Heavy section EB welds face the NDT treatment

TWI Bulletin, November/December 1991

Bryan Kenzie graduated from the University of Nottingham in pure and applied physics in 1983. He was sponsored by TI Research Laboratories whom he joined in 1976. His work here involved research and development of a wide variety of measurement and monitoring equipment including the design and development of EMAT transducers for non-contact ultrasonic testing applications.

In 1986 he joined TWI's Non-Destructive Testing Department where he is currently leader of the Applications Section. His special interests include NDT for pipelines and offshore installations, NDT ofpower beam welds and development of automated ultrasonic testing techniques.

Having graduated in 1986 from Cambridge University with a degree in metallurgy and materials science, Bruce Dance joined TWI in 1987. He is now Senior Research Metallurgist in the Electron Beam Department where he has been involved in many research projects addressing the development and application of high power electron beam welding, including fade-out procedures, and electron beam weld microstructure and toughness in C-Mn steels, as well as numerous application studies.

More recently, he has been investigating other areas, including further use of electron beams for surfacing and surface modification, and demagnetisation techniques for use prior to electron beam welding of thick section components.

Radiography and ultrasonic inspection are often applied to EB welds in heavy section steel, but what exactly are their capabilities? Bryan Kenzie and Bruce Dance report.

Different welding processes characteristically produce different types of weld defect. This inevitably means that non-destructive testing (NDT) techniques, procedures and defect acceptance criteria need to be carefully specified taking into account joint type, welding process and service requirements.

This is particularly true for electron beam (EB) welding, which, since the mid-1970s, has been recognised as an alternative method for welding very thick section material. EB welding has been steadily improving in terms of power, quality and consistency and undoubtedly its application will become more widespread in future. Of particular benefit is EB welding's unique capability of single pass welding of these thick steels at very high rates (2m ² of joint area per hour).

Various NDT techniques and procedures are routinely applied to joints in heavy section steel but are they appropriate for high power EB welds? Experience to date suggests that this is not always the case; significant improvements in detection, sizing and characterisation of defects in EB welds are required because of the generally unfavourable orientation and morphology of these defects. The aim of work described here is to assess the capabilities of various NDT techniques which can be applied to EB welds in heavy section steel.

EB weld defect specimens

To assess the capabilities of the NDT techniques available, a series of six thick section (125-150mm) steel EB weld specimens was prepared, each EB weld containing deliberately introduced defects of various types. These included gross porosity or voids, fine porosity, fade-out region defects, missed joints and solidification cracks. The specimens were approximately 600mm long by 900mm wide, an example is shown in Fig.1.

Specimens were generally fabricated by the following route:

1. An EB weld containing deliberate defects was made in thick section material.
2. A slice of 25-35mm thickness containing the weld was removed from the original sample.
3. This slice was radiographed in the transverse direction to verify the presence of intended defects and obtain more accurate details about them. A radiograph facsimile obtained from specimen W15 is shown in Fig.2
4. The slice containing the weld plus defects was then welded between two steel plates by EB welding using optimum parameters. The extension welds being defect free (verified by 0° ultrasonic scanning), the resulting specimen is equivalent to a single defective butt weld between two large steel plates. A photomacrograph of a transverse section from a trial weld containing fine porosity is shown in Fig.3

NDT techniques assessed

High energy X-radiography

Radiography is perhaps the most common inspection method for EB welds in material thickness up to about 75mm where there is access to both sides of the weld and where a full volumetric inspection is required. Above this thickness,with conventional equipment, exposure times become very long and definition and sensitivity to small defects are reduced.

To obtain reasonable exposure times for 125mm welds a high energy radiographic facility is required, such as the Van de Graaf X-ray generator used for this work. This equipment provided 2.4MeV at 200µA yielding exposure times around 140sec. All six EB defect specimens were radiographed through thickness for comparison with the ultrasonic technique results. A facsimile of the radiograph obtained from specimen W15 is shown in Fig.4.

Conventional ultrasonics

Conventional or 'manual' ultrasonics using hand held probes and a flaw detector with A-scan display is often used in place of, or in addition to, radiography for inspection of EB welds in thick section material. With the exceptionof cases where there is access to a surface that is parallel to the weld, the shear wave angle probe technique is most often used, exactly as for arc fusion welds.

Conventional ultrasonic inspection was carried out at TWI on all six EB defect specimens by qualified NDT operators. Access was allowed to top and bottom surfaces but not to end faces parallel to the weld. The results were recordedon plan and side elevation sketches to facilitate easy comparison with other techniques. An example of the side elevation sketch for specimen W15 is shown in Fig.5.

Advanced ultrasonics

A variery of ultrasonic techniques was used to inspect the EB defect specimens with data being collected and displayed by either P-scan or Zipscan computerised ultrasonic inspection systems. The techniques assessed included:

• 0° compression wave scans from a surface parallel to the weld;
• 45, 60 and 70° shear wave angle beam scans; -Tandem 45° reflection technique;
• Tandem 45° obscuration or 'shadow' technique;
• Time-of-flight diffraction (TOFD) technique.

To enable the two tandem 45° probe techniques to be carried out with data collection, a tandem probe scanning jig was specially designed and constructed. This device is shown in Fig.6, and is also applicable for use with conventional ultrasonic flaw detectors. A feature of this jig is that the probes are uniquely configured to examine the weld plane alone. This is ideal for narrow EB welds, and may also be of benefit with narrow welds made by other techniques.

In all cases a hard copy print-out of the inspection results was obtained enabling the different techniques to be reliably compared.

An example of a P-scan image obtained for W15 using the tandem 45° reflection technique is shown in Fig.7. Note that in this image the Top view represents the side elevation of the EB weld and the Side view represents the plan elevation. The Echo view is a histogram of the maximum amplitude seen at each position along the weld length direction.

Results and discussion

The high power (2.4MeV) X-radiography results show that, in this thickness of steel (125-150mm), small EB weld defects such as fine porosity and fade-out region porosity are not always detected. However, larger defects, for example,voids and favourably orientated planar defects such as centre line solidification cracks are much more readily detected, but information about their nature, through wall extent and position is not obtained.

The conventional ultrasonic results again showed that larger voids and cracking were generally well detected, although some smaller voids were missed. There were some interpretation difficulties; for example, the columnar nature of gross voids caused by loss of molten metal from the weld root in one specimen was not clearly identified. Small defects such as fine porosity were generally not detected by conventional ultrasonics and as a consequence of this there were very few false indications.

The advanced ultrasonic techniques showed that the choice of evaluation or display level is extremely important to correct interpretation of results. Reference test sensitivity (OdB) for the particular techniques was determined using the following calibration reflectors:

1. 0° scans - the amplitude of a 2mm flat bottomed hole at the appropriate range;
2. Shear wave angle beam scans - a distance amplitude correction (DAC) curve obtained using 3mm side drilled holes;
3. Tandem 45° reflection technique - the amplitude of a 3mm side drilled hole at the appropriate range;
4. Tandem 45° shadow technique - the amplitude of the back wall echo;
5. TOFD technique - a surface breaking notch of half-wall height (62.5mm) set to full screen height.

Initially, for all except the TOFD technique, a commonly used evaluation level of reference test sensitivity (OdB) plus 14dB gain added was used to display data. This was not generally found to be optimum. In most cases a less sensitive display level was recorded. The average optimum display levels obtained for each technique, rounded to convenient values, are indicated in the Table. Note that for shear wave angle beam scans, optimum display level sensitivity was found to reduce as the probe angle increases. Also, in general, to detect very small defects such as porosity, a higher level of display sensitivity is required - perhaps 6-12dB more sensitive than in the Table. However, a consequence of this is that the size of large defects such as voids appears grossly overestimated.

Optimum evaluation/display sensitivity levels* for various ultrasonic techniques for inspection of EB welds

* With respect to the methods of reference sensitivity calibration in this investigation.

The 0° compression wave scans, not surprisingly, gave the most consistent results. However, this technique, which uses scanning from a surface parallel to the weld, is not often possible. The 45, 60 and 70° shear wave angle beam scans, which are most commonly used in practice, generally gave poor results. It was noted that the larger angles, particularly 70°, gave the poorest results. This is somewhat surprising, since it might be expected that using a probe angle closer to normal incidence with the joint interface would yield better results. Presumably the effect of long ranges and beam spread has an overridingly detrimental effect.

The tandem 45° reflection technique using the special scanning jig gave good results and is considered to be a good alternative to a scan where access for the latter is not possible. The tandem 45° obscuration technique only occasionally gave reasonable results and was difficult to interpret because of amplitude variations in the back wall echo which should, theoretically, stay constant in the absence of defects.

The TOFD results showed that this technique can be sensitive to small defects such as porosity and can provide useful location and sizing information, but interpretation of defect type and significance is extremely difficult since indications are only obtained from the edges of defects.

There is difficulty in assessing the performance of NDT techniques because at present allowable defect types and sizes embodied within acceptance criteria in current standards have been developed for arc fusion welds and are therefore not entirely appropriate for EB welds. What is needed is an alternative set of acceptance criteria based on quality control specifically for EB welds.

Conclusions

Preliminary conclusions and recommendation from this work on NDT of thick section EB welds are:

• High power radiography is only sensitive to more significant volumetric defects and favourably orientated planar defects. Additionally the through wall extent, position and nature of these defects are not always apparent.
• Where access is possible to a surface parallel to the weld, the 0° compression wave technique is the optimum ultrasonic technique to use.
• Where access is only possible from beam entry and/or beam exit surfaces, the tandem 45° reflection technique is the optimum ultrasonic technique provided that a suitable probe scanning jig is available.
• If there is no alternative to use of shear wave angle beam probes, use of large angle probes, such as 70°, should be avoided and great care should be taken in choosing appropriate test sensitivity and evaluation/display levels to avoid false indications and overestimating the size of the defects.
• It is recommended to be extremely diligent with use of marked datum points and agreed co-ordinate systems since for EB welds it is very easy for an unfamiliar NDT operator to confuse beam entry (weld cap) and beam exit (weld root) surfaces as well as the scan direction.
• Further work is necessary to quantify the capabilities of the various NDT techniques to size and characterise EB weld defects. In line with this, defect acceptance criteria appropriate for quality control of EB welds should be developed.

It is intended to include a more detailed account of this work in a report to Industrial Members on completion of the programme.