·Table of Contents
Lack of fusion in welded jointsGabriel RIHAR
Welding Institute, Ptujska 19, SI-1000 Ljubljana
Our studies were to provide as much data as possible on lack of fusion. It was investigated with which welding processes lack of fusion is most frequent, what its position in a welded joint is and what its physical properties are. It was also studied what kind of indications the lack of fusion gives with different non-destructive methods.
Testing was carried out with typical welded joints. Metallographic and mechanical testing was also used in order to make a comparison with non-destructive testing. Various types of lack of fusion were classified into characteristic groups, i.e. the types including voids and non-metallic inclusions and the types of pure lack of fusion which cannot be detected by non-destructive testing.
Key words: welding, NDT, weld defects, lack of fusion.
It is known from the welding practice that the lack-of-fusion defects often occur in a welded joint. Many a failure of a welded structure thus occurs due to undetected lack of fusion in welded joints.
Lack of fusion is one of the most serious weld defects. It produces the notch effect. There are, however, no efficient non-destructive methods available for its detection.
If lack-of-fusion defects and cracks are compared, it is evident that much more attention is paid to cracks than to lack of fusion although lack of fusion is as serious a defect as a crack. In the welding literature numerous papers deal with cracks but very few with lack of fusion. Only some older documents of the International Institute of Welding were found to report on lack of fusion [1-4]. It was, therefore, decided to study lack of fusion more closely. It was to be found out where it occurs most frequently, under which conditions it forms and how it can be detected.
With regard to the position of the lack-of-fusion defects in a weld, three types of lack of fusion are distinguished :
As to the appearance of the fracture face, one distinguishes the lack of fusion due to unmelted oxide inclusions and the lack of fusion due to melted oxide inclusions. The lack-of-fusion defects due to unmelted oxide inclusions consist of oxides and non-metallic inclusions. Lack of fusion, of which three types, i.e. IIW references 4011, 4012, and 4013, are distinguished in a standard should not be mixed up with lack of penetration, i.e. IIW reference 402 . The defects located at the surface are efficiently detected by a visual inspection. Lack of penetration inside the weld, however, can be detected by X-ray or ultrasonic inspection methods.
As to the possibility of detecting, different types of lack of fusion can be classified into two groups, i.e., the one in which lack of fusion includes voids or non-metallic inclusions which can be detected by non-destructive methods, and the one in which the lack of fusion shows no discontinuity in the material since it is a structural defect and thus cannot be detected by non-destructive methods.
The pure lack of fusion is a structural defect. In this case the molten metal sticks to the parent metal which has not melted enough during welding. A joint between the solid phase and the liquid one forms. It is like a brazed joint. This type of lack of fusion cannot be detected by non-destructive testing methods but with a microscopic inspection. A straight fusion line indicates that there may be the lack of fusion between the parent metal and the weld. The inter-run lack of fusion is even more hidden. It can be detected only by an accurate microscopic inspection with a 50-times magnification. An example of the pure lack of fusion is shown in Fig. 1.
|Fig 1: Pure lack of fusion between the final run and the parent metal. a) macrograph: x3.5; micrograph: x100.|
Because of internal stresses produced during weld solidification and cooling, the faces sticking to each other will separate. A void having a width of only some hundredths of a millimeter forms. This gap in the weld is very muck like a crack. It can, however, be detected by non-destructive testing methods. Such a type of lack of fusion is difficult to distinguish from a crack. An example of the open lack of fusion is shown in Fig. 2.
|Fig 2: Open lack of fusion between the central and final runs. a) macrograph, x3.5; b) micrograph: x100.|
Where the lack of fusion is there are very often also oxides and non-metalic inclusions. Such a case is shown in Fig. 3. If the oxide layer does not melt, the inclusions are uniformly distributed across the entire surface of the lack-of-fusion defect. If they melt, however, the non-metallic inclusions become spherical.
|Fig 3: Inclusions at the faces sticking together. a) The macrograph shows lack of fusion between the central and final runs; b) The micrograph shows inclusions at the faces sticked together.|
The lack of fusion is usually to be found at the weld inside. It rarely reaches the final runs or the root run. Location of typical types of lack of fusion are shown in Fig. 4.
|Fig 4: Lack of fusion in a weld: lack of side-wall fusion (above), lack of inter-run fusion (below).|
Account should be taken that the lack of fusion may appear in gas-shielded arc welding with a consumable electrode and gas welding. There is a great chance of the appearance of lack of fusion in downhill welding.
It is very useful to obtain data on the weld structure and welding conditions prior to welding. Information should be sought whether any mechanical and metallographic examinations of the welded joints to be examined have been performed.
For detection of the lack of fusion, the ultrasonic examination is the most suitable of all the available non-destructive testing methods. All types of lack of fusion, with the exception of the pure lack of fusion, can be detected. Our investigations showed that considerably large lack-of-fusion defects give weak ultrasonic indications which are in practice considered minor acceptable defects. A considerably poor reflection of ultrasonic waves is obtained due to:
|Fig 5: Ultrasonic examination of the lack of fusion between the parent metal and the weld metal. a) examination from all four scanning directions; b) examination from the cover layer.|
Because of the above, weak, repeating indications should also be considered when inspecting welds in which the lack of fusion is suspected. The ultrasonic waves should be directed, if possible, perpendicular to the sticking faces. It can be supposed that the lack of fusion may appear at the edge of the parent metal. In the case when the access of ultrasonic waves is possible from all four sides, the direct path as shown in Fig. 5a is used. The examination can be accomplished only from the cover layer. In this case the single bounce technique is used to examine the upper part of the weld (Fig. 5b).
The penetrant testing methods are used primarily when tightness of welded joints is to be ensured, e.g. in testing of tanks. They permit detection of the lack of fusion reaching the surface.
All indications at the edge of runs indicate a possibility of the lack of fusion. Special attention should be paid to fillet welds. The lack-of-fusion defect existing in a weld may open when loaded and produce leaking of a tank.
The lack of fusion is a typically welder's fault. By monitoring the welding process an experienced supervisor can find the formation of lack of fusion. Attention should be paid to the position of the weld pool which must not run ahead of the arc and fill the weld groove. Attention should be paid whether the welder sufficiently melts the joint edges. The lack of fusion can be successfully detected by the visual inspection during welding. The visual inspection after welding is not efficient.
|© AIPnD , created by NDT.net|||Home| |Top||