FIELD OF INVENTION
The present invention relates to a method of inspection of stub to header weld joints in steam generators by Phased Array Ultrasonic Testing (PAUT) with simultaneous use of multiple sound waves. More particularly, the invention relates to the inspection method of Phased Array Ultrasonic Testing (PAUT) to detect defects like lack of side wall fusion (LOF), lack of root penetration (LOP), cracks and inclusions in the stub to, header partial penetration welds. This also relates to the technique of employment of Phased Array (PA) ultrasonic probe with suitable wedge and placement on the stub to detect longitudinal, transverse and oblique defects in welds.
BACKGROUND OF THE INVENTION
Steam generator / Boiler headers are manufactured on a large scale for thermal power plant applications. Recently number of failures are noticed in the area of boiler tubes especially stub to header weld joints during either erection & commissioning or operation. The reason could be either the weld defects are not detected during fabrication by using proper Non Destructive Evaluation (NDE) Technique or failing due to ligament cracking. Dye Penetrant Testing (DPT) and Magnetic Particle Testing (MPT) are being used to detect surface / subsurface defects only. Inter pass quality check is

also not possible as welds need time for cooling and heat treatment. Conventional Ultrasonic Testing (UT) is not effective due to their fixed angles of probes, i.e., normal or angle beam probes and less accessibility to move UT probes. PAUT technique is a versatile Non Destructive test (NDT) method which generates multiple sound waves and can be employed simultaneously, can be effectively used for detection of volumetric / internal defects and inspection of complex shapes. Each header consists of, on average, 100 stubs for steam generation in super heater area. The thickness of the welding comprising root layer, filler layer and capping layer.
Stub-to-Header welds typically contain defects such as slag inclusion, cracks, cluster porosity, lack of root penetration (LOP) and lack of side wall fusion (LOF). Cracks can be initiated at weld root, where the inclusions / defects generally exist. Presently Magnetic particle inspection is carried out after completion of entire welding. MPT is effective to detect surface and subsurface defects only and not for volumetric defects which lie beyond 3mm depth. Therefore, a specialized PAUT technique was developed after number of lab trials on mock up weld samples with known artificial defects. During trials, multiple linear PA probes, different probe locations and linear and sectorial waves with different focal laws were tried to get best possible test parameters. This invention can now be used for reliable detection of defects in stub to header weld joints of steam generators.

Prior to this invention some of the patents were referred. In the Pat No. US 7428842B2, describes about use of Phased Array Ultrasonic testing for detection of defects in turbine components which are highly complex in shape and difficult to detect defects / cracks in those parts like blades, discs and blade attachments. But this patent does not cover simultaneous use of multiple sound waves and also applicability of PAUT testing to tubular or welded components.
Non-destructive testing of tubes and pipes. In particular, detection of longitudinal, transverse and oblique defects in tube walls using a phased array ultrasonic system is described in Pat No. US 2004/0016299A1. This patent also talk about virtual rotation of sound beams but not about use of multiple beams and also about weld inspection.
PAUT testing method is employed for detection of defects in welds of two different pipes is described in Pat No. US7412890B1. The method includes mounting at least one ultrasonic phased array probe including at least one transducer inside a nuclear reactor. This invention is applicable only to tube to tube welds by using a single beam.

In the present invention, use of PAUT technique for inspection of stub to header welds in boilers is described. Multiple sound waves generated by single probe is used to cover different areas of the weld to save time and also to improve detectability of the defects.
SUMMARY OF THE INVENTION
The invention relates to the Non Destructive testing technique, Phased Array Ultrasonic Testing (PAUT) using a linear array probe with a number of elements employed for detection of longitudinal, transverse and oblique defects / flaws / cracks in welds of stub to header in boilers. One PA probe having the capability of generating multiple sound waves in multiple directions with different focal depths and with combination of compatible PA system is moved circumferentially on stub tube along the weld tip. The PA probe is made to move manually by hand or with the designed suitable scanner such that it fits to the stub tube outer diameter. The invention relates to the selection of PA ultrasonic probe with a maximum no of elements such that the generated multiple sound waves are hitting the reflectors and coming back to PA ultrasonic probe. More number of Piezo electric crystals in multiple of 8 to max 256 can be used. Here in this case 16 elements are used. This invention relates to that the

present technique is used for detection of internal defects like LOF, LOP, cracks, slag inclusions, clustered porosity, etc. in stud to header partial penetration welds.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of inspection of stub to header weld joints in steam generators by phased array ultrasonic testing (PAUT) with simultaneous use of multiple sound waves which is capable of detection / identification of internal / volumetric defects like lack of side wall fusion (LOF), lack of root penetration (LOP), cracks, flaws ad slag inclusions and clustered porosity.
Another objection of the invention is to propose a method of inspection of stub to header weld joints in steam generators by phased array ultrasonic testing (PAUT) with simultaneous use of multiple sound waves, which is able to detect longitudinal, transverse and oblique defects.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1.: Schematic diagram describing the overall view of stub to header weld joints in a typical manner with cross section position on header with possible cutting of stubs.

Fig.2: Schematic diagram showing the cross section 1-1 with details of header outer diameter, header thickness, stub tube outer diameter (OD), stub thickness.
Fig.3: Schematic diagram showing the scan plan of liner array multiple sound waves, probe placement, weld tip, sound waves legs (A, B and C), cross section of stub showing weldment between stub and header.
Fig.4: Schematic diagram showing the scan plan of sectorial array multiple sound waves, probe placement, weld tip, cross section of stub showing weldment between stub and header with little angular view.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
No of boiler / steam generator tube leakages are reported in recent times either in operation or during erection and commissioning. These leakages may be due to the presence of cracks, LOF, LOP slag inclusion, etc. in the welds of stub to headers at manufacturing stage and grown over a period of time or ligament cracks which probably initiated and grown during operation and opened up lead to leakages. These leakages would be very effective to the utility in the form of unplanned outage of the

generating units. This is possible only when the defects are not detected in proper way during manufacturing by employing proper NDE techniques. DPT and MPT are not effective to the defects in volumetric nature. Conventional Ultrasonic testing also not effective due to fixed probe angles. Phased Array Ultrasonic Testing (PAUT) is gainfully employed to assess the soundness of the weld joints of stub (6) to header (11) weld joints in steam generators / boilers by detecting internal defects. By detecting these internal defects by PAUT method, the leakage of tubes of boiler can be reliably avoided. The method of PAUT technique, usage of PAUT transducers and the positioning of transducers on stub from the weld joint is explained in this invention. By employing this technique the internal defects can be detected very easily with high reliability.
This invention relates to the placement of PA probe 9 on the side of stub tube 6 wall which is very much important for transmission and reception of sound wave 10, 12 and 13 in phased manner and with desired angles and desired focal depth with different combinations of the sound waves. The plurality of sound waves ensures the chances of detection of internal defects in weld 8 of stub to headers in boilers. The internal defects lie in between 2nd (B) and 3rd (C) legs of the sound waves in linear as well as in sectorial scans of shear waves. The LOF of stub side and internal defects like cracks and slag inclusions will be detected by linear scan. LOP and oblique defects will be detected using sectorial scans with the reflections from these defects. The scan angles are

prefixed with desired orientation to transmit and received the multiple sound waves from liner as well as oblique internal defects. Three types of multiple sound beams from multiple group of piezo electric elements were made to generate three different fixed angles to transmit and receive the reflections from the internal defects. One PA probe 9 having the capability of generating sound waves 10, 12 and 13 in multiple directions with different focal depths and with combination of compatible PA system is moved circumferentially on stub 6 away from the weld 8. In 1st step, the PA probe 9 is made to move manually by hand or with the designed suitable scanner such that it fits to the stub tube 6 outer diameter (OD). In 2nd step, a group of three sound waves are generated in the PA probe 9, i.e., one (1) linear and two (2) sectorial. The linear array sound wave 10 with 60O refracted angle are generated in linear triggering manner such that the resulted linear waves are hitting the surface of internal defects and reflecting back to the PA probe 9. Yet again, the sound waves 12 generated from the same PA probe 9 in sectorial scan with angles (ranging from 350 to 700) such that the waves are hitting on the LOP and oblique cracks / defects and receiving at same PA probe 9. Yet again, the sound waves 13 are generated from same PA probe 9 in sectorial scan with angle (ranging from 40° to 50°) such that the multiple waves are hitting the surface of LOS and oblique defects on the side of stub in the weld and reflecting back to the PA probe 9.

Yet again, the 9 is selected with a maximum no of elements such that the generated sound waves 10, 12 and 13 are hitting the reflectors and coming back to PA probe 9. The size of PA probe 9 is selected such that the maximum contact with suitable wedge with minimum possible foot print on outer surface of stub tube 6 OD for transmitting the sound waves 10, 12 and 13 in the desired directions in to stub tube 6.
Yet again, the PA probe 9 is placed on the stub tube 6 OD surface in such a way that the sound waves 10, 12 and 13 are travelling towards weld and reflecting back from defects to PA probe 9.
The invention relates to the technique employed for detection of longitudinal, transverse and oblique defects / flaws / cracks in welds 8 of stub to header in boilers is transmit and receive technique.
Yet again, the PA probe 9 is moved in circumferential direction on OD surface of stub tube 6 in either clock wise or anti clock wise directions such that the beam covers entire volume of weld with all the three groups of array ultrasonic sound waves 10, 12 and 13 in a stepped manner with an increment such that the over lapping of at least 10% scan.

The invention relates the PA probe 9 movement in axial direction on OD surface of 6 stub diameter away from weld tip 7 and an increment of such that the entire weld is covered in minimum no of scans with possible chances for defects detection.

WE CLAIM
1. A method of inspection of stub to header weld joints in steam generators by
phased array ultrasonic testing (PAUT) with simultaneous use of multiple sound waves, the said method comprising;
placing a phase array (PA) probe (9) on side tube (6) wall to transmit and received sound wave (10, 12 and 13) in phased manner with desired angles and focal depth with different combinations of sound waves to detect internal defects in weld (8) of stub to headers in boilers;
moving the said probe (9) circumferentially on stub (6) away from a weld (8) firstly manually by hand or with the designed scanner that fits to the stub tube (6) outer diameter;
generating a group of three sound waves in the said PA probe (9), the said sound waves being one linear (10) and two sectional (12 and 13) when the linear array sound wave (10) with fixed refracted angle generated in linear triggering manner such that the resulted linear waves hitting the surface of internal defects and reflecting back to the PA probe (9);

generating a sound waves (12) from the PA probe (9) in sectional scan with fixed angles such that the waves hitting on the LOP and oblique cracks/defects and received back to PA probe (9);
generating sound waves (13) from the PA probe (9) in sectorial scan with fixed angle such that the multiple waves hitting the surface of LOP and oblique defects on side of stub (6) in the weld (8) and reflecting back to the PA probe (9);
wherein,
the PA probe (9) is moved in circumferential direction on OD surface of stub tube (6) in either clockwise or anticlockwise direction so that beam covers entire volume of weld with all the three groups of array ultrasonic sound waves (10, 12 and 13) in a stepped manner with an increment resulting a scan overlap of at least 10%.
2. The method as claimed in claim 1, wherein three types of multiple sound beams
from multiple group of piezo electric elements generating three different fixed angles to transmit and receive the reflections from the internal defects, wherein the three fixed angles are linear array wave (10) with 600 refracted angle, sound wave (12) with angles ranging from 350 to 700 and sound waves (13) with angles ranging from 400 to 500.

3. The method as claimed in claims 1 and 2, wherein the scan angles are prefixed with desired orientation to transmit and receive multiple sound waves from linear as well as oblique internal defects.