECT DETECTION. ................ 86 4.5.1 Lack of fusion. ............................... 87 4.5.2 Porosity or inclusions. .......................... 88 Contents C3 4.5.3 Cracks ...................................... 90 4.5.4 Other defects. ................................ 91 5. DISCUSSION ............................................ 92 5.1 DIFFICULTIES IN DEFECT DETECTION CAUSED BY THE HIGH TEMPERATURES OF WELDING. ......... 92 5.1.1 Signal time. .................................. 92 5.1.2 Signal amplitude and gain control. ................. 94 5. 1.3 Grain growth and phase transformation. ............ 97 5.2 NOISE AND NOISE REDUCTION. .................... 100 5.2. 1 Signal processing. ............................. 101 5.3 RELIABILITY OF DEFECT DETECTION DURING WELDING. ..................................... 105 5.3.1 Restrictions caused by high welding temperatures. .... 106 5.3.2Some limitations of contact coupling and the mechanical scan. ...................................... 107 5.3.3 Some limitations of on-line defect detection. ........ 108 5.3.4 The reliability of detecting defects during welding ..... 109 5.4 STRATEGY LIMITATIONS. ......................... 112 6. CONCLUSIONS . 114 7. FURTHER WORK RECOMMENDATIONS .................... 117 8. ACKNOWLEDGEMENTS . 119 REFERENCES . 120 FIGURES Contents C4 APPENDICES: Appendix A: Expression for the energy passed through the interface in a perspex simulation block . Al Appendix B: Programs written for the system . A3 Appendix C: Publication ................................. A52 Page 1 1. INTRODUCTION The application of ultrasonic sensors in welding process monitoring and control has become an important area and significant achievements have been made in ultrasonic seam tracking and penetration control[1-10j. On the other hand, ultrasonic inspection of weld quality has long been employed as one of the most frequently used methods in industry, especially in the area of thick plate weld inspection and 'on-site' service inspection. However, the use of ultrasonic sensors to detect defects as they form in the interior of the weld pool whilst welding is in progress has received less attention[ll]. Some defects might occur during welding due to the complexities of the welding processes and the unpredictable factors. Therefore, welds must be inspected along the joint to be sure that the weld is free of defects and that the weld structure can still meet the demands of ultimate integrity and strength, even though some defects exist in the weld. If the defects could be detected as soon as they occur by on-line defect detection, then similar defects might be reduced or avoided thereafter if automatic welding parameters modification could be suitably found and applied immediately. In addition, if the defect is out of an acceptable region and a repair is necessary, then the weld process could be halted in order to carry out repairs. As a result, repair costs will be minimised, this could be very significant in many cases, especially when the defect is located in the root run of a thick plate weld[12]. It is well known that conventional ultrasonic weld inspection is performed after the welding process is terminated, i.e., after the severe thermal gradients have given way to a lower and more equal background temperature and ultrasonic inspections are usually carried out manually[13]. Thus the major difference between conventional ultrasonic inspection and the in-process defect detection is the existence of high welding temperatures with steep gradients and the impossibility of a repetitive test to increase signal to noise ratio in the 'in-process' defect detection. The high temperature could greatly influence signal time and severely Introduction 2 reduce signal amplitude, the two most important ultrasonic parameters in ultrasonic weld inspection. Therefore, temperature effects must be borne in mind in the development of welding defect detection ultrasonically. Furthermore, the existence of large austenite dendritic grains in the weld (before the phase transformation during cooling process) could, more or less, generate problems which were often encountered in the austenite weld inspection process. Austenite grains in welds could scatter ultrasound and produce the noise named "grass" and even skew the ultrasonic beam path [14, 15] . A principal objective of this work was the development of a computer controlled welding defect detection system and the demonstration of the feasibility to detect defects concurrent with welding by employing the system. Moreover, the overall effects of the high welding temperature and the possibility of the temperature effects compensation or reduction was investigated experimentally.